Some geophysical implications of the deformation and metamorphism of the Ivrea zone, northern Italy

Tectonophysics, Elsevier

147

182 (1990) 147-160

Science Publishers

B.V., Amsterdam

Some geophysical implications of the deformation and metamorphism of the Ivrea zone, northern Italy E.H. Rutter Geology Department, (Received

* and K.H. Brodie

*

Imperial College, London S W7. 2BP (United Kingdom)

February

17, 1989; revised version accepted

July 26, 1989)

ABSTRACT Rutter, E.H. and Brodie, K.H., 1990. Some geophysical implications of the deformation and metamorphism of the fvrea zone, northern Italy. In: D.M. Fountain and A. Boriani (Editors), The Nature of the Lower Continental Crust. Tectonophysics, 182: 147-160. The Ivrea zone is commonly cited as an upthrust and steepened section through the lower continental crust, being contiguous, via the geophysical “Ivrea body”, with the present day lower crust beneath the PO basin. The rocks presently exposed were buried to lower crustal depths through the Variscan orogeny and were gradually exhumed to mid-crustal depths through Permian and early Mesozoic times as a result of crustal extension. They were emplaced into their present position during the Alpine orogeny. An important time marker in the deep-crustal evolution of the rocks was the development of a coarse-grained, equigranular texture, following the attainment of the amphibolite and granulite facies metamorphic peak. This effectively separates earlier, pervasive, synmetamorphic deformation from later, highly localized plastic deformation associated with crustal extension and progressive cooling. The geophysical properties of the section would have been markedly different during each of these two stages of development. In the later stage the granulite facies rocks remained relatively dry and, when buried, would have been characterized by the high seismic velocities that they display under pressure in the laboratory. During the prograde (erogenic) part of the history, seismic velocities would have been substantially depressed through the effects of elevated pore water pressure, particularly at high temperatures.

Introduction

physical properties whilst they were subjected to the prograde part of their deep crustal history compared to the retrograde part, and that this will have affected how they would have been sensed by seismic methods.

The Ivrea zone of northern Italy is widely held to represent an upended crustal section that has spent a portion of its history as part of the middle and lower continental crust. It is inferred to be contiguous with the present day lower crust beneath body”

the PO basin via the geophysical (Giese, 1968; Berckhemer, 1969).

Summary of the deformation/metamorphism tions in the lower part of the Ivrea zone

“Ivrea In this

Although

note we discuss aspects of the deformation and metamorphism of the rocks of the higher-grade part of the section. In particular, we point out that these rocks must have displayed rather different

* Present

address:

Manchester

Geology

Department,

The

on our observations in the western part of the Ivrea zone, in the Valle d’Ossola (Brodie and Rutter, 1987a), and it accords with the observations of other authors (e.g., Schmid, 1967;

University,

0 1990 - Elsevier Science Publishers

of deformation

events in the Ivrea zone is com-

plex, in broad terms a relatively simple sequence of events can be identified. This sequence is based

Ml3 9PL, U.K.

0040-1951/90/$03.50

in total the sequence

and metamorphic

rela-

B.V.

E H. RUTTFR

148

Hunziker

and Zingg,

1980; Zingg,

1983; Schmid et

A very important attainment libration tial

demarcation

of a high under

stress.

quence

conditions

into

mineral

the southeast.

This

individual

NE-SW,

further

of metres,

to decimetric roughly

on the

and mafic on the

and banded

scale within is usually

parallel

of the Ivrea zone. A coarse

to

which

sometimes

on ultramafic

rock type. The banding

and trends gation

rocks

that are interlayered

scale of tens to hundreds on a centrimetric

developed,

equilibration,

grain-sizes

rocks and paragneisses

of the se-

side of the Ivrea zone)

textural

in equant

part

facies

order of 1 cm, is imposed

equi-

of low or zero differen-

assemblages

amphibolite

was the

textural

In the highest-grade facies

resulted

event

temperature

(at the northwestern

granulite passing

an

steep,

to the elonmineral

linea-

tion, generally plunging at about 30” to the NE, is pervasively developed in the plane of the banding, and is defined by elongate clusters of mineral grains (Schmid, 1967; see fig. 3 in Brodie and Rutter, 1987a). Throughout the region this high temperature, isotropic texture is superimposed on ductile deformation ranging

obliterated

through

metamorphic

al., 1987).

features, especially tight to isoclinal folds in wavelength from centimetric to kilo-

metric. The “southern probably are usually

antiform”

of Schmid (1967)

falls into the latter category. coaxial

with the mineral

These folds lineation.

In

places the mineral lineation can be seen to be an intersection lineation. For example, about 100 m south from the survey trigonometric point 363, which lies 1 km east from Premosello village, relatively open, metre-scale folds in gar-cpxopx-plag granulites, metre-scale mineral

picked out by coarse, centibanding, display an axial

surface foliation, also indicated by mineral banding (variations in modal proportions of garnet and plagioclase). The intersection of the folded surfaces with the axial planar foliation ciearly gives rise to the lineation here. Elsewhere it may be modified by grain-growth from a true mineral stretching lineation. Nowhere does the deformation that preceded the thermal peak appear to be intensely localized into faults or shear zones, although it must be borne in mind that such zones may have been

region

may

straining ated

grain-growth

peak. have

It seems, suffered

K.H

BRODIE

at and after the therefore,

that

a pervasively

event or series of events,

this

ductile

perhaps

associ-

the progression

of the regional

meta-

and the expulsion

of metamorphic

water

with

morphism

AND

from the rock pile. Following

the textural

to zero deviatoric

equilibration

stress conditions

mal peak of metamorphism, formation nized, zones.

low

two subsequent

de-

events or groups of events can be recog-

both

centrate

under

after the ther-

characterized

deformation The

first

by a tendency

into localized

of these

involved

high-temperature

plastic

nents

and amphibolite

of granulite

assemblages,

particularly

deformation

does

been accompanied

not

isochemical,

deformation

of compofacies mineral

in metabasic appear

by mineral

to con-

shear or fault

rocks. The

generally

to have

assemblage

changes,

but the most strained rocks show evidence of limited ionic exchange between phases (Brodie, 1981). In the case of the shear zone at the eastern end of Anzola quarry, Brodie (1981) used such compositional changes to infer that the shearing occurred under conditions of near-peak metamorphic grade. Elsewhere, hot-work microstructures that tions

formed

under

water

vapour

too low to cause metamorphic

pressure

condi-

retrogression

in the shear zones points to deformation high-temperature conditions ( > 500 o C). deformation

and

dynamic

under Plastic

recrystallization

was

concentrated into feldspars and quartz gneisses, and into olivine in the ultrabasic

in the rocks.

Amphiboles

and pyroxenes

ity. Garnet

appears

to have

only by brittle fracture. phases were carried about plagioclase in the basic

show limited

plastic-

deformed,

if at all,

Clasts of these mafic in a matrix of flowing granulite facies rocks

(Brodie and Rutter, 1985). Localization of deformation into shear zones appears to be associated with extreme grain-size reduction of the softest phase, resulting in the formation of augen mylonites. In the metabasic rocks the softest phase is plagioclase, which is generally present in a sufficient modal proportion to ensure a contiguous load-supporting framework. Commonly, the fine-grained matrix displays an intense crystallographic preferred orientation,

I)EFORMATI(1N/M~TAMORPHlSM

but no systematic intense

fabric

grain-size

OF IVREA

study

and

the mylonite

there

shape fabric, optical in strike

made.

is a certain

strain,

revealed

dates

a mineral

lineation,

the shear

direction.

This

colinear

attainment

sense is revealed cally

from centi-

rotated

of the metamo~hic by shear

bands

porphyroblasts

that prepeak. Shear

the mylonite. by the sense of

of the host rock at the

margin

of the shear zone. Using such features,

taking

into account

the effects

formation, Brodie and these high-strain zones, microstructures, form zones that consistently

ap-

and asymmetri-

within

and reliably

of the banding

to

is generally

with the lineation

but most consistently rotation

well foliainferred

and

of subsequent

de-

Rutter (1987a) showed that characterized by hot work a congruent set of fault extend the rock pile (rela-

et al. (1987)

events and

subvertical

into

the

its present

tilting

granulite

radiometric

and Strona-Ceneri

zones.

of the pre-Variscan

it is becoming

increasingly

facies metamorphic

conditions

range of mineral

ages that are available

shear

zone with the

The youngest d’ossola features, ture shear lite-bearing

section, include

deformation

events

in the Valle

which cut or distort the older the generation of low-tempera-

zones, the formation of pseudotachyfault and breccia zones and the large

(3 km half-wavelength) axial surface passes

antiformal structure through the summit

whose of P.

Proman (the “northern antiform” of Schmid, 1967). The antiform has a box-fold profile (Schmid et al., 1987; Brodie and Rutter, 1987a) and appears to be at least partly accommodated by layer-parallel, low-temperature shear zones. These are recognizable as such because deformation was accompanied by, and at these lower temperatures probably dependent upon, ingress of water which resulted in the formation of local greenschist facies

of the

clear

that

were at-

are gener-

ally interpreted to record the cooling and depressurization of the rock pile through the Permian period

and

Hodges

into

the Mesozoic

and Fountain,

era (Zingg,

1983;

1984). It seems likely

that

the high-temperature extensional shear zones described above were active during this time (not necessarily simultaneously), thinning which facilitated

producing the crustal the cooling (Handy,

The Ivrea zone as a deep crustal section

extensional

Whatever

tained in the structurally lower part of the section in association with Variscan erogenic activity. The

Pogallo fault zone. which in part separates the Ivrea zone from the lower grade Strona---Ceneri is a major

events

history

1987; Brodie et al., 1989).

same movement picture as the high-temperature shear zones of the lower part of the Ivrea zone.

(1984) evidence

and tectonic

tive to the regional banding). Hodges and Fountain (1984) and Handy (1987) have shown that the

zone,

to its pre-

and Fountain

available

for the timing of metamorphic

rock pile,

regional

attitude.

the then

the complexities

the

upper-

of the

structure

Zingg (1983) and Hodges

in the Ivrea

argue

was connected

of the Ivrea zone during

and the metamorphic

reviewed

in width

display

proximately

erogenic position

sently

to ca. 20 m. They are generally

record

Alpine crustal

(see fig. 3 of

range

Schmid

of these features

with the emplacement

banding

plastic-

assemblages.

that the formation

layering

in the paragneisses.

ted and

mineral

and variations

ity is most evident metres

of

a mineral

1987a). Such pervasive

belts

Outside

amount

by

Brodie and Rutter, The mylonite

of

149

IMPLICATIONS

features

strain

of the regional

of an

to amount

has yet been

belts

plastic

GEOPHYSlCAL

of the occurrence

its relationship

reduction

pervasive

ZONE:

The Ivrea

zone

as presently

piece of the deep crust,

although

exposed

is not a

it may connect

directly, via the inclined slab of the buried “Ivrea body” (Geise, 1968; Berkhemer, 1969) with the lower

crust

and

upper

mantle

beneath

the

PO

basin. The emplacement of the Ivrea zone into its present position involved a certain amount of deformation, which means that it is inappropriate to rotate simply the section through 90” in order to obtain previously the Ivrea ture, we through sumably burial.

a lower crustal

section,

as has been done

(Fountain and Salisbury, 1981). To use zone as a model for lower crustal strucmust deduce its structure and properties its high-temperature history, which precorresponds to its history of deepest

The Ivrea zone probably lay in the deep, possibly lowermost continental crust through the peak of the Variscan orogeny and during the subse-

E.H. RUTTER

150

quent period of crustal

thinning

and cooling.

(1983) reviews geothe~omet~ of the Ivrea zone and, despite variation

and error,

of the lower buried

part

all of the sources of

of the Ivrea

to pressures

zone

on the order

of 700” C. An

reconstruct only sional Rutter

mylonite,

to

strained

than the overlying

This has been done by Brodie

(Fig.

temperature ological

olivine

and

rotating

and

(several

to the

existed

during

formed character

lith-

tectonic

can then be seen.

contact

granulites

the village

exposure

metabasic

with

granulites

occur

time.

of

1988a).

lens of limited in the Ivrea

the petrologic

suggests

sheared intensely

and Brodie,

an ultrabasic

Triassic

of

that the

more

such lenses

in

at the

It is unfortunate

apparently

it is simply

thickness

exposed

for it is an intensely

zone), it might represent

of the high-

faults to the regional

seen,

the lower Ivrea zone (Rutter Unless

of the P.

the layering

1). The relationship

extensional

layering

the effects

is not

K.H. BRODIE

antiform

near

that this unit is of such limited base

be made

type

is a spine1 dunite.

in

can

rock

of the Valle d’ossola

Premosello,

of 800 to 1000

exten-

folding

lowest

the core of the Proman

temperatures

the end of the post-Variscan by removing

structurally

bottom

been

section

(1987a),

horizontal

have

of the crustal

period.

Proman

attempt

the appearance

toward

The

the section,

it seems likely that the rocks

MPa (30 to 40 km) and reached excess

Zingg

and geobarometry

AN,,

intensely

de-

that it was brought

into

the

Its

Moho as it

overlying

metabasic

along a shear zone at a low angle to the

4 1

f z 0

STRONA-CENERI

tONE

%7 KEY

Pogallo Fault

Metasedimentary

Schist & Gneiss

Metagabbro; hornblende dominant Garnet-Pyroxene Ultrabasic

predom. pyroxanits

High Temperature

SPINEL

Horizontal -

10 Fig. 1. Restored have appeared direction

geological

cross-section

at the end of its period

on the high-temperature

(looking of crustal

Vertical

PEAIDOTITE

(thicknsr

Granulite

Shear Zones

unknown)

Scale

south) of the lower part of the Ivrea zone in the Valle d’Ossola extension

(probably

shear zones. The principal

during

Triassic

time). The section

shear zones are indicated

Rutter,

1987a).

by heavy,

is drawn

dashed

section, parallel

lines. (After

as it would with the slip Brodie

and

DEFORMATION/METAMORPHISM

layering,

perhaps

excising

ness

of the rocks

crust.

We cannot

granulites d’Ossola

OF IVREA

infer, attained

therefore,

thick-

continental that

Ivrea zone in the Valle

tectonostratigraphic

period,

position

151

IMPLICATIONS

(a) Deformation and metamorphism after the metamorphic peak

the basic

their metamorphic

prior to the start of the extensional same

GEOPHYSICAL

unknown

of the lowermost

of the lowermost section

some

ZONE:

that

During retrogressive

in the

accompany

we see

shear

zones.

mation

in strain-rate,

starting The lower continental can only be probed

crust at the present

remotely,

typically

time

by seismic

reflection and refraction, electrical conductivity, gravity and magnetic methods. However, from field and ~crostructural observations and the aid of existing experimental data on

with rock

properties we can make some inferences about how these rocks might have appeared during the deep crustal part of their history. The high seismic reflectivity that appears to be so characteristic of the lower crust in many continental areas poses no problems if the rock types of the Ivrea zone are typical. The alternation of and metasedimentary layers of hundreds thickness and many kilometres of lateral

extent (Fig. l), together

with occasional

ultramafic

lenses, could give rise to the commonly reported layering of seismic reflection profiles (Hale and Thompson, 1982; Mooney and &ocher, 1987). The mechanical properties of the rocks of the section would have been very sensitive to the particular stage in its metamorphic history, and this can also be expected to have had significant effects on seismic velocities. We can consider separately the behaviour expected (a) during the extensional period following the thermal peak, and (b) during the pervasively may have accompanied phism.

ductile deformation that the prograde metamor-

Intrusive igneous activity and partial melting of particular rock types may have been locally important in the sequence, both before and after the metamorphic peak. In the following discussion, however, we omit for clarity a consideration of the additional complexities to which it might give rise.

defor-

this type of deformation simulation. to have

vance

for natural

deformation

under

tions,

but

problem

experimental

with data

is

Constitutive

from experiments are likely

the

iso-

available

materials

available

not

from the effects of dif-

to experimental

flow laws obtained

could

extensional

isochemical,

was the only Apart

so dry that

of the

Hip-temperature

plasticity

amenable

were effects

development

mechanism.

ferences

Rock properties during the deep crustal part of the history

the rocks

metamorphic the

mineralic

them now.

metabasic of metres

this time

peak,

on suitable some

rele-

such condi-

almost

all of

at present

the

is that

it

examines the flow of rocks over only limited

relatively coarse-grained (ca. 20%) increments of

strain. In nature, during tic deformation, strain shear zones (i.e., faults)

large increments of plasbecomes localized into characterized by intense

grain refinement (e.g., White, 1976). Thus the particular flow characteristics of these microstructurally-modified fault rocks, rather than intracrystalline plastic flow of their protoliths, may effectively determine

the

rheological

characteristics

lower crust under dry conditions.

of the

These rocks may

deform by one of a number of possible grain-size sensitive deformation mechanisms (Kirby, 1985), or fabric-softening

(White

et al., 1980)

may

be

important in stabilizing the localization when an intense preferred orientation has developed. There have

to date been few experimental studies of related to microstructural change, espe-

softening

cially for silicate rocks (but see Kronenberg and Tullis, 1984; Cooper and Kohlstedt, 1984; Tullis and Yund, 1985; Karat0 Brodie, 1988c). Most laboratory rocks under high temperatures

studies pressure

et al., 1986; Rutter

and

of seismic velocity of (and sometimes high

also) have been

carried

out on dry

rocks (e.g., Fountain, 1976; Kern, 1978; Kern and Schenk, 1988). It has frequently been noted that there is an excellent correspondence between the laboratory determined velocities in rock types such as occur in the Ivrea zone and velocities measured for the lower crust in relatively stable continental areas (e.g., Fountain, 1976; Hall, 1986). Meissner

E.H. RUTTER

152

(1986)

summarizes

depth (V-z) of

P-wave

crustal

velocity

velocities

means

number

(V,)

with

with

the

determinations

depth

results

of

dry

rocks

on

that pore water pressure

in the

lower

of velocity-

which show a steady increase

in the range 6.5-7.8

correspondence velocity

a large

sections

crust.

This

and

lower

km/s.

This

laboratory probably

is commonIy

is consistent

observed

lack of retrogression

granulites

of the lower Ivrea zone.

with

sures as a result

Rock deformation ways

during

by Brodie crystallizing grained sensitive

prior

to the

flow processes

elevation

of activity

nents of the vapour

extent

field observations the observed

it is not clear to what

pervasively

ductile

tion of the Ivrea zone that occurred attainment

of

accompanied to what

the

metamo~hic

deforma-

prior peak

to the actually

is defined

pressure

pressure. In effective of physical property

peak. The same of microstrucdeformed rocks

much lower peak temperatures

series or (e.g., in parts of the Scottish Dalradian the more external parts of the western Alps). in these simpler

and very common

tions it can often be argued of the rocks was directly metamorphic

deformation.

evi-

that have attained

However,

to cataclastic

and

libration at or after the thermal effects plague the interpretation tures from much less intensely

heating

gress of metamo~hic

that the deformability

related

to the progressive

and in particular reactions

Brodie and Rutter, 1985). Sills and Tarney (1984)

situa-

to the pro-

(e.g., Fyfe,

interpreted

1976;

the inter-

pressure,

pressure

metamorphism

(a,)

minus

aid

in-

that the

chemical

phases.

tance

Microstructural

super-

may

compoplasticity,

The transient

may, through

static

(P,)

of fine-

grain-growth

of particular

re-

grain-size

phase may facilitate

lithostatic

it is earlier.

dence of deformation mechanisms is effectively wiped out by the mineralogical and textural equi-

existence

changes

of grain-refined

of the effective

actively

and it is possible

rise of pore fluid pressure

the progressive

extent

volume

reac-

(reviewed

may permit until

plasticity,

particularly From

1985). Within

products

Solid-phase

in several

reactions

rocks, the transient

reaction

venes.

and metamorphism

during

of metamorphic

dehydration

and Rutter,

tracrystalline (b) ~~fo~mution

reactions

can be facilitated

the progress

tions, particularly

the

m~t~~lorph~~ peak

K.H. BRODIE

the metamo~hism.

low

in the anhydrous

of dehydration

AND

lowering

lower the resisEffective

iitho-

as the lithostatic

np, where

p is pore

fluid

stress laws for different types the value of n varies. Pore

pressure affects the resistance of rocks to cataclastic deformation such that n = 1. Elastic moduli are affected by pore pressure whereas the ductility of rocks

such that n -C 1, is modified such

that n > 1. This mechanical effect of pore pressure will affect not only dehydrating rocks but, through permeation,

adjacent

rock

masses

not

actively

being transformed. Unlike

the case of intracrystalline

dry rocks, it is rather stitutive

more difficult

flow laws for rocks being

plasticity

to write condeformed

ing a prograde metamo~hic reaction, but likely that the transient mechanical weakening be extreme.

in durit is will

Some of the above types of effect have

leaved metabasic and metasedimentary rocks of the Ivrea zone as having originated as a Palaeozoic accretionary complex. In such a case almost all of the component rock types are likely to have contained hydrous minerals which would have yielded

been demonstrated in experimental studies (e.g., Raleigh and Paterson, 1965; Heard and Rubey, 1966; Gordon, 1971; Murrell and Ismail, 1976; Murrell, 1985; Sammis and Dein, 1974; Hobbs, 1981; Rutter and Brodie, 1988b, c; Brodie and

pore water during progressive regional metamorphism. It is not entirely clear how much of the

Rutter, studies

metabasic component of the granulites was intrusive into the lower crust and therefore initialfy anhydrous, but the close spatial association with the paragneisses during metamorphism means that through permeation all of the components of the rock pile may have experienced high pore pres-

and Knipe, 1978; Rubie, 1983; Brodie and Rutter, 1985, 1987b). In the appendix we examine the rate of rise of pore pressure resulting from a dehydration reaction under constant mean stress conditions, but assuming for simplicity that no loss of pore fluid

1987a) or inferred from microstructural of naturally deformed rocks (e.g., White

UEFORMATION/METAMORPHISM

occurs,

and neglecting

in the particular still

OF IVREA

ZONE-

the role of partial

system considered.

substantial

GEOPHYSICAL

effective

Whilst

confining

153

IMPLICATIONS

melting

strain-rate

there is

grain-size

enhancement sensitive

reaction

pressure,

this

products.

effect

might

be largely

flow of transiently We have recently

experimentally

of the disaggregated increasingly necessary only

to produce

mass.

of microcracks

considered

accompanied

the

structure

and

(cataclastic

by a porosity

Total

would

flow) is

increase

dehydration

in the appendix

of 1% is

However,

of the

of rock

be able to fill

about 10% porosity with fluid at about 800 MPa, X = 1. This will be able to admit cataclastic flow at

800

very

low stress

levels,

low-temperature emphasized E a 600

mental

3

Mu + 0 = Ksp

+

SIII+ Ii,0

The elevation nies dehydration

0

Temperature

Fig. 2. Graph muscovite

of P,,,,,

+ quartz

lated as described constant

versus

= K-feldspar

for the reaction

+ sillimanite

+ water,

and the shapes

The approximate

constant

rate of total

dehydration deflected required

begins upwards

path

through

a constant

pressure

been

heat fhrx (dQ/dt)

and a

(dP,,,,,/dt)).

When path

because

is now

part

of the

through

heat

input

When the path reaches

and excess (relative

VP, permeability

inter-

specific vofume

(at X = 0.5 in this case) the P-T

to drive the reaction.

dissipated

to the assump-

the dehydration

increase

curve, it follows this curve (path segment is complete

for

form of a possi-

val is also shown by a bold line (for a constant of water = 0.0014 m3/kg,

calcu-

boundaries

of these are subject

tions given in the appendix. ble pressure-temperature

Equilibrium

( PH2,) and X ( = Pn,o/P,,,,,)

values of pore pressure

are shown,

800

‘C

temperature

in the appendix.

and (qualitatively)

to vary over the dehydration

is

the X = 1

R) until the reaction

to available permeation.

have

yet been

of rock deformation reactions

than

the

but it must be no experi-

accompanied

in which the microstruc-

over the interval

of a dehydration

of pore pressure that accompareactions is necessary for the

escape of evolved fluid, in order that the reaction(s) can proceed to completion. The permeability of crystalline rocks is very sensitive to effective pressure (Brace et al., 1968; Brace, 1980) (Fig. 2) and

, 700

600

500

there

lower

strength,

tural evolution has been followed. In Fig. 2 we show qualitatively the change in mechanical properties expected reaction.

200

probably

uniaxial

that

studies

by dehydration

aL 3 ki? c; z ‘i 400 c

how

of reaction

the generation percent.

flow

amount

h = 1 at a porosity

1% of the rock

Brodie,

to become

of the grain-boundary

several

has

The

and

cataclastic

is likely

disaggregation necessarily

1ooc

grains

important.

about

demonstrated

(Rutter

1988b). As X -+ 1, where h = p/a”,

through

fine-grained

void space) Inset

strain-rate interval.

curves

fluid show

are likely

the attainment

of pore

pressures

close

to litho-

static are probably required to open sufficient cracks and pores to allow the escape of evolved fluids

upwards

and/or

1983; Murrell, 1985). The time duration

laterally

(Etheridge

of elevation

pressure will be controlled that dehydration reactions

et al.,

of pore

fluid

by the heat flux, given are typically endother-

mic. The peak pore pressure governed by competition with

attained drainage

will be through

permeation. The strength of the rock (resistance to cataclastic flow when X is high) will be governed largely by the X value, but the rate of strain will be determined by the reaction rate, which depends in turn on the heat flux and rate of fluid loss. Various studies have shown that both P and S wave velocities are significantly lowered as pore

E.H. RUITER

154

fluid

pressure

(Todd

approaches

and Simmons,

reduction cally

1972; Christensen,

of perhaps

be expected

possible

more than as h

consequences

In Figs.

of VP with depth

Some extrapolation its range

has

apparent,

gradient

made.

The

at pressures

and is more pronounced

1

lower

to lower and

thermal

tenta-

pressure

on VP follow

on

with n typically

tended

to increase

The expected rock

types

effective

outside

have

of a

above,

used

(Fig.

these

for the granite.

law

in the range

0.8

that n

magnitude. on VP for

in Figs. 3a and b for the two using

pressure

curves

d). This

the fact

that

= a,(1 - RX).

We

to estimate

VP vs. depth

3c and

pressure

pressure

calculated

lithostatic

the form of

700 MPa is

and

of pore

(1972) reported

of pore

at en-

(Kern

with pore pressure

effects

n = 0.85 are shown

is shown,

pressures

the effects

an effective

to 0.9. Todd and Simmons

K.H. BRODIE

be suppressed

gradients

All data

(i.e. VP=f(P,))

d the

beginning

above

shift

gradients

1981).

and an

data

would

Richter,

of 20” C/km.

of experimental

been

inversion

at

3c and

inversion

The

3a and b show experimen-

a geothermal

velocity

tirely

typi-

unity.

on VP for a dry granite

variation

assuming

thermal

10% might

approaches

respectively.

consequent

1984). A

higher

(1978) on the effect of tempera-

ture and pressure amphibolite

This

of this are explored

tively in Fig. 3. Figures tal data of Kern

pressure

the lithostatic

AND

qualitatively

for different

involves

the

X values hazards

of

7,0 [a) GRANITE

/

J L.5

,/’

0

,,’ 200

,I

,,800

600 IMPal

LOO Llthostatlc Pressure

Llthostak

Pressure

6.5.

(MPaJ

__-----_

_ __ - -x=1.0

---___

#’

Lkpt h iK m

) at

AMPHIBOLITE

35 MPa:Kgm

(Km)

4.5 0

200

LOO Lilhoslatic

Fig. 3. Compressional with

Depth 1p

L5

depth

along

lilies = experimental using an effective

600 Pressure

wave velocity

a geothermal

gradient

and

1978); dashed

how

1978) for a granite this might

lines = isothermat

law with n = 0.85, and neglecting

Curves are shown for two different along a geothermal

temperatures

of 20 o C/km

200

LOO Lithostatlc

(V,) data (Kern,

gradient

data (Kern, pressure

0

600

(MPa)

and an amphibolite

be influenced

by pore

VP vs. total-pressure

any differences

600 Pressure

pressure.

curves calculated

in behaviour

20 o C and zero pore pressure,

gradient

of 35 MPa/km.

to facilitate

comparison

800

(MPa)

used to illustrate

water between

how VP might

(a, b) Continuous from the experimental

the dry and water saturated

in each case. (c, d) VP versus depth curves calculated

and with a geobaric

at 35MFWKm 2p

The continuous with (a) and (b).

vary

curved data states.

as in (a) and (b), but for points curve in each case is that for

DEFORMATION/METAMORPHISM

extrapolation

OF IVREA

beyond

the pressure

in existing

experimental

peratures,

and

ZONE:

GEOPHYSICAL

range

covered

data and to elevated

neglects

velocity

tem-

differences

be-

tween dry and water saturated rocks. Up to about h = 0.5 there is little effect, but as X approaches

unity,

of V,, especially effects

are more marked

the amphibolite, the

there is a marked

at the higher

form

pressures. enhanced carried

for the granite

and are simply

of the They

dilatancy

for

a consequence

of

at low

curve

to be exacerbated

if the experiments a non-zero

ime, and comparable

The

than

VP vs. pressure are likely

out under

depression

temperatures.

deviatoric

by

were to be stress reg-

effects are to be expected

for

shear wave velocities (Christensen, 1984). From the above inferences for a granite, the expected variation of VP over a dehydration reaction interval is superimposed on Fig. 2. Regions of active prograde metamorphism

and

155

IMPLICATIONS

history. During the prograde history the presently observable rocks were probably further from the Moho

in a tectonically

later displaced

thickened

closer to it during

part of the tectonic

history.

crust,

and were

the extensional

The electrical

proper-

ties of these rocks are also likely to have been very different

in the two environments.

grade

metamorphism

been

very conductive

pore water pressure

as a result

the extensional

period,

imagine

the dry rocks under

dilatancy.

it is difficult

a large effective

being anything

pro-

to have

of the elevated

and accompanying

During

static pressure

During

the rocks are likely

to

litho-

but highly resistive.

Summary and conclusions The crustal

history

of the rocks

of the lower

part of the Ivrea zone is dominated by: (a) The effects of pervasive deformation and prograde metamorphism of a pile of intercalated

associated ease of deformation in mobile belts are therefore likely to be marked by both low P and S

metasedimentary and metabasic rocks, reaching a thermal peak during the Variscan orogeny, and

wave velocities.

leading

may display fact that reactions

At any instant

several

such zones,

different rock over different

a crustal

section

according

to the

types may take part in temperature intervals.

growth

to the development textures

reduction

tectonically

into mylonitic

of the present

day con-

relaxed

mineral

stress conditions.

deformation of the now effecrock pile by isochemical, high-

temperature intracrystalline plasticity. Microstructural evolution in the direction of grain-size

High pore fluid pressures have been invoked as a possible explanation for the low-velocity zones sometimes observed in the I/-z structures of active sections

under

(b) Extensional tively anhydrous

of isotropic

led to weakening bands

and strain

but without

localization

ingress

of water

tinental crust (e.g. Meissner, 1986; Mueller, 1977). Alternative explanations also exist for low-velocity

and associated metamorphic retrogression, at least in the earlier part of the extensional history. The

zones (Meissner, 1986). The simplest of these for explaining substantial velocity inversions are (a)

metabasic

the

fact

that

active

zones

tend

to have

higher

geothermal gradients that, as shown above, can lead to velocity inversion and, (b) the stacking of overthrust sheets can result in the emplacement of typically higher velocity material of the hinterland of an erogenic zone onto lower velocity sediments

and

metasedimentary

rocks

became

tectonically juxtaposed against olivine rocks possibly of the upper mantle. The crustal extension was associated rocks.

with the post-erogenic

cooling

of the

Subsequently, localized deformation and metamorphic retrogression effects were associated with the emplacement of the Ivrea zone into its present upper crustal position.

of the foreland. The latter situation, of course, is likely to result in prograde metamorphism of the buried slab. All of these effects therefore tend to

The remotely sensible properties of these rocks are likely to have been very different during these

compound one-another. From the foregoing we infer that the lower part of the Ivrea zone during the prograde part of the

two episodes of their lower crustal history. During prograde metamorphism as pressurized pore water was being evolved, the rocks probably formed part

metamorphic cycle would have presented dramatically different seismic and mechanical properties compared with its post-metamorphic lower crustal

of a low velocity zone within the middle to lower part of a thickened continental crust. Subsequently, the dehydrated rocks would have been

156

E.H. RUTTER

characterized

by the high

reflectivity

typical

velocities

of the lower

day non-erogenic

and

crust

seismic

in present

regions.

Research

Mark Handy we thank liminary

by U.K. Natural

En-

grant GR3/5636.

We

from discussions

with

on the geology of the Ivrea zone, and

him

anonymous

Council

considerably

draft

for critical

comments

of this paper.

on a pre-

Kip Hodges

referee provided

helpful

and an

and construc-

tive reviews. Appendix: Evolution

of pressurized

the isobaric

order

pressure

P,d T

normal

temperature change

of reaction

solid phases change

at pressure

P (MPa)

T (K)

and temperature enthalpy

volume

change

change

of reaction

water

neglect

the

melting

will begin

dration

in heat

capacity

of assemblage

during

reaction

of an idealized

fluid is permitted.

evolution

reaction.

fact

that,

begin

during in this

during

in

in pore those

dehydration,

the course systems,

required

of the dehydehydration (not

subjected

to an

at temperatures

lower

for dehydration

PHIO = P,,,,,.

we

system,

interfaces

in rocks

pressure,

In

of pres-

particular

solid-solid

spaces)

confining

effects

We

have

under

the

demonstrated

that a water activity exists at the site of reaction of a value less than that which would exist if the pressure

were equal

to the confining

pres-

sure. Whilst recognising the problems of this approach, for simplicity we assume that the activity of water

at the dehydration

sites

is that

which

rock density

corresponds to the pore pressure, which initially is less than P,,,,,. This approach is identical to the

water density

treatment

specific

heat of the rock

porosity

activity

thermal

expansion

coefficient

amount

of heat

at constant

quired

to dehydrate

totally

mass of water tinit volume

released

of water temperature

salt re-

unit volume of rock

by total dehydration

of

of rock

Pore fluid pressure markedly affects many physical properties of rocks, including strength, seismic velocity, permeability and electrical conductivity. In order to obtain qualitative insight into these effects, we estimate here the evolution pore

We

for the reaction:

At least in some mineral

water

of reaction

PH20/P,“,.4 water compressibility

of

that

this experimentally for a serpentinite breakdown reaction (Rutter and Brodie, 1988b). This requires

time entropy

are correct

surized

condition

stress

dehydration

to focus on the mechanical

than

mean intergranular

slow

rate-limiting.

rock, in which the proportions

and where no loss of evolved

effective

pore water

BRODIE

mu + qtz = kspar + sill + H,O

merely

HZ0

are not overall

+ quartz

reactions

of terms

K.H.

it is sufficiently

examine

fluid during a

dehydration reaction

P

provided

kinetics

of these minerals

This work was supported have benefitted

flux,

reaction muscovite

Acknowledgements

vironment

heat

AND

pressure

with

temperature

and

time

through a dehydration reaction when water is the only fluid phase component present. (a) The dehydration

reaction

of dehydration is lowered

(Barnes

(Greenwood, From

and

in which water

by the addition Ernst,

1963)

of a dissolved or an

inert

gas

1961).

the thermodynamic

data

set of Powell

(1978) we have constructed the P,ota,-PH,O-temperature diagram for this reaction (Figs. 1 and A.l). We have neglected the fact that andalusite is the stable aluminium silicate polymorph at the lower end of the P,,,,, scale, and the effects of variations from local equilibrium arising from deviatoric stress and the existence of pore spaces. Equilibrium constructed

boundaries for PH,() = constant from the Clapeyron relation:

A&x,, (0.1, T) Ay(O.1,

Because dehydration reactions are endothermic, the progress of the reaction is determined by the

equilibria

in which the effects

298)

of pressure

were

(1) and temperature

DEFORMATION,‘METAMORPHlSM

OF IVREA

ZONE:

GEOPHYSICAL

157

IMPLICATIONS

dominated

by water,

for any dehydration

and

is assumed

reaction

(Powell,

= 0.013

kJ

1978). From

the Pu,o = constant curves, the set of curves for X = constant = PH,JPlot,, were constructed (Fig. 2). An alternative

representation

of these data are

shown in Fig. A.1, as curves of PHzO versus

T.

(b) Pore pressure rise Progressive

heating

of the

onset of the dehydration

800

reaction

AP n,o=PAT=----

P

600

prior

causes

to the a rise of

(APHzo) given by:

the pore water pressure 2 2

rock

AQ

a

P

(3)

PsCp

in which it is assumed that porosity is constant and sufficiently small that the effective specific

0, I a

heat

is dominated

temperature

400

Using

by the solid

rise corresponding P-V-T

to estimate

600

700 Temperature

Fig. A.l. Graph ture

T

for

according pressure

of pore water pressure

the

muscovite+

= pore pressure

of constant

(continuous

total pressure

of reaction

ranging

(dashed

begins

curves).

Examples

constant

to 0.002

m3/kg)

when one of these curves

for the appropriate

PH20/Plotat

V = 0.0014

m3/kg

curve,

dehydration

interval

specific of water.

values

are given

prior

to the

volumes

meets the dehydration

curve

ratio. This is illustrated

which

has been

(V,

Dehydration

extended

for the over

the

(for Ptotal = 800 MPa). Using the VP-P-T

curves of Fig. 3 for a granite, vary over the temperature

we also show how VP is likely to

interval

loss of pore pressure

of dehydration,

prior to attainment

on AV, have been assumed

assuming

no

properties,

insignificant,

examples

rise are shown

of the

in Fig. A.l,

no fluid loss.

When the onset

-T conditions required for Pn,o-%,,I of reaction are reached (and assuming

no substantial temperature overstep is required for the reaction to proceed, a reasonable assumption for a dehydration reaction), PHzo must rise with T along the equilibrium boundary. Each increof pore pressure

is therefore

brought

about

in part by thermal expansion and the associated temperature rise, and in part by fluid evolved through sumed

reaction constant

(Fig. heat

A.2). As a result, flux, dQ/dt,

the as-

is partitioned

into a part (Q2) that drives

the temperature

and

the reaction.

a part

(Q,)

that

drives

rise Hence

temperature rises with time more slowly than prior to the onset of the reaction. Let AP, be the incremental isothermal pore pressure rise associated with a small amount of reaction that:

of X = 1.

AQ.

assuming

ment

calculated

curve) and various

rise with temperature

for different

from 0.0011

PHzO versus temperadehydration

made. Curves are shown for total

of the rate of pore pressure start

quartz

to the assumptions

600

‘C

AT is the

data for water (Fyfe et al., 1978)

the fluid

rate of pore pressure 200

rock.

to heat input

that consumes

a quantity

of heat AQ,, so

(4

and:

AS,,,,,(O.l, 7’) = AS,,,,,(O.l,298)+ &dT

The heat of reaction 7: is:

at constant

temperature,

(21 AH(O.1, in

which

the

change

in

heat

capacity,

r,

is

T) = AH(O.l,

298) + jT r dT 298

(5)

E.H. RVTTER

and if A is defined

K.H.

BRODIE

as:

a QoGprP

(

A= N- p l/(1

AND

1

(10)

xcpps

+ A) is the proportion

required

of the total heat input

to drive the reaction.

The rate of temperature the start of the reaction,

rise with time prior (dT/dt)‘,

(11)

TFig. A.2. Schematic temperature

illustration

of a part of the PHI0

curve (see Fig. A.l) for a dehydration

with constant

total

pressure

at constant

total

heat flux must be sufficiently

slow that reaction

control

rate and

the overall

minimized.

For

minimized

by the typically

Each increment pressure

reaction

dehydration

Equilibrium

is restored

the relative

partitioning

from

which,

the heat required A,H(O.l,

overstepping

large entropy

of reaction.

is the total heat flux. Hence the rate

of temperature

rise during

the reaction

(dT/dt)”

is:

is

is also

tends

to stop

the reaction.

increment

AT, which

rise APa. In this way pore pressure curve.

This geometry

of the heat input

and driving

using

kinetics do not

overstepping

by temperature

the equilibrium

reaction

where dQ/dt

flux. The

dT dt

”

(-1

1 dQ = dt C,p,(l +A)

(12)

evolves fluid that causes a pore

AP,, which

also causes a pore pressure can rise along

relatively

of dehydration

increment

so that

reactions,

versus

equilibrium heat

to

is given by:

between

the temperature

the data at 700°C

Assuming a constant heat the rate of rise of pore water prior to the start of the reaction

is given by:

controls

driving

BCX

the

(13)

PC,P,

rise.

set of Powell

flux (dQ/dt = B), pressure with time

(1978)

Using (6) and (12), during

the reaction:

NB

is:

= C,P,(~

973) = 569400 kJ rnp3

The fraction of the total weight of the initial rock mass released as water is 3.92% (106 kg mp3), which corresponds approximately to 10.5% of the volume at 500 MPa and 700°C. Let the incremental pore pressure rise AP2 be associated with the quantity of heat AQ2 as de-

Prescription

(14)

+A)

of the heat flux (or the initial

temperature rise) allows the temperature/time pore pressure/time histories to be calculated.

rate of or

The physical quantities OL, p and p, depend upon fluid pressure and temperature, particularly at low pressures. The integration of equations (11)

fined above. The total rise of pore pressure AP, + APz is related to the temperature increment AT

through (14) over large pressure and temperature ranges therefore requires knowledge of these vari-

(Fig. A.2) by:

ations

AP,+AP,=NAT

(6)

where N is the slope of the equilibrium A.2). Hence from (3): AP,=AT[N-(a//3)]

curve (Fig. (7)

ditions,

and, subject

to appropriate

the heat flow equation

boundary must

con-

be satisfied.

It is useful however, to obtain some idea of the value of A. At 700 MPa and 700°C: p-2.5~10-~

MPa-‘;

pr = 850 kg mp3;

therefore:

;=_245MPaOl_4C?;

which describes the partition and temperature rise. From eqn. (8):

AQ, + AQ, At

= s[l+(N-;)Q$?/]

of heat into reaction

(9)

C, - 10m2 kJ MPa-‘; p, = 2700 kg me3; Q, = 570,000 kJ rnp3. Taking porosity to be l%, H = 1.27. Thus the thermal input to the dehydrating mass is ap-

DEFORMATION,‘METAMORPHlSM

proximately

equally

OF IVREA

divided

dration

and temperature

heating

is reduced

ZONE:

between

GEOPHYSICAL

driving

dehy-

rise, and the time rate of

during

the reaction

by a factor

l/2.27.

159

IMPLICATIONS

ity of granite

under

high pressure.

Res., 73:

Brodie,

K.H.,

1981.

composition

Variation

with

in amphibole

deformation.

and

plagioclase

Tectonophysics,

78: 385-

402. Brodie,

(c) Fluid loss

K.H.

between

and

Rutter,

deformation

E.H.,

been assumed,

no loss of evolved

and for simplicity

the behaviour to a constant

crease

in X from

permeability a factor

heat

of about

rock volume

flux. In reality,

0 to 1 causes

of a granite

rate of fluid

sub-

an increase

of by

1000 (Brace et al., 1968). If the

production

cannot

through

match

reaction

the

and

X

slightly greater than unity, hydraulic may cause a markedly greater increase

in effective permeability. The reaction cannot go to completion unless the evolved fluid permeates away, an action which also extracts heat from the system. It is during the interval when pore pressure is at its maximum that the most profound effects on the physical properties of the rock mass will be produced. Modelling such reaction accompanied

by permeation

particular

geometric

and a length present

requires

specification

of a

configuration

of the rock pile

scale. It lies beyond

the scope of the

paper.

of basic

and D.C. Rubie (Editors), ics, Textures

an in-

at room temperature

rate of fluid loss by permeation becomes fracturing

has

we have consid-

of a small

jected

fluid

Springer,

and

Brodie, K.H. and Rutter, faulting

of water of 0.0014 m3 kg-‘).

In: A.B. Thompson Reactions;

Adv.

Phys.

Kinet-

Geochem.,

E.H., 1987a. Deep crustal

K.H. and Rutter,

fine-grained

4.

extensional

Italy. Tectonophysics,

phism:

E.H., 1897b. The role of transiently

reaction

natural

products

in syntectonic

and experimental

examples.

metamor-

Can.

J. Earth

Sci., 24: 554-564. Brodie,

K.H.,

Rex, D. and Rutter,

deep crustal Italy.

extensional

E.H.,

faulting

In: M.P. Coward,

1989. On the age of

in the Ivrea zone. northern

D. Dieterich

tors), Alpine Tectonics.

and R.G. Park (Edi-

Geol. Sot. London,

Spec. Publ., 45:

203-210. Cooper,

R.F. and Kohlstedt,

ture of olivine

D.L.., 1984. Rheology

basalt

partial

melts.

1984.

Pore

pressure

and struc-

J. Geophys.

Res., 91:

9315-9323. Christensen,

N.I.,

structure. Etheridge,

Geophys.

J.R. Astron.

and

formation. Fountain,

during

regional

J. Metamorph.

Geol.,

R.H.,

northern

Italy:

evidence

seismic

of the continental

velocities

of rock sam-

M.H., 1981. Exposed

the continental

petrology

and Strona-Ceneri

33: 145-165.

D.M. and Salisbury,

structure,

and de-

1: 205-226.

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crustal

1983. The role

metamorphism

D.M., 1976. The Ivrea-Verbano

zones,

oceanic

Sot., 79: 411-424.

M.A., Wall, W.J. and Vernon,

of the fluid phase

crust:

and evolution.

cross-sec-

implications

for crustal

Earth

Sci. Lett.,

Planet.

56: 263-277. Fyfe, W.S., 1976. Chemical Trans.

aspects of rock deformation.

R. Sot. London,

the Earth’s

Crust,

Giese, P., 1968.

Elsevier,

Die struktur und

logischen

Deutung.

der

A.B., 1978. Fluids

verschiedener,

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Fyfe, W.S., Price, N.J. and Thompson,

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140: 193-212. Brodie,

tions through

volume

rocks.

in the Ivrea zone of northern

Fountain,

Using the estimates of the variation of VP with temperature, pressure and pore pressure shown on Fig. 3, we have shown on Fig. A.1 the estimated variation of VP with the progression of the dehydration reaction indicated (for an initial specific

the

with special refer-

Berlin, pp. 138-179.

crust-new

velocities

On

Metamorphic

Deformation.

ples. Tectonophysics,

(d) Seismic

1985.

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In the foregoing, ered

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der Inter-

petrographisch-geo-

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Mitt.,

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255-260. Barnes,

H.L. and Ernst, W.G.,

hydrothermal

fhrids-the

1963. Ideality system

and ionization

MgO-H,O-NaOH.

in Am.

J. Sci., 261: 129-150. Berckhemer,

H., 1969. Direct

rocks.

evidence

for the composition

Tectonophysics,

of crystalline

of

8: 97-105.

and argillaceous

Int. J. Rock Mech. Min. Sci.. 17: 241-251.

Brace, W.F., Walsh,

R.B.,

J.B. and Frangos,

W.T., 1968. Permeabil-

1971. Observation

high pressures Geophys.

the lower crust and the Moho. Brace, W.F., 1980. Permeability

Gordon,

Greenwood, total

H.J.,

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water

under

mantle.

J.

NaAlSi,O,-H,O-argon:

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Res., 66: 3923-3946.

Hale, L.D. and Thompson, character

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