Fault reactivation during Mesozoic extension in eastern offshore Canada

Tectonophysics, 173 (1990) 567-580 Elsevier Science Publishers

567

B.V., Amsterdam

- Printed

Fault reactivation

BEATRICE

in The Netherlands

during Mesozoic extension offshore Canada

de VOOGD

*, CHARLOTTE

E. KEEN

in eastern

and WILLIAM

A. KAJ’

Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2 (Canada,1 (Received

September

1,1988;

accepted

March 6.1989)

Abstract De Voogd,

B., Keen,

Canada.

C.E. and Kay,

Continents

profiles

now straddle

that document

the Hibemia

each other near the western

picture

of basement

appears

to follow a pre-existing

Hibernia

structures.

1990. Fault C. Wright,

reactivation J.C. Dooley

reactivation

basin

characterized

by numerous,

This detailed been formed Mesozoic

occupies

The basin-bounding

fault

which

fault system. More precisely,

Grand

dipping

the reactivated

extensional

the central

most of the Bonavista reflective

study of the Hibernia

within

region suggests

hanging

marks

the eastern

of

that the narrow

elongated

sheets. Similar

shelf. The pre-Mesozoic Orogen

The Late Palaeozoic history of the eastern Canadian continental margin is poorly known by comparison with the European conjugate. Palaeozoic or older structures are however likely to be present and may have played a role in the formation of the rifted Mesozoic basins that now occupy this passive margin. This paper presents new deep multichannel seismic reflection data where

de GCologie,

The profiles

edge of the Bonavista

the Bonavista part

Canada.

giving a three-dimensional

the Jeanne

a Palaeozoic

thrust

platform

d’Arc Basin just ncrth platform,

of

where it his not

fault. A laterally

of the crust

extensive

is quite reflective

Ecole Normale

75231 Paris, Cedex 05, France.

0 1990 Elsevier Science Publishers

B.V

basins of the Grand observations

compressional

and may be of Variscan

structures

0040-1951/90/$03.50

offshore

Seismic Probing

fault. Four deep s:ismic

off eastern

d’Arc Basin, thereby

The deeper

walls of older thrust

basins of the U.K. continental

DCpartement

in eastern

(Editors),

.md is

packages.

Banks lie along the Appalachian

24 rue Lhomond,

extension

Kennett

Banks

the fault that bounds

was probably

platform.

Introduction

address:

Mesozoic

fault as a normal

of the Grand

edge of the 22 km deep Jeanne

The seismic data show that this feature

Palaeozoic(?)

SupCrieure,

of a thrust

oil field region

can be traced on the seismic data along strike to the south, beneath

been reactivated.

* Present

during

and B.L.N.

Tectonophysics, 173: 567-580.

and their Margins.

New seismic data are presented reflection intersect

W.A..

In: J.H. Leven. D.M. Finlayson,

Banks may have

have been made for the structures

documemed

on

age.

which pre-date

the Mesozoic

extension

are imaged. The seismic data reported here were recorded in 1987 in the Hibernia region of the Grand Banks off eastern Canada (lines 87-1 and 87-2, Figs. 1 and 2) and complement the data collected in 1985 (de Voogd and Keen, 1987). The Mesozoic basins and the deep structure of the continental margin were the main focus of these two surveys that straddled the region between the Bonavista platform to the west, and the rifted continental margin to the east. Results concerning the continentocean transition are presented by Keen and de Voogd (1988). The 1985 profiles revealed crustal-scale structures associated with the Meso-

I

60°

44OW

52O

-P-p\'-p---

-i

1,52O N /, \84-2

1,O!

! 48O

85-l

IU \

\85-4

i’

-440

'85-2

I

52O

I

440

Fig. 1. Location of deep multichannel seismic reflection profiles (bold lines) on the eastern Canadian margin. Major sedimentary basins are stippled; box indicates area of Fig. 2.

zoic basins (Keen et al., 1987). The basins are bounded by large, moderately dipping normal faults that terminate in the lower crust or at Moho depth. Besides theses normal faults, several intra-basement dipping reflectors were imaged and one goal of the 1987 survey was to constrain their origin. Tectonic framework The area of study (Figs. 1 and 2) lies along the Caledonian/Appalachian erogenic zone. Basement beneath the central Grand Banks consists of rocks of the Avalon terrane of the Appalachian orogen (Haworth and Lefort, 1979; Willlams and Hatcher, 1983). This pre-Mesozoic basement includes Hadrynian and possibly older crystalline rocks, and may be overlain by Palaeozoic sedimentary or metasedimentary sequences similar to those described by Durling et al. (1987). As rifting of the North Atlantic progressed northward, several episodes of extension of dif-

fering orientations dissected the preYMesozoic basement. The Grand Banks first experienced extensional tectonics in Triassic time, followed by rifting in Jurassic and Early Cretaceous time (Enachescu, 1987; Tankard and Welsink, 1987). The end of Mesozoic rifting is recorded by the-prominent Late Cretaceous “breakup unconformity”, or series of unconformities, collectively termed the Avalon Unconformity (Jansa and Wade, 1975). The Jeanne d’Arc Basin (Fig. 2) is one of the major Mesozoic rift basins. It lies on the northern Grand Banks, at the point where the direction of extension changed through time from east-west to northeast-southwest. The basin is exceptionally deep, over 22 km, and is bounded on the west by the Bona&a platform. Industry seismic data have delineated the shape of the basin-bounding fault system, the Murre-Mercury faults. The Mercury sub-basin is of Cretaceous age, whereas a thick section of Jurassic and older deposits has been documented in the Jeanne d’Arc Basin (Enachescu, 1987). The jagged e&e of the Bonavista platform in the Hibemia region may result from

FAULT

RELAC-I’IVATION

DURING

MESOZOK

Fig. 2. Model of depth to basement location software)

of deep multichannel

EXTENSION,

showing

major deep structures

seismic reflection

of the deep seismic profiles features

E. OFFSHORE

profiles.

and selected

near the edge of the Bonavista

It is constrained

released

569

CANADA

industry

by three-dimensional

lines. Note

are not shown here. Dot is location

that secondary

of the Hibemia

platform ray-tracing faults

an discussed (Sierra

in text, with

Gesphysics

as well as many

Inc.

small-scale

oil field.

transfer faulting oblique to the normal faults that bound the basin (Tankard and Welsink, 1989).

summarized by Keen et al. (1990). Processing included pre-stack deconvolution and FK filtering, and post-stack migration. The seismic: sections

Seismic reflection data

and line drawings shown here (Figs. 3-:3) have no vertical exaggeration for a velocity of 5 km s-l. We shall first describe lines 87-1 and 87-2, and discuss their ties with lines 85-3 and 85-4 (Fig. 2). These ties provide key evidence needed to fuel our discussion on reactivation of pre-existing structures.

The 1987 data were recorded by Geophysical Services Inc. (GSI), Calgary, Alberta, and processed under contract by Western Geophysical. Acquisition and processing parameters are similar to the 1985 survey (Kay and Keen, 1988) and

H I)l.

0

\‘OotiO

I I .x1

FAULT

REAC’I’IVATION

DURING

MESOZOIC

EXTENSION.

E. OFFSHORF

CAYADA

B r>E VOOCiI>

BONAVISTA

(a)

ET AI.

PLATFORM

15

20 KM

line 87-1 (migrated)

f&l

BONAVISTA

JEANNE

PLATFQFtM

15

D’ARC 6ASlN

- ‘-

_--

20 KM

15

Fig. 4. Line-drawing interpretation of migrated stacks of the new seismic profiles presented here (lines 87-1 and 87-22 Se&ted examples of the actual data are shown in Figs. 3,5-8. Triangles indicate ties with released industry seismic lines used to constrain the interpretation; ties between the deep profiles are also shown. (a) Line 87-l; dashed tine at about 3 s is drawn immediately below the prominent seismic marker labeled B in Figs. 6,7, and 8. (b) Line 87-2, &owing the western edge of the Jeanne d’Arc Baain north of the Hibernia oil field. K indicates the Cretaceous section that fills the Mmzzry sub-basin; J indicates the thick Jurassic pile present in the Jeanne d’Arc Basin.

Line 87-l is confined to the Bonavista platform, where the post-rift sediments were initially thought to lie directly on top of basement. Line 87-2 starts on the platform, runs eastward across the Mercury sub-basin (Enachescu, 1987) and crosses line 85-3 in the Jeanne d’Arc Basin (Fig. 2). Post-rift sediments overlie the Avalon unconformity, a prominent seismic marker labelled A in Figs. 4, 5 and 7. It separates Upper Cretaceous sediments from underlying Jurassic/Triassic deposits or basement (Enachescu, 1987). The uppermost prominent reflector is the “base- Tertiary” un~nfor~ty (labeled T), which is readily distinguished from the Avalon unconformity in the Jeanne d’Arc Basin (eastern half 87-2, Figs. 4b and 7). These two unconformities coalesce on the

flanks of the basin. Below the Avalon unconformity, the basin-fill consists of a thick, faulted, syn-rift section of Triassic to Lower Cretaceous deposits. Sedimentary structures within the Jertnrze d’Arc Basin itself will not be further discussed here (see Enachescu, 1987; Keen et al., 1987; Tankard and Welsink, 1987). The Murre-Mercury fault system that bounds the western side of the Jeanne d’Arc Basin (Fig. 2) is clearly imaged on line 87-2 (Figs. 4b and 7). Inferred fault plane reflections can be wed to at least 10 s (or about 25 km depth) on the seismic section. These faults are the focus of this paper, and their geometry and origin are discussed in the next section. Prominent reflective packages are observed in

FAULT

REACTIVATION

DURING

MESOZOIC

EXTENSION.

E. OFFSHORE

the deeper part of the crust on lines 87-1 and 87-2 (Figs.

7 and

different

8). Distinct

orientations

reflective

are imaged

“layered”

lower crust (McGeary,

flections

occur

throughout

bands,

and intersect the Bonavista

Moho is well defined lower crustal

rather

than

than

Individual

a

evenly

events

released

38 km) and appears

zontal.

data

sub-hori-

85-4 give

a

basement velocity of 6.1 km s-i and confirm that Moho coincides with the base of the reflective lower crustal unit (Reid and Keen, 1988; I. Reid, 1988). Moho is poorly imaged pers. commun.,

lines, the west

d’Arc Basin (Enachescu. a deeper

to the Murre

1987;

reflection

unknown

fault image.

origin

The loca-

to tie with reflection

H on line 85-3. The intersecting displayed

industry

fault bounding

tion of line 87-2 was chosen

unit (Figs. 4, 5. 7 and 8). It is fairly line

with

the Murre

Keen et al., 1987). In addition, runs parallel

by the base of the reflective

along

85-3, along imaged

(H, Figs. 3a and 5) of previously

the reflection

deep (13 s or about Refraction

Line clearly

side of the Jeanne

are

each other. platform

Deep fault geometry and basement structures

of

1987). These re-

rather

the lower crust.

often dipping Beneath

in

packages

573

CANADA

in Fig. 3 demonstrate

seismic that

sections

13 ties with

the Mercury fault to the north. Assuming a locally planar reflector, and taking an average basement velocity of 6.1 km s-l, it can be calculated that reflector H strikes the east.

NlO”E

and dips about

30” to

beneath the Jeanne d’Arc Basin. On line 87-2, Moho may correspond to a weak, “pulled-down”

To further constrain deep reflector geometry in the absence of crustal-scale three-dimensional pro-

arrival

filing and depth migration, forward modelling techniques were applied. Three-dimensional mod-

at about

14 s (Figs.

4b and

suggesting a flat Moho beneath of the basin.

7), thereby

the western

edge

els of major geological

BONAVISTA

markers

(base Tertiary,

top

PLATFORM MURRE FAULT

87-l

line 85-3 (migrated) Fig. 5. Migrated vertical reflective

dashed

stack of line 85-3, showing line indicates

packages

underneath

the location the Bonavista

the western

edge of the Jeanne

of a small jog in the trend platform

are interpreted

d’Arc

Basin in the vicinity

of line 85-3 (located as thrust

in Figs. 6, 7, and 8.

faults.

of the Hibernia

in Fig. 2). Highlighted.

Reflections

between

oil field. A

easterly

dipping

1.5 and 3 s are better seen

574

Jurassic, top basement, intra-basement reflectors to be studied) were created, and synthetic seismic sections

generated

for comparison

Three-dimensional Sierra

ray-tracing

Geophysics

Inc.

was performed

modelling

industry

with the data.

blicly

available

lines

strain

the interpretation

crust.

Fig. 2 shows the depth

with

software.

were used

in the shallow

Pu-

to con-

part of the

to basement

model

tions that terminate within basement, beneath the Avalon unconformity. At depth, they appear to merge with the uppermost reflectors of the lower crust, at about below,

these

represent

pre-rift

In summary, platform whose

that the dipping

formation

be a primary

event

coming

pping planar horizon trace of the Mercury

H (Figs. 3 and 5) must from

which projects fault (Fig. 2).

an easterly

di-

to the surface

One of the most prominent features mapped in this area is a basement ridge trending north-northeast away from the Bonavista platform and plunging under the basin (Fig. 2). It is imaged on line 87-2 (Figs. 4b and 7) as well as on industry lines (Enachescu, 1987). This ridge is narrow and flanked on either side by deep troughs filled with Mesozoic sediments. Its general attitude is well constrained by the seismic data except across its northeastern edge, where coverage is sparse. On line 85-3 (Fig. 5), H appears as one of a series

of easterly

dipping

intra-basement

reflec-

thrust

pfigin,

packages

As discussed

are inferred

edge of the Bonavista

to a complex

fault

system,

as is shown

below,

predates

of the Mesozoic

Jeanne

d’Arc

The basement

to

slices.

the jagged

corresponds

which best fit the data used. This study confirmed reflection

8 s (or 22 km depth). reflective

ridge that separates

the Basin.

the Cretaceous

Mercury sub-basin from the Jeanne d’Arc Basin proper trends about 20” away from the Murre fault. A Palaeozoic(?)

basin on the Bonavista platform

Basin-like features are clearly imaged on the Bonavista platform, well beneath the base-Tertiary unconformity which was previously thought to rest on Precambrian Avalon Terrane rocks. A prominent reflector (B, Figs. 4, 6, 7 and 8) is observed about 1.5 s beneath the breakup unconformity. In places, it is difficult to distinguish from multiples,

but its existence

is unequivocal

on

n7

Fig. 6. Close-up view of the top left portion of seismic section in Fig. 5. Note the folded appearance of horizon B at the intersection of the profile with line 87-l.

10

Fig. 7. Migrated

15

1

5

BJ

Moho

S E c 0 N D S

T1

0

stack of line 87-2 showing

the western

87-1 (Fig. 8).

d’Arc Basin, in the vicinity

M_ercp-y \\I Fault

edge of the Jeanne

line 87-2 (migrated)

Bonavista Platform

of the Mercury

\\r

Cretaceous

Murre c- I. t-ault

sub-basin.

Note reflector

10km B, better

imaged

0x1 line

15

10

5

‘A

IT

0

Jeanne d’Arc Basin

wl

Y

Bonavista Platform

S E

c 0 N D S

Moho 15 line 87-l (migrated) Fig. 8. Migrated

stack of northern

areas of the Bonavista

platform

half of line 87-l. and discussed

IOkm

northern half

7’ is base Tertiary;

in text. Note dipping

B is the prominent

and subhorizontal

event observed

reflective

packages

at about

I 3 s over iarge

in the middle

and lower

crust.

line 87-1 (Fig. 8). It is a prominent seismic marker, that can be traced for large distances on the Bonavista platform. It appears subhorizontal on line 87-1, as well as on industry lines that run roughly parallel to the edge of the platform. On east-west trending profiles, such as line 85-3, B is locally deformed in broad antiforms (Fig. 6). At the intersection of line 87-l and 85-3 (Figs. 5 and 6), there is little doubt that B caps the east-dipping reflections that border the Murre fault to the west. The section between B and the base Tertiary event T is poorly imaged. Inspection -of released industry seismic lines indicates a highly deformed and poorly reflective sequence. There is some sparse and often unclear evidence that this sequence has been compressively deformed (Fig. 6). Nowhere could tilted blocks or extensional features be documented. This sequence appears to be evidence of a wide, relatively shallow sedimentary

basin covering at least part of the Bonavista platform. Evidence for the existence of a sedimentary basin on the eastern half of the Bonavista platform was published by Reid and Keen (1988) on the basis of seismic refraction experiments. Enachescu (1987) describes the area to the north as underlain by Palaeozoic metasediments, and Enachescu (1989) discusses thrust faults and folding of these Palaeozoic and underlying Precambrian rocks from industry seismic lines. No deep well has been drilled on the platform, though Palaeozoic m&sediments were encountered in a small rotated basement block about 150 km northwest of the area of study (Enachescu, 1989). Palaeozoic sedimentary rocks, often folded and deformed, have been mapped in the western half of the platform (Durling et al., 1987). These observations, together with the broad gravity low that occupies this area of the Bonavista platform (En-

FAULT

K!3ACTIVATION

DURING

achescu,

1987), provide

existence

of a basin.

MESOZOIC

further

EXTENSION,

E. OFFSHORE

evidence

for the

CANADA

577

Mesozoic

reactivation.

phy of the Jeanne the Mercury

Reactivation of pre-existing structures Reactivation structures explain

crustal

sional

basins

often difficult gle normal deep

of

pre-existing

as normal

faults

features

associated

(e.g. Hillis

and

to document.

Day,

reflection

profiles

exten-

tion of Palaeozoic

1987),

but is

normal

interest

because

worldwide

models

and Boutilier,

of extensional

have

faulting

north

of the basement

with

provided direct evidence that the upper crust has been extended along low to moderate-angle normal faults (e.g. Allmendinger et al., 1983). It is also a critical issue in current attempts to develop numerical

further

On the European

to

The origin of low-an-

faults is of particular

seismic

invoked

not include

and the Murre

fault may

as suggested ridge

mapped

(Bassi

1988).

conjugate and older

margin, thrusts

faults has been proposed

the formation on BIRPS,

of several

ECORS

to accommodate

Mesozoic

basins

and other profiles

No

such

interpretations

have

been

documented

prior to this study concerning the G-and Banks basins, though several authors have suggested that

Mercury Cretaceous normal fault (Fig. 2) ties to the south of the Mercury sub-basin with an intra-

Tankard

1989). Because

Fig.

5) along

which

no

displacement of Mesozoic age is observed beneath the Bonavista platform. Furthermore, this reflector H, as well as weaker events beneath the platform, are associated with a Palaeozoic(?) sequence that has apparently been compressively deformed (B, Fig. 6). Therefore, we suggest these easterly dipping faults,

reflectors

are Palaeozoic

which have undergone

or older

various

of

and

Welsink,

had been in(Keen, 1989; the direc-

tions of the Mesozoic extensional episodes are unlikely to be pe~endicular to the strike of the pre-existing structures, complex motion and oblique dip-slip movement will result, giving rise to variations

in structural

style within

the region.

The European connection Middle

thrust

amounts

et

al., 1983: Brewer and Smythe, 1984; BIRPS and ECORS, 1986; Cheadle et al., 1987; Stein, 1988), though some of these interpretations have been disputed (e.g. Pinet et al., 1987; Wernicke, 1986).

evolution structures

(H,

imaged

(Brewer

basin formation and fluenced by inherited

reflector

reactiva-

as low-angle

The data presented in this paper document reactivation of a pre-existing fault system. The

basement

by the in this

study (Fig. 2).

compressional

is often

palaeogeogra-

d’Arc Basin would

sub-basin,

have extended presence

The Jurassic

Europe

to Late Palaeozoic and

eastern

/

North

orogenies

of western

America

have

been

IBERIA /

Fig. 9. Reconstruction indicates

of the North

zone of Appalachian 1988); prominent

Atlantic

deformation;

magnetic

at anomaly dashed

MO time, taken from Srivastava

heavy lines are Variscan

ridges on the Grand

Banks are stippled;

trends

et al. (1988). Hatched

(Lefort

FC is Flemish

and Haworth,

pattern

approximately

1984; Hutchinson

Cap; box is area of study (Fig. 2).

et al.,

57X

correlated on the basis of geophysical data (Lefort. 1983; Lefort and Haworth, 1984). The deep seismic

the two segments of the basin bounding fault system (Murre and Mercury faults) are different.

profiles

This geometry

Gulf

presented of Maine

Orpheus

here, along with profiles (Hutchinson

Graben

(Marillier

recent

profile

(Keen

et al., 1990),

Late

across

Palaeozoic

Mesozoic

passive

The Variscan

et al.,

provide

structures margin

1988).

et al., 1989)

the southern

Grand

the first

and

a

Banks

images

in the vicinity

of

of the

front is relatively as the northern

well defined

a 700-800 Carboniferous

km wide zone affected tectonics. The Variscan

boundary

in

by Late belt of

by thrust and nappe

struc-

Variscan tectonics in western Europe, but little is known about its extent in North America, and a of models

have been

proposed

(Arthaud

and Matte, 1977; Gardiner and Sheridan, 1981; Haworth and Jacobi, 1983). Seismic profiling on the western European continental shelf imaged Palaeozoic or older faults and suggest that these are Variscan thrust faults reactivated during Mesozoic crustal extension (Cheadle et al., 1987; Hillis and Day, 1987). The inferred thrust system imaged near the edge of the Bonavista platform may also be a Variscan feature, related to those on the European

has enabled

grid of deep seismic which

us to map the pertinent

in three dimensions,

clearly

using the excellent

and industrial

shows

“dormant”

portions

favourable

geometries

both

the

seismic

lines,

reactivated

of the Mercury near major

and

fault.

Other

basin-bounding

to extend

our investigations

to other areas.

of

tures and wrench-fault tectonics dated essentially between 380 and 280 Ma ago (Matte, 1986; Matte and Him, 1988). Much has been published about

variety

reflectors

faults must surely exist and it would be instructive

Europe

is characterized

the

(Fig. 9).

northwestern

Europe

in the

margin.

Conclusions

Acknowledgements We thank S. Srivastava and J. Verhoef for the use of one of their figures and A. Edwards for discussions. G. Stockmal, M. Keen, G. Bassi, S.L. Klemperer and an anonymous reviewer made useful comments on earlier versions of the manuscript. Synthetic seismic modelling of basement features was performed

Basin bounding fault of Mesozoic age, the Murre-Mercury fault system, reactivates crustalscale, easterly-dipping thrusts. A laterally extensive Palaeozoic(?) basin occupies most of the Bonavista platform. While reactivation of thrust faults to produce extensional faults has often been suggested as a controlling factor in the development of extensional basins on the continental margins of western Europe and eastern North America, there are few instances where reactivation can be doeumented. In the Jeanne d’Arc region the trend of

of Geonova

Enter-

logical Survey of Canada. B. de V. was funded by a National Sciences and Engineering Research Council

(NSERC)

post-doctoral

ship. Geological No. 47688.

Survey

Visiting

of Canada

Fellow-

Contribution

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