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The synchronism of crustal thickening and low-pressure facies metamorphism in the Mount Isa Inlier, Australia 2. Fast convective thinning of mantle lithosphere during crustal thickening
Tectonophysics,
165 (1989) 191-218
Elsevier Science Publishers
191
B.V., Amsterdam
- Printed
in The Netherlands
The synchronism of crustal thickening and low-pressure facies metamorphism in the Mount Isa Inlier, Australia 2. Fast convective thinning of mantle lithosphere during crustal thickening RAMON Department
of Geology, Australian
(Received
February
J.H. LOOSVELD National
23,1988;
University,
revised version
*
Canberra, A.C. T. 2601 (Australia)
accepted
December
15,1988)
Abstract Loosveld,
R.J.H.,
1989. The synchronism
Inlier, Australia.
2. Fast convective
of crustal thinning
thickening
of mantle
and low-pressure
lithosphere
facies metamorphism
during’crustal
thickening.
in the Mount
Isa
Tectonophysics,
165:
191-218. The Mount
Isa Inlier is, like some other northern
by an anti-clockwise thickening study,
(
+AP,
two possible
quantitatively immediately delamination,
P-T-t
path:
i.e. regional,
+ AT), and were overprinted models
evaluated before
for the synchronism
using
latter results in prograde
low-P
event,
thickening
due
Early to Middle
low-P
of essentially
of regional,
low-P
to e.g. lithospheric
facies metamorphism
by (convective) during
crustal
isobaric
fold belts, characterized
assemblages cooling
facies metamorphism
finite-difference
accompanied
Proterozoic
facies metamorphic
by a phase
a one-dimensional,
the thickening
and (2) crustal
Australian
prograde
and crustal
technique:
(1) extreme
extension,
magmatic
thinning
thickening.
grew during
crustal
(AP - 0, -AT).
thickening
elevation activity,
of the mantle
are
of isotherms
or crust-mantle
lithosphere.
It is the preferred
In this
Only
the
model for the Mount
Isa Inlier.
Introduction
developed
In the companion paper (Loosveld, Part I, 1989), the simultaneous development of low-P facies metamorphism ratios, characterized stability
field,
(i.e. low metamorphic P/T by the andalusite-sillimanite
Miyashiro,
1973) and
a phase
crustal thickening in the central Soldiers Cap Group belt, eastern Mount Isa Inlier (Fig. l), Australia, was demonstrated. A direct effect of crustal thickening alone, however, must be the general increase of P/T ratios (resulting in many erogenic situations in high-P facies phism). If steep metamorphic gradients
* Present
address:
Koninklijke/Shell
Box 60, 2280 AB Rijswijk, 0040-1951/89/$03.50
Explor.
metamorhave only
Prod. Lab., P.O.
The Netherlands.
0 1989 Elsevier Science Publishers
B.V
structural
positions,
like those
high in a nappe sheet, the anomalously high temperatures can be explained by fast erosion (decompression) of the thickened crust and/or heating induced by the overlying nappe Moderately
of
in certain
high metamorphic
are not restricted
to these
gradients, structural
by the sheets. however,
positions
in
the Mount Isa Inlier (Wilson, 1973; Hill et al., 1975; Derrick et al., 1977; Derrick, 1980; Hamilton, 1985; Oliver and Wall, 1987; Reinhardt and Hamilton, in prep; Oliver et al., in prep.). They prevail throughout the Soldiers Cap Group. A remaining problem is to find a heat source which could explain the overall steep metamorphic gradients during the thickening of the crust. A wide variety of fold belts has undergone low-P facies metamorphism, similar to that of the Mount Isa Inlier. Examples can be found in the
192
The deeper
in the crust,
phic conditions tive
is the heating.
concluded 20”
the later peak metamor-
are attained,
that Archaean
mes were similar
and the more effec-
England
and
continental
Bickle
(1984)
thermal
regi-
thermal
regi-
to those at present.
The hypothesis
that Precambrian
mes are similar to those at present does not, however, comprehensively address the problem of the lack of relict high-P Precambrian. eclogites,
glaucophane
kyanite-bearing Lepontine 40”
: Fig. 1. Distribution to Middle Inliers
are
Murphy;
of Australian
Proterozoic numbered:
4-Tennant
-Halls
Creek;
Leopold
Mobile
Musgrave;
Archaean
fold belts (after 1 -Coen; Creek;
Belt;
I4--Crawler;
II -Paterson; I5-Mount
and Early
et al., 1987).
Z-Georgetown;
S-Arnhem;
8 -Granites-Tans;
cratons
Etheridge 6-Pine
3Creek;
9 -Arunta; IZ-Capricorn; Painter;
7
10 -King I_?--
16-Willyama;
I7-Albany-Fraser.
phism,
+
jadeite
rocks, although moderate-i’/
in the
in the Western schists,
overprinted
moderate-P
are in places still recognizable
Alps, and by the
metamor(Frey et al.,
1974). P-T-t paths are clockwise in the P-T field (Fig. 2, trajectory “A”. Note: Abbreviations are defined
in Appendix
paths are generated els, which simulate
I ). Such clockwise
P-T-t
in virtually all numerical modcrustal or lithosphe~c thicken-
ing (e.g., England and Thompson, 1984; Davy and Gillet, 1986; Thompson and Ridley, 1987). With certain favourable parameterisations in these models,
Hercyno-type erogenic belts of Zwart (1967), but more commonly in the Archaean (granite-green-
facies assemblages
For example,
geotherms
will transect
the low-P
facies
(andalusite-sillima~te) field for some period of time. They can only do so, however, during postthickening decompression ( - A P, 4 AT), unless a combination of extreme parameters is assumed, e.g. an initial geotherm, which already transects the andalusite-sillimanite field, very low strain
stone) terranes (Grambling, 1981; Green, 1981) and Proterozoic fold belts (Grambling, 1981; Lambert, 1983). This secular trend seems to reflect the overall cooling of the Earth through geological
rates, low conductivities, and a high radioactive heat generation. If tectonic and thermal regimes of
time. England and Richardson (1977) and England and Bickle (1984) however, pointed out that
the Precambrian the Phanerozoic.
peak
of the Western Alps, the Precambrian trajectories should resemble their Alpine
metamorphic
conditions
reflect
points
transient and polychronous rnet~o~~c than on steady-state geothermal gradients.
on
rather Thus,
the apparent time-dependence of peak-metamorphic conditions should be attributed to erosionrate-dependent thermal relaxation of a tectonically thickened crust: radioactive selfheating in a slowly eroding thickened crust may lead to overprinting of early high-P facies assemblages by moderate-to low-P facies assemblages (e.g. in the European Alps, the overprinting of the Eo-Alpine high-P facies metamorphic event by the Lepontine moderate-P, moderate-T event). Depending on reaction kinetics, even (near) total disappearance of the early high-P facies assemblages may occur.
fold belts were similar to those of or even more specifically, to those P-T-f counter-
parts and also be clockwise. In the northern Australian Early to Middle Proterozoic inliers (Fig. l), however, the low-P facies metamorphism is prograde and “compressive” ( + A P, + AT), and succeeded by a phase of cooling. Cooling was isobaric to slightly decompressive, as in the Mary Kathleen Fold Belt in the Mount Isa Inlier (Reinhardt and Hamilton, in prep.), the Arunta Inlier (Warren, 19X3), and the Broken Hill Block (Philips and Wall, 1981; Hobbs et al., 1984) or compressive at first, as in the Olary Block/Willyama Province (Clarke et al., 1987). The resulting P-T-t paths are anti-clock-
193
tic constraints, elled
the same
numerically,
finite-difference
code (details
2, 3 and 4). The thermal
there
are
magmatism,
low-P
thickening:
therms
immediately
600
Fig.
2. P-T
diagram
showing
geotherms,
transecting
the
radioactive
heat production
three
kyanite
before
All
(A) dist~bution
100 km;
of 3 W K-t
D,,,V, i =lO
m-‘.
Curve
between
18 and 22 km depth
tion
3 additionally
of 4.1 pW/m3.
stability “A”,
and retrograde tories
“B”
which
characterize
“Alpine-type”
and
“c”
are
Reinhardt
the
reaction
(retrograde)
history
Proterozoic
is common
history.
Trajec-
P-T-Z
paths,
of the northern
Clarke
et al., 1987):
+ sillimanite
(prograde)
also elsewhere
(e.g., Gemuts,
1971, for the Halls Creek Mobile Belt). The cooling these anti-clockwise
P-T-t
sive, isobaric
wise (Fig. 2). Similar
repre-
fold betts (respectively
in prep.;
paths
can be slightly
+
segment
of
decompres-
or slightly compressive.
anti-clockwise
the thickening
extension,
event,
magmatic
delamination,
ac-
and
(2)
discarded.
is reliable, The
the former
results
may
to Mid-Proterozoic
mechanism
can be
be applicable
to all
fold belts
Australia, and to prograde, terranes in general.
low-P
of northern metamorphic
Tectonic m~hanisms metamo~hism
that lead to low-P facies
trajectory
metamorphism
metamorphic
andalusite
respective
P-T-t
facies
the anti-clockwise
and Hamilton, series
and
of iso-
granite
heat produc-
metamorphism,
the metamorphic
Early to Middle
after
kyanite
(1971). high-P
moderate-T/moderate-P
sents a clockwise,
Australian
Holdaway
of km;
Ao,curvr 3 = 2
a 4 km thick
tripiepoint
by prograde
s =15
elevation
thickening accompanied by convective of the mantle lithosphere. These two
database Early
length
km, D,,,,,
with a radioactive
The AlzSiG,
fields are after
characterized
reflects
a
by A =
heat distribution),
Ao,curve z = 2 pW/m3,
pW/m3.
have
defined
and a lithospheric
km, D,,,.,,, a =15
A O.cuwe 1 = 2.5 pW/m”.
conductive three
crust-mantle
may during
models are compared with the existing geological database of the Mount Isa Inlier. Assuming this
1°C)
steady-state
Aae- x/D ( D = the length scale for radiogenic a conductivity
crustal thinning
800
field.
or
which
facies metamo~hism
due to, e.g., lithosphetic tivity,
In general,
mechanisms
(1) extreme
de-
as well as some
are simulated.
possible
crustal
effects of lithocrust-mantle
thickening,
thereof, two
yield regional,
Temperature
of which are in Ap-
thinning,
and crustal
will be mod-
one-dimensional,
spheric
combinations
.----+
a
pendices tachment
400
scenarios
using
P-T-t
To explain the anomalously low P/T ratios and the subsequent isobaric cooling, one first needs to investigate three obviously possible mechanisms (Fig. 3B, C, D): (l)syn- to slightly pre-metamorphic lithospheric extension (McKenzie, 1978: Wickham
and
Oxburgh,
magmatic
events
(Wells,
1985, 19X7), (2) various 1980; Bohlen,
(3) crust-mantle detachment welling of hot asthenospheric
1987), and
coupled with the upmaterial to the base
of the crust (Bird, 1978, 1979; Bird and Baumgardner. 1981; Houseman et al., 1981; Lister et al., in press). As the prograde metamorphism is conpaths
were suggested by Bohlen (1987) for Precambrian granulite terranes as, e.g., those in the Adirondacks (N.Y., U.S.A.), Bamble (Norway), Namaqualand (South Africa), Southern India and West Uusimaa (Finland). In this paper, I will first explore, in a qualitative way, the main tectono-thermal scenarios to which low-P facies metamorphism has been attributed. Then, in order to obtain physically realis-
temporaneous it is necessary
with a phase of crustal thickening, to discuss as a fourth possibility the
thermal effects of crustal thickening accompanied by an anomalously high basal heat flow. Such high basal heat flow may be caused by convective thinning of the mantle lithosphere (Fig. 3E. F), as advocated by Houseman et al. (1981). A mechanism not pursued here, is the late-stage extension of a (thermally partially equilibrated) thickened crust (England, 1982; Houseman and
194
t1 PRF,-M?STAMORPHIC
to
1250
0 0
A
F
Temperature
Fig. 3. The various correspond
tectonic
respectively
pre-metamorphic conductive
(f,),
geotherm
models
E and within
similar
to that of an extremely a thinned
in the upper replaced
crust
series
F are syn-metamorphic
C. The geotherm
and within
experiment
an undeformed
lithosphere.
thrusting
tested in this paper (B-F),
to numerical
mantle granite,
(“C)
within
thinned mantle
lithosphere
lithosphere.
and homogeneous
by hot asthenospheric
material
lb,
(tz).
lc,
lithosphere.
M-Moho; crust,
2b (also
that,
which
(the Houseman
within
an abruptly
homogeneous.
Models
1). The situations
A-asthenosphere.
within
is underlain
within
or entirely
Table
L-lithosphere;
D. The geotherm
E. The geotherm below
2a and
at t = 0. B. The geotherm
an undisturbed
mantle
which all add heat to the upper lithosphere.
la,
an undisturbed which
thickened
material.
is intruded crust,
The complete
D are
A. The steady-state
crust and abruptly
by asthenospheric a crust
B, C, D, E and F B, C and
is
by a “recumbent”
with either
mantle
thinned
This situation thickening
lithosphere
et al., 1981, model). F. As E, but with a mid-crustal
granite.
by
has been
195
England,
1986; Platt,
fold belts of northern metamorphism (Hobbs
1986)
Australia
is coeval
+
as in the Proterozoic the low-P
with
crustal
et al., 1984; Etheridge
C
B
Ten
D
F
E
facies
shortening
et al., 1987; Loos-
veld, 1989). Metamorphism
due to lithospheric
Steep metamorphic areas,
are attributed
ing (McKenzie, Le Pichon
gradients, by many
1978; Le Pichon
et al., 1982;
1985, 1987). Uniform
extension which affect large
to continental
rift-
and Sibuet,
1981;
Wickham
stretching
and
Oxburgh,
of the lithosphere
extension) can lead to astheno(“ pure-shear” spheric diapirism (or vice versa), and hence to partial melting of the lower crust, the emplacement of granodiorites in the middle crust and to
L: Lithosphere A: Asthenosphen crust-mantle kunday a1 35 km stippled area npresents anam of heat added to lithosphere and topmost asthenosphen Fig.
4.
Various
mantle geotherm
and the geotherm
line) represents and topmost
peratures
geotherm
of modern
rift zones (Bridwell
Heat
flow studies
and Potzick,
1981;
Lachenbruch, 1979; Lachenbruch and Sass, 1977; Mohr, 1982; Morgan, 1982) agree well with this model. In contrast to the classical model of symmetrical and uniform stretching of the lithosphere (the pure-shear model), the recognition of major asymmetrical detachment fault/ shear zones in the Basin
and
Range
province
(Davis
et al., 1980;
results
is linear.
neous
shear
Kenzie-model. pure
shear
pure
thinning
A-asthenosphere.
mantle
lithosphere
horizon,
with
of the mantle
the proposal of asymmetric continental extension models, the “ simple shear” models (Wernicke, 1985; Lister et al., 1986, in press). These explain
dipping,
in the mid-crust Complete delamination
excision
(Voorhoeve
and replacement
asthenospheric
material
(Lister
and
et al.-
instantaneous et al.of the
here
dipping,
at the base 1988).
horizon.
detachment of the crust
E. Instantaneous
lithosphere
by means
with the detachment
and Houseman-model,
of the mantle
(Mc-
(Lister
of the mantle
detachment
lithosphere
and instantaneous
lithosphere
Houseman-model,
simple shear thinning
in
and instanta-
simple shear thinning
of a planar,
the detachment
The
are given
lithosphere
of the mantle by means
and
A-F
of the complete
C. 300% homogeneous
thinning
within
conductive
A. 100% homogeneous
thinning
(hold
to the lithosphere
heat production
of modes
model, in press). D. Instantaneous
planar,
cay process with a time constant of approximately 60 Ma (McKenzie, 1978; Jarvis and McKenzie, 1980; Voorhoeve and Houseman, 1986) if heat is transferred by one-dimensional conduction only. Relaxation is faster if advection of fluids and/or lateral heat flow play a role. England and Thomp-
Radioactive
1978). B. 100% homogeneous
in press). shear
the steady-state
after extension
of heat added
modelling
Fig. 6A-F.
model,
are shown
area between
L-lithosphere;
respectively pure
The crust-
is left out here, so the steady-state
of the thermal
(Voorhoeve
Both models of extension, however, have their limitations. Thermal relaxation of an instantaneously thinned lithosphere is an exponential de-
asthenosphere.
extension. boundaries
immediately
the amount
the lithosphere
Wernicke, 1981; Wernicke and Burchfiel, 1982; Lister et ai., 1986; Lister and Davis, 1989) led to
the geographic offset between areas of active (supra-)crustal extension and areas affected by the highest geothermal gradients, thus providing an explanation for the absence of extensional structures in rocks characterized by low-P facies assemblages.
of instantaneous
for all modes. The cross-hatched
the generation of a condensed series of isograds. McKenzie (1978) quantified the relaxation of temafter such extension.
modes
and lithosphere-asthenosphere
lithosphere,
of the mantle
of a here
1988). F.
i.e. crust-mantle lithosphere
(Lister et al.-model.
by hot
in prep.).
son (1984) therefore, concluded that the thermal profiles of passive continental margins, compressed 60 Ma after their formation, would not differ noticeably from those resulting from compression of steady-state geotherms. For extension by means of a simple detachment zone, the lithospheric heat input is approximately a factor two smaller than for the pure-shear McKenzie model (Fig. 4 and Voorhoeve and Houseman, 1988); thus, the thermal perturbation is also smaller. In the Mount Isa Inlier, this poses a formidable problem, because, although the geochronological dates on the three extensional events and on the
196
prograde
metamorphic
no extensional the
event are widely bracketed,
structures
> 100 Ma
metamorphism.
time
have been recognized
interval
A direct
prior
for
to prograde
link between
the exten-
sional events and the prograde low-P facies metamorphism is therefore questionable and one might be tempted
to not
here. In general, extension
this line
might
character
of research
if the time gap between
and metamorphism
Ma, extension facies
pursue
however,
is narrower
not only explain
of the metamorphism
not necessarily
andalusite-sillimanite
also subsequent
isobaric
than 60
but
crustal rift can be affected by (1) lateral heat flow, aided by the generation of small-scale convection cells in the asthenosphere under a rift margin due to the steep horizontal temperature gradients (Buck, 1986) and (2) the thinning of the mantle lithosphere under the undisturbed crust, either by pure shear as in the asymmetric extension model of Lister et al. (1986, in press), or by simple shear as in the asymmetric extensional models of Wernicke (1985) and Voorhoeve and Houseman (1988). Most asymmetric extension models predict a horizontal offset between that part of the lithosphere, which is most attenuated, and that part of the upper crust, in which rift structures are developed: anomalously steep geothermal gradients, due develop
crust),
will
upper plate”
crust (the hanging wall, i.e. the “upper of Lister et al., 1986). This upper plate
situation is simulated heat flow complication. Heating
in a relatively
( * lower
here,
omitting
undisturbed
the lateral
by intrusion of magmas
Crustal thickening by magmatic accretion will result in very high relative temperatures in all levels of the crust (Wells, 1980) and, temporarily, very steep geothermal gradients above the intrusion (100-500 o C/kbar, Bohlen, 1987). Rocks above the intruded material will heat isobarically, and subsequently, during the thermal relaxation,
differentiated
granites
and cooling,
continue
role in the thermal
because
they
heat-producing anomalously
elements high heat
metamorphic
granites
may
be
balance
enriched
in
(HPE). In this study, the production in the pre-
of the Mount
Isa Inlier
will
be calculated. Wyborn entiation.
oped locally. The thermal evolution of tectonically undisturbed crustal areas adjacent to a late supra-
lithosphere
area,
(although facies),
Highly
to play an important of an
the Mount
cooling.
of the mantle
isobarically.
can, even after emplacement
the low-P
A possible scenario could be that late-tectonic, and as yet unrecognized, asymmetric rifts devel-
to the thinning
cool
et al. (1988) argue that the granites Isa Inlier result from a two-step As magmatic
equilibration
of a highly
events
represent
perturbed
of
differthe re-
thermal
(or
pressure) gradient, these granites must be the result of two successive thermal perturbations. Discussing the first thermal perturbation, Wyborn et al. (1988) mention the possibility of both extensional and compressional events, which can lead to mantle melting, and, mainly because primary magmas from the mantle are denser at crustal depths than crustal material (Herzberg et al., 1983) to underplating at the base of the crust. These underplates the major, Inlier. Crust-mantle
could I-type
subsequently be the source of batholiths of the Mount Isa
detachment
In most cases of underplating, exchange of cold, dense lithosphere by hot, less dense asthenosphere, plays a role (Wyborn et al., 1988). Such substitution can be explained in four ways: (1) by conductive thinning of the mantle lithosphere, due to a heat flow perturbation (thermal “plume”); (2) by “delamination”, i.e. the vertical separation of crust and upper mantle by the bending of a coherent slab of dense mantle lithosphere, whose tip sinks into the less dense and hotter asthenosphere (Bird, 1978, 1979; Bird and Baumgardner, 1981); (3) by convective thinning of the mantle lithosphere during crustal thickening (Houseman et al., 1981); (4) by juxtaposition of hot asthenospheric material in the footwall of a major throughgoing (extensional) detachment zone, and cooler lithospheric or crustal material in its hanging wall. Crust-mantle delamination (2) and convective thinning (3) are driven by the gravitationally unstable layering of relatively dense mantle lithosphere over less dense asthenosphere. Independent
197
of the mode
of substitution
of the mantle
sphere by the asthenosphere, anomalously Etheridge
high
crust-mantle
Proterozoic Inlier, event
Orogeny
fold belts of northern
however,
differs than
from
the
tion has been simulated sional and compressional
that extenexplains to Middle The
Mount
1860
Ma
Isa old
Of the six factors Mount
Isa Inlier.
lead
AT = 0). The subsequent erosion.
Possible
substitu-
thermal
relaxation
extenfacies
conditions and subsequent isobaric cooling, they cannot explain the synchroneity of the prograde low-P facies metamorphism and a pervasive phase of crustal thickening (+ A P, + AT). One is forced to re-examine the effects of thickening on the profile. Brady (1982) heat transfer in an oro-
genie setting, and summarized the factors leading to lower than normal P/T ratios, and possibly to in such an environment:
(1) synmetamorphic regional (2) a very strongly insulating (3) transport of hot rocks to tectonic movements or by rapid
post-tectonic
(4) a mantle heat flux triple the normal (5) a very thick radioactive crust;
value;
(6) a volatile flux equivalent to one rock volume of fluid per 5000 yrs. Additionally, Reitan (1968a, 1968b, 1969) Graham and England (1976) Molnar et al. (1983) and Werner (1985) considered frictional heating in tectonically active areas, such as subduction and overthrust terranes. Frictional heat can condense isotherms locally (and even lead to locally inverted metamorphic geotherms), but cannot lead to a widespread, significant temperature increase. Furthermore, there are no indications that frictional heat played a more prominent role in the
metamorphism. compression,
heating-up
by isostatic melting
Ineither
more heating
uplift
(+ A P,
phase will be and
consequent
in the lower crust by this
is therefore
(Miyashiro,
normally
1973;
Richardson, 1977). Clockwise P-T-t trajectories result. to factor
late-
England
to and
(Alpino-type)
2 (the unusually
insulat-
ing sediment pile), Jaupart and Provost (1985) considered conduction of heat across the Main Central Thrust of the Himalayas. They concluded that the location of young leucogranites at the top of the basement sheet can be explained by the heterogeneous vertical thermal conductivity distribution, i.e. a low conductivity in the upper sediments versus a high conductivity in the crystalline basement. There is no evidence, ever, that a similarly lain the Proterozoic during
insulating sequence inliers of northern
lower how-
has overAustralia
the metamorphism.
Factor
plutonism; sediment pile; shallow levels by erosion;
facies
or with slightly
metamor-
there
1, 2, 3 and 4 in the out in the introduc-
stead, it will lead to progressive adiabatically,
in orogens
paths.
as such will not generally
low-P
With respect
evolving temperature-depth modelled one-dimensional
As pointed
facies
with some of the extenmodels.
for factors
thickening
to coeval
than
P-T-t
listed by Brady (1982)
is no direct evidence tion, crustal
terranes
by clockwise
lithosphere
It is argued in this paper that, although sion and magmatism can result in low-P
stability
Proterozoic
accompanied
Crustal thickening
andalusite
Australian characterized
is at least
tectonism
mantle
crust.
Australia.
in the
the low-P
In this paper,
the
in the 1860 Ma
in the Early
in that the extensional
litholeads to
delamination
Ma old metamorphism
100 Ma older phism.
in
facies metamorphism
old Barramundi 1550
temperatures
et al. (1987) have suggested
sion-triggered the low-P
it invariably
3 (rapid
upward
transport)
only plays a
role if the upward transport is faster than thermal relaxation. As mentioned (Loosveld, 1989) folds in the
Mount
Isa
isoclinal
and
upright
Inlier and
are generally the lower
tight
P/T
to
ratios
might be expected in the major antiforms. Even though andalusite-sillimanite blastesis is synchronous with this folding event, such a structurallycontrolled P/T ratio distribution is not documented and thermal relaxation must be faster than deformation. Indeed, for upright folding with reasonable strains and strain rates, this must be generally the case: thermal relaxation of a lateral perturbation of periodicity L,, with L, the halfwavelength of a fold, has a time constant of Lf/?r2~ (with K the thermal diffusivity), so that, e.g., for L = 10 km and K = 10ph m*/s, the thermal time constant is only = 0.3 Ma. Widespread uplift
by erosion
and/or
tectonic
denudation
do
198
not
have
facies
to be considered
metamorphism
either
as the low-P
is synchronous
to crustal
thickening,
and the post-compressional
segment
the P-T-t
path is essentially
(Reinhardt
isobaric
of
by Oliver et al. (1987) and Oliver and Wall (1987). In this
study,
crustal
single-pass
(factor
4)
in rocks
1.6 Ga ago was 20 to 40% higher
it is now
Etheridge
C,“/C,”
1975; Davies, Channelized tion phism,
1980; Turcotte mantle
for local,
than
= 104; McKenzie
and
Weiss,
and Schubert,
1982).
heat flow can be an explana-
prograde
but a constantly
low-P
facies
elevated
basal
experiments
metamorheat
flow
simulating
will, in simplified
fluid
flow and
(the role of convection,
and Hamilton, in prep.). The rate of mantle heat production (assuming
numerical
thickening
below
3-6
form, include
its advection
i.e. multi-pass
of heat fluid flow,
km is still contentious;
et al., 1984, versus Wood
e.g.,
and Walther,
1986). I will also evaluate convergence
a special case of continental
or thickening,
which combines
ening of the crust with (convective) subcrustal
lithosphere.
This
thick-
thinning
of the
case was argued
for
will not affect the sense of rotation of the P-T-f path (clockwise versus anti-clockwise). Disregarding factors 1, 2, 3 and 4 leaves radio-
by Houseman et al. (1981) for a lower lithosphere and asthenosphere with the same constant viscos-
active selfheating
stratified, viscous Newtonian authors reason that as the
and heat transfer
by fluid advec-
tion processes as the most important factors. England and Thompson’s (1984) and Davy and Gillet’s (1986) studies on the thermal balance erogenic zones are in this regard the most
of de-
tailed. England and Thompson’s (1984) experiments quantitatively incorporated radioactive selfheating after an instantaneous crustal/lithospheric thickening event, and qualitatively discussed fluid also omitted
ity, and by Fleitout
dense
and Froidevaux
lithospheric
mantle
(1982) for a
lithosphere. These relatively cool and thickens,
it
is
sub-
merged into the underlying, hot and less dense asthenosphere. Thus, its gravitational instability is increased, overlying
and it may (partially) detach from the crust (and uppermost mantle litho-
advection. Davy and Gillet (1986) fluid flow from the numerical code,
but introduced multiple time-dependent thrusting events. Both models can explain low-P facies decompressive metamorphism (- AP, k AT) by the combination of slow uplift and an increased radiogenie heat supply in the thickened crust. Instantaneous thickening of a crust with a normal geothermal gradient is unlikely, though, to result in prograde andalusite-sillimanite blastesis during crustal thickening. P-T-t paths from the numerical experiments are invariably Mount Isa Inlier, nevertheless, limanite blastesis is prograde
clockwise. In the andalusite and siland compressive
(Reinhardt and Hamilton, in prep.; Loosveld, 1989) and is followed by essentially isobaric cooling (Reinhardt and Hamilton, in prep.): the P-T-t paths are anti-clockwise. Deformation, however, can increase permeabilities and fluid flow can become an additional mechanism of heat transfer (Fyfe et al., 1978; Etheridge et al., 1983, 1984; Ferry, 1984; Bickle and McKenzie, 1987). In the central Mount Isa Inlier. large-scale fluid flow has been documented
1
2
Fig. 5. Five schematic accompanied
by progressive
lithosphere.
overlying
crustal
thickening
lithosphere
hot
continues,
and
more
the downgoing is detached
2)
later
convection
less dense
lithospheric
of
of the
cold and dense
litho-
belt is submerged
into
asthenosphere. material
litho-
At the onset at the base
as the relatively mountain
of the mantle
conductive
As thickening
is swept
sideways
from the crust (stage 4). If convection
the remaining
Houseman
thinning
a meta-stable
thickening
into
plume (stage 3), until the entire (?) lithosphere
material
continuous these
(stage
5
crustal
asthenosphere.
root of the incipient
relatively
spheric
convective
convecting
is enforced
4
of progressive
Stage 1 represents
sphere
spheric
3
stages
continues stages
of crustal
and extremely stages
under
of astheno-
the base of the crust during thickening
(stage
5). then
a
high basal heat flow will accompany
of crustal
thickening.
et al. (1981). Vertical
After
scale greatly
a model exaggerated.
by
199
sphere)
to sink
into
the asthenosphere
(Fig.
5).
Houseman
et al. (1981)
argued
in favour
of the
detachment
of the entire
mantle
lithosphere
from
the crust,
exposing
the lower
spheric
temperatures.
similar
to the crust-mantle
as proposed to be resolved: combination ening grade
advection
selfheating lithosphere
andalusite-sillimanite
tially isobaric
in a thicklead
blastesis?
to proAnd
sec-
conditions is the prograde stage succeeded by essen-
cooling,
all anti-clockwise
have does a
of heat by fluid flow, and
of the mantle
ond, under what andalusite-sillimanite
Bird and
questions
what conditions
of radiogenic
crust,
thinning
first, under
is
process
1979) and
(1981). The following
thus giving rise to an over-
P-T-t
gated.
only
a few of these need
As thermal
the entire
generally
melting
to be investi-
after
thickening
leads
pressional
geotherm,
combination carded Id,
le
sphere
those and/or
and
lf),
of this
here. Also dis-
combinations
with upwelling
magmatism
are envisaged
active
to
to syn-com-
the possibility
asthenosphere since
upwelling
tectonically
and
(No. 2c) is discarded
are
magmatism
of
to late-
in the lower crust, such melt-
ing will have no effect on the early-
setting.
isolated
and
(Nos.
astheno-
to take place in a
Here, only the thermal
effect of asthenosphere upwelling is considered, regardless of the mechanics of replacement of cold lithosphere
by
3-20 (Table
1) combine
mental
path?
relaxation
lithosphere
post-tectonic
to astheno-
this situation
delamination
by Bird (1978,
Baumgardner
crust
Thermally,
straints,
series
hot 1 and
asthenosphere.
Possibilities
the components 2, and
of experi-
are more
complex,
Results of the numerical experiments
involving more assumptions and free parameters. Because of the complexity, and in order to evaluate
In all reported cases of low-P phism in the northern Australian
isolated rather than randomly combined processes, possibilities 3-20 are also left out. This leaves an early extension with or without asthenosphere up-
facies metamorProterozoic fold
belts, the prograde metamorphism is coeval with a period of crustal thickening. Hence, the thermal
welling
and/or
magmatism,
followed
by compres-
to the
sion (Nos. la, lb and lc), and compression combined with asthenosphere upwelling, with or
anomalously high temperatures must have been interacting with the overall stretching of the geo-
without magmatism (Nos. 2a and 2b). These five possibilities are depicted in Fig. 3B-F.
therm caused by crustal thickening. Combining the various pre- to syn-metamorphic modes of crustal heating with syn-metamorphic crustal thickening will therefore be essential. Table 1 lists the possible combinations. Due to geological con-
The first group of numerical experiments deals with two events, an early crustal extension with optional asthenosphere upwelling and magmatism, followed by a phase of compression (Table 1: series 1). The second group is concerned with one
effects
of any
TABLE
1
Possible
combinations
mechanism
that
has
led
of events, which lead to low-P
facies metamorphism
Exp. series
Pre-metamorphic
Syn-metamorphic
Fig.
la:
extension
(pure shear)
(compression)
3B
la:
extension
(simple shear)
(compression)
lb*
ext. + asthenosphere
Icompression)
3c
lC*
ext. + magmatism
(compression0
3D
Id
magmatism
(compression)
upwelling
le
asthenosphere
If
magm. + asthenosphere
upwelling
(compression) upwelling
(compression)
2a*
asthenosphere
upwelling
2b*
_
asthenosphere
upw. + magm. + compression
2c
_
magmatism
3-20 Combinations (explanation
variations marked in text).
by an asterisk
are considered
possible
+ compression
3E 3F
+ compression of 1 and 2 series
for the Australian
Proterozoic
fold belts
and
are modelled
here
200
B rooo horn. thinning of entire lithosphere
100% extension of mantle lithosphere
E B8(@ G
44kmm::
-
6OOf, F t
1okm..
200 y
6km..
0
0
,‘,‘.‘*“‘.“‘.‘. 10
20
o
2km
30
40
so
60
70
+
80
.+
90
.,.,.,.~.,.~.~.~.~*~
10
0
100
20
30
40
50
60
70
+
time[Mal
I
300% extension of mantle lithosphere
80
90
100
timeIMa1
detachment at 40 km
44km:: 36km PEl 22km 18 km:: 14 km lOkIn” 6km..
, zkmt 0
0 .‘.‘.‘.I. 10
20
30
40
50
60
70
+
80
90
"0
100
10
20
30
40
50
60
70
+
time[Mal
80
90
F
I
L
detachment at 20 km
EL200
0 .‘.‘.I. 10
20
30
40
50
60
70
---+
Fig. 6. Temperature-time undisturbed.
7;=, = 1350 o C. Experiment mm’; K = 1.2. 10e6
km with
thinned
A,, = 4.1 pW/m3
parameters: m2/s;
experiments,
I
.
10
to Voorhoeve
pre-thinning
A, = 2.5 pW/m3;
is also reflected
lithospheric
I
.
20
,
.
30
I
I
40
50
lines represent
.
I
I
60
70
and Houseman,
respectively
the 10, 18, and 26 km level, using Holdaway’s
heat
extension
thickness distribution.
F, the crust
crustal
4 km thick granitic of heat
100
remains
has excised
thickness
slab between
in the crust
the temperature
domains
90
the
33 km thick; at the base of the
is 700 km; pre-thinning Transfer
.
is 100 km, the dip of the
1988). In E, the detachment
In F the crust is effectively
I
80
time[Mal
in Fig. 4. In B, C, D and
D = 10 km. A pre-thinning,
in the radiogenic
only. The three horizontal
The depth/time
depicted
overlies the upper mantle.
conduction
of 2700 kg/m3.
0
100
by a factor 2. For D and E, the horizontal
is 12” and the throw is 21 km (comparable
K = 3W K-’
I
+
for the six “extensional”
plots
lower crust, such that the upper crust directly crust,
90
complete excision of mantle lithosphere
time[Ma]
In A, the crust is homogeneously
detachment
80
R
ot
zkm
0
100
time [Ma]
after
is 35 km; 18 and 22 r = 0 is by
levels at which either andalusite or sillimanite becomes stable at and assuming an upper crustal density (1971) Al,SiO, reaction conditions,
in which either andalusite
or sillimanite
is stable are dotted.
201
tectonic
event, i.e. Houseman
consisting coeval
of crustal
thickening
asthenosphere
magmatism
(series 2). In all experiments, conservation
solved
a simple,
finite
difference,
(details
and
of energy
BASIC
in appendices
are typical margins
are unlikely
to be found
the one-
An abrupt
is
iterative,
code on a MacintoshTM
extension
series 1: lithospheric
superposed
mal steady-state
Whether
andalusite/
during
dates
an extensional
variables location
event,
event,
are
which post-
will depend
on slightly
conductive relaxation
on such
as the amount and style of extension, the within the extensional setting, the time
interval between extension and compression, and the ratio of the compressional strain rate to the thermal relaxation rate. Additional heat sources may result from crust-mantle delamination or magmatic intrusion. The abrupt change of the extension is degeotherm by “instantaneous”
upper
The thinning is absent tion
when
by a factor of 2 steeper-than-nor-
geothermal
gradients,
conditions
(cooling),
however,
of the HPE-enriched
upper
crust
simulating
plate
situa-
of asymmetric
the upper
extensional
models,
(3) thinning of the lithosphere by movement along a planar, dipping detachment horizon (Voorhoeve and Houseman, 1988). Loosveld and Etheridge (in prep.) argue that andalusite-sillimanite conditions will generally only result from lithospheric extension (without magmatism and/or delamination) if extension factors equal or exceed 3. Such large extension
a
other hand, the heat input to the lithosphere (represented by the shaded area in Fig. 4) is, at the same amount of crustal extension, smaller in the asymmetric extensional models than in the symmetric
“pure
shear” and
lithospheric
Houseman,
thinning
tions. If thermal relaxation is attributed duction alone, even an “instantaneous”,
(1) homogeneous “pure shear” thinning of the entire lithosphere after McKenzie (1978); (2) homogeneous “pure shear” thinning of the mantle lithosphere while not deforming the crust (the “ upper plate” of Lister et al.‘s models, in press);
where
thinned mantle lithosphere underlies an undisturbed crust (Figs. 4B, C, D), or where the mantle lithosphere is completely excised (Fig. 4F). On the
respectively. The steady-state conductive in the experiments of Fig. 6 is relatively
Extension alone: experiment series la Three modes of-instantaneous-lithospheric extension were simulated:
will of the
crust by a factor 2.
(Voorhoeve
it includes a cooled, 4 km thick, HPE-enriched granitic slab between the 18 and 22 km levels (representing conditions of the Mount Isa Inlier).
(Fig.
thinning
picted in simplified form in Figs. 4A-F. Figures 6A-F give the thermal relaxation on ten depth levels associated with the situations in Figs. 4A-F geotherm “hot”, as
rifts.
plane strain “ pure shear”
be fast, aided by the concomitant
thinning
sillimanite-conditions
a compressional
et al., 1982), but
in intra-continental
result in sillimanite
HPE-enriched stable
parts of passive
of the entire lithosphere
(@ = 2)
will generally
2 and 3).
(Le Pichon
homogeneous,
6A). Thermal Experiment
for the outward
continental
with
equation
one-dimensional,
factors
optional
combined
upwelling
dimensional with
et al.‘s (1981) model,
adding these competing still effectively buffers
1988;
Fig.
models 4). On
effects, the lower crust basal heat flow fluctuato con300%
pure-shear extension of the mantle lithosphere, immediately followed by conductive cooling, will not always lead to sillimanite grades in the middle crust (Fig. 6C). Simple shear experiments (experiment series la,) yield results even further removed
from sillimanite
cept for those situations zone 6E).
happens
to transect
conditions
(Fig. 6D), ex-
in which the detachment the middle
crust
(Fig.
Whether or not andalusite-sillimanite conditions are reached will strongly depend on the steepness of the pre-extension, steady-state geotherm, and on the mode of extension, but, independent of these factors, these conditions will not be stable for more than 10 to 15 Ma. (Figs. 6A, E, and F.) As extension will not be instantaneous, T/P ratios will in fact be lower than those of Fig. 6. In all modes of extension, thermal relaxation is fast; after 50 Ma, temperatures in the middle crust are only slightly higher than the original steadystate ones. Additional heat transfer mechanisms,
202
other than conduction, thermal
would result in even faster
within
the mantle
laxation
relaxation.
paths
lithosphere
Under mantle
+ delamination: an “upper-plate”
lithosphere
pletely
can
experiinent series lb passive margin, the
be
attenuated
or com-
excised (Lister et al., 1986, in press). In the
latter
case, hot asthenospheric
material
the lower crust (Fig. 4F). This situation lated
by making
the pure
shear
is simu-
extension
factor
“freezing”
of the asthenospheric
“hot
mantle
mode”
and
delamination
1981). The “hot
of Bird
mode”
and
“cotd mode” delamination
t
e.skm-. Ybn-. 4skmm-. 39km_. 33 km 27km
i------_ ~21km
Pkm-. 3km-.
0 0
10
20
30
40 +
B
50
time [Ma]
I
first 1 Ma
in the “cold mode”
boundary
is shifted back to lithosphere
im-
delamination and in the crust by conduction only, the
heat transfer
andalusite-sillimanite
field may, depending
on the
steady-state conductive geotherm, conductivity and the thickness of the crust above the detach-
in the experiment depicted in Fig. 6F, in which the mantle lithosphere (including the 36 km depth level) is detached, sillimanite conditions were reached at the 26 km level between 2 and 12 Ma
lmo , Tcmp
here by keep-
ment horizon, be reached at the deeper crustal levels (compare Fig. 6F with 7A, and Bird, 1979);
15 km” -
crust-
Baumgardner,
is simulated
the base of the pre-extension mediately after delamination. Considering “cold mode”
Temp
(respec-
mode”
at the base of the crust con-
the lower temperature
[Cl loo0
material
the “cold
stant for a given time, whereas
1500
material both with
ongoing convection in the anomalously shallow asthenospheric material, and with immediate
ing the temperature
A
delamina-
of asthenospheric
to the base of the crust were calculated
tively
underlies
large. Re-
after such crust-mantle
tion with the upwelling Extension
infinitely
“hot mode”
after the instantaneous delamination. The experiment depicted in Fig. 7A, on the other hand, in
(Cl NJ0 4skm 39km 33km 27 km
t 500
Fig. 7. Temperature-time periments.
T = 281-281eeX”’
3km
0
0
10
20
30
40 -time
The initial
50
T=1350°C km. T,,,
[Ma]
m2/s;
+7.13x,
(in A) and 1300°C
stable
km level. The dotted domain heat
t
blastesis.
in the crust
after
crust-mantle
crust. 20
30
40 --+
time
[Ma]
50
upwelled
at respectively or sillimanite No
crust-mantle
crossor
the 9, 15, 21 and 27 the time/depth
is stable. Transfer
of
only. A. Cold
andalusite/sillimanite
delamination
for 1 Ma,
at the base of the crust is kept at 1300 o C rigorous
asthenospheric
convection
material
C. Hot mode crust-mantle
the temperature
is:
rc=10V6
either sillimanite
t = 0 is by conduction
for 1 Ma, thus simulating newly
at which
delamination.
B. Hot mode
i.e. the temperature
K-r;
The four horizontal
fields in B and C represent
in which andahtsite
(Cl 1000
ex-
(in B & C) for 391x<150 K=2Wm-’
the temperatures
becomes
“delamination”
in all three experiments
D = 15 km, A, = 2.5 pW/ms.
andalusite
mode
for three
with x in km, for x I 36 km, and
(at150km)=1350°C;
lines represent
Temp
plots temperature
of heat in the
at the base
delamination
of the
for 20 Ma, i.e.
at the base of the crust is kept at 1350°C 20 Ma.
for
203
which the detachment and in which ductive
geotherm
lima&e
at similar
in the upper
the experiment in the
time-gap actual
steady-state, did not depth
crust
to trigger
levels.
between time
the Mount
12 Ma.
sillimanite This
Isa Inlier,
and
Peak be-
but are growth.
In
is only sta-
implies
the compressional
of andalusite/
sil-
are reached
of Fig. 6F, sillimanite
first
con-
yield
10 and 20 Ma after delamination,
not high enough ble
is at 37.5 km depth,
cooler
was chosen,
conditions
temperatures tween
horizon
a slightly
that
the
felsic magmas elements igneous
slab
slab with a radioactive
an order of magnitude granites
are powerful
Intrusion crust
higher.
of such granites
growth
heating
this “cold
mode”
cooling and
heat production
of the magma cooling
of the country
the other
therms during crustal thickening, it is unlikely that “cold mode” delamination alone can explain the
temperatures. The abundance of pre-metamorphic, riched granites in the Mount Isa Inlier
To obtain time
higher crustal
intervals,
another
and
temperatures heat
source
regional
Australian for longer has
to be
sought, either some form of “hot mode” upwelling of the asthenosphere, or immediate rising of mag-
Proterozoic
1987) justified effects cumbent”
cause
fold belts;
intrusions.
Wyborn
et al.,
of the thermal
Seismic,
masses
high
HPE-en(and other
heat
data have led to the concept batholithic
is
anomaly,
anomalously
a closer examination
of granitic
and gravity
effect
by the HPE-distribution
will permanently
the
and concomitant
brought
synchroneity of crustal thickening low-P facies metamorphism.
or upper
the first one
lamination could not have exceeded 12 Ma. Considering the progressive, vertical stretching of iso-
which
of
heat sources.
in the middle
metamorphism),
about
by an
Such HPE-enriched
effects:
subsequent
rock (contact
that a mid-
is replaced
and permanent
has two thermal
sillimanite
in heat-producing
It is not uncommon
or upper-crustal
transient
de-
(HPE).
is more complex,
enriched
crustal
event, i.e. the in
the heat balance
as they are commonly
flow
of “re-
(Hamilton
and
matic products. Hot mode delamination will lead to the expansion of andalusite-sillimanite conditions to higher crustal levels. In Fig. 7, three experiments with respectively 0, 1 and 20 Ma hot mode delamination are depicted. This figure shows
within these granites (A,,) of 2.5, 5.0 and 7.5 pW/m3. Taking into account the metamorphic grades of the granites in the Mount Isa Inlier, they
that, as expected, hot mode delamination would result in a very powerful heat source at the base of
must have been at mid-crustal levels during metamorphism. I have assumed the top of the granites
the crust.
Myers, 1967). Here, we use 3, 6, 9 and 12 km thick slabs of granite, and average heat productions
at 18 km. Figure presence
of such
8 shows the thermal a differentiated
effect of the granite
in the
Extension •t magmatic events: experiment series lc In many numerical experiments which simulated various modes of extension, the temperature
rock pile. The temperature change (T= ,/T,= decreases linearly with distance to the granite.
in the lower crust exceeded the solidus temperature for some time. After a 300% pure-shear extension of the mantle lithosphere for example, the
plained by transient thermal pulses caused by extensional events, magmatism or cold mode delamination, if these events are synchronous with,
temperature at the 30 km level is briefly raised to a maximum of 660” C (Fig. 6C), so that melting and subsequent advective heat transport may take
or immediately predate, the metamorphism. Lower and mid-crustal rocks will then for a limited time remain anomalously hot: in the case of magmas in
place. Furthermore, pressure-release melting of the mafic upper mantle and subsequent rise of mafic melts may trigger more lower crustal melting. The thermal perturbation of a mafic magma emplaced at a depth of 15 km and with a thickness of 10 km is, however, only short-lived (Wickham and Oxburgh, 1985). With fractionated
extension and/or cold mode delamination a few tens of million years at the most. For the low-P facies metamorphism to be prograde (t AT), the time-gap has to be even narrower. Prograde and compressive low-P metamorphism ( + AT, + A P) cannot be explained by any extensional event.
i)
From experiment series 1, it can be concluded that low-P facies metamorphism can only be ex-
the order of a few million
years, and in the case of
204
T/To
slightly
decreasing
Hamilton, t’“t
by conduction reasonable
1.5-
l
.
1.0
@
3
6
itself,
drastically
increase
duction
12 15 to top of granite [km]
9 ---+distance
parameters
One
transferred
ratios
ratios
during
should,
thickening
conclude
alone (e.g. advection
flow), or that crustal
is for all
slower than the lithospheric
P/T
must
and
relaxation
lithosphere
that
by a more effective
an increased
(Reinhard
Fig. 2). As thermal
of a thickening
thickening decrease.
12km
l
P/T
in prep.;
however, rather
either
process
heat
is
than con-
of a single-pass
thickening
than
is associated
fluid with
basal heat flow.
The effect of single-pass fluid flow The effective fluid flux, W, is normally
deduced
from D’Arcy’s ically inferred
Law, or calculated fluid/rock ratios.
from geochemAs, at present,
there
reliable
on
are
few
estimates
fluid/rock
ratios and original rock permeabilities (in Early to Mid-Proterozoic fold belts), 1 use an alternative d3km
3
6
12 15 to top of granite [km]
9 ~distance
T/To
C
approach. Walther and Orville the amount of post-diagenetic products
during
regional
E*
A~=75
Fig.
m3km
’
l.OI
’
8. The long-term
HPE-enriched T,= ,/T,= granite
granite
_, is plotted for various
2.5 pW/m3; Experiment
’
6
3
(B):
on versus
thicknesses A,
parameters:
tive geotherm
9 +distance
thermal
before
effect
the
.
’
’
’
12 to top of grani:e’[km]
of the intrusion
overlying
country
’
of a rocks.
the distance
to the top of the
of the granitic
slab. (A): A,, =
= 5.0 pW/m3;
(C):
A,, = 7.5pW/m3.
L = 150 km; the steady-state intrusion
is defined
conduc-
by T,,,, = 1350 o C,
A,=2.5~W/m3,D=15kmandK=2WK-‘mm’.Thetop of the intrusion region
is in each case at 18 km depth.
of the plots represents
temperatures
and is therefore
Experiment
metamorphism
of an
average pelite as being 12 vol.%. Assuming a thickness, h, in meters of pelitic material, and assuming a constant fluid-flow rate during deformation, and disregarding dergoing the transition
1.5
(1982) calculated devolitalization
above
The top left the solidus,
unstable.
series 2: crustal thickening
The first segment of the P-T-t path deduced for the Mount Isa Inlier shows compression with
the fact that rocks unfrom andalusite-bearing
assemblages to sillimanite-bearing assemblages would already have been relatively dry, W, the effective fluid flow (in m/s) above this pelitic layer is approximated as (O.l2h)/(time of deformation in seconds). This method is not only simple to incorporate
in the numerical
code,
but it
also ensures that no unrealistic amount of fluids is being withdrawn from the crust. The effective fluid-flow the wide
rates obtained in this way fall within range of fluid flows calculated with
D’Arcy’s Law (Etheridge et al., 1984). The result of constant fluid-flow rate during deformation is a constant extra heat input on any crustal level above the pelitic layer during deformation. By generously assuming h as 20 km, the final amount of expelled fluid has a column thickness of 2.4 km. In Fig. 9, the effect of the steady expulsion of this amount of fluid during two experiments (curves I and 3), involving constant strain-rate deformation is shown and compared with two identical experiments (curves 2 and 4)
205
sphere,
when the basal
heat flow decreases
again,
the crust
may cool isobarically
or slightly
decom-
pressively
(if end of convection
coincides
with the
end
of crustal
thickening),
end of convection thickening).
is prior
or compressively
(if
to the end of crustal
Thus, an overall
anti-clockwise
P-T-t
path may result. Houseman
2.0 150
350
250
during
of upwards
progressive,
directed
homogeneous
strain
numbers
I and 3 result from experiments
advection
rate.
component
3, W= 2.5.10-‘* ments without 10mh m2/s; thickness
numbers
T,,
D =15
km;
is 35 km;
thickening,
P-T-t
post
s-’
thermal
a fluid
m/s;
for comparison.
K=2W
kg/m3.
in exp.
from experi-
K-’
m-t;
Strain
(duration
only. No decompression
the
crustal
rate,
i, of
of thickening (duration
thermal
instantaneous
formulated
thickening,
the time,
t,, of the peak of kinetic
coincides
with the time of detachment
mal boundary
layer
from
energy,
the upper
rigid
lo-30
Ma. This probably
the order of the time-scale event.
Houseman
experimental
runs
of a crustal
et al. (1981) with
did
progressive
not
include
deformation,
be subjected to asthenospheric temperatures ing the (later) stages of thickening.
that the
(convective)
The only remaining model that can explain compressive and prograde (+ AP, + AT), regional, low-P facies metamorphism is one with directly links crustal thickening with a contemporaneous, anomalously high, basal heat flow, as for example Houseman et al.‘s (1981) model, which combines crustal thickening with convective thinning of the mantle lithosphere (Fig. 5). This model can result in the prograde transection of the
is in
thickening
that the base of the crust may well
The viability of anti-clockwise P-T-t a result of Houseman et al.‘s (1981)
without the fluid flow. One may conclude advection-of-heat effect is only minor.
layer:
log( to) is inversely proportional to f. The time of the peak of kinetic energy, t,, ranged in their experiments between 4 and 88 Ma, with the bulk
of
accompanied
which
of the ther-
but concluded
relaxa-
the
f, and
is
after thickening.
Crustal thickening and associated thinning of the mantle lithosphere
et al.‘s (1981) study
between
of the runs between
a=
fend =1.6;
the initial
3 and 4 was 10m’5.3 s-’
relaxation
a
incorporating
is 30 Ma). At the end of thickening,
tion was by conduction
with shown,
is 150 km: = 2700
flow
paths
A, = 2.5 PW me3;
1 and 2 was 10-‘4.3
fluid
2 and 4 result
=1350°C;
thickness
30 Ma) ( of experiment thickening
four
the fluid flow, and are plotted
lithospheric
experiment
the
(in exp. 1, W= 2.5.10-”
m/s);
In all experiments initial
From
single-pass
crustal
constant
relation
TempKl
+ Fig. 9. The effect
550
450
dur-
paths model
as is
tested here by a few experiments with widely varying parameters. Crustal thickening was approximated as being homogeneous throughout the crust. The finite difference grid was fixed to the medium
and
thus deformed
with thickening
(the
change in timestep-spacing being quadratically proportional to the change in depthstep-spacing). Strain rates were kept constant in each given experiment; value between 10p’4.3 and 10-‘5-3 ss’ were used. The finite amount of thickening was by a factor 1.6 (total time of thickening therefore varies between 3 and 30 Ma). Convective thinning of the mantle lithosphere was simulated by a sudden shift of the lower boundary condition: T= 1250-135O’C is shifted to the base of the (deforming) crust. Thus, a situation is simulated in
andalusite-sillimanite field during crustal thickening, if thickening is slow relative to fast (= early)
which the complete
convective thinning of the upper mantle. Then, the condensing of isotherms in the crust due to increased basal heat flow will compete with the stretching of isotherms, caused by the (progressive) crustal thickening. Hence, the temperature on a certain depth level may rise, and the overall geothermal gradient may steepen. Subsequently, at the end of convection in the upwelled astheno-
from the crust (thermally, this model is identical to crust-mantle delamination). At the end of convection, the lower boundary condition was abruptly shifted back to the base of the pre-deformation lithosphere. Cooling at the end of crustal thickening is purely isobaric if no erosion or tectonic denudation takes place. Those experiments characterized by a relatively fast convective
mantle
lithosphere
is detached
206
thinning,
low
strain
rates
and
long
Geological setting of the Mount Isa Inlier
convection
times at the base of the crust yield anti-clockwise P-T-t
paths (Fig. 10).
A recent
review of the geological
history
Mount Isa Inlier has been presented (1986, 1987). The main tectono-thermal summarized inliers
in Table
unconformably
sequences,
2. Pre-1875 underlie (Glikson
rick, 1982; Ellis and Wyborn, Blake et al., 1985). There oceanic main
Ma basement
two or three cover
which have been interpreted
rift and sag sequences
crust
was formed
rift sequences
have
of the
by Blake events are
as ensialic
et al., 1976; Der1984; Sweet, 1985;
are no indications during been
that
any stage. The deposited
during
Del, the first extensional event, and De3, the third extensional event. D,, has been dated by its felsic
Too
Fig. 10. Seven P-T-t geneous ment
crustal
Strain
thickening
was
thickening, me’
only
after
m2/s;
after
crustal
includes
of 4.1 pW/m3
sent the Proterozoic thickening
is 30 Ma;
s-’
W= 7.6.10m12
( of experiment
m/s);
tion of thickening sive substitution was
boundary
is Ma;
(duration
approximated
tdc,. In experiments
m/s);
of
e of experiments
m/s).
abruptly
Strain
se’ (duration
is 10 Ma;
7 is 10m’4.3 s-’
lithosphere
by
22 km to repre-
Isa Inlier.
of thickening
W= 2.5.10~”
of the mantle
condition
is thought
of the Mount
4.5 and 6 is 10~‘4.“2
is 25 km. The
with a radiogenic
1, 2 and 3 is 10-‘5.3 W = 2.5. lo-l2
or
lithospheric
the 18 and
distribution
situation
(, of experiments
between
kg/m’;
no erosion
(dura-
The progres-
by asthenospheric shifting
the
lower
(T = 1350 o C) to the base of the crust
at
1, 4 and 7, t r - 0 Ma; in experiment
2,
tdc, = 7.5 Ma. in experiment
3 t d ‘-15
tdct = 2.5 Ma; m expenment’
get. tde,= 5 Ma. In all seven at the base of the crust, after the
experiments,
the temperature
substitution was
held
of mantle constant
thickening. shifted
back
Then,
at
lithosphere
Ma; in experiment
by asthenospheric
T= 1350 o C until
the lower boundary
to its original thickening
position,
the end
condition at the base
lithosphere.
mal faults, all affecting the first cover and large amounts of metamorphosed dykes and “A”-
and “Y-type
5,
material, of crustal
was abruptly of the pre-
granites
sequence, dolerite (Passchier.
1986a; Holcombe et al., 1987; Pearson et al., 1987; Loosveld. in press). Its age is estimated at = 1750 Ma, as it affects
K = 2.5
= 2700
thickness
a 4 km thick granite
in
crustal
thickening.
the initial
is 100 km; the initial crustal
material
during
pas,
thickening;
levels. This heat source
rate,
replacematerial.
Heat transfer
advection
of 0 and 1350 o C (T,,,,);
denudation
production
depth
and
K = 1.2.10-6
conditions
stable geotherm heat
homo-
by temporary
in each experiment.
by conduction
K-‘;
boundary thickness
Tew ICI
from progressive,
fend = 1.6; D = 10 km; A,,= 2.5 pW/m’;
Parameters:
tectonic
----+
700
by asthenospheric
by conduction
and
600
accompanied
lithosphere
rates are constant
the crust
W
paths resulting
of the mantle
500
400
300
200
volcanics at 1779 + 9 Ma (Page, 1978. 1983) De3 at 1678 k 1 (Page, 1978). A second, but only locally recognized, extensional event is manifested in the central and eastern parts of the inlier by extensional duplexes, high-angle (to bedding) nor-
the sag sequence
of DC1
(Passchier, 1986a; Loosveld, in press). but does not affect = 1740 Ma old granites in the zone of extensional D,, structures (R.W. Page, pers. commun., 1988). Major I-type granite suites intruded the upper crust at 1860, 1800, 1740-1720, 1670 and 1500 Ma (Page, 1978; Nisbet et al., 1983; Blake, 1987). The first compressional sequences, D,i, was an,
event to affect the cover as yet only
locally
re-
cognized, thrusting event, resulting in imbricate stacks of thrust sheets (Bell, 1983; Loosveld and Schreurs, 1987) fold nappes and bedding-parallel LS-fabrics (Loosveld, 1989). One DC, shear zone in the west of the inlier has been dated, giving an age of 1610 f 13 Ma (Page and Bell, 1986). The subsequent DC2 event was a phase of strong, E-W directed, coaxial shortening, resulting in upright, tight, N-S trending folds, vertical axial-plane foliations, and vertical extension lineations. DC,-shortening is penetrative over the inlier and amounts to 35-55% (thickening the crust by a factor of between 1.5 and 2.2). Page and Bell (1986) dated a
207
TABLE
2
thermal
Main tectonic Time/Period
events of the Mount Phase
Events
(Ma) 187551850
Dee/“co
1780
De,
Barramundi
Orogeny/Kalka-
doon igneous
relaxation
of the extensional
events,
are
thick (in the order of 5 km) and are pre-metamor-
Isa Inlier
phic, there cannot
be a direct
extensional
and
events
link between
the (low-P
facies)
these meta-
morphism.
event (basement)
Major extensional
event
Implications of models for the Mount Isa Inlier
(1st “ rift + sag” sequence) = 1750
De2
Local extensional
1680-1670
D,
Deposition sequence
1610
DC1
1550
DC1 Dc3+
Penetrative
E-W shortening+
reg. metamorphism
Various
contractional
structures Intrusion
= 1500
Lithospheric
voluminous
metamorphic
post-
granites
Dates after Page (1978, 1983) and Page and Bell (1986).
Asymmetric
extension
Following
and
arguments
(Wilson, 1973; Jaques et al., 1986b; Reinhardt and Hamilton,
1982; Passchier, in prep.; Loos-
veld, 1989). Post-D,, deformation and western parts in the inlier
is in the central generally char-
by steep faults and shear zones, trending
extending
for thermal
McKenzie’s
relaxation
(1978)
after a pure-shear
extensional event, the three documented rifting events of the Mount Isa Inlier, of 1780, 1750 and 1680 Ma, are unlikely to be related to the low-P facies metamorphism at 1600-1540 Ma, primarily because events
D,, shear zone at 1544 5 12 Ma. Studies of porphyroblast blastesis generally confirm the synchroneity of D,. and the regional metamorphism
acterized
thinning
in the west
Local (?) thrusting low-P
1510-1450
structures
2nd “ rift + sag”
the
time
gap
between
and metamorphism
the
extensional
is too wide. The same
thermo-mechanical arguments refute the suggested relation (Hobbs et al., 1984) between the regional, prograde, low-P facies metamorphism and ensialic rifting at approximately (R.W.
Page, pers. commun.,
Hill Block, Australia. The fact that crustal many
cases
1988) in the Broken
thinning
an asymmetric
at 1600 Ma 1700 Ma
proves
process,
to be in
resulting
in
030 o N (dextral) and 310 o N to 340 o N (sinistral). Specific parts of the inlier are affected by late N-S shortening, and NNW-trending, upright folds. The latter have been dated at 1510 k 13 Ma (Page and Bell, 1986). A large population of K-Ar dates of 1450-1500 Ma (Richards et al., 1963) is
two different, but complementary plate halves, cannot overcome this problem. The main problem
interpreted the crust
models explain lateral upper crustal extension
offsets between areas of and areas affected by low-
uplift.
P facies metamorphism,
on the scale of the com-
P-T-t data indicate a poly-metamorphic, anti-clockwise path: sillimanite grows usually after andalusite (during DC._,),whereas kyanite replaces sillimanite and cordierite in late retrograde shear
plete inlier (400 X 200 km), normal faulting would have to be developed at least in some areas: even
zones (Reinhardt and Hamilton, in prep.). Similar metamorphic observations have been made in other Australian Early to Middle Proterozoic inliers. especially in the Willyama (Phillips and Wall, 1981; Hobbs et al., 1984; Clarke et al., 1987) Halls Creek (Gemuts, 1971) and Arunta (Warren, 1983) mobile belts (Fig. 1). It is important to realize that, since the sag deposits, resulting from
wall plates
as reflecting and cessation
a widespread cooling of of Ar diffusion during
with any model incorporating lute asymmetrical extension is the general absence of young (post1670 Ma) rift sediments and young extensional structures. Although the asymmetric extension
more so, since transfer faults normally compartmentalize the extensional terrane, so that hanging and
foot
wall
plates
are juxtaposed
along strike (Gibbs, 1984; Bosworth, Etheridge, 1987; Lister et al., in press).
1985;
Magmatic events Magmatism results from a primary perturbation of a depth-temperature profile, and, more importantly here, its effect is only secondary on
208
TABLE
3
Concentration compared
of heat-producing
to the worldwide
elements
average
Worldwide
Wppm)
3
3 8.5
4.2
The percentages production
high Ca
17
K ,O(% under
in the three
2.5
the batholith
can be calculated.
names
Data supplied
largest
pre-metamorphic
batholiths
of the Mount
Isa Inlier,
as
in granites
average
low Ca
Th(ppm)
(HPE)
concentration
represent
Sybella
Wonga
(3.14%)
(1.1%)
8
8
5
35
46
25
5.15
5.42 the surface
by L.A.I. Wyborn
Kalkadoon
coverage (Bureau
of the respective of Mineral
4.28 granites,
so that a weighed
average
heat
Resources).
this perturbation. There is no question that during, or after, thickening of the crust felsic lower crust may melt, and that during extension deeperseated, mafic, pressure-release melting may occur.
Crust-mantle detachment Figure 6F shows that sillimanite-grade conditions can be reached when the crust is abruptly “underplated” by hot asthenospheric material,
However, whatever pulse of magmatic
which,
the mode of magmatism, heat is short-lived and
the T/P
ratios will only be raised temporarily. In the Mount Isa Inlier, there is no evidence for syn- or immediately pre-metamorphic rising melts. The additional thermal effect of mid-crustal, pre-metamorphic granites enriched in HPE will be permanent, however. Table 3 gives concentrations of the HPE in the main pre-metamorphic granites of the Mount Isa Inlier. The concentration of HPE in Mount Isa’s granites is anomalously high; it yields an average radiogenic heat production of 4.1 pW/m3. This is 52% higher than the worldwide average (2.68 pW/m3 according to Turcotte and Schubert, 1982). Figure 2 shows the effect of a granitic slab between 18 and 22 km with a heat production of 4.1 pW/m3 on the steady-state conductive geotherm. It will raise the temperature in the 3-5 kbar interval by 50-75°C which is not enough, though, to transect the andalusite/ sillimanite field. Pre-metamorphic HPE-enriched granites are widespread elsewhere in the northern Australian Early to Middle Proterozoic fold belts (Wyborn et al., 1987). The presence of such granites alone can explain steep average geothermal gradients, but not the anti-clockwise P-T-t path. Also, no major pre-peak metamorphic granites are within 20 km (surface-) distance to the Soldiers Cap Group, whereas it too is affected by the low-P facies metamorphism.
after
simulates
upwelling,
cools
by conduction.
in simplified
form
the
It
extension-trig-
gered delamination models for Proterozoic fold belts, proposed by Kroner (1983) and Etheridge et al. (1987). Such models, however, cannot be applied to the Mount Isa Inlier for the same reasons that models incorporating only extension cannot be applied: the time gap between extension/ delamination and low-P facies metamorphism is too wide. Hot mode delamination, associated the documented extensional events, would
with miti-
gate this objection. However, there is little evidence for anomalously steep geothermal gradients in the 120 Ma time gap between extension and metamorphism: there is no record of major crustal magmatism in this time span, nor is D,i generally characterized by low-P facies assemblages. In contrast, in both the Mount Isa Inlier and the Willyama Block (Fig. l), metamorphic grades increased during crustal thickening (respectively Reinhardt and Hamilton, in prep., and Hobbs et al., 1984). Any causal relation between the documented extensional events in the inliers and the at least 100 Ma younger phases of low-P facies metamorphism
is
therefore
invalid.
A
variation,
favoured here, is the coupling of crust-mantle detachment with compressional tectonics. Crustal thickening and concomitant thinning of the mantle lithosphere
(convective)
The main attraction of this model is that convective thinning of the mantle lithosphere is syn-
209
chronous
with thickening
with the period Also,
heat
remain
flow
high
thickening.
at the
during Thus,
perturbation
of the crust
of low-P
base
brought
magmatism,
isotherms
considerable
time.
thus
crust
phase
about may
by stay
On the other
may
elevated hand,
for
with this model.
a ap-
None
insuperable.
First, it is unlikely crystallization
of sillimanite
of crustal
thickening,
magmatic
events)
the crustal
that this model
would
thickening
can lead to
the early stages
during
since
the delamination
have to predate
and strain
.( f
most of
rates would
thus
have to be very low. In the Mount Isa Inlier, however, andalusite/ sillimanite assemblages are coeval with the early stages of Dc2. If thinning of the mantle lithosphere was solely the result of Dcz, strain rates in this phase must have been unrealistically low (< = 10-‘5s-‘) and detachment of the upper crust relatively fast. It is more likely that a major thickening event preceded Dc2, i.e. D,, was not just of local importance, but triggered thinning of the mantle lithosphere. D,, thrusting has now been documented in the western part of the inlier (Bell, 1983) in the central part (Loosveld and Schreurs, 1987) and in the eastern part (Loosveld, 1989, this respect that is characterized mation. whereas is marked
and in press). It is noteworthy in the early crustal thickening, Dcl, by pre-dominantly brittle deforongoing crustal thickening, DC*,
by dramatically
higher
temperatures
and predominantly
model-dependent),
deformation.
This
is re-
garded as typical for the Early to Middle Proterozoic fold belts of northern Australia (Etheridge et al., 1987). Intrusion of the volumetrically huge 1500 Ma old granites, the Williams and Naraku Batholiths, clearly postdates Dc2. The phase of the Naraku Granite
main, coarse-grained gives a (Sm/Nd)T,,,
source age of 1637 Ma; the T,, source age of the Williams Batholith ranges from either 1530 to 1620 Ma, or, and more probably, from 1630 to 1720 Ma (Wyborn et al., 1988). Thus, the source ages for the main phases of the Williams and Naraku Batholiths are close to the deformation
events
during
the source
crustal
ages
thickening
accompanied
which
1610 k
the chronological
(and
the
may have been
mantle
huge
by major amounts
of
source rocks were created
in the lower crust or just
under
et al., 1988). A concur-
the crust (Wyborn
rence of crustal
thickening
would
strongly
point
(1981)
model
mantle
lithosphere
and such mantle
events
towards
Houseman
et al.‘s
for (convective)
instability
of the
during
compressional
tecton-
ics. The
second
problem
lies in the post-tectonic
history. Generally, crustal thickening must lead to syn- to post-erogenic uplift and erosion. In the Mount straints isobaric
Isa Inlier, however, petrological conindicate compression followed initially by Hamilton,
in
prep.). An explanation lies in the different constants for erosion and thermal relaxation:
time the
cooling
time constants
(Reinhardt
for erosion
and
(X: 60-300
Ma;
En-
gland and Richardson, 1977) are generally much greater than thermal time constants (heat transfer by conduction only: L2/m2~. in the order of tens of Ma). Thirdly, the common absence in the sillimanite (K-feldspar) zone of signs of nearby anatexiswhich, considering the steep metamorphic gradients, should have occurred immediately below this metamorphic
zone-can
by the Coulomb-Navier (opening) migrate,
ductile
is still very small
period(s)
and
there
database very
thermal
extension
ages for D,, and (less so) D,,, respectively 13 and 1544 &- 12 Ma. Although
of crustal
the short-lived
pear to be three problems is, however,
of the
the entire unlike
(and
facies metamorphism).
fractures are unlikely
potentially fracture through
be explained theory:
which
to be vertical
tensile
melt
can
in a regional
compressional regime. With overall high differential stress levels, melt migration will be severely constrained in all directions; all, it will do so horizontally.
if melt migrates
at
Conclusions General conclusions are: (1) Cooled granitoids, enriched in U, Th and K, substantially contribute to the local heat production. (2) Prograde and compressive metamorphism (+AT, +AP) cannot be attributed to any premetamorphic extensional event.
210
(3) After elimination
of other options,
ing of the crust with associated ning of the mantle possible
lithosphere
explanation
for
(convective) remains
(Convective)
thinning
sphere
has to be fast with respect thickening.
Conclusions Isa Inlier Proterozoic
period. concerning
probably
Australian
(1) The P-T-r
other
to Middle
isobaric cooling. In the Mount Isa Inlier, the low-P facies metamorphism is contemporaneous of pervasive
crustal
thickening.
magmatism preevent by at least
100 Ma. (2) Granitoids.
in U, Th and K, sub-
enriched
Isa Inlier, convective
lithosphere
might
high until
the end of Dcz.
Acknowledgements
The receipt
of an Australian
Additional provided
financial by
the
National
is gratefully and
logistical
Bureau
and assigned
support
of Mineral
(metamorphic petrology), Lesley Wyborn (granites) and particularly Mike Etheridge (general overview).
Reviews
by Mike Rickard,
N. Rast and
M.P. Ryan led to improvements in the paper. 1 am specially grateful to Ineke de Bruyn for her stubborn
assistance
with the programming.
notations
A B’
radiogenic
heat production
of granite
(2.5-7.5)
Au
radiogenic
heat production
at surface
(2-3)
values 10 me W/m’
10mh W/m’
c
concentration
(‘1
specific heat capacity
of fluid
4000 J kg
’ K ’
(‘, D
specific heat capacity
of solid
1000-1300
J kg
radiogenic-heat upward
the progressive
velocity (here. erosion
HPE
heat-producing
amount
the finite amount
K
thermal
L
lithospheric
concentration)
scale length (A = A,,e- ‘j”)
E
lo-15
rate)
of thickening;
conductivity
’ K ’
km
O-2 km/Ma f = e “. in case of constant
strain rate
of thickening elements
1.5-2.2
(mainly
U. Th, K)
of solid
thickness
2.0-3.0
W mm
100-200
km
’ K ’
&!xGr)_, ( P\c’,) r
stability
t
time in seconds
tdrt
time of crust-mantle
fmd
time at end of thickening
r,
intrusion
T, T rlux
liquidus
r,
solidus temperature
factor:
r = kAt/Ax* detachment
= k/h’ during
temperature temperature
temperature
at the base of the lithosphere
0.4 (by definition progressive
was
Resources,
Geology and Geophysics, Canberra, and the Broken Hill ,Proprietary Co. Ltd., Brisbane. The research benefited from discussions with Greg Houseman (numerical techniques), Steve Lonker
Abbreviations,
(e.g. CnK initial potassium
Univer-
acknowledged.
Appendix 1
J JAI
trig-
event. Tempera-
tures at the base of the crust remained approximately
thinning
have been
gered by DC,, the early thrusting
sity Ph.D. Scholarship
for the Early
Extension and pre-metamorphic date this major crustal thickening
(3) In the Mount of the mantle
to Middle
Proterozoic fold belts of Australia are markedly anti-clockwise, made up of a segment of prograde, low-P facies metamorphism (compressive heating; + AT, + A P), followed by a segment of essentially
with D,,, a phase
to the local heat production.
the Mount
Early
fold belts) are:
paths
contribute
at the base
high for a considerable
specifically
(and
litho-
to the progres-
Temperatures
of the crust have to remain part of the erogenic
P-T-f
of the mantle
stantially
thin-
as the only
anti-clockwise
paths.
sive crustal
thicken-
cruatal
thickening 750&900 Dc 800-1050
oC
12OC-1350°C 700-900
oc
here)
:11
fluid velocity
Vr
w=
V,@
effective
2.5.10m"-2.5.10m"
fluid flux
x
depth axis (m), positive downwards
P
the finite amount
bt=rh'L'/~
timestep
A.x=hL
depthstep
i
strain rate (constant)
cp
porosity
K = K/C &xc,,)
thermal
h
time constant
P,
specific density
of solid
27OOC3300 kg/m’
Pf
specific density
of fluid
1000 kg/m’
1.5-m
of extension
m/s
(locally)
0.04-2.0 Ma
(set)
2-4 km
(m)
diffusivity
I
10-151
_10~14~s lom’_lom~ 4.10~7P1.2.10
of solid
for erosion
(Ma) (E = he-"'Ed)
Appendix 2
50-500
‘m’s
Ma
ing experiments, a Lagrangian reference frame was generally used, such that the solid advection
The “conservation
factor can be omitted. The third factor:
of energy” equation
We start with the one-dimensional conservation of energy equation after, e.g., Carslaw and Jaeger (1959):
(1) where
T is temperature,
t is time,
c, is the heat
capacity of the solid, p, is the specific density of the solid, and K is the thermal conductivity (p,, c, and K are assumed constant in each given experiment). Transfer of heat is considered cal direction, x, only.
in the verti-
for radiogenic
heat
production
in the
crust, + A. A is assumed to decrease exponentially with depth or to be constant (see “Initial Conditions”, Appendix 3). Because of the common enrichment of heat-producing elements in fractionated granites, a different +A,, is assigned to the heat production in a granite. The second factor: -
EPA
$
accounts for the reference e.g., the top erosion level, rock system (negative for
porosity;
V, fluid
erosion of the upper surface where frame is fixed relative to that surface, boundary condition is held at the with E, the upward velocity of the with respect to the erosion level upward movement). In the thicken-
velocity
(negative
for upward
flow); ct heat capacity of the fluid; pr specific density of the fluid. W = V,@ = effective fluid flux. Equation
(1) expanded
with factors
1. 2 and 3,
3T -=.s+&-(E+MW)g
at
Depending on the physical processes that need to be modelled, various factors can be added to the right hand side of equation (1). The first accounts
is the advective component for upward single-pass fluid flow in a frame fixed to the rock pile, with @
(2)
\ \
with
K =
K/p,c,,
(p,c,)/(p,c,), Crank-Nicolson order)
the thermal can be (implicit,
diffusivity
and
M =
solved employing the finite-difference, second
algorithm:
PI.,+1- 7).,)/At =
ww/+,.,+,-2T.,+,
+ 7;-,.,+,
+
7;+,,,- 27;.,+ L&Ax’
+ A/P,L.,
-
(EC MW)(7;+1., - 7;.,wx
(e.g., Smith, 1978; Gerald and Wheatley, 1984). If one defines: r = tcAt/Ax’. r becomes the stability factor to the solution of the finite difference equation. Using this implicit, second order accurate scheme, the solution will be unconditionally stable: here, r is continuously 0.4. After grouping subscripts, eqn. (3) becomes:
212
recorded
(1 + 0.4)7;.,+, =
further
by Vitorello assumed
WT+,.,+, + T-1.,+1+ L.,) + AAT,‘p\c\
(Pollack
+ [i-0.4+
to calculate
and Chapman,
W m-’ Km’ (Schatz
(E+MW)AT/Ax]7;,,
and Pollack (1980). We have
an A, of 2.0, 2.5, or 3.0 pW/m’
various
1977) and a K of 2 or 3
and Simmons,
1972)
steady-state
in order
conductive
geo-
therms: + [0.2 - (E + MW)AT/Ax]
T+ ,,,
(4) T=
so that:
A@
r/D
K
Tr.,+, = {0.2(7Y+,,,+,
+ 71P,.,,,
+ TT;,.,)
+
+ AAT/p,c, + [0.2 - (E + MW)AT/Ax]
/1.4
TIndY- (A,D’/K) L
X
L, the thickness of the continental i.e. that part of the mantle which
7),,} (5)
against strained.
solid-state convection, For thermal effects
is on
lithosphere, is stabilized poorly conthe crust, the
lithospheric thickness is of importance. because the time-scale is proportional to L’. The litho-
Appendix 3 Model parameters Boundary conditions. The boundary quisites are made up of the temperature
prereat the
surface, fixed at 0°C and the temperature at the base of the thermal lithosphere (100-200 km), T,,,,. which ranges from 1200 to 1350” C (constant in a given experiment). Initial conditions. With respect to the distribution of heat-producing elements (HPE) in the upper crust, two geotherm endmembers are used here: HPE can either be homogeneously distributed over the upper crust or their concentration exponentially
K
with x in km.
7;+,.,
+ [l - 0.4 + (E + MW)AT/‘Ax]
decreases
A@ ~
---e
with depth.
The latter dis-
tribution could be the result of progressive fractionation of magmas (Lachenbruch, 1970) and upward migration of fluids with dissolution-precipitation of the HPE (Albarede, 1975; Moorbath, 1978). This exponential distribution is more realistic, since it preserves the observed linear relation between surface heat flow and surface radiogenic heat production under differential erosion within a heat flow province (Lachenbruch, 1970). A = A,,e ‘/D, with A,, the radiogenic heat production at the surface, and D the characteristic length scale for the distribution of HPE, in this case for the decrease of A with depth. Two different characteristic lengths, D, are used here (10 or 15 km). Both fall within the spread of characteristic lengths
sphere-asthenosphere boundary is probably gradational, but is here. rather arbitrarily. represented by an isotherm. Lithosphere thickness estimates based on heat flow studies (e.g., Pollack and Chapman, 1977) indicate an increase of thickness with tectonic age. Jordan and Sipkin and Jordan (1976) anomalously
short
travel-times
(1978, 1979, 1981) on the basis of the of ScS-waves
un-
der Precambrian shields (an average 4 s shorter than under oceans) and on geochemical evidence from kimberlites, favoured 200-400 km thick rigid mantle roots under these shields. They suggested that basalt-depletion of the upper mantle results in a chemical boundary layer with a higher viscosity, while thickening
of the depleted
with
the large
time
nesses under
explains
Precambrian
upper mantle
tectospheric
shields. Woodhouse
thickand
Dziewonski (1984) also argued for high values for L. associating mantle structures. up to 350 km depth, with surface tectonic expressions (ridges, shields). Richardson et al. (1984) argued, on the basis of diamond inclusions, for a lithospheric thickness of at least 180 km beneath Archaean shields. On the other hand, the thermal time constants for the lithosphere, which are proportional to L2. indicate a much thinner lithosphere, i.e. approximately 125 km (Parsons and Sclater, 1977; Sclater et al.. 1980). Also, the half-life of heat production for a chondritic model Earth is approximately
2 Ga, implyinp
q higher
basal
heat
213
flux
and
Middle
a thinner
lithosphere
in the
L is ranged
Proterozoic.
Early
to
here between
and 200 km.
Neglected parameters
(p,c,)EaT/ax
is the advected
E, the characteristic
relative
heat
flux, with
vertical velocity of the medium
Inevitably,
to plane x = 0. This is the rate of denuda-
tion of an otherwise
undeforming
tion can also be tectonic, are, however,
by means
no indications
in the
climate,
Mount
resistivity
Isa
of extensional
E
erosion
of
types, drainage patterns and surface thus on the rigidity of the lithosphere.
the
on rock
height, and All of these
history. ments
and
clear
relations
parameter
erosion-rates, as reported
values
by Hollister
range (1975,
function. such that, following Carson and Kirby (1972) England and Richardson (1977) and and England
(1979):
E = XEd e-r:h with
X a time constant
eroded sequence. crustal thickening however, essentially denudation
for the erosional
Ed the thickness
with unit Ma and
process
of the finally
Erosion started at the end of the period. In the Mount Isa Inlier,
the initial
post-tectonic
isobaric, so that factor is justified.
cooling
an omission
period
the thermal
of physical
mal conductivity,
1982) and 0.1 mm/yr (= 10 km/100 Ma). We have generally opted for the exponential erosion
Richardson
and
is
of the
the
heating,
extensional
offer
a particular
history.
Here,
I ne-
such as the therdeformation
experiments, absorbed
(4)
dur-
frictional
by devolatiliza-
(1) The
omission
of lateral
heat
transfer
is
acceptable, because the experiments simulate large-scale processes (the same anti-clockwise P-T-t paths are documented or inferred for Australian Proterozoic fold belts widely apart). On these scales, the lateral compared to the vertical temperature negligible. (2) The temperature
gradients
can safely
be assumed
dependence of c, Q’, K, and p on and pressure is negligible compared
to the dependence on lithology. Here, no specific lithology has been incorporated (except crust-
temperatures than even instantaneous
tend = 30 Ma) to tsnd = 3 Ma). spaced over the factor” r of 0.4, 0.4 L2/(2500 K). lop6 m*/s, At =
between
tion reactions:
son (1987) mentions characteristic fluid permeation rates (V,) of 4 mm/y (= 4 km/Ma = 1.3 . 1Op’0 m/s). Here, similar values are used, calcu-
from 2.5. lo-l2 (for f= 1.6 and 2.5 . lo-” m/s (for f= 1.6 and Fifty depthsteps were equally lithosphere, L. Using a “stability this implies a timestep, At, of With, e.g., L = 150 km and K = 0.11 Ma.
experi-
simplifications
parameters
and (5) energy
mantle distinction), occurrence of the metamorphism.
lated by taking W as (2400)/t,,, in m/s, thus simulating a constant devolatilization of a 20 km thick pelitic sequence (12 vol.% fluids) during deformation. This yields values of W ranging
most
metamorphic
numerical
(3) progressive
fluid flux, equals V,@ where V, is the fluid velocity and Q is the porosity. Thomp-
W, the effective
and
glect, for the reasons presented below, (1) lateral heat transfer, (2) the depth- and temperature-de-
ing
2 mm/yr,
rendering necessary
general
influencing
for a particular
from the
tially with time, or to be constant. the time-independent
known
have to be,
Factors
are numerous,
futile,
pendence
In the case of
experiments
simplified.
processes
Far
make generalizations about E almost impossible. Generally, E is taken either to decay exponen-
between
be,
are poorly
there
depends
numerical
should
metamorphic
for such late exten-
Inlier).
against
and
solid (denuda-
forces, in which case E is depth-dependent; sion
Appendix 4
100
(3) Progressive
because of the widespread to-be-modelled low-P facies extension
will result
in lower
instantaneous extension. Since extension cannot explain the
low-P Inlier,
facies metamorphism in the Mount Isa simulating progressive extension is superfluous. (Experiment series 2, on the other hand, does incorporate progressive thickening.) (4) Frictional heating (in W/m’); Qr = CT&,
(5,
is stress, and i is strain
rate)
is neglected here as studies by Reitan (1968a. 1968b, 1969) Graham and England (1976) and Werner (1985) show that its influence is limited.
214
Frictional
melting
opment
can possibly
of pseudotachylytes,
cool, dry, crystalline 1972;
lead to the devel-
Sibson,
inverted
and especially
rocks (McKenzie
1975;
Maddock,
metamorphic
mic; e.g., the conversion mately
plus
47 kJ/kg,
production
Locally
zones can result, but only if
reactions
to sillimanite
and Brune,
1983).
most strain is taken up in narrow Dehydration
so in
zones.
K-feldspar
plus quartz
which offsets
approxi-
a radiogenic during
of
Walther and Orville (1982) showed that the devolatilization reactions, which take place during the metamorphism of an average pelitic rock, absorb approximately
168 kJ/kg,
whereas
some 210
kJ/kg is needed to heat one kilogram of pelitic rock from 400-600°C the assumed temperature interval dition
for such
devolatilization.
Geol. Sot. S. Afr., 89: 253-262.
Blake.
D.H.,
1987.
environs, Miner.
Resour.,
Thus,
the ad-
of:
W..
Brady,
at 3
200
1985.
for the
heat required for endothermic devolatilization reactions during prograde metamorphism. However, as we are here concerned with he heat the entire lithosphere, the factor is proximated by a low value of A,. The a factor for endothermic reactions
balance of simply apomission of affects all
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Elsevier Science Publishers
191
B.V., Amsterdam
- Printed
in The Netherlands
The synchronism of crustal thickening and low-pressure facies metamorphism in the Mount Isa Inlier, Australia 2. Fast convective thinning of mantle lithosphere during crustal thickening RAMON Department
of Geology, Australian
(Received
February
J.H. LOOSVELD National
23,1988;
University,
revised version
*
Canberra, A.C. T. 2601 (Australia)
accepted
December
15,1988)
Abstract Loosveld,
R.J.H.,
1989. The synchronism
Inlier, Australia.
2. Fast convective
of crustal thinning
thickening
of mantle
and low-pressure
lithosphere
facies metamorphism
during’crustal
thickening.
in the Mount
Isa
Tectonophysics,
165:
191-218. The Mount
Isa Inlier is, like some other northern
by an anti-clockwise thickening study,
(
+AP,
two possible
quantitatively immediately delamination,
P-T-t
path:
i.e. regional,
+ AT), and were overprinted models
evaluated before
for the synchronism
using
latter results in prograde
low-P
event,
thickening
due
Early to Middle
low-P
of essentially
of regional,
low-P
to e.g. lithospheric
facies metamorphism
by (convective) during
crustal
isobaric
fold belts, characterized
assemblages cooling
facies metamorphism
finite-difference
accompanied
Proterozoic
facies metamorphic
by a phase
a one-dimensional,
the thickening
and (2) crustal
Australian
prograde
and crustal
technique:
(1) extreme
extension,
magmatic
thinning
thickening.
grew during
crustal
(AP - 0, -AT).
thickening
elevation activity,
of the mantle
are
of isotherms
or crust-mantle
lithosphere.
It is the preferred
In this
Only
the
model for the Mount
Isa Inlier.
Introduction
developed
In the companion paper (Loosveld, Part I, 1989), the simultaneous development of low-P facies metamorphism ratios, characterized stability
field,
(i.e. low metamorphic P/T by the andalusite-sillimanite
Miyashiro,
1973) and
a phase
crustal thickening in the central Soldiers Cap Group belt, eastern Mount Isa Inlier (Fig. l), Australia, was demonstrated. A direct effect of crustal thickening alone, however, must be the general increase of P/T ratios (resulting in many erogenic situations in high-P facies phism). If steep metamorphic gradients
* Present
address:
Koninklijke/Shell
Box 60, 2280 AB Rijswijk, 0040-1951/89/$03.50
Explor.
metamorhave only
Prod. Lab., P.O.
The Netherlands.
0 1989 Elsevier Science Publishers
B.V
structural
positions,
like those
high in a nappe sheet, the anomalously high temperatures can be explained by fast erosion (decompression) of the thickened crust and/or heating induced by the overlying nappe Moderately
of
in certain
high metamorphic
are not restricted
to these
gradients, structural
by the sheets. however,
positions
in
the Mount Isa Inlier (Wilson, 1973; Hill et al., 1975; Derrick et al., 1977; Derrick, 1980; Hamilton, 1985; Oliver and Wall, 1987; Reinhardt and Hamilton, in prep; Oliver et al., in prep.). They prevail throughout the Soldiers Cap Group. A remaining problem is to find a heat source which could explain the overall steep metamorphic gradients during the thickening of the crust. A wide variety of fold belts has undergone low-P facies metamorphism, similar to that of the Mount Isa Inlier. Examples can be found in the
192
The deeper
in the crust,
phic conditions tive
is the heating.
concluded 20”
the later peak metamor-
are attained,
that Archaean
mes were similar
and the more effec-
England
and
continental
Bickle
(1984)
thermal
regi-
thermal
regi-
to those at present.
The hypothesis
that Precambrian
mes are similar to those at present does not, however, comprehensively address the problem of the lack of relict high-P Precambrian. eclogites,
glaucophane
kyanite-bearing Lepontine 40”
: Fig. 1. Distribution to Middle Inliers
are
Murphy;
of Australian
Proterozoic numbered:
4-Tennant
-Halls
Creek;
Leopold
Mobile
Musgrave;
Archaean
fold belts (after 1 -Coen; Creek;
Belt;
I4--Crawler;
II -Paterson; I5-Mount
and Early
et al., 1987).
Z-Georgetown;
S-Arnhem;
8 -Granites-Tans;
cratons
Etheridge 6-Pine
3Creek;
9 -Arunta; IZ-Capricorn; Painter;
7
10 -King I_?--
16-Willyama;
I7-Albany-Fraser.
phism,
+
jadeite
rocks, although moderate-i’/
in the
in the Western schists,
overprinted
moderate-P
are in places still recognizable
Alps, and by the
metamor(Frey et al.,
1974). P-T-t paths are clockwise in the P-T field (Fig. 2, trajectory “A”. Note: Abbreviations are defined
in Appendix
paths are generated els, which simulate
I ). Such clockwise
P-T-t
in virtually all numerical modcrustal or lithosphe~c thicken-
ing (e.g., England and Thompson, 1984; Davy and Gillet, 1986; Thompson and Ridley, 1987). With certain favourable parameterisations in these models,
Hercyno-type erogenic belts of Zwart (1967), but more commonly in the Archaean (granite-green-
facies assemblages
For example,
geotherms
will transect
the low-P
facies
(andalusite-sillima~te) field for some period of time. They can only do so, however, during postthickening decompression ( - A P, 4 AT), unless a combination of extreme parameters is assumed, e.g. an initial geotherm, which already transects the andalusite-sillimanite field, very low strain
stone) terranes (Grambling, 1981; Green, 1981) and Proterozoic fold belts (Grambling, 1981; Lambert, 1983). This secular trend seems to reflect the overall cooling of the Earth through geological
rates, low conductivities, and a high radioactive heat generation. If tectonic and thermal regimes of
time. England and Richardson (1977) and England and Bickle (1984) however, pointed out that
the Precambrian the Phanerozoic.
peak
of the Western Alps, the Precambrian trajectories should resemble their Alpine
metamorphic
conditions
reflect
points
transient and polychronous rnet~o~~c than on steady-state geothermal gradients.
on
rather Thus,
the apparent time-dependence of peak-metamorphic conditions should be attributed to erosionrate-dependent thermal relaxation of a tectonically thickened crust: radioactive selfheating in a slowly eroding thickened crust may lead to overprinting of early high-P facies assemblages by moderate-to low-P facies assemblages (e.g. in the European Alps, the overprinting of the Eo-Alpine high-P facies metamorphic event by the Lepontine moderate-P, moderate-T event). Depending on reaction kinetics, even (near) total disappearance of the early high-P facies assemblages may occur.
fold belts were similar to those of or even more specifically, to those P-T-f counter-
parts and also be clockwise. In the northern Australian Early to Middle Proterozoic inliers (Fig. l), however, the low-P facies metamorphism is prograde and “compressive” ( + A P, + AT), and succeeded by a phase of cooling. Cooling was isobaric to slightly decompressive, as in the Mary Kathleen Fold Belt in the Mount Isa Inlier (Reinhardt and Hamilton, in prep.), the Arunta Inlier (Warren, 19X3), and the Broken Hill Block (Philips and Wall, 1981; Hobbs et al., 1984) or compressive at first, as in the Olary Block/Willyama Province (Clarke et al., 1987). The resulting P-T-t paths are anti-clock-
193
tic constraints, elled
the same
numerically,
finite-difference
code (details
2, 3 and 4). The thermal
there
are
magmatism,
low-P
thickening:
therms
immediately
600
Fig.
2. P-T
diagram
showing
geotherms,
transecting
the
radioactive
heat production
three
kyanite
before
All
(A) dist~bution
100 km;
of 3 W K-t
D,,,V, i =lO
m-‘.
Curve
between
18 and 22 km depth
tion
3 additionally
of 4.1 pW/m3.
stability “A”,
and retrograde tories
“B”
which
characterize
“Alpine-type”
and
“c”
are
Reinhardt
the
reaction
(retrograde)
history
Proterozoic
is common
history.
Trajec-
P-T-Z
paths,
of the northern
Clarke
et al., 1987):
+ sillimanite
(prograde)
also elsewhere
(e.g., Gemuts,
1971, for the Halls Creek Mobile Belt). The cooling these anti-clockwise
P-T-t
sive, isobaric
wise (Fig. 2). Similar
repre-
fold betts (respectively
in prep.;
paths
can be slightly
+
segment
of
decompres-
or slightly compressive.
anti-clockwise
the thickening
extension,
event,
magmatic
delamination,
ac-
and
(2)
discarded.
is reliable, The
the former
results
may
to Mid-Proterozoic
mechanism
can be
be applicable
to all
fold belts
Australia, and to prograde, terranes in general.
low-P
of northern metamorphic
Tectonic m~hanisms metamo~hism
that lead to low-P facies
trajectory
metamorphism
metamorphic
andalusite
respective
P-T-t
facies
the anti-clockwise
and Hamilton, series
and
of iso-
granite
heat produc-
metamorphism,
the metamorphic
Early to Middle
after
kyanite
(1971). high-P
moderate-T/moderate-P
sents a clockwise,
Australian
Holdaway
of km;
Ao,curvr 3 = 2
a 4 km thick
tripiepoint
by prograde
s =15
elevation
thickening accompanied by convective of the mantle lithosphere. These two
database Early
length
km, D,,,,,
with a radioactive
The AlzSiG,
fields are after
characterized
reflects
a
by A =
heat distribution),
Ao,curve z = 2 pW/m3,
pW/m3.
have
defined
and a lithospheric
km, D,,,.,,, a =15
A O.cuwe 1 = 2.5 pW/m”.
conductive three
crust-mantle
may during
models are compared with the existing geological database of the Mount Isa Inlier. Assuming this
1°C)
steady-state
Aae- x/D ( D = the length scale for radiogenic a conductivity
crustal thinning
800
field.
or
which
facies metamo~hism
due to, e.g., lithosphetic tivity,
In general,
mechanisms
(1) extreme
de-
as well as some
are simulated.
possible
crustal
effects of lithocrust-mantle
thickening,
thereof, two
yield regional,
Temperature
of which are in Ap-
thinning,
and crustal
will be mod-
one-dimensional,
spheric
combinations
.----+
a
pendices tachment
400
scenarios
using
P-T-t
To explain the anomalously low P/T ratios and the subsequent isobaric cooling, one first needs to investigate three obviously possible mechanisms (Fig. 3B, C, D): (l)syn- to slightly pre-metamorphic lithospheric extension (McKenzie, 1978: Wickham
and
Oxburgh,
magmatic
events
(Wells,
1985, 19X7), (2) various 1980; Bohlen,
(3) crust-mantle detachment welling of hot asthenospheric
1987), and
coupled with the upmaterial to the base
of the crust (Bird, 1978, 1979; Bird and Baumgardner. 1981; Houseman et al., 1981; Lister et al., in press). As the prograde metamorphism is conpaths
were suggested by Bohlen (1987) for Precambrian granulite terranes as, e.g., those in the Adirondacks (N.Y., U.S.A.), Bamble (Norway), Namaqualand (South Africa), Southern India and West Uusimaa (Finland). In this paper, I will first explore, in a qualitative way, the main tectono-thermal scenarios to which low-P facies metamorphism has been attributed. Then, in order to obtain physically realis-
temporaneous it is necessary
with a phase of crustal thickening, to discuss as a fourth possibility the
thermal effects of crustal thickening accompanied by an anomalously high basal heat flow. Such high basal heat flow may be caused by convective thinning of the mantle lithosphere (Fig. 3E. F), as advocated by Houseman et al. (1981). A mechanism not pursued here, is the late-stage extension of a (thermally partially equilibrated) thickened crust (England, 1982; Houseman and
194
t1 PRF,-M?STAMORPHIC
to
1250
0 0
A
F
Temperature
Fig. 3. The various correspond
tectonic
respectively
pre-metamorphic conductive
(f,),
geotherm
models
E and within
similar
to that of an extremely a thinned
in the upper replaced
crust
series
F are syn-metamorphic
C. The geotherm
and within
experiment
an undeformed
lithosphere.
thrusting
tested in this paper (B-F),
to numerical
mantle granite,
(“C)
within
thinned mantle
lithosphere
lithosphere.
and homogeneous
by hot asthenospheric
material
lb,
(tz).
lc,
lithosphere.
M-Moho; crust,
2b (also
that,
which
(the Houseman
within
an abruptly
homogeneous.
Models
1). The situations
A-asthenosphere.
within
is underlain
within
or entirely
Table
L-lithosphere;
D. The geotherm
E. The geotherm below
2a and
at t = 0. B. The geotherm
an undisturbed
mantle
which all add heat to the upper lithosphere.
la,
an undisturbed which
thickened
material.
is intruded crust,
The complete
D are
A. The steady-state
crust and abruptly
by asthenospheric a crust
B, C, D, E and F B, C and
is
by a “recumbent”
with either
mantle
thinned
This situation thickening
lithosphere
et al., 1981, model). F. As E, but with a mid-crustal
granite.
by
has been
195
England,
1986; Platt,
fold belts of northern metamorphism (Hobbs
1986)
Australia
is coeval
+
as in the Proterozoic the low-P
with
crustal
et al., 1984; Etheridge
C
B
Ten
D
F
E
facies
shortening
et al., 1987; Loos-
veld, 1989). Metamorphism
due to lithospheric
Steep metamorphic areas,
are attributed
ing (McKenzie, Le Pichon
gradients, by many
1978; Le Pichon
et al., 1982;
1985, 1987). Uniform
extension which affect large
to continental
rift-
and Sibuet,
1981;
Wickham
stretching
and
Oxburgh,
of the lithosphere
extension) can lead to astheno(“ pure-shear” spheric diapirism (or vice versa), and hence to partial melting of the lower crust, the emplacement of granodiorites in the middle crust and to
L: Lithosphere A: Asthenosphen crust-mantle kunday a1 35 km stippled area npresents anam of heat added to lithosphere and topmost asthenosphen Fig.
4.
Various
mantle geotherm
and the geotherm
line) represents and topmost
peratures
geotherm
of modern
rift zones (Bridwell
Heat
flow studies
and Potzick,
1981;
Lachenbruch, 1979; Lachenbruch and Sass, 1977; Mohr, 1982; Morgan, 1982) agree well with this model. In contrast to the classical model of symmetrical and uniform stretching of the lithosphere (the pure-shear model), the recognition of major asymmetrical detachment fault/ shear zones in the Basin
and
Range
province
(Davis
et al., 1980;
results
is linear.
neous
shear
Kenzie-model. pure
shear
pure
thinning
A-asthenosphere.
mantle
lithosphere
horizon,
with
of the mantle
the proposal of asymmetric continental extension models, the “ simple shear” models (Wernicke, 1985; Lister et al., 1986, in press). These explain
dipping,
in the mid-crust Complete delamination
excision
(Voorhoeve
and replacement
asthenospheric
material
(Lister
and
et al.-
instantaneous et al.of the
here
dipping,
at the base 1988).
horizon.
detachment of the crust
E. Instantaneous
lithosphere
by means
with the detachment
and Houseman-model,
of the mantle
(Mc-
(Lister
of the mantle
detachment
lithosphere
and instantaneous
lithosphere
Houseman-model,
simple shear thinning
in
and instanta-
simple shear thinning
of a planar,
the detachment
The
are given
lithosphere
of the mantle by means
and
A-F
of the complete
C. 300% homogeneous
thinning
within
conductive
A. 100% homogeneous
thinning
(hold
to the lithosphere
heat production
of modes
model, in press). D. Instantaneous
planar,
cay process with a time constant of approximately 60 Ma (McKenzie, 1978; Jarvis and McKenzie, 1980; Voorhoeve and Houseman, 1986) if heat is transferred by one-dimensional conduction only. Relaxation is faster if advection of fluids and/or lateral heat flow play a role. England and Thomp-
Radioactive
1978). B. 100% homogeneous
in press). shear
the steady-state
after extension
of heat added
modelling
Fig. 6A-F.
model,
are shown
area between
L-lithosphere;
respectively pure
The crust-
is left out here, so the steady-state
of the thermal
(Voorhoeve
Both models of extension, however, have their limitations. Thermal relaxation of an instantaneously thinned lithosphere is an exponential de-
asthenosphere.
extension. boundaries
immediately
the amount
the lithosphere
Wernicke, 1981; Wernicke and Burchfiel, 1982; Lister et ai., 1986; Lister and Davis, 1989) led to
the geographic offset between areas of active (supra-)crustal extension and areas affected by the highest geothermal gradients, thus providing an explanation for the absence of extensional structures in rocks characterized by low-P facies assemblages.
of instantaneous
for all modes. The cross-hatched
the generation of a condensed series of isograds. McKenzie (1978) quantified the relaxation of temafter such extension.
modes
and lithosphere-asthenosphere
lithosphere,
of the mantle
of a here
1988). F.
i.e. crust-mantle lithosphere
(Lister et al.-model.
by hot
in prep.).
son (1984) therefore, concluded that the thermal profiles of passive continental margins, compressed 60 Ma after their formation, would not differ noticeably from those resulting from compression of steady-state geotherms. For extension by means of a simple detachment zone, the lithospheric heat input is approximately a factor two smaller than for the pure-shear McKenzie model (Fig. 4 and Voorhoeve and Houseman, 1988); thus, the thermal perturbation is also smaller. In the Mount Isa Inlier, this poses a formidable problem, because, although the geochronological dates on the three extensional events and on the
196
prograde
metamorphic
no extensional the
event are widely bracketed,
structures
> 100 Ma
metamorphism.
time
have been recognized
interval
A direct
prior
for
to prograde
link between
the exten-
sional events and the prograde low-P facies metamorphism is therefore questionable and one might be tempted
to not
here. In general, extension
this line
might
character
of research
if the time gap between
and metamorphism
Ma, extension facies
pursue
however,
is narrower
not only explain
of the metamorphism
not necessarily
andalusite-sillimanite
also subsequent
isobaric
than 60
but
crustal rift can be affected by (1) lateral heat flow, aided by the generation of small-scale convection cells in the asthenosphere under a rift margin due to the steep horizontal temperature gradients (Buck, 1986) and (2) the thinning of the mantle lithosphere under the undisturbed crust, either by pure shear as in the asymmetric extension model of Lister et al. (1986, in press), or by simple shear as in the asymmetric extensional models of Wernicke (1985) and Voorhoeve and Houseman (1988). Most asymmetric extension models predict a horizontal offset between that part of the lithosphere, which is most attenuated, and that part of the upper crust, in which rift structures are developed: anomalously steep geothermal gradients, due develop
crust),
will
upper plate”
crust (the hanging wall, i.e. the “upper of Lister et al., 1986). This upper plate
situation is simulated heat flow complication. Heating
in a relatively
( * lower
here,
omitting
undisturbed
the lateral
by intrusion of magmas
Crustal thickening by magmatic accretion will result in very high relative temperatures in all levels of the crust (Wells, 1980) and, temporarily, very steep geothermal gradients above the intrusion (100-500 o C/kbar, Bohlen, 1987). Rocks above the intruded material will heat isobarically, and subsequently, during the thermal relaxation,
differentiated
granites
and cooling,
continue
role in the thermal
because
they
heat-producing anomalously
elements high heat
metamorphic
granites
may
be
balance
enriched
in
(HPE). In this study, the production in the pre-
of the Mount
Isa Inlier
will
be calculated. Wyborn entiation.
oped locally. The thermal evolution of tectonically undisturbed crustal areas adjacent to a late supra-
lithosphere
area,
(although facies),
Highly
to play an important of an
the Mount
cooling.
of the mantle
isobarically.
can, even after emplacement
the low-P
A possible scenario could be that late-tectonic, and as yet unrecognized, asymmetric rifts devel-
to the thinning
cool
et al. (1988) argue that the granites Isa Inlier result from a two-step As magmatic
equilibration
of a highly
events
represent
perturbed
of
differthe re-
thermal
(or
pressure) gradient, these granites must be the result of two successive thermal perturbations. Discussing the first thermal perturbation, Wyborn et al. (1988) mention the possibility of both extensional and compressional events, which can lead to mantle melting, and, mainly because primary magmas from the mantle are denser at crustal depths than crustal material (Herzberg et al., 1983) to underplating at the base of the crust. These underplates the major, Inlier. Crust-mantle
could I-type
subsequently be the source of batholiths of the Mount Isa
detachment
In most cases of underplating, exchange of cold, dense lithosphere by hot, less dense asthenosphere, plays a role (Wyborn et al., 1988). Such substitution can be explained in four ways: (1) by conductive thinning of the mantle lithosphere, due to a heat flow perturbation (thermal “plume”); (2) by “delamination”, i.e. the vertical separation of crust and upper mantle by the bending of a coherent slab of dense mantle lithosphere, whose tip sinks into the less dense and hotter asthenosphere (Bird, 1978, 1979; Bird and Baumgardner, 1981); (3) by convective thinning of the mantle lithosphere during crustal thickening (Houseman et al., 1981); (4) by juxtaposition of hot asthenospheric material in the footwall of a major throughgoing (extensional) detachment zone, and cooler lithospheric or crustal material in its hanging wall. Crust-mantle delamination (2) and convective thinning (3) are driven by the gravitationally unstable layering of relatively dense mantle lithosphere over less dense asthenosphere. Independent
197
of the mode
of substitution
of the mantle
sphere by the asthenosphere, anomalously Etheridge
high
crust-mantle
Proterozoic Inlier, event
Orogeny
fold belts of northern
however,
differs than
from
the
tion has been simulated sional and compressional
that extenexplains to Middle The
Mount
1860
Ma
Isa old
Of the six factors Mount
Isa Inlier.
lead
AT = 0). The subsequent erosion.
Possible
substitu-
thermal
relaxation
extenfacies
conditions and subsequent isobaric cooling, they cannot explain the synchroneity of the prograde low-P facies metamorphism and a pervasive phase of crustal thickening (+ A P, + AT). One is forced to re-examine the effects of thickening on the profile. Brady (1982) heat transfer in an oro-
genie setting, and summarized the factors leading to lower than normal P/T ratios, and possibly to in such an environment:
(1) synmetamorphic regional (2) a very strongly insulating (3) transport of hot rocks to tectonic movements or by rapid
post-tectonic
(4) a mantle heat flux triple the normal (5) a very thick radioactive crust;
value;
(6) a volatile flux equivalent to one rock volume of fluid per 5000 yrs. Additionally, Reitan (1968a, 1968b, 1969) Graham and England (1976) Molnar et al. (1983) and Werner (1985) considered frictional heating in tectonically active areas, such as subduction and overthrust terranes. Frictional heat can condense isotherms locally (and even lead to locally inverted metamorphic geotherms), but cannot lead to a widespread, significant temperature increase. Furthermore, there are no indications that frictional heat played a more prominent role in the
metamorphism. compression,
heating-up
by isostatic melting
Ineither
more heating
uplift
(+ A P,
phase will be and
consequent
in the lower crust by this
is therefore
(Miyashiro,
normally
1973;
Richardson, 1977). Clockwise P-T-t trajectories result. to factor
late-
England
to and
(Alpino-type)
2 (the unusually
insulat-
ing sediment pile), Jaupart and Provost (1985) considered conduction of heat across the Main Central Thrust of the Himalayas. They concluded that the location of young leucogranites at the top of the basement sheet can be explained by the heterogeneous vertical thermal conductivity distribution, i.e. a low conductivity in the upper sediments versus a high conductivity in the crystalline basement. There is no evidence, ever, that a similarly lain the Proterozoic during
insulating sequence inliers of northern
lower how-
has overAustralia
the metamorphism.
Factor
plutonism; sediment pile; shallow levels by erosion;
facies
or with slightly
metamor-
there
1, 2, 3 and 4 in the out in the introduc-
stead, it will lead to progressive adiabatically,
in orogens
paths.
as such will not generally
low-P
With respect
evolving temperature-depth modelled one-dimensional
As pointed
facies
with some of the extenmodels.
for factors
thickening
to coeval
than
P-T-t
listed by Brady (1982)
is no direct evidence tion, crustal
terranes
by clockwise
lithosphere
It is argued in this paper that, although sion and magmatism can result in low-P
stability
Proterozoic
accompanied
Crustal thickening
andalusite
Australian characterized
is at least
tectonism
mantle
crust.
Australia.
in the
the low-P
In this paper,
the
in the 1860 Ma
in the Early
in that the extensional
litholeads to
delamination
Ma old metamorphism
100 Ma older phism.
in
facies metamorphism
old Barramundi 1550
temperatures
et al. (1987) have suggested
sion-triggered the low-P
it invariably
3 (rapid
upward
transport)
only plays a
role if the upward transport is faster than thermal relaxation. As mentioned (Loosveld, 1989) folds in the
Mount
Isa
isoclinal
and
upright
Inlier and
are generally the lower
tight
P/T
to
ratios
might be expected in the major antiforms. Even though andalusite-sillimanite blastesis is synchronous with this folding event, such a structurallycontrolled P/T ratio distribution is not documented and thermal relaxation must be faster than deformation. Indeed, for upright folding with reasonable strains and strain rates, this must be generally the case: thermal relaxation of a lateral perturbation of periodicity L,, with L, the halfwavelength of a fold, has a time constant of Lf/?r2~ (with K the thermal diffusivity), so that, e.g., for L = 10 km and K = 10ph m*/s, the thermal time constant is only = 0.3 Ma. Widespread uplift
by erosion
and/or
tectonic
denudation
do
198
not
have
facies
to be considered
metamorphism
either
as the low-P
is synchronous
to crustal
thickening,
and the post-compressional
segment
the P-T-t
path is essentially
(Reinhardt
isobaric
of
by Oliver et al. (1987) and Oliver and Wall (1987). In this
study,
crustal
single-pass
(factor
4)
in rocks
1.6 Ga ago was 20 to 40% higher
it is now
Etheridge
C,“/C,”
1975; Davies, Channelized tion phism,
1980; Turcotte mantle
for local,
than
= 104; McKenzie
and
Weiss,
and Schubert,
1982).
heat flow can be an explana-
prograde
but a constantly
low-P
facies
elevated
basal
experiments
metamorheat
flow
simulating
will, in simplified
fluid
flow and
(the role of convection,
and Hamilton, in prep.). The rate of mantle heat production (assuming
numerical
thickening
below
3-6
form, include
its advection
i.e. multi-pass
of heat fluid flow,
km is still contentious;
et al., 1984, versus Wood
e.g.,
and Walther,
1986). I will also evaluate convergence
a special case of continental
or thickening,
which combines
ening of the crust with (convective) subcrustal
lithosphere.
This
thick-
thinning
of the
case was argued
for
will not affect the sense of rotation of the P-T-f path (clockwise versus anti-clockwise). Disregarding factors 1, 2, 3 and 4 leaves radio-
by Houseman et al. (1981) for a lower lithosphere and asthenosphere with the same constant viscos-
active selfheating
stratified, viscous Newtonian authors reason that as the
and heat transfer
by fluid advec-
tion processes as the most important factors. England and Thompson’s (1984) and Davy and Gillet’s (1986) studies on the thermal balance erogenic zones are in this regard the most
of de-
tailed. England and Thompson’s (1984) experiments quantitatively incorporated radioactive selfheating after an instantaneous crustal/lithospheric thickening event, and qualitatively discussed fluid also omitted
ity, and by Fleitout
dense
and Froidevaux
lithospheric
mantle
(1982) for a
lithosphere. These relatively cool and thickens,
it
is
sub-
merged into the underlying, hot and less dense asthenosphere. Thus, its gravitational instability is increased, overlying
and it may (partially) detach from the crust (and uppermost mantle litho-
advection. Davy and Gillet (1986) fluid flow from the numerical code,
but introduced multiple time-dependent thrusting events. Both models can explain low-P facies decompressive metamorphism (- AP, k AT) by the combination of slow uplift and an increased radiogenie heat supply in the thickened crust. Instantaneous thickening of a crust with a normal geothermal gradient is unlikely, though, to result in prograde andalusite-sillimanite blastesis during crustal thickening. P-T-t paths from the numerical experiments are invariably Mount Isa Inlier, nevertheless, limanite blastesis is prograde
clockwise. In the andalusite and siland compressive
(Reinhardt and Hamilton, in prep.; Loosveld, 1989) and is followed by essentially isobaric cooling (Reinhardt and Hamilton, in prep.): the P-T-t paths are anti-clockwise. Deformation, however, can increase permeabilities and fluid flow can become an additional mechanism of heat transfer (Fyfe et al., 1978; Etheridge et al., 1983, 1984; Ferry, 1984; Bickle and McKenzie, 1987). In the central Mount Isa Inlier. large-scale fluid flow has been documented
1
2
Fig. 5. Five schematic accompanied
by progressive
lithosphere.
overlying
crustal
thickening
lithosphere
hot
continues,
and
more
the downgoing is detached
2)
later
convection
less dense
lithospheric
of
of the
cold and dense
litho-
belt is submerged
into
asthenosphere. material
litho-
At the onset at the base
as the relatively mountain
of the mantle
conductive
As thickening
is swept
sideways
from the crust (stage 4). If convection
the remaining
Houseman
thinning
a meta-stable
thickening
into
plume (stage 3), until the entire (?) lithosphere
material
continuous these
(stage
5
crustal
asthenosphere.
root of the incipient
relatively
spheric
convective
convecting
is enforced
4
of progressive
Stage 1 represents
sphere
spheric
3
stages
continues stages
of crustal
and extremely stages
under
of astheno-
the base of the crust during thickening
(stage
5). then
a
high basal heat flow will accompany
of crustal
thickening.
et al. (1981). Vertical
After
scale greatly
a model exaggerated.
by
199
sphere)
to sink
into
the asthenosphere
(Fig.
5).
Houseman
et al. (1981)
argued
in favour
of the
detachment
of the entire
mantle
lithosphere
from
the crust,
exposing
the lower
spheric
temperatures.
similar
to the crust-mantle
as proposed to be resolved: combination ening grade
advection
selfheating lithosphere
andalusite-sillimanite
tially isobaric
in a thicklead
blastesis?
to proAnd
sec-
conditions is the prograde stage succeeded by essen-
cooling,
all anti-clockwise
have does a
of heat by fluid flow, and
of the mantle
ond, under what andalusite-sillimanite
Bird and
questions
what conditions
of radiogenic
crust,
thinning
first, under
is
process
1979) and
(1981). The following
thus giving rise to an over-
P-T-t
gated.
only
a few of these need
As thermal
the entire
generally
melting
to be investi-
after
thickening
leads
pressional
geotherm,
combination carded Id,
le
sphere
those and/or
and
lf),
of this
here. Also dis-
combinations
with upwelling
magmatism
are envisaged
active
to
to syn-com-
the possibility
asthenosphere since
upwelling
tectonically
and
(No. 2c) is discarded
are
magmatism
of
to late-
in the lower crust, such melt-
ing will have no effect on the early-
setting.
isolated
and
(Nos.
astheno-
to take place in a
Here, only the thermal
effect of asthenosphere upwelling is considered, regardless of the mechanics of replacement of cold lithosphere
by
3-20 (Table
1) combine
mental
path?
relaxation
lithosphere
post-tectonic
to astheno-
this situation
delamination
by Bird (1978,
Baumgardner
crust
Thermally,
straints,
series
hot 1 and
asthenosphere.
Possibilities
the components 2, and
of experi-
are more
complex,
Results of the numerical experiments
involving more assumptions and free parameters. Because of the complexity, and in order to evaluate
In all reported cases of low-P phism in the northern Australian
isolated rather than randomly combined processes, possibilities 3-20 are also left out. This leaves an early extension with or without asthenosphere up-
facies metamorProterozoic fold
belts, the prograde metamorphism is coeval with a period of crustal thickening. Hence, the thermal
welling
and/or
magmatism,
followed
by compres-
to the
sion (Nos. la, lb and lc), and compression combined with asthenosphere upwelling, with or
anomalously high temperatures must have been interacting with the overall stretching of the geo-
without magmatism (Nos. 2a and 2b). These five possibilities are depicted in Fig. 3B-F.
therm caused by crustal thickening. Combining the various pre- to syn-metamorphic modes of crustal heating with syn-metamorphic crustal thickening will therefore be essential. Table 1 lists the possible combinations. Due to geological con-
The first group of numerical experiments deals with two events, an early crustal extension with optional asthenosphere upwelling and magmatism, followed by a phase of compression (Table 1: series 1). The second group is concerned with one
effects
of any
TABLE
1
Possible
combinations
mechanism
that
has
led
of events, which lead to low-P
facies metamorphism
Exp. series
Pre-metamorphic
Syn-metamorphic
Fig.
la:
extension
(pure shear)
(compression)
3B
la:
extension
(simple shear)
(compression)
lb*
ext. + asthenosphere
Icompression)
3c
lC*
ext. + magmatism
(compression0
3D
Id
magmatism
(compression)
upwelling
le
asthenosphere
If
magm. + asthenosphere
upwelling
(compression) upwelling
(compression)
2a*
asthenosphere
upwelling
2b*
_
asthenosphere
upw. + magm. + compression
2c
_
magmatism
3-20 Combinations (explanation
variations marked in text).
by an asterisk
are considered
possible
+ compression
3E 3F
+ compression of 1 and 2 series
for the Australian
Proterozoic
fold belts
and
are modelled
here
200
B rooo horn. thinning of entire lithosphere
100% extension of mantle lithosphere
E B8(@ G
44kmm::
-
6OOf, F t
1okm..
200 y
6km..
0
0
,‘,‘.‘*“‘.“‘.‘. 10
20
o
2km
30
40
so
60
70
+
80
.+
90
.,.,.,.~.,.~.~.~.~*~
10
0
100
20
30
40
50
60
70
+
time[Mal
I
300% extension of mantle lithosphere
80
90
100
timeIMa1
detachment at 40 km
44km:: 36km PEl 22km 18 km:: 14 km lOkIn” 6km..
, zkmt 0
0 .‘.‘.‘.I. 10
20
30
40
50
60
70
+
80
90
"0
100
10
20
30
40
50
60
70
+
time[Mal
80
90
F
I
L
detachment at 20 km
EL200
0 .‘.‘.I. 10
20
30
40
50
60
70
---+
Fig. 6. Temperature-time undisturbed.
7;=, = 1350 o C. Experiment mm’; K = 1.2. 10e6
km with
thinned
A,, = 4.1 pW/m3
parameters: m2/s;
experiments,
I
.
10
to Voorhoeve
pre-thinning
A, = 2.5 pW/m3;
is also reflected
lithospheric
I
.
20
,
.
30
I
I
40
50
lines represent
.
I
I
60
70
and Houseman,
respectively
the 10, 18, and 26 km level, using Holdaway’s
heat
extension
thickness distribution.
F, the crust
crustal
4 km thick granitic of heat
100
remains
has excised
thickness
slab between
in the crust
the temperature
domains
90
the
33 km thick; at the base of the
is 700 km; pre-thinning Transfer
.
is 100 km, the dip of the
1988). In E, the detachment
In F the crust is effectively
I
80
time[Mal
in Fig. 4. In B, C, D and
D = 10 km. A pre-thinning,
in the radiogenic
only. The three horizontal
The depth/time
depicted
overlies the upper mantle.
conduction
of 2700 kg/m3.
0
100
by a factor 2. For D and E, the horizontal
is 12” and the throw is 21 km (comparable
K = 3W K-’
I
+
for the six “extensional”
plots
lower crust, such that the upper crust directly crust,
90
complete excision of mantle lithosphere
time[Ma]
In A, the crust is homogeneously
detachment
80
R
ot
zkm
0
100
time [Ma]
after
is 35 km; 18 and 22 r = 0 is by
levels at which either andalusite or sillimanite becomes stable at and assuming an upper crustal density (1971) Al,SiO, reaction conditions,
in which either andalusite
or sillimanite
is stable are dotted.
201
tectonic
event, i.e. Houseman
consisting coeval
of crustal
thickening
asthenosphere
magmatism
(series 2). In all experiments, conservation
solved
a simple,
finite
difference,
(details
and
of energy
BASIC
in appendices
are typical margins
are unlikely
to be found
the one-
An abrupt
is
iterative,
code on a MacintoshTM
extension
series 1: lithospheric
superposed
mal steady-state
Whether
andalusite/
during
dates
an extensional
variables location
event,
event,
are
which post-
will depend
on slightly
conductive relaxation
on such
as the amount and style of extension, the within the extensional setting, the time
interval between extension and compression, and the ratio of the compressional strain rate to the thermal relaxation rate. Additional heat sources may result from crust-mantle delamination or magmatic intrusion. The abrupt change of the extension is degeotherm by “instantaneous”
upper
The thinning is absent tion
when
by a factor of 2 steeper-than-nor-
geothermal
gradients,
conditions
(cooling),
however,
of the HPE-enriched
upper
crust
simulating
plate
situa-
of asymmetric
the upper
extensional
models,
(3) thinning of the lithosphere by movement along a planar, dipping detachment horizon (Voorhoeve and Houseman, 1988). Loosveld and Etheridge (in prep.) argue that andalusite-sillimanite conditions will generally only result from lithospheric extension (without magmatism and/or delamination) if extension factors equal or exceed 3. Such large extension
a
other hand, the heat input to the lithosphere (represented by the shaded area in Fig. 4) is, at the same amount of crustal extension, smaller in the asymmetric extensional models than in the symmetric
“pure
shear” and
lithospheric
Houseman,
thinning
tions. If thermal relaxation is attributed duction alone, even an “instantaneous”,
(1) homogeneous “pure shear” thinning of the entire lithosphere after McKenzie (1978); (2) homogeneous “pure shear” thinning of the mantle lithosphere while not deforming the crust (the “ upper plate” of Lister et al.‘s models, in press);
where
thinned mantle lithosphere underlies an undisturbed crust (Figs. 4B, C, D), or where the mantle lithosphere is completely excised (Fig. 4F). On the
respectively. The steady-state conductive in the experiments of Fig. 6 is relatively
Extension alone: experiment series la Three modes of-instantaneous-lithospheric extension were simulated:
will of the
crust by a factor 2.
(Voorhoeve
it includes a cooled, 4 km thick, HPE-enriched granitic slab between the 18 and 22 km levels (representing conditions of the Mount Isa Inlier).
(Fig.
thinning
picted in simplified form in Figs. 4A-F. Figures 6A-F give the thermal relaxation on ten depth levels associated with the situations in Figs. 4A-F geotherm “hot”, as
rifts.
plane strain “ pure shear”
be fast, aided by the concomitant
thinning
sillimanite-conditions
a compressional
et al., 1982), but
in intra-continental
result in sillimanite
HPE-enriched stable
parts of passive
of the entire lithosphere
(@ = 2)
will generally
2 and 3).
(Le Pichon
homogeneous,
6A). Thermal Experiment
for the outward
continental
with
equation
one-dimensional,
factors
optional
combined
upwelling
dimensional with
et al.‘s (1981) model,
adding these competing still effectively buffers
1988;
Fig.
models 4). On
effects, the lower crust basal heat flow fluctuato con300%
pure-shear extension of the mantle lithosphere, immediately followed by conductive cooling, will not always lead to sillimanite grades in the middle crust (Fig. 6C). Simple shear experiments (experiment series la,) yield results even further removed
from sillimanite
cept for those situations zone 6E).
happens
to transect
conditions
(Fig. 6D), ex-
in which the detachment the middle
crust
(Fig.
Whether or not andalusite-sillimanite conditions are reached will strongly depend on the steepness of the pre-extension, steady-state geotherm, and on the mode of extension, but, independent of these factors, these conditions will not be stable for more than 10 to 15 Ma. (Figs. 6A, E, and F.) As extension will not be instantaneous, T/P ratios will in fact be lower than those of Fig. 6. In all modes of extension, thermal relaxation is fast; after 50 Ma, temperatures in the middle crust are only slightly higher than the original steadystate ones. Additional heat transfer mechanisms,
202
other than conduction, thermal
would result in even faster
within
the mantle
laxation
relaxation.
paths
lithosphere
Under mantle
+ delamination: an “upper-plate”
lithosphere
pletely
can
experiinent series lb passive margin, the
be
attenuated
or com-
excised (Lister et al., 1986, in press). In the
latter
case, hot asthenospheric
material
the lower crust (Fig. 4F). This situation lated
by making
the pure
shear
is simu-
extension
factor
“freezing”
of the asthenospheric
“hot
mantle
mode”
and
delamination
1981). The “hot
of Bird
mode”
and
“cotd mode” delamination
t
e.skm-. Ybn-. 4skmm-. 39km_. 33 km 27km
i------_ ~21km
Pkm-. 3km-.
0 0
10
20
30
40 +
B
50
time [Ma]
I
first 1 Ma
in the “cold mode”
boundary
is shifted back to lithosphere
im-
delamination and in the crust by conduction only, the
heat transfer
andalusite-sillimanite
field may, depending
on the
steady-state conductive geotherm, conductivity and the thickness of the crust above the detach-
in the experiment depicted in Fig. 6F, in which the mantle lithosphere (including the 36 km depth level) is detached, sillimanite conditions were reached at the 26 km level between 2 and 12 Ma
lmo , Tcmp
here by keep-
ment horizon, be reached at the deeper crustal levels (compare Fig. 6F with 7A, and Bird, 1979);
15 km” -
crust-
Baumgardner,
is simulated
the base of the pre-extension mediately after delamination. Considering “cold mode”
Temp
(respec-
mode”
at the base of the crust con-
the lower temperature
[Cl loo0
material
the “cold
stant for a given time, whereas
1500
material both with
ongoing convection in the anomalously shallow asthenospheric material, and with immediate
ing the temperature
A
delamina-
of asthenospheric
to the base of the crust were calculated
tively
underlies
large. Re-
after such crust-mantle
tion with the upwelling Extension
infinitely
“hot mode”
after the instantaneous delamination. The experiment depicted in Fig. 7A, on the other hand, in
(Cl NJ0 4skm 39km 33km 27 km
t 500
Fig. 7. Temperature-time periments.
T = 281-281eeX”’
3km
0
0
10
20
30
40 -time
The initial
50
T=1350°C km. T,,,
[Ma]
m2/s;
+7.13x,
(in A) and 1300°C
stable
km level. The dotted domain heat
t
blastesis.
in the crust
after
crust-mantle
crust. 20
30
40 --+
time
[Ma]
50
upwelled
at respectively or sillimanite No
crust-mantle
crossor
the 9, 15, 21 and 27 the time/depth
is stable. Transfer
of
only. A. Cold
andalusite/sillimanite
delamination
for 1 Ma,
at the base of the crust is kept at 1300 o C rigorous
asthenospheric
convection
material
C. Hot mode crust-mantle
the temperature
is:
rc=10V6
either sillimanite
t = 0 is by conduction
for 1 Ma, thus simulating newly
at which
delamination.
B. Hot mode
i.e. the temperature
K-r;
The four horizontal
fields in B and C represent
in which andahtsite
(Cl 1000
ex-
(in B & C) for 391x<150 K=2Wm-’
the temperatures
becomes
“delamination”
in all three experiments
D = 15 km, A, = 2.5 pW/ms.
andalusite
mode
for three
with x in km, for x I 36 km, and
(at150km)=1350°C;
lines represent
Temp
plots temperature
of heat in the
at the base
delamination
of the
for 20 Ma, i.e.
at the base of the crust is kept at 1350°C 20 Ma.
for
203
which the detachment and in which ductive
geotherm
lima&e
at similar
in the upper
the experiment in the
time-gap actual
steady-state, did not depth
crust
to trigger
levels.
between time
the Mount
12 Ma.
sillimanite This
Isa Inlier,
and
Peak be-
but are growth.
In
is only sta-
implies
the compressional
of andalusite/
sil-
are reached
of Fig. 6F, sillimanite
first
con-
yield
10 and 20 Ma after delamination,
not high enough ble
is at 37.5 km depth,
cooler
was chosen,
conditions
temperatures tween
horizon
a slightly
that
the
felsic magmas elements igneous
slab
slab with a radioactive
an order of magnitude granites
are powerful
Intrusion crust
higher.
of such granites
growth
heating
this “cold
mode”
cooling and
heat production
of the magma cooling
of the country
the other
therms during crustal thickening, it is unlikely that “cold mode” delamination alone can explain the
temperatures. The abundance of pre-metamorphic, riched granites in the Mount Isa Inlier
To obtain time
higher crustal
intervals,
another
and
temperatures heat
source
regional
Australian for longer has
to be
sought, either some form of “hot mode” upwelling of the asthenosphere, or immediate rising of mag-
Proterozoic
1987) justified effects cumbent”
cause
fold belts;
intrusions.
Wyborn
et al.,
of the thermal
Seismic,
masses
high
HPE-en(and other
heat
data have led to the concept batholithic
is
anomaly,
anomalously
a closer examination
of granitic
and gravity
effect
by the HPE-distribution
will permanently
the
and concomitant
brought
synchroneity of crustal thickening low-P facies metamorphism.
or upper
the first one
lamination could not have exceeded 12 Ma. Considering the progressive, vertical stretching of iso-
which
of
heat sources.
in the middle
metamorphism),
about
by an
Such HPE-enriched
effects:
subsequent
rock (contact
that a mid-
is replaced
and permanent
has two thermal
sillimanite
in heat-producing
It is not uncommon
or upper-crustal
transient
de-
(HPE).
is more complex,
enriched
crustal
event, i.e. the in
the heat balance
as they are commonly
flow
of “re-
(Hamilton
and
matic products. Hot mode delamination will lead to the expansion of andalusite-sillimanite conditions to higher crustal levels. In Fig. 7, three experiments with respectively 0, 1 and 20 Ma hot mode delamination are depicted. This figure shows
within these granites (A,,) of 2.5, 5.0 and 7.5 pW/m3. Taking into account the metamorphic grades of the granites in the Mount Isa Inlier, they
that, as expected, hot mode delamination would result in a very powerful heat source at the base of
must have been at mid-crustal levels during metamorphism. I have assumed the top of the granites
the crust.
Myers, 1967). Here, we use 3, 6, 9 and 12 km thick slabs of granite, and average heat productions
at 18 km. Figure presence
of such
8 shows the thermal a differentiated
effect of the granite
in the
Extension •t magmatic events: experiment series lc In many numerical experiments which simulated various modes of extension, the temperature
rock pile. The temperature change (T= ,/T,= decreases linearly with distance to the granite.
in the lower crust exceeded the solidus temperature for some time. After a 300% pure-shear extension of the mantle lithosphere for example, the
plained by transient thermal pulses caused by extensional events, magmatism or cold mode delamination, if these events are synchronous with,
temperature at the 30 km level is briefly raised to a maximum of 660” C (Fig. 6C), so that melting and subsequent advective heat transport may take
or immediately predate, the metamorphism. Lower and mid-crustal rocks will then for a limited time remain anomalously hot: in the case of magmas in
place. Furthermore, pressure-release melting of the mafic upper mantle and subsequent rise of mafic melts may trigger more lower crustal melting. The thermal perturbation of a mafic magma emplaced at a depth of 15 km and with a thickness of 10 km is, however, only short-lived (Wickham and Oxburgh, 1985). With fractionated
extension and/or cold mode delamination a few tens of million years at the most. For the low-P facies metamorphism to be prograde (t AT), the time-gap has to be even narrower. Prograde and compressive low-P metamorphism ( + AT, + A P) cannot be explained by any extensional event.
i)
From experiment series 1, it can be concluded that low-P facies metamorphism can only be ex-
the order of a few million
years, and in the case of
204
T/To
slightly
decreasing
Hamilton, t’“t
by conduction reasonable
1.5-
l
.
1.0
@
3
6
itself,
drastically
increase
duction
12 15 to top of granite [km]
9 ---+distance
parameters
One
transferred
ratios
ratios
during
should,
thickening
conclude
alone (e.g. advection
flow), or that crustal
is for all
slower than the lithospheric
P/T
must
and
relaxation
lithosphere
that
by a more effective
an increased
(Reinhard
Fig. 2). As thermal
of a thickening
thickening decrease.
12km
l
P/T
in prep.;
however, rather
either
process
heat
is
than con-
of a single-pass
thickening
than
is associated
fluid with
basal heat flow.
The effect of single-pass fluid flow The effective fluid flux, W, is normally
deduced
from D’Arcy’s ically inferred
Law, or calculated fluid/rock ratios.
from geochemAs, at present,
there
reliable
on
are
few
estimates
fluid/rock
ratios and original rock permeabilities (in Early to Mid-Proterozoic fold belts), 1 use an alternative d3km
3
6
12 15 to top of granite [km]
9 ~distance
T/To
C
approach. Walther and Orville the amount of post-diagenetic products
during
regional
E*
A~=75
Fig.
m3km
’
l.OI
’
8. The long-term
HPE-enriched T,= ,/T,= granite
granite
_, is plotted for various
2.5 pW/m3; Experiment
’
6
3
(B):
on versus
thicknesses A,
parameters:
tive geotherm
9 +distance
thermal
before
effect
the
.
’
’
’
12 to top of grani:e’[km]
of the intrusion
overlying
country
’
of a rocks.
the distance
to the top of the
of the granitic
slab. (A): A,, =
= 5.0 pW/m3;
(C):
A,, = 7.5pW/m3.
L = 150 km; the steady-state intrusion
is defined
conduc-
by T,,,, = 1350 o C,
A,=2.5~W/m3,D=15kmandK=2WK-‘mm’.Thetop of the intrusion region
is in each case at 18 km depth.
of the plots represents
temperatures
and is therefore
Experiment
metamorphism
of an
average pelite as being 12 vol.%. Assuming a thickness, h, in meters of pelitic material, and assuming a constant fluid-flow rate during deformation, and disregarding dergoing the transition
1.5
(1982) calculated devolitalization
above
The top left the solidus,
unstable.
series 2: crustal thickening
The first segment of the P-T-t path deduced for the Mount Isa Inlier shows compression with
the fact that rocks unfrom andalusite-bearing
assemblages to sillimanite-bearing assemblages would already have been relatively dry, W, the effective fluid flow (in m/s) above this pelitic layer is approximated as (O.l2h)/(time of deformation in seconds). This method is not only simple to incorporate
in the numerical
code,
but it
also ensures that no unrealistic amount of fluids is being withdrawn from the crust. The effective fluid-flow the wide
rates obtained in this way fall within range of fluid flows calculated with
D’Arcy’s Law (Etheridge et al., 1984). The result of constant fluid-flow rate during deformation is a constant extra heat input on any crustal level above the pelitic layer during deformation. By generously assuming h as 20 km, the final amount of expelled fluid has a column thickness of 2.4 km. In Fig. 9, the effect of the steady expulsion of this amount of fluid during two experiments (curves I and 3), involving constant strain-rate deformation is shown and compared with two identical experiments (curves 2 and 4)
205
sphere,
when the basal
heat flow decreases
again,
the crust
may cool isobarically
or slightly
decom-
pressively
(if end of convection
coincides
with the
end
of crustal
thickening),
end of convection thickening).
is prior
or compressively
(if
to the end of crustal
Thus, an overall
anti-clockwise
P-T-t
path may result. Houseman
2.0 150
350
250
during
of upwards
progressive,
directed
homogeneous
strain
numbers
I and 3 result from experiments
advection
rate.
component
3, W= 2.5.10-‘* ments without 10mh m2/s; thickness
numbers
T,,
D =15
km;
is 35 km;
thickening,
P-T-t
post
s-’
thermal
a fluid
m/s;
for comparison.
K=2W
kg/m3.
in exp.
from experi-
K-’
m-t;
Strain
(duration
only. No decompression
the
crustal
rate,
i, of
of thickening (duration
thermal
instantaneous
formulated
thickening,
the time,
t,, of the peak of kinetic
coincides
with the time of detachment
mal boundary
layer
from
energy,
the upper
rigid
lo-30
Ma. This probably
the order of the time-scale event.
Houseman
experimental
runs
of a crustal
et al. (1981) with
did
progressive
not
include
deformation,
be subjected to asthenospheric temperatures ing the (later) stages of thickening.
that the
(convective)
The only remaining model that can explain compressive and prograde (+ AP, + AT), regional, low-P facies metamorphism is one with directly links crustal thickening with a contemporaneous, anomalously high, basal heat flow, as for example Houseman et al.‘s (1981) model, which combines crustal thickening with convective thinning of the mantle lithosphere (Fig. 5). This model can result in the prograde transection of the
is in
thickening
that the base of the crust may well
The viability of anti-clockwise P-T-t a result of Houseman et al.‘s (1981)
without the fluid flow. One may conclude advection-of-heat effect is only minor.
layer:
log( to) is inversely proportional to f. The time of the peak of kinetic energy, t,, ranged in their experiments between 4 and 88 Ma, with the bulk
of
accompanied
which
of the ther-
but concluded
relaxa-
the
f, and
is
after thickening.
Crustal thickening and associated thinning of the mantle lithosphere
et al.‘s (1981) study
between
of the runs between
a=
fend =1.6;
the initial
3 and 4 was 10m’5.3 s-’
relaxation
a
incorporating
is 30 Ma). At the end of thickening,
tion was by conduction
with shown,
is 150 km: = 2700
flow
paths
A, = 2.5 PW me3;
1 and 2 was 10-‘4.3
fluid
2 and 4 result
=1350°C;
thickness
30 Ma) ( of experiment thickening
four
the fluid flow, and are plotted
lithospheric
experiment
the
(in exp. 1, W= 2.5.10-”
m/s);
In all experiments initial
From
single-pass
crustal
constant
relation
TempKl
+ Fig. 9. The effect
550
450
dur-
paths model
as is
tested here by a few experiments with widely varying parameters. Crustal thickening was approximated as being homogeneous throughout the crust. The finite difference grid was fixed to the medium
and
thus deformed
with thickening
(the
change in timestep-spacing being quadratically proportional to the change in depthstep-spacing). Strain rates were kept constant in each given experiment; value between 10p’4.3 and 10-‘5-3 ss’ were used. The finite amount of thickening was by a factor 1.6 (total time of thickening therefore varies between 3 and 30 Ma). Convective thinning of the mantle lithosphere was simulated by a sudden shift of the lower boundary condition: T= 1250-135O’C is shifted to the base of the (deforming) crust. Thus, a situation is simulated in
andalusite-sillimanite field during crustal thickening, if thickening is slow relative to fast (= early)
which the complete
convective thinning of the upper mantle. Then, the condensing of isotherms in the crust due to increased basal heat flow will compete with the stretching of isotherms, caused by the (progressive) crustal thickening. Hence, the temperature on a certain depth level may rise, and the overall geothermal gradient may steepen. Subsequently, at the end of convection in the upwelled astheno-
from the crust (thermally, this model is identical to crust-mantle delamination). At the end of convection, the lower boundary condition was abruptly shifted back to the base of the pre-deformation lithosphere. Cooling at the end of crustal thickening is purely isobaric if no erosion or tectonic denudation takes place. Those experiments characterized by a relatively fast convective
mantle
lithosphere
is detached
206
thinning,
low
strain
rates
and
long
Geological setting of the Mount Isa Inlier
convection
times at the base of the crust yield anti-clockwise P-T-t
paths (Fig. 10).
A recent
review of the geological
history
Mount Isa Inlier has been presented (1986, 1987). The main tectono-thermal summarized inliers
in Table
unconformably
sequences,
2. Pre-1875 underlie (Glikson
rick, 1982; Ellis and Wyborn, Blake et al., 1985). There oceanic main
Ma basement
two or three cover
which have been interpreted
rift and sag sequences
crust
was formed
rift sequences
have
of the
by Blake events are
as ensialic
et al., 1976; Der1984; Sweet, 1985;
are no indications during been
that
any stage. The deposited
during
Del, the first extensional event, and De3, the third extensional event. D,, has been dated by its felsic
Too
Fig. 10. Seven P-T-t geneous ment
crustal
Strain
thickening
was
thickening, me’
only
after
m2/s;
after
crustal
includes
of 4.1 pW/m3
sent the Proterozoic thickening
is 30 Ma;
s-’
W= 7.6.10m12
( of experiment
m/s);
tion of thickening sive substitution was
boundary
is Ma;
(duration
approximated
tdc,. In experiments
m/s);
of
e of experiments
m/s).
abruptly
Strain
se’ (duration
is 10 Ma;
7 is 10m’4.3 s-’
lithosphere
by
22 km to repre-
Isa Inlier.
of thickening
W= 2.5.10~”
of the mantle
condition
is thought
of the Mount
4.5 and 6 is 10~‘4.“2
is 25 km. The
with a radiogenic
1, 2 and 3 is 10-‘5.3 W = 2.5. lo-l2
or
lithospheric
the 18 and
distribution
situation
(, of experiments
between
kg/m’;
no erosion
(dura-
The progres-
by asthenospheric shifting
the
lower
(T = 1350 o C) to the base of the crust
at
1, 4 and 7, t r - 0 Ma; in experiment
2,
tdc, = 7.5 Ma. in experiment
3 t d ‘-15
tdct = 2.5 Ma; m expenment’
get. tde,= 5 Ma. In all seven at the base of the crust, after the
experiments,
the temperature
substitution was
held
of mantle constant
thickening. shifted
back
Then,
at
lithosphere
Ma; in experiment
by asthenospheric
T= 1350 o C until
the lower boundary
to its original thickening
position,
the end
condition at the base
lithosphere.
mal faults, all affecting the first cover and large amounts of metamorphosed dykes and “A”-
and “Y-type
5,
material, of crustal
was abruptly of the pre-
granites
sequence, dolerite (Passchier.
1986a; Holcombe et al., 1987; Pearson et al., 1987; Loosveld. in press). Its age is estimated at = 1750 Ma, as it affects
K = 2.5
= 2700
thickness
a 4 km thick granite
in
crustal
thickening.
the initial
is 100 km; the initial crustal
material
during
pas,
thickening;
levels. This heat source
rate,
replacematerial.
Heat transfer
advection
of 0 and 1350 o C (T,,,,);
denudation
production
depth
and
K = 1.2.10-6
conditions
stable geotherm heat
homo-
by temporary
in each experiment.
by conduction
K-‘;
boundary thickness
Tew ICI
from progressive,
fend = 1.6; D = 10 km; A,,= 2.5 pW/m’;
Parameters:
tectonic
----+
700
by asthenospheric
by conduction
and
600
accompanied
lithosphere
rates are constant
the crust
W
paths resulting
of the mantle
500
400
300
200
volcanics at 1779 + 9 Ma (Page, 1978. 1983) De3 at 1678 k 1 (Page, 1978). A second, but only locally recognized, extensional event is manifested in the central and eastern parts of the inlier by extensional duplexes, high-angle (to bedding) nor-
the sag sequence
of DC1
(Passchier, 1986a; Loosveld, in press). but does not affect = 1740 Ma old granites in the zone of extensional D,, structures (R.W. Page, pers. commun., 1988). Major I-type granite suites intruded the upper crust at 1860, 1800, 1740-1720, 1670 and 1500 Ma (Page, 1978; Nisbet et al., 1983; Blake, 1987). The first compressional sequences, D,i, was an,
event to affect the cover as yet only
locally
re-
cognized, thrusting event, resulting in imbricate stacks of thrust sheets (Bell, 1983; Loosveld and Schreurs, 1987) fold nappes and bedding-parallel LS-fabrics (Loosveld, 1989). One DC, shear zone in the west of the inlier has been dated, giving an age of 1610 f 13 Ma (Page and Bell, 1986). The subsequent DC2 event was a phase of strong, E-W directed, coaxial shortening, resulting in upright, tight, N-S trending folds, vertical axial-plane foliations, and vertical extension lineations. DC,-shortening is penetrative over the inlier and amounts to 35-55% (thickening the crust by a factor of between 1.5 and 2.2). Page and Bell (1986) dated a
207
TABLE
2
thermal
Main tectonic Time/Period
events of the Mount Phase
Events
(Ma) 187551850
Dee/“co
1780
De,
Barramundi
Orogeny/Kalka-
doon igneous
relaxation
of the extensional
events,
are
thick (in the order of 5 km) and are pre-metamor-
Isa Inlier
phic, there cannot
be a direct
extensional
and
events
link between
the (low-P
facies)
these meta-
morphism.
event (basement)
Major extensional
event
Implications of models for the Mount Isa Inlier
(1st “ rift + sag” sequence) = 1750
De2
Local extensional
1680-1670
D,
Deposition sequence
1610
DC1
1550
DC1 Dc3+
Penetrative
E-W shortening+
reg. metamorphism
Various
contractional
structures Intrusion
= 1500
Lithospheric
voluminous
metamorphic
post-
granites
Dates after Page (1978, 1983) and Page and Bell (1986).
Asymmetric
extension
Following
and
arguments
(Wilson, 1973; Jaques et al., 1986b; Reinhardt and Hamilton,
1982; Passchier, in prep.; Loos-
veld, 1989). Post-D,, deformation and western parts in the inlier
is in the central generally char-
by steep faults and shear zones, trending
extending
for thermal
McKenzie’s
relaxation
(1978)
after a pure-shear
extensional event, the three documented rifting events of the Mount Isa Inlier, of 1780, 1750 and 1680 Ma, are unlikely to be related to the low-P facies metamorphism at 1600-1540 Ma, primarily because events
D,, shear zone at 1544 5 12 Ma. Studies of porphyroblast blastesis generally confirm the synchroneity of D,. and the regional metamorphism
acterized
thinning
in the west
Local (?) thrusting low-P
1510-1450
structures
2nd “ rift + sag”
the
time
gap
between
and metamorphism
the
extensional
is too wide. The same
thermo-mechanical arguments refute the suggested relation (Hobbs et al., 1984) between the regional, prograde, low-P facies metamorphism and ensialic rifting at approximately (R.W.
Page, pers. commun.,
Hill Block, Australia. The fact that crustal many
cases
1988) in the Broken
thinning
an asymmetric
at 1600 Ma 1700 Ma
proves
process,
to be in
resulting
in
030 o N (dextral) and 310 o N to 340 o N (sinistral). Specific parts of the inlier are affected by late N-S shortening, and NNW-trending, upright folds. The latter have been dated at 1510 k 13 Ma (Page and Bell, 1986). A large population of K-Ar dates of 1450-1500 Ma (Richards et al., 1963) is
two different, but complementary plate halves, cannot overcome this problem. The main problem
interpreted the crust
models explain lateral upper crustal extension
offsets between areas of and areas affected by low-
uplift.
P facies metamorphism,
on the scale of the com-
P-T-t data indicate a poly-metamorphic, anti-clockwise path: sillimanite grows usually after andalusite (during DC._,),whereas kyanite replaces sillimanite and cordierite in late retrograde shear
plete inlier (400 X 200 km), normal faulting would have to be developed at least in some areas: even
zones (Reinhardt and Hamilton, in prep.). Similar metamorphic observations have been made in other Australian Early to Middle Proterozoic inliers. especially in the Willyama (Phillips and Wall, 1981; Hobbs et al., 1984; Clarke et al., 1987) Halls Creek (Gemuts, 1971) and Arunta (Warren, 1983) mobile belts (Fig. 1). It is important to realize that, since the sag deposits, resulting from
wall plates
as reflecting and cessation
a widespread cooling of of Ar diffusion during
with any model incorporating lute asymmetrical extension is the general absence of young (post1670 Ma) rift sediments and young extensional structures. Although the asymmetric extension
more so, since transfer faults normally compartmentalize the extensional terrane, so that hanging and
foot
wall
plates
are juxtaposed
along strike (Gibbs, 1984; Bosworth, Etheridge, 1987; Lister et al., in press).
1985;
Magmatic events Magmatism results from a primary perturbation of a depth-temperature profile, and, more importantly here, its effect is only secondary on
208
TABLE
3
Concentration compared
of heat-producing
to the worldwide
elements
average
Worldwide
Wppm)
3
3 8.5
4.2
The percentages production
high Ca
17
K ,O(% under
in the three
2.5
the batholith
can be calculated.
names
Data supplied
largest
pre-metamorphic
batholiths
of the Mount
Isa Inlier,
as
in granites
average
low Ca
Th(ppm)
(HPE)
concentration
represent
Sybella
Wonga
(3.14%)
(1.1%)
8
8
5
35
46
25
5.15
5.42 the surface
by L.A.I. Wyborn
Kalkadoon
coverage (Bureau
of the respective of Mineral
4.28 granites,
so that a weighed
average
heat
Resources).
this perturbation. There is no question that during, or after, thickening of the crust felsic lower crust may melt, and that during extension deeperseated, mafic, pressure-release melting may occur.
Crust-mantle detachment Figure 6F shows that sillimanite-grade conditions can be reached when the crust is abruptly “underplated” by hot asthenospheric material,
However, whatever pulse of magmatic
which,
the mode of magmatism, heat is short-lived and
the T/P
ratios will only be raised temporarily. In the Mount Isa Inlier, there is no evidence for syn- or immediately pre-metamorphic rising melts. The additional thermal effect of mid-crustal, pre-metamorphic granites enriched in HPE will be permanent, however. Table 3 gives concentrations of the HPE in the main pre-metamorphic granites of the Mount Isa Inlier. The concentration of HPE in Mount Isa’s granites is anomalously high; it yields an average radiogenic heat production of 4.1 pW/m3. This is 52% higher than the worldwide average (2.68 pW/m3 according to Turcotte and Schubert, 1982). Figure 2 shows the effect of a granitic slab between 18 and 22 km with a heat production of 4.1 pW/m3 on the steady-state conductive geotherm. It will raise the temperature in the 3-5 kbar interval by 50-75°C which is not enough, though, to transect the andalusite/ sillimanite field. Pre-metamorphic HPE-enriched granites are widespread elsewhere in the northern Australian Early to Middle Proterozoic fold belts (Wyborn et al., 1987). The presence of such granites alone can explain steep average geothermal gradients, but not the anti-clockwise P-T-t path. Also, no major pre-peak metamorphic granites are within 20 km (surface-) distance to the Soldiers Cap Group, whereas it too is affected by the low-P facies metamorphism.
after
simulates
upwelling,
cools
by conduction.
in simplified
form
the
It
extension-trig-
gered delamination models for Proterozoic fold belts, proposed by Kroner (1983) and Etheridge et al. (1987). Such models, however, cannot be applied to the Mount Isa Inlier for the same reasons that models incorporating only extension cannot be applied: the time gap between extension/ delamination and low-P facies metamorphism is too wide. Hot mode delamination, associated the documented extensional events, would
with miti-
gate this objection. However, there is little evidence for anomalously steep geothermal gradients in the 120 Ma time gap between extension and metamorphism: there is no record of major crustal magmatism in this time span, nor is D,i generally characterized by low-P facies assemblages. In contrast, in both the Mount Isa Inlier and the Willyama Block (Fig. l), metamorphic grades increased during crustal thickening (respectively Reinhardt and Hamilton, in prep., and Hobbs et al., 1984). Any causal relation between the documented extensional events in the inliers and the at least 100 Ma younger phases of low-P facies metamorphism
is
therefore
invalid.
A
variation,
favoured here, is the coupling of crust-mantle detachment with compressional tectonics. Crustal thickening and concomitant thinning of the mantle lithosphere
(convective)
The main attraction of this model is that convective thinning of the mantle lithosphere is syn-
209
chronous
with thickening
with the period Also,
heat
remain
flow
high
thickening.
at the
during Thus,
perturbation
of the crust
of low-P
base
brought
magmatism,
isotherms
considerable
time.
thus
crust
phase
about may
by stay
On the other
may
elevated hand,
for
with this model.
a ap-
None
insuperable.
First, it is unlikely crystallization
of sillimanite
of crustal
thickening,
magmatic
events)
the crustal
that this model
would
thickening
can lead to
the early stages
during
since
the delamination
have to predate
and strain
.( f
most of
rates would
thus
have to be very low. In the Mount Isa Inlier, however, andalusite/ sillimanite assemblages are coeval with the early stages of Dc2. If thinning of the mantle lithosphere was solely the result of Dcz, strain rates in this phase must have been unrealistically low (< = 10-‘5s-‘) and detachment of the upper crust relatively fast. It is more likely that a major thickening event preceded Dc2, i.e. D,, was not just of local importance, but triggered thinning of the mantle lithosphere. D,, thrusting has now been documented in the western part of the inlier (Bell, 1983) in the central part (Loosveld and Schreurs, 1987) and in the eastern part (Loosveld, 1989, this respect that is characterized mation. whereas is marked
and in press). It is noteworthy in the early crustal thickening, Dcl, by pre-dominantly brittle deforongoing crustal thickening, DC*,
by dramatically
higher
temperatures
and predominantly
model-dependent),
deformation.
This
is re-
garded as typical for the Early to Middle Proterozoic fold belts of northern Australia (Etheridge et al., 1987). Intrusion of the volumetrically huge 1500 Ma old granites, the Williams and Naraku Batholiths, clearly postdates Dc2. The phase of the Naraku Granite
main, coarse-grained gives a (Sm/Nd)T,,,
source age of 1637 Ma; the T,, source age of the Williams Batholith ranges from either 1530 to 1620 Ma, or, and more probably, from 1630 to 1720 Ma (Wyborn et al., 1988). Thus, the source ages for the main phases of the Williams and Naraku Batholiths are close to the deformation
events
during
the source
crustal
ages
thickening
accompanied
which
1610 k
the chronological
(and
the
may have been
mantle
huge
by major amounts
of
source rocks were created
in the lower crust or just
under
et al., 1988). A concur-
the crust (Wyborn
rence of crustal
thickening
would
strongly
point
(1981)
model
mantle
lithosphere
and such mantle
events
towards
Houseman
et al.‘s
for (convective)
instability
of the
during
compressional
tecton-
ics. The
second
problem
lies in the post-tectonic
history. Generally, crustal thickening must lead to syn- to post-erogenic uplift and erosion. In the Mount straints isobaric
Isa Inlier, however, petrological conindicate compression followed initially by Hamilton,
in
prep.). An explanation lies in the different constants for erosion and thermal relaxation:
time the
cooling
time constants
(Reinhardt
for erosion
and
(X: 60-300
Ma;
En-
gland and Richardson, 1977) are generally much greater than thermal time constants (heat transfer by conduction only: L2/m2~. in the order of tens of Ma). Thirdly, the common absence in the sillimanite (K-feldspar) zone of signs of nearby anatexiswhich, considering the steep metamorphic gradients, should have occurred immediately below this metamorphic
zone-can
by the Coulomb-Navier (opening) migrate,
ductile
is still very small
period(s)
and
there
database very
thermal
extension
ages for D,, and (less so) D,,, respectively 13 and 1544 &- 12 Ma. Although
of crustal
the short-lived
pear to be three problems is, however,
of the
the entire unlike
(and
facies metamorphism).
fractures are unlikely
potentially fracture through
be explained theory:
which
to be vertical
tensile
melt
can
in a regional
compressional regime. With overall high differential stress levels, melt migration will be severely constrained in all directions; all, it will do so horizontally.
if melt migrates
at
Conclusions General conclusions are: (1) Cooled granitoids, enriched in U, Th and K, substantially contribute to the local heat production. (2) Prograde and compressive metamorphism (+AT, +AP) cannot be attributed to any premetamorphic extensional event.
210
(3) After elimination
of other options,
ing of the crust with associated ning of the mantle possible
lithosphere
explanation
for
(convective) remains
(Convective)
thinning
sphere
has to be fast with respect thickening.
Conclusions Isa Inlier Proterozoic
period. concerning
probably
Australian
(1) The P-T-r
other
to Middle
isobaric cooling. In the Mount Isa Inlier, the low-P facies metamorphism is contemporaneous of pervasive
crustal
thickening.
magmatism preevent by at least
100 Ma. (2) Granitoids.
in U, Th and K, sub-
enriched
Isa Inlier, convective
lithosphere
might
high until
the end of Dcz.
Acknowledgements
The receipt
of an Australian
Additional provided
financial by
the
National
is gratefully and
logistical
Bureau
and assigned
support
of Mineral
(metamorphic petrology), Lesley Wyborn (granites) and particularly Mike Etheridge (general overview).
Reviews
by Mike Rickard,
N. Rast and
M.P. Ryan led to improvements in the paper. 1 am specially grateful to Ineke de Bruyn for her stubborn
assistance
with the programming.
notations
A B’
radiogenic
heat production
of granite
(2.5-7.5)
Au
radiogenic
heat production
at surface
(2-3)
values 10 me W/m’
10mh W/m’
c
concentration
(‘1
specific heat capacity
of fluid
4000 J kg
’ K ’
(‘, D
specific heat capacity
of solid
1000-1300
J kg
radiogenic-heat upward
the progressive
velocity (here. erosion
HPE
heat-producing
amount
the finite amount
K
thermal
L
lithospheric
concentration)
scale length (A = A,,e- ‘j”)
E
lo-15
rate)
of thickening;
conductivity
’ K ’
km
O-2 km/Ma f = e “. in case of constant
strain rate
of thickening elements
1.5-2.2
(mainly
U. Th, K)
of solid
thickness
2.0-3.0
W mm
100-200
km
’ K ’
&!xGr)_, ( P\c’,) r
stability
t
time in seconds
tdrt
time of crust-mantle
fmd
time at end of thickening
r,
intrusion
T, T rlux
liquidus
r,
solidus temperature
factor:
r = kAt/Ax* detachment
= k/h’ during
temperature temperature
temperature
at the base of the lithosphere
0.4 (by definition progressive
was
Resources,
Geology and Geophysics, Canberra, and the Broken Hill ,Proprietary Co. Ltd., Brisbane. The research benefited from discussions with Greg Houseman (numerical techniques), Steve Lonker
Abbreviations,
(e.g. CnK initial potassium
Univer-
acknowledged.
Appendix 1
J JAI
trig-
event. Tempera-
tures at the base of the crust remained approximately
thinning
have been
gered by DC,, the early thrusting
sity Ph.D. Scholarship
for the Early
Extension and pre-metamorphic date this major crustal thickening
(3) In the Mount of the mantle
to Middle
Proterozoic fold belts of Australia are markedly anti-clockwise, made up of a segment of prograde, low-P facies metamorphism (compressive heating; + AT, + A P), followed by a segment of essentially
with D,,, a phase
to the local heat production.
the Mount
Early
fold belts) are:
paths
contribute
at the base
high for a considerable
specifically
(and
litho-
to the progres-
Temperatures
of the crust have to remain part of the erogenic
P-T-f
of the mantle
stantially
thin-
as the only
anti-clockwise
paths.
sive crustal
thicken-
cruatal
thickening 750&900 Dc 800-1050
oC
12OC-1350°C 700-900
oc
here)
:11
fluid velocity
Vr
w=
V,@
effective
2.5.10m"-2.5.10m"
fluid flux
x
depth axis (m), positive downwards
P
the finite amount
bt=rh'L'/~
timestep
A.x=hL
depthstep
i
strain rate (constant)
cp
porosity
K = K/C &xc,,)
thermal
h
time constant
P,
specific density
of solid
27OOC3300 kg/m’
Pf
specific density
of fluid
1000 kg/m’
1.5-m
of extension
m/s
(locally)
0.04-2.0 Ma
(set)
2-4 km
(m)
diffusivity
I
10-151
_10~14~s lom’_lom~ 4.10~7P1.2.10
of solid
for erosion
(Ma) (E = he-"'Ed)
Appendix 2
50-500
‘m’s
Ma
ing experiments, a Lagrangian reference frame was generally used, such that the solid advection
The “conservation
factor can be omitted. The third factor:
of energy” equation
We start with the one-dimensional conservation of energy equation after, e.g., Carslaw and Jaeger (1959):
(1) where
T is temperature,
t is time,
c, is the heat
capacity of the solid, p, is the specific density of the solid, and K is the thermal conductivity (p,, c, and K are assumed constant in each given experiment). Transfer of heat is considered cal direction, x, only.
in the verti-
for radiogenic
heat
production
in the
crust, + A. A is assumed to decrease exponentially with depth or to be constant (see “Initial Conditions”, Appendix 3). Because of the common enrichment of heat-producing elements in fractionated granites, a different +A,, is assigned to the heat production in a granite. The second factor: -
EPA
$
accounts for the reference e.g., the top erosion level, rock system (negative for
porosity;
V, fluid
erosion of the upper surface where frame is fixed relative to that surface, boundary condition is held at the with E, the upward velocity of the with respect to the erosion level upward movement). In the thicken-
velocity
(negative
for upward
flow); ct heat capacity of the fluid; pr specific density of the fluid. W = V,@ = effective fluid flux. Equation
(1) expanded
with factors
1. 2 and 3,
3T -=.s+&-(E+MW)g
at
Depending on the physical processes that need to be modelled, various factors can be added to the right hand side of equation (1). The first accounts
is the advective component for upward single-pass fluid flow in a frame fixed to the rock pile, with @
(2)
\ \
with
K =
K/p,c,,
(p,c,)/(p,c,), Crank-Nicolson order)
the thermal can be (implicit,
diffusivity
and
M =
solved employing the finite-difference, second
algorithm:
PI.,+1- 7).,)/At =
ww/+,.,+,-2T.,+,
+ 7;-,.,+,
+
7;+,,,- 27;.,+ L&Ax’
+ A/P,L.,
-
(EC MW)(7;+1., - 7;.,wx
(e.g., Smith, 1978; Gerald and Wheatley, 1984). If one defines: r = tcAt/Ax’. r becomes the stability factor to the solution of the finite difference equation. Using this implicit, second order accurate scheme, the solution will be unconditionally stable: here, r is continuously 0.4. After grouping subscripts, eqn. (3) becomes:
212
recorded
(1 + 0.4)7;.,+, =
further
by Vitorello assumed
WT+,.,+, + T-1.,+1+ L.,) + AAT,‘p\c\
(Pollack
+ [i-0.4+
to calculate
and Chapman,
W m-’ Km’ (Schatz
(E+MW)AT/Ax]7;,,
and Pollack (1980). We have
an A, of 2.0, 2.5, or 3.0 pW/m’
various
1977) and a K of 2 or 3
and Simmons,
1972)
steady-state
in order
conductive
geo-
therms: + [0.2 - (E + MW)AT/Ax]
T+ ,,,
(4) T=
so that:
A@
r/D
K
Tr.,+, = {0.2(7Y+,,,+,
+ 71P,.,,,
+ TT;,.,)
+
+ AAT/p,c, + [0.2 - (E + MW)AT/Ax]
/1.4
TIndY- (A,D’/K) L
X
L, the thickness of the continental i.e. that part of the mantle which
7),,} (5)
against strained.
solid-state convection, For thermal effects
is on
lithosphere, is stabilized poorly conthe crust, the
lithospheric thickness is of importance. because the time-scale is proportional to L’. The litho-
Appendix 3 Model parameters Boundary conditions. The boundary quisites are made up of the temperature
prereat the
surface, fixed at 0°C and the temperature at the base of the thermal lithosphere (100-200 km), T,,,,. which ranges from 1200 to 1350” C (constant in a given experiment). Initial conditions. With respect to the distribution of heat-producing elements (HPE) in the upper crust, two geotherm endmembers are used here: HPE can either be homogeneously distributed over the upper crust or their concentration exponentially
K
with x in km.
7;+,.,
+ [l - 0.4 + (E + MW)AT/‘Ax]
decreases
A@ ~
---e
with depth.
The latter dis-
tribution could be the result of progressive fractionation of magmas (Lachenbruch, 1970) and upward migration of fluids with dissolution-precipitation of the HPE (Albarede, 1975; Moorbath, 1978). This exponential distribution is more realistic, since it preserves the observed linear relation between surface heat flow and surface radiogenic heat production under differential erosion within a heat flow province (Lachenbruch, 1970). A = A,,e ‘/D, with A,, the radiogenic heat production at the surface, and D the characteristic length scale for the distribution of HPE, in this case for the decrease of A with depth. Two different characteristic lengths, D, are used here (10 or 15 km). Both fall within the spread of characteristic lengths
sphere-asthenosphere boundary is probably gradational, but is here. rather arbitrarily. represented by an isotherm. Lithosphere thickness estimates based on heat flow studies (e.g., Pollack and Chapman, 1977) indicate an increase of thickness with tectonic age. Jordan and Sipkin and Jordan (1976) anomalously
short
travel-times
(1978, 1979, 1981) on the basis of the of ScS-waves
un-
der Precambrian shields (an average 4 s shorter than under oceans) and on geochemical evidence from kimberlites, favoured 200-400 km thick rigid mantle roots under these shields. They suggested that basalt-depletion of the upper mantle results in a chemical boundary layer with a higher viscosity, while thickening
of the depleted
with
the large
time
nesses under
explains
Precambrian
upper mantle
tectospheric
shields. Woodhouse
thickand
Dziewonski (1984) also argued for high values for L. associating mantle structures. up to 350 km depth, with surface tectonic expressions (ridges, shields). Richardson et al. (1984) argued, on the basis of diamond inclusions, for a lithospheric thickness of at least 180 km beneath Archaean shields. On the other hand, the thermal time constants for the lithosphere, which are proportional to L2. indicate a much thinner lithosphere, i.e. approximately 125 km (Parsons and Sclater, 1977; Sclater et al.. 1980). Also, the half-life of heat production for a chondritic model Earth is approximately
2 Ga, implyinp
q higher
basal
heat
213
flux
and
Middle
a thinner
lithosphere
in the
L is ranged
Proterozoic.
Early
to
here between
and 200 km.
Neglected parameters
(p,c,)EaT/ax
is the advected
E, the characteristic
relative
heat
flux, with
vertical velocity of the medium
Inevitably,
to plane x = 0. This is the rate of denuda-
tion of an otherwise
undeforming
tion can also be tectonic, are, however,
by means
no indications
in the
climate,
Mount
resistivity
Isa
of extensional
E
erosion
of
types, drainage patterns and surface thus on the rigidity of the lithosphere.
the
on rock
height, and All of these
history. ments
and
clear
relations
parameter
erosion-rates, as reported
values
by Hollister
range (1975,
function. such that, following Carson and Kirby (1972) England and Richardson (1977) and and England
(1979):
E = XEd e-r:h with
X a time constant
eroded sequence. crustal thickening however, essentially denudation
for the erosional
Ed the thickness
with unit Ma and
process
of the finally
Erosion started at the end of the period. In the Mount Isa Inlier,
the initial
post-tectonic
isobaric, so that factor is justified.
cooling
an omission
period
the thermal
of physical
mal conductivity,
1982) and 0.1 mm/yr (= 10 km/100 Ma). We have generally opted for the exponential erosion
Richardson
and
is
of the
the
heating,
extensional
offer
a particular
history.
Here,
I ne-
such as the therdeformation
experiments, absorbed
(4)
dur-
frictional
by devolatiliza-
(1) The
omission
of lateral
heat
transfer
is
acceptable, because the experiments simulate large-scale processes (the same anti-clockwise P-T-t paths are documented or inferred for Australian Proterozoic fold belts widely apart). On these scales, the lateral compared to the vertical temperature negligible. (2) The temperature
gradients
can safely
be assumed
dependence of c, Q’, K, and p on and pressure is negligible compared
to the dependence on lithology. Here, no specific lithology has been incorporated (except crust-
temperatures than even instantaneous
tend = 30 Ma) to tsnd = 3 Ma). spaced over the factor” r of 0.4, 0.4 L2/(2500 K). lop6 m*/s, At =
between
tion reactions:
son (1987) mentions characteristic fluid permeation rates (V,) of 4 mm/y (= 4 km/Ma = 1.3 . 1Op’0 m/s). Here, similar values are used, calcu-
from 2.5. lo-l2 (for f= 1.6 and 2.5 . lo-” m/s (for f= 1.6 and Fifty depthsteps were equally lithosphere, L. Using a “stability this implies a timestep, At, of With, e.g., L = 150 km and K = 0.11 Ma.
experi-
simplifications
parameters
and (5) energy
mantle distinction), occurrence of the metamorphism.
lated by taking W as (2400)/t,,, in m/s, thus simulating a constant devolatilization of a 20 km thick pelitic sequence (12 vol.% fluids) during deformation. This yields values of W ranging
most
metamorphic
numerical
(3) progressive
fluid flux, equals V,@ where V, is the fluid velocity and Q is the porosity. Thomp-
W, the effective
and
glect, for the reasons presented below, (1) lateral heat transfer, (2) the depth- and temperature-de-
ing
2 mm/yr,
rendering necessary
general
influencing
for a particular
from the
tially with time, or to be constant. the time-independent
known
have to be,
Factors
are numerous,
futile,
pendence
In the case of
experiments
simplified.
processes
Far
make generalizations about E almost impossible. Generally, E is taken either to decay exponen-
between
be,
are poorly
there
depends
numerical
should
metamorphic
for such late exten-
Inlier).
against
and
solid (denuda-
forces, in which case E is depth-dependent; sion
Appendix 4
100
(3) Progressive
because of the widespread to-be-modelled low-P facies extension
will result
in lower
instantaneous extension. Since extension cannot explain the
low-P Inlier,
facies metamorphism in the Mount Isa simulating progressive extension is superfluous. (Experiment series 2, on the other hand, does incorporate progressive thickening.) (4) Frictional heating (in W/m’); Qr = CT&,
(5,
is stress, and i is strain
rate)
is neglected here as studies by Reitan (1968a. 1968b, 1969) Graham and England (1976) and Werner (1985) show that its influence is limited.
214
Frictional
melting
opment
can possibly
of pseudotachylytes,
cool, dry, crystalline 1972;
lead to the devel-
Sibson,
inverted
and especially
rocks (McKenzie
1975;
Maddock,
metamorphic
mic; e.g., the conversion mately
plus
47 kJ/kg,
production
Locally
zones can result, but only if
reactions
to sillimanite
and Brune,
1983).
most strain is taken up in narrow Dehydration
so in
zones.
K-feldspar
plus quartz
which offsets
approxi-
a radiogenic during
of
Walther and Orville (1982) showed that the devolatilization reactions, which take place during the metamorphism of an average pelitic rock, absorb approximately
168 kJ/kg,
whereas
some 210
kJ/kg is needed to heat one kilogram of pelitic rock from 400-600°C the assumed temperature interval dition
for such
devolatilization.
Geol. Sot. S. Afr., 89: 253-262.
Blake.
D.H.,
1987.
environs, Miner.
Resour.,
Thus,
the ad-
of:
W..
Brady,
at 3
200
1985.
for the
heat required for endothermic devolatilization reactions during prograde metamorphism. However, as we are here concerned with he heat the entire lithosphere, the factor is proximated by a low value of A,. The a factor for endothermic reactions
balance of simply apomission of affects all
tectono-thermal models in this comparative study equally, and is thus of secondary importance.
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