Dipping shear zones and the base of the crust in the Appalachians, offshore Canada

Tecronophysics, Elsevier

173 (1990) 581--593

Science Publishers

581

B.V., Amsterdam

- Printed

in The Netherlands

Dipping shear zones and the base of the crust in the Appalachians, offshore Canada JEREMY HALL ‘, GARRY QUINLAN and ’

KEEN

MARILLIER

Geuwence

September

Neu$oundiand

3X7 (Canada]

Centre, Geological Survey of Canada, Bedford inairute (Received

’

2

CentreforEarth Resources Research. Department ofEarth Sciences, Memorial Universi!, of St. John’s, Nfr. AIB

2 Aiiantx

I, FRAN$DIS

CHARLOnE

of ~~ea~a~ra~~y,

1, 1988: accepted

Dar~maa~h, N. S. 817Y 4A 2 (Ccmada)

April 3, 1989)

Abstract Hall,

J.. Quinlan,

G., Marillier,

Appalachians, (Editors),

offshore

Seismic Probing

The seismological concentration They appear

character

into the mantle. detachment

mobile

which appear

We interpret

of continental

especially

during

the reflectors

blocks

deep seismic

Tectonophysics,

sub-horizontal

towards

migration. a mid-crustal

zones which of the Iapetus

B.L.N.

in the Kennett

173: 581-593.

from eastern

by ray-trace

of the crust and

ramp

Canada

“bright” through

an unusual

at the base of the

They are not point

It is suggested Ocean.

includes

reflectors

band,

diffrxtors.

and disappear

the lower crust that the reflectors

from

a

were

but may have been subject

to

strike-slip.

At the northwestern margin of the mobile a foreland composed of a Grenville basement

pro-

Publishers

the base

J.C. Dooley

belt offshore

at the final closure

patterns with offshore extrapolation of terranes mapped onshore has permitted a collision history to be constructed which accounts for minor variations along strike, and the Z-shaped swing of the northern margin of the mobile belt as it crosses the Gulf of St. Lawrence (Stockmal et al., 1987; Marillier et al., 1988). 0 1990 Elsevier Science

and

to dip through

as shear

files have been recorded across the Palaeozoic Appalachian orogen (Fig. 1) between Nova Scotia and the continental margin northeast of Newfoundland (Keen et al., 1986, 1987; Marillier et al., 1988). Correlation of deep crustal reflective

0040-r 95 1/90/$03.50

zones

C. Wright,

set of shears in the mantle.

late Carboniferous

reflection

shear

Finlayson,

upwards

Characteristics of deep seismic reflection profiles across the App~achians, offshore eastern Canada Since 1984 several

Dipping

has been characterised

25 and 45”. but flatten

to a diffuse and less reflective

by the collision

later reactivation,

D.M.

and their Margins.

reflectors

of these structures

mid-crustal

C., 1990.

Leven,

of the Appalachian

to dip at between

downwards

Keen,

In: J.H.

of Continents

of northwest-dipping

crust. The geometry

caused

F. and

Canada.

B.V.

belt, with

a Lower Palaeozoic shelf and basin cover (the Humber Miogeocline) has been overridden by slices of that foreland and cover, and additionally by ophiolitic remnants of the former Iapetus Ocean, to form the Humber Zone (Williams, 1978). A relatively undisturbed foreland basin the Anticosti Basin, lies further northwest below the waters of the Gulf of St. Lawrence and on Anticosti Island (Fig. I). To the southeast, the Dunnage terrane consists of a structurally complex assortment of rocks of oceanic affinity and is succeeded, further southeast again, by a similarly complex Gander terrane of rocks of continental or arc affinity. thought to originate from the southern

The

northwesterly

ville foreland is characteristic

of the

depths

observed Grenville foundland NEWFOUNDLAND

(Wilson,

block

places et al..

structures

caused by the Acadian “Central” Grenville

block foreland,

dipping

structures

to New-

Marillier

have been

are

belt and

adjacent

collision with

its

at lower

in the mobile 1986;

against

edge and

belt of NW Europe

immediately

(Keen

belt

1966). However.

northwesterly

at various

1988). These

Newfoundland

Appalachian

in the Caledonide

and Greenland cruslal

of the Gren-

in western

of the style of the Laurentian

of the whole continuation

overthrusting

observed

et al.,

interpreted

as

of the sub-Gander

the

basement

to

the

the

Dunnage

terrane

regarded as an upper crustal allochthon (Keen et al., 1986; Stockmal et al., 1987). Thus the northwesterly

overthrusting

(Taconic)

telescoping

its former

passive

is

viewed

of the lapetus

margin,

whereas

as

an

early

Ocean

onto

the northwest-

erly dipping structures are considered to be later formed, at the final closure of lapetus (ColmanSadd, 1982), though some reactivation of the shallower thrusts may also have occurred at this time.

Fig. 1. Map showing location of seismic lines referred to in text, in relation to Newfoundland and mainland Canada, and to the Appalachian mobile belt. The latter is contained between dashed lines marked BE% (Baie Verte line) and DF (Dover Fault). GRUB marks the Gander River ultrahasic belt which separates the principal components of the mobile belt- the Dunnage (D) and Gander (G) terranes. The margins of the belt are marked by the Humber Miogeocline (H) and the Avdon (A) terrane. The dotted line marks the edge of the known Appalachian deformation in the Gulf of St. Lawrence (GSL), north of which lies a forefand basin named after Anticosti (AC) Island. Thick parts of seismic tines denote those sections used here: note that they lie along the strike of the NW margin of the Appalachian mobile belt. NS = Nova Scotia. Lower map shows location of upper map in the Appalachian orogen (shown with diagonal lining, to include adjacent miogeoclinal margins and suspect terranes).

margins of Iapetus (Wil~ams, 1978). 3eyond the Gander terrane, the Avalon terrane has direct correlatives in NW Africa and is considered to be a former part of that continental block (O’Brien et al., 1983).

The deep northwesterly dipping structures are observed only below the Anticosti Basin, the Humber Miogeocline and the western Dunnage Zone. This has been interpreted to indicate that they are contained in deep Grenville basement, though the parallelism with Appalachian surface structure suggests basement

an Appalachian origin. The here is a promontory block

from the foreland transform

to the southwest

fault, responsible

Grenville separated

by a former

for the offset in strike

of the mobile belt shown by the Z-shaped across the Gulf of St. Lawrence (Stockmal 1987).

swing et al.,

Structures interpreted as shear zones that cut the base of the crust are rare in the published record of deep seismic reflection data. The more common pattern recognised is of shear zones soling into the lower crust, and this has been used to suggest that the lower crust may behave as a zone of decoupling between more brittle upper crust and mantle (Matthews and Cheadle, 1986). The northwesterly dipping structures observed around Newfoundland are unusual in that they appear to dip through the sub-horizontal reflectors interpreted as the Moho, and demand further

DIPPING

SHEAR

ZONES

AND

THE

BASE

OF THE

CRUST

IN THE

APPALACHIANS

attention. In the following sections we review the geometry of the structures, and present a scenario for their evolution. Geometry of the northwesterly dipping reflections

The dipping reflectors are observed on deep seismic linesshot for the AtIantic Geoscience Centre, Gealogical Survey of Canada, as part of the Frontier Geoscience Project. The lines contribute to LITHOPROBE, Canada’s national programme of lithospheric transects. The dipping reflectors occur on the highlighted sections (Fig. 1) of seismic Iines 1984/l, 2 (Keen et al., 1986) and 1986/3 (Marillier et al., 1988). Parts of these lines have been reprocessed on the CONVEX mini-supercomputer, at Memoriat University, using the Merlin SKS software. Data were acquired from 30-fold stack of 6320 in3 (7780 in3 for 1986/3) air-gun shots, recorded on a 120-channel streamer with a group interval of 25 m. Line 1984/2 provides the closest links with deep structure to the south and is the primary focus of this section. Line AGC 1984/2

The unmigrated

section of 170 km of line in Fig. 2, The upper crustal section (0-4 s two-way time) has few primary reflections, partly because of the deleterious effects of a series of strong multiples caused by a hard sea-bed. Multi-zone deconvolution has considerably aided resolution of the deeper section, but the shallow section remains si~ficantly affected by variable focussing of multiple energy caused by slight curvature of the sea bed. The primary reflections in the shallow section (Fig. 2b, events labelled c), discerned through the multiple energy, may relate to known Carboniferous basins, and also to possible shallow-dipping shears related to the inward-dipping margins of the Dunnage terrane. These margins are the Baie Verte-Brompton line (Williams and St.-Julien, 1982; Fig. 2b, event Iabelled BVf,) in the northwest, and tke Gander River ultrabasic belt. (event labelied GRUB) line (Jenness, 1958) in the southeast. At mid-crustal depths (abaut 6-8 s two-way time) a series of discontinuous, sub-horizontal ref984/2

is shown

583

flectors (Fig. Zb, events labelled d) may represent parts of the detachment at the base of the Dunnage zone. Below this level he the reflectors of principal concern here. The lower section is characterised by two groups of reflections. Sub-horizontal reflectors concentrated between 9 and 12 s appear as they do in many deep continental crustal profiles, and by analogy we assume that they lie in the lower crust and that the deepest sub-horizontal primaries (Fig. 2b, events labelled m) lie close to the base of the crust. If they are actually at the base of the crust, it varies in depth across the section, and these variations may relate to offset across the second set of reflectors: those which dip predominantly to the northwest. It should be noted that the signalto-noise ratio of these deep data is nor. especialiy high, and the possibility exists that other subhorizontal reflectors are present but unable to break up through the noise level. The commonality of the pattern here with that in line 1986/3, in which the signal-to-noise ratio is rather better, leads us to suppose that we do observe t.he base of a fower crustaf layered zone in 1984/2. The dipping reflectors are widely distributed along this profile but their density is */ariable, a particularly strong band being present in the central part of the section displayed. The reflectors give the appearance of levelfing out upwards towards the mid-crust. This convex upward shape is similar to that expected from point diffractors (cf. Fig. 2c) but variations in dip, including occasional tendencies to level out downwards (Fig. 2b, events labelled f > suggest that these are reflections. Migration of deep reflections is well known to be unreliable. Migration velocities somewhere between true and infinity should be applied, dependent on the strike of the reflectors relative to the orientation of the profile. Quite different velocities may be required to properly migrate events, from surfaces of different strik.e, in the same part of the reflection section. In 2D migration that is impossible to achieve. By and large, migration with an estimated velocity function results in an unavoidable smearing of the data. An example of part of the deep data from line 1984/2 migrated with a velocity of 6500 m s’“-’ is given in Fig. 3, In this example, note that the sub-horizon-

Fig. 2. (a) Unmigrated seismic section of part of AGC lute 19X4/2. This section is displayed with strong bias to cmphasise the more prominent events: some significant events he below the threshold of the display. (b) The same section with interpretation. Reflectors (’ show the shallow structure of several Carboniferous basins, probably separated by faults. Events beiow 3 s two-way time (TWT) are those picked for tine migration. The dashed tines indicate arbitrary ghost horizons used to create models of orustal velocity tncreasmg wrtt? depth. Sub-horiLonta1 picks include the deepest identifiable (m), which are therefore interpreted to lie at the base of the crust. Reflectors d are interpreted as parts of a mid-crustal detachment on which the overlying Dunnage terrane rests. Dipping reRectors include some with changes of curvature with depth, e.g. at points marked f. The L-shaped corners marh the location of seismic sections shown in Fig. 3. The Dunnage terrane is contained between the Baie Verte Line ( BVL) and the GRUS line. the location of which is off the southeast end of the section and approximately where the picked mid-crustal dipping reflector at the southeast end of the section projects to the surface.

DIPPING

SHEAR

ZONES

AND

THE

BASE

OF THE

CRUST

IN THE

585

APPALACHIANS

and

115 km has some

above and

the deeper also above

scend

through

the dipping

towards

some

the base

C Fig. 2 (continued).

many

velocity

(c) Unmigrated

point

diffractions

in (a) and (b), for several depths of 6 km s-‘.

of the dipping

diffractors

to

which

de-

and, with di-

but this is not maintained

show

a tendency

of the section

.o level out

(Fig

2b, events

that the events are undiffractors, which would

consistently

convex

upwards

sig-

15 s

would be imaged crustal

rise

reflectors

reflectors

labelled f). This shows likely to arise from point give

segments

tail off towards the base of The dips of these events tend

downwards

throughout:

but lies wholly

sub-horizontal

the crustal

minishing amplitude, the recorded section. to increase

structure,

short

because

Comparison

events they

change

do

of curvature

have

AGC 84.2

5km

,

UNMIGRATED

~

with (a) shows that

in {a) cannot not

as they

and a mean

arise

the

same

from

point

consistent

with offset.

tal reflections retain, as they should, the same positions on the migrated and un~grated profiles (except that reflection terminations are smeared laterally

and suggest a lateral

The problem then is to migrated dipping reflectors where they might migrate Despite

several

iterations

extent

beyond

real).

ascertain where the “come from” and to

with different with

velocities.

a sequence

of

migration velocities, the likely migrated positions of the dipping reflectors remain rather obscure, mainly because of their broad distribution (Fig. 3b) and the consequent difficulty of isolating particular events and thereby tracking their movement as migration velocity is altered. A much more fruitful procedure is to pick key events and migrate them. This so called “line drawing migration” has the distinct advantage of providing a very direct indication of how each event is displaced in the migration process. We have used the Merlin SIMPAC software on the CONVEX to migrate the deep events picked in Fig. 2b. Several migrations have been used, and the results are summa~sed in Fig. 4. On the unmigrated profile (Fig. 4a), a series of sub-horizontal reflector segments are identified, the deepest of which may be the base of the crust. A major mid-crustal reflecting band between 70

Fig. 3. Unmigrated

and migrated

sections

Fig. 2, showing

the smearing

dipping

and sub-horizontat

reflectors

6500 m s-l).

Despite

the smearing,

close to the base of the crust, dipping

effects

of the same part of

of migration ones (section sub-horizontal

still appear

shear zones (e.g. u-CJ to b-b). Arrows significant

dipping

on both migrated

and sub-horizontal

at

reflectors,

to be offset indicate reflectors.

across

the more

natures,

as indicated

on the synthetic

diffractions

between the domains of unmigrated time, migrated depth and migrated time. To show the most direct

(Fig. 2~). The SIMPAC

modelling

tom into short segments ray

constructions, AGC

NW

package

breaks

and uses normal

as appropriate,

84/2

0 ’

15

s

between

profiles,

the

migrated

depth

the

original

transformations to migrated

are

and

migrated

taken

through

time. To carry out the

UNMIGRATED

I

I

I

I

160

120

80

,

_ ~_..___ .---.-

___.____,.,--_-_ .._.-- ---.--

5 i-

comparison

to transform

40

0

reflec-

or image

1

I

I

.~-----

I

L

1

AGC NW

8412

SE

km

#&ORATED 160

0

SE

km

t_______-__-__-__-____--_-_-__-_-_-_--_-_-_---__-_--_______--_____--_____--_____-_6300

I

ms-’

E

-.._.zd 20

-

km

b I_

15 sJ

~~_~_~__ I AGC

NW

8412

MIGRATED

0

i

01

40 I

i

80 t

I

120 I

%

4000

160 I

rn*-’

kmSE I-

_--.-----__

I 20 km

15 s

,

I

i

Fig. 4. (a) Line drawing derived directly from picked, unmigrated section of Fig. 2. (b, c) Migrated line drawings us@

velocities

shown. Each migrated event is identified by three lines: the centre hne is the migrated position for the velocities given, white the outer lines define 95% confidence limits on the migrated position on the assumption that the lowest velocity layer has an associated uncertainty of 10%. The limits are constructed from a set of migrations derived by Monte Carlo perturbation of the lower crustal velocity given, where perturbations have a standard deviation of 10% from that given.

DIPPING

SHEAR

ZONES

transformations distribution.

AND

THE

requires seismic

knowledge

control

al., 1966) but indicates through

OF THE

This is only poorly

cal refraction section

BASE

the continental

of velocity basement.

data

to first-arrival

APPALACHIANS

here. Lo(Dainty

continental

crust, the rarity of good shallow available

IN THE

of the velocity

known

is sparse

a normal

with some increase

CRUST

et

crustal

downwards In the upper

reflectors

refractions

reduces recorded

on the streamer. Apparent velocities tend to range around 4-5 km s-‘, with velocities at the lower end of the range over the Carboniferous basins,

587

reflector,

the velocity

the reflecting

immediately

segments

that the RMS deviation for all such segments defines

a new,

of the velocity

perturbed

model.

segment

limits implied

such

variations is 10%. This

Thirty

and migrated.

lines for each migrated

each of

randomly,

from 6.3 km s-’

were thus generated confidence

above

is varied

models

The two outer

represent

the 95%

by the 30 solutions.

Fig-

ure 4b shows the expected increase in uncertainty as the dip increases. It also shows some variations, even

for the sub-horizontal

segments,

on whether

flat cracks

above that intersected segment, rather than retaining the velocity of the earlier segment. This effect

and joints

close up, we assume

a gen-

eral increase of velocity with depth, and characterise this very simply by a change from a 6 km thick uppermost

crustal

layer with a velocity

of 4

km s-‘. For the rest of the crustal section, several alternatives have been tried to illustrate the spread of solutions. Two simple but extreme examples are assumptions of constant velocity of 6 and 7 km s-l for the section below 3 s two-way travel time. Preliminary migrations not illustrated here show that the dipping are largely

events migrate to the southeast and confined within the lower crust, but

that some still cut the base of the crust. Those lying between 70 and 115 km remain below the mid-crustal reflector, except when migrated at 7 km s-t where they intersect the mid-crustal reflecting band. If we assume that dip section then this reconstruction unlikely,

because

of the intersection.

the profile is a is geologically We interpret

this to suggest that the velocity of 7 km s-’ assumed for the continental basement is too high. These preliminary of likely soiutions, stration-illustrated

migrations encompass the range but a more convincing demonhere-is obtained through the

SIMPAC package by taking other, less extreme, velocity dist~butions and perturbing them to allow a range of solutions to be generated in Monte Carlo fashion. The results of applying this process are shown in Fig. 4b and c. In Fig. 4b, the deep crustal velocity is taken to be a constant 6.3 km SK’. Having obtained the appropriate solution, given by the middle of the three lines defining the migrated position of each

rays just do, or just

dependent

which may extend to depths of 3 km or so. Given that crystalline rocks always show increase of velocity with depth in the top few kilometres as

an overlying segment. ray then being subject

is equivalent

to adding

do not, intersect

Intersection results in the to the velocity immediately

some random

velocities

to

the lower crust in a laterally incoherent way, and might rather well be a realistic model :‘or a lower crust characterised by lateral inhomogeneity. The results show that the dipping segments lie predominantly within the lower crust. However, the sub-horizontal

reflectors

remain

discrete

and

the dipping reflectors may run between their terminations, suggesting that the shear zones (which we interpret them to be) continue into the mantle but with diminishing reflectivity. It should be recalled (see Fig. 2) that some of I.he picked segments continue to the lower limit of the original seismic section and may continue further. They are artificially truncated time and, despite having

by the 15 s recording low amplitude at that

point, must continue to greater depth. Figure 4c shows results from similar except that the crustal velocity

processing,

distribution

like that often encountered, in having crustal basement velocity of 6.0 km s-’

is more an upper underlain

by a lower crustal layer of 6.7 km s-‘. In this case only the velocity of the 6.7 km s-l layer has been perturbed in the Monte Carlo modeliing. Because of the reduced depth extent of the perturbed layer the spread of the 95% confidence limits is rather smaller than in Fig. 4b. Otherwise the two figures are very similar. The fact that most of the reflections appear to originate in the crust or close to its base justifies the exclusion of mantle velocities from consideration in the modelling. Many other velocity effects could have been

t

58X

NW

AGC 84.1

Fig. 5. Unmigrated

seismic section of line AGC 1984/l

cut by northwest-dipping

shears. Arrows

Ii41

20 km

showing indicate

features

similar to line 1984/2:

the more significant

modelled. However, we believe that the variations illustrated here are enough to give a good general guide to the interpretation of the northwest dipping events and their re~ations~p with the horizontal layering. The most obvious conclusion is that the dipping reflectors cut through the lower crust, and some possibly pass through the base of the crust, to disappear in the mantle. At their upper ends some of them may sole as they flatten out upwards into the mid-crustal events. The reflectors in the mid-crust at distances along the profile between 80 and 110 km and between 130 and 160 km on the migrated profiles may be shear zones which carry the displacement across from the northwest dipping shears along the mid-crust and off the southeast end of the section. One possibility is that the reflector between 130 and 160 km continues as a shear zone upward to link with the surface Gander River Ultrabasic Belt, which forms the boundary between Dunnage and Gander terranes.

dipping

I. I-.-r 41

SE

deep crustal

and sub-horizontal

sub-horizontal

layering

reflectors.

Line AGC 1984/l

This section of the 1984 data (Fig. 5) shows similar features to that of the 1984/2 data, though the noise level in 1984/l is distinctly higher. On this line, the northwest dipping events are below the surface exposure of the Humber Miogeocline and thus almost certainly he within the. Grenville basement. Since the geometry and amplitude ~of the events are similar to those of 1984/2, we assume the same conclusions apply: that the dipping events are lower crustal with possibly limited extension into the mantle. Though the reflectors lie within Grenville basement, their position in relation to similar features on the other lines considered here suggests that their age is related to Appalachian events, rather than earlier. Line AGC 198613

This line runs northwestwards across the Gulf of St. Lawrence from the Bay of Islands in west-

DiPPlNCi

0

SHF.AR

ZONES

AND

THE

BASE

OF THE

CRUST

IN THE

APPALACHIANS

0

(Fig. 6a) shows northwesterly dipping structures with strong amplitude at several places at the base

event. We interpret the age as Acadian, based on arguments presented by Stockmal et al. (1987) and on the geometrical continuity of the structures

of the crust. The line drawing

with the Gander

ern Newfoundland

that

(Fig.

the dipping

structures

A) which appears erly dipping

dipping

pre-Taconic,

linking

to the Grenville

1987).

Consequently

reflective

end of the sec-

et al., dipping

perhaps

Acadian

AGC 6613

possibly

(viz. Green

caused

Taco&

zone

or

by a

younger

Ultrabasic

structures enough

makes is

boundary

of

migration

a terrane the

section

southeast

a number (Fig.

side

on line 1984/2

of events

7a) and

process and velocity

I

I

used

from the as used

in Fig. 4c. In this case it is of note

SE

1

I

120

km

I

1

I

t

MIGRATED

NW 40

0

SE

80

120

-

20

ms-’

6000

ms-’

6700

ms-’

b shown in Fig. 6. Deepest sub-horizontal

to base of crust. (b) Line drawing migration of (a) with 95% confidence

from a set of random perturbations

km

4000

km

Fig. 7. (a) Selected reflectors picked from unrnigrated data of same part of AGC line 1986/3 obtained

the

the same

distribution

a

reflectors (m) may correspond

of

that there is no tendency for the reflectors to flatten out downwards. This results in their migra-

80

40

0

AGC 6613

dipping

boundary-not

to be assertive.

We have selected unmigrated

of the southeast

this

seen

Belt. It is not

UNMIGRATED

NW 0

River

clear that the truncation

zone is regarded

northwesterly

must be younger,

late-Grenvillian,

southeast-

structure,

Front the

one (marked

the bright

to the northwest

as a Grenville,

section

(Fig. 6b) confirms

tails of the mid-crustal

This southeasterly

structures

seismic

include

to truncate

which continues tion.

1). The

knits on the migrated position

(with standard deviation of 10% from the mean) of the deepest velocity given. A is the

major dipping reflecting zone referred to in the text.

DIPPING

SHEAR

ZONES

AND

THE

tion (Fig. 7b) causing pression retain

rather

of the events.

results in less distant a broadly

migrated

events,

BASE

OF THE

greater

Their

migration,

similar

CRUST

lateral

relatively

with the un-

also clearer

that some of these events

the mantle:

this can

here because

the crustal

higher amplitude.

com-

low dip

the case with 1984/2. be more

APPALACHIANS

so that the events

relationship

unlike

IN THE

remain

assertively

reflectivity

It is in

stated

is of generally

The same conclusions

are drawn

as in the case of line 1984/2.

591

The sense strably

is sometimes normal.

is indeterminate.

Predominantly

movement

during

followed

erous sion

strike-slip

collision,

and perhaps

in the Mesozoic,

could

demon-

The extent

strike-slip

by Carbonif-

also modest have

of

reverse exten-

produced

the

variety of apparent displacements seen now. A particularly unusual feature of these structures

is their

ramping

from a mid-crustal the common

through

detachment.

view (e.g. Kusznir

the

lower

crust

This is contrary

to

and Park, 1986) of

continental crust as characterised by a brittle upper crust and mantle separated by a more ductile

Discussion

On three places of AGC deep seismic data around Newfoundland, northwesterly dipping reflectors are associated with the lower crust, may cut the base of the crust, and then die out in the mantle. They are interpreted as shear zones. Several questions are posed by this, relating to the age, sense of movement and mechanical behaviour of the crust. The structures

of movement

reverse, occasionally

are observed

in an area which

has strong correlations with the Appalachian mobile belt: they occur in a zone correlating with the position

of the Grenville

basement

margin,

either where it is known

near to its SE to be present

or

where it can readily be inferred to be so. The zone does not extend southwest of the major bend in the northern margin of the mobile belt in the Gulf of St. Lawrence (Fig. 1). For these reasons, it is likely that the event is related to some stage of the development of the mobile belt. As mentioned above, one member of the set truncates mid-crustal structures thought to be pre-Appalachian. They are a more broadly distributed set of structures than are usually associated with late-erogenic strike-slip, and they occur in an area which has no appreciable extensional strain as occurs in the post-erogenic history of the adjacent continental margin. Thus we interpret the structures as primarily caused by dip-slip motion late in the history of the Appalachian belt, probably Acadian, associated with the final continental collision. Such events would have involved quite large energy, but distributed over many shear zones, each of which need not necessarily have large displacement across it.

lower crust, the ductility being induced greater temperature of a layer with broadly

by the similar

composition to that of the upper crust. The oversimple generalisation of that model may more typically represent extensional cond: tions with their enhanced temperature gradients; the lower temperature gradients typical of convergence may allow a variety of other styles of crustal deformation, in which variations of lithology, or just anisotropy of strength, may be enough to cause localised detachment. The broad styles of deformation now recognised

as typical

of the northern

margins

of the Variscides in France (Matte and Hirn, 1988) and Germany (DEKORP, 1988) show shear zones ramping through upper and lower crusts with detachments

at the mid-crust

and

near

the

Moho. In the case examined here, the former is recognised but not the latter: the shears appear to cut straight through the base of the crust. Their lack

of, or lower,

reflectivity

in the

mantle

is

probably caused by the splitting of the large shear zones in the crust as they enter the mantle-the strain being divided among many narrower shear zones, few of which merge enough at seismic wavelengths to produce the constructive ference necessary for significant reflectance.

inter-

Conclusions Northwesterly dipping reflectors are observed in the deep crust of the Appalachian mobile belt around Newfoundland. Their geometry cannot be explained by point diffraction. The reflectors are interpreted (Acadian)

as shear zones caused at the final collision of continental blocks during

PRE-TACONIC

LITHOSPHERE \____“_

ASTHENOSPHERE -----I

100

km

TACONIC -%I -, ACADIAN

+ + I?

GRENVtLLE

d cl

DUNNAGE

y GANDER izl x

Fig. 8. Cartoon showing postulated development of the northwest-dipping shears, developed From figs. 3 and 4 of Stockma et al. (1987) by the inclusion of mid-Grenville bright zone as reactivated to form the upper member of a lower crustal duplex. FF = Grenville Front (note break in section indicating GF is further northwest than scaled on this cartoon), PM * Grenvilk passive margin sedimentary wedge, AB = Anticosti foreland basin, 10 7 Iapetus Ocean. Scale is approximate.

the closure of the Iapetus Ocean. The shears may have been reactivated during late- to post-orogenie strike-slip, and again during IMesozoic extension. The extent of the reflectors into the mantle is uncertain, though the base crustal reflectors still appear offset implying through-going shears. The fading of the reflectors in the mantle may be caused by their splitting into small unreflective shears in the more brittle rocks of the mantle. The patterns of shingling of continental and oceanic fragments result in a “flake” form of their edges, with apparent back-thrusting at depth below foreland-directed thrusting near surface, as interpreted elsewhere in the Appalachian-Caledonide belt by Hall et al. (1984). Figure 8 illustrates the supposed process and is developed from Stockmal et al. (1987) to include some of the relationships suggested here, including reactivation of aid structure not all of which is yet proven.

This paper is LITHOPROBE Contribution No. 96 and Geological Survey of Canada Contribution No. 24089. Reprocessing and line-drawing migra-

tion were carried out on the seismic processing facility at Memorial University and funded through an industrial research chair by NSEKC and Petro-Canada. References Colman-Sadd, S.P., 1982. Two stage continental collision and plate driving forces. Tectonophysics, 90: 263-282. Dainty, A.M., Keen, C.E., Keen, M.J. and Blanchard, J.E., 1966. Review of geophysical evidence on the crust and upper mantle structure on the eastern seaboard of Canada. Am. Geophys. Union, Geophys. Monogr., 10: 349-369. DEKORP, 1988. Results of the DEKORP 4/KTB Oberpfah deep seismic reflection ~nv~tigatjons. J. Geophys.. 62: 69-101. Green, A.G.. Milkemit, B., Spencer, C., Morel, P., Davidson, A. and Teskey, D., 1987. Seismic reflection profihng across the Grenville Front: lower crustal reflections extend to the surface. Abstr., 19th Gen. Assoc. Int. Union Geod. Geophys., 1: 64. Hall, J., Brewer. J.A., Matthews, D.H. and Warner, h&R,, 1984. Crustal structure across the Caledonides. from the WINCH seismic reflection profile: influences on the evolution of the Midland Valley of Scotland. Trans. R. Sot. Edinburgh, Earth Sci., 75: 97-109. Jenness, SE., 19.58. Geology of the Gander River ultrabasic belt. Newfoundland. Geol. Surv. Newfoundland, Rep. II.

DIPPING

SHEAR

ZONES

AND

THE

BASE

Keen, C.E.. Keen, M.J., Nichols, Colman-Sadd, Wiliiams, profile

H. and Wright, the

CRUST

IN THE

B., Reid, I., Stockmal,

S.P., O’Brien,

across

OF THE

S.J., Miller,

APPALACHIANS

G.S.,

H., Quinlan,

G.,

J., 1986. A deep seismic reflection

northern

Appalachians.

Geology.

14:

141-145. Keen,

Stockmal,

Mudford, Lithoprohe

northeast Park,

(Editors),

Nature

F., Keen, H.,

Crustal

Appalachians: Can. J. Earth Matte,

section

Tectonics, Matthews. from

J. Hall

with the Himalayas.

Brown (Editors),

Reflection

tive. Am. Geophys. O’Brien,

S.J., Wardle,

deformation. and

C.E..

Stockmal,

In:

K. Wedepohl

Continental

and

G.S.,

S.P. surface

implications

and

Crust.

of the

Quinlan,

G., WilS.J.,

1989.

of the Canadian

of deep seismic reflection

data.

.

Sci., 26: 305-321 Variscan

Union,

crust

and tectonic

in western

France.

and

the Caledonides

M.J.,

1986.

and Variscides

Deep

reflections

west of Britain

and

terrane

and I_.

A Global

Perspec-

Ser., 13: 5-19.

King, A.F..

1983. The Avalon

in the Appalachian

Williams,

S.P., Keen,

G., 1987. Collision plate

Appalachians.

orogen

in

tectonic

C.E., O’Brien.

along an irregular

interpretation

S.J.

margin:

of the Canadian

Can. J. Earth Sci., 24: 1098-1107.

H., 1978. Geological

Appalachians:

development

its bearing

In: D.R.

Bowes

and

in Northwestern

of the northern

on the evolution B.E. Leake Britain

cf the British

(Editors},

Crustal

and Adjacent

Regions.

Geol. J., Spec. Issue, 10: l-22. Williams,

H. and St.-Julien,

ton line: early Canadian ern

Paleozoic

Appalachians. Major Structural

Appalachians.

Geol.

P., 1982. The Baie Verte-Brompcontinent-ocean In:

int,:rface

P. St.-Julien

ard

Zones and Faults Assoc.

Can..

in the

J. Beland

in the North-

Spec.

Pap..

24:

177-207. Wilson.

Cheadle,

Geodyn.

R.J. and

G.S., Colman-Sadd,

and Quinlan,

(Editors), signature

7: 241-155. D.H.

Stockmal,

Isles.

In: M. Barazangi

Seismology:

Geol. J.. 18: 185-222.

Evolution

O’Brien,

zonation

Canada.

a regional

lithosphere

Spec. Publ., 24: 79-93.

P. and Wirn, A., 1988. Seismic

cross

of

results from

Sci.. 24: 1537-1549.

of the Lower

Colman-Sadd,

structure

G. and

and evolution

1986. Continental

Carswell.

Geol. Sot. London, liams,

structure

role of lower crustal

D.A.

The

H., Quinlan,

of Newfoundland:

R.G.,

the critical

Dawson,

Marillier,

Welsink,

East. Can. J. Earth

N.J. and

strength:

G.S.,

B.. 1987. Deep crustal

the rifted margin

J.B.

comparison

zone: a pan-African

C.E.,

Kusznir.

593

J.T.,

Nature.

1966. Did the Atlantic

211: 676-681.

close and

then

reopen?