Download extended basement beneath the intracratonic rifted basins of the

CANADIAN
JOURNAL
OF EXPLORATlON
GEOPHYS,CS
VOL. 24, NO. 1 ,J”NE 1988,. P, 48-65
EXTENDED BASEMENT BENEATH THE INTRACRATONIC
OF THE GRAND BANKS OF NEWFOUNDLAND’
RIFTED BASINS
MICHAEL E. ENACHESCU~
Keen. 1979: Wilson, 1981). The Cambro-Ordovician passive margin of western Newfoundland was transformed
into an active zone of convergence during OrdovicianSilurian time and then one of oblique slip during
Devonian-Carboniferous time (Haworth, 1975; Strong,
1980; Williams and Hatcher, 1982). The Paleozoic orogenie phases (Taconic. Acadian, Hercynian) resulted in
the accretion of the Avalon and Meguma terranes that
now partly underlie the Grand Banks (Williams, 1979:
Schenk. 1981).
During the Mesozoic, the Grand Banks were involved
in North Atlantic rifting and ocean opening (McWhae,
19X1: Wade, 1981). As a result of continental stretching,
a series of intracontinental and plate-margin basins were
formed (Enachescu. 1986; Grant et al., l986a, b;
Enachescu, 19X7; Keen et al., 1987a; Tankard and
Welsink, 1987, 1988). This paper focuses on the nature
and behavior of the pre-Mesozoic rocks which formed the
Grand Banks basement and are present today on the platforms and ridges and beneath the Mesozoic basins.
The studied area extends north, east and south of
Newfoundland, covering over one million km2 (Figure I).
This coincides partially with water depths shallower than
the 200-m bathymetric contour (the actual Grand Banks),
but also includes the deeper water regions around Orphan
Knoll, Flemish Pass, Flemish Cap and the surrounding
slope and rise zones.
ABSTRACT
The Lecto”icandSlrUCtUrsl
frameworkof the GrandBanksof
Newfoundland (East Coast Canada)comprisesa series of
intracramnic basins and ridges developed during repeated
Mesozoicrifting periods.Recentindustryseismicliner anddeep
seismicdatatLithoprok) allow for the recognitionof coherent
reftectionsof inrrabasement
provenance.The Precambrianand
Paleozoicracksof the *valon zone.which form the coreof the
upperCOntlnental
crust,underwentcxle”siYeblock faulting and
rotationsat theonsetof the LateTriassic-EarlyJurassicphaseof
rifting. Theextensionof the continentalCNS~
initially look place
in a northwest-southeast
direc&m. on easterlydipping normal
faults.Oneof therefaultsevolvedinto a listric mepafault.which
detachestn a deepcrustalzoneandunderliesmostof the Grand
BanksStr”Et”ra,e,emen,r.Its associated
syntheticandan,itile,ic
faultsboundthe other strwturd provinces,basinsandridgesof
the shelf.Theseismiclinesrecordedon the slopeandrise.in the
continent-ocean
mnsition zone.suggestextendedflood hasalts
of oceanicextractionlying on attentuatrdand subsidedcontinentalbasement.
This impliesthat in the transitionzonethe ~0”~
tinentalbasementis not “lost“ but only intrudedandcoveredby
new oceaniccrust, and that this zone must he accountedfar
whenpaleocontinental
reconsfructions
aremade.Reactivationof
older PrecambrianandPaleozoiczonesof weaknessasrift-triggeringfaults is questionedandexamplesof deviationsfrom prr~
vious geologic grain are offerrd. The mantle beneathmajor
abandoned
rifts is postulatedto be significantlycontaminated
by
continentalcrust materials.so that a firm contactbetweenthe
crustandmantle- the MohorovitiC(Moho) disconrinuity~ is
temporarilydestroyed.
REGIONAL GEOLOGY
INTRODUCTION
Conventional plate tectonics demonstrates that the
Grand Banks of Newfoundland occupied a pivotal position during the last two cycles of plate reorganization in
the northern hemisphere (Wilson, 1966; Haworth and
The present-day
complex regional geology off
Newfoundland reflects a lengthy and complicated active
and passive margin history. Briefly, the area (Figure I)
consists of:
~Presented
at the 7th InternationalConferenceon BnsementTectonics.Kingston.Ontario.August, 1987.Manuscriptreceivedby the Editor March 10.
,988:revisedmanuscriptreceivedMay 3. ,988.
WuskyOil OperationsLtd., P.O.Box 6525,Station‘D‘, Calgary,Albena TZPSC?7
I am gratefulto University of BucharestprofessorsLiviu Constantinescu.
St&n Calotaand Andrei Soare.who introducedme to geophysicalcrustal
studies.I wish to thankmy Huskycolteagues,especiallyGerry Smee.RichardBaudreauandHugh Wishart.RichardHiscottof MemorialUniversity.
andmy wife, for their supportduring my recoveryfrom an accidentandfor their reviewsof this paperandvaluablesuggestions.
Collaborativework on
Lithoprobe&cl wasfacilitatedby Boberr andPatPurcdl of Huskyan* by CharlotteKeenan* TonyEdwardsOf,k .4tklnricGeoscience
Centre,Wh”
atsoprovideddeepreflectionlinesand~onsm~tivesuggestions.
Gilbert Boillot kindly providedhis NorthAtlantic extensionalmodel.Thanksgo ah to
Brian Eyres,JenniferGall. DianaGillrie andGina Coninx for their dedicateddrafringand typing. HuskyOil and it6 pannera,Bow Valley tnduslrira
Ltd.. Petro-Canada
Inc.. NoreenEnergyResources
andParenallowedthe useof seismicreflectionlines,which wererecordedandprocessed
by GSI.
48
Fig. 1. Map showing location of the studied area, bathymetry and main geological provinces: 1) onshore extension of the AppalachianAvalon zone, including basement platforms, ridges and isolated continental blocks such as Flemish Cap and Ophan Knoll; 2) Mesozoic
abandoned rift system, including Jeanne d’Arc, Flemish Pass and South Whale Basins: 3) Newfoundlandtransform margin to the swth; 4)
true divergent margin, floored by transitional and oceanic crust to the east and 5) Avalon uplift, an Early Cretaceous northwest~southeast
basementarch affecting the central and southern Grand Banks.
I, An offshore extension of the Appalachian orogen,
including the Bonavista platform and tw” isolated
continental blocks, Flemish Cap and Orphan Knoll
(Haworth and Keen, 1979; King et al., 1985, 1986;
Pars”” et al., 1985);
2. A system of Mesozoic rift basins, including the
Jeanne d’Arc and Flemish Pass Basins ~ targets of
hydrocarbon exploration (Jansa and Wade. 197.5;
Enachescu, 1986, 1987; Grant et al., 1986b:
Tankard and W&ink, 1987, 1988);
3. A southern margin of transform origin, known as
the Newfoundland transform zone, along which
North America separated from Africa (Grant, 1977;
Sullivan and Keen, 1978; Keen and Haworth,
1985,;
4. A true divergent margin located east of the Grand
Banks, beyond both Flemish Cap and Orphan Knoll
(Keen et al., 1977; Sullivan, 1983; Enachescu,
1987: Keen and de Voogd, 1988).
The timing of the main geological events and a simplified stratigraphy for this region are presented in Table I.
In this paper “the basement” is defined as the sequence
of rocks which formed the peneplaned crat”nic crust prior
to the Mesozoic cycle of ocean opening, i.e., the
Hadrynian and possibly older crystalline rocks and the
Paleozoic metasedimentary and sedimentary groups
(Table I). Due to successive phases of rifting t” form
Tethys, the North Atlantic and the Labrador Sea, this continental basement was subjected to extension, resulting in
intense faulting and major subsidence.
The c”re of the Avalon tectonostratigraphic unit consists of Hadrynian, and probably older, crystalline rocks,
similar in character to pan-African
erogenic belts
(O’Brien et al., 1983; King et al., 1986). The uppermost
Precambrian and lower Paleozoic metamorphic. metasedimentary, sedimentary and volcanic rocks of the Avalon
terrane and the overlying middle-upper Paleozoic sedimentary succession of the Horton, Windsor, CansoRiversdale and Picto-Cumberland Groups are well documented and widely known in the Maritimes (Howie and
Bass, 1975; Schenk, 197X; Williams and Hatcher, 1982,
1983; O’Brien et al., 1983).
Previous geological and geophysical studies showed
that similar Hadrynian, lower Paleozoic and upper
CENOZOIC
WtTHtN PANGEA
ACCRETION OF
AVALON & MEGUMA
STABLE PLATFORh
ewestrial red-beds,
slates. turbidites,
bimodal volcanics
OROGENIES
HADRYNIAN
PAN-AFRICAN
ISLAND VOLCANISM
STACKING OF
gneisslc basement
Table 1. Geohistory and simplified stratigraphy
of the Grand Banks.
are present offshore
on the Grand Banks
areas (Jacobi and Kristoffersen,
1981;
Haworth and Jacobi, 1983: King et al., 1986).
Paleozoic
and
rocks
adjacent
DATA BASE
Different research and industry sources provide us with
a data base of information on the nature and distribution
of basement rocks on the continental shelf. This information is contained in well-history reports, GSC reports and
papers, master’s and doctorate theses, COGLA files and
scientific papers included in the reference list. The information can be classified as either geological or geophysical and a summary is as follows.
Geological information
Physical and lithological
properties of Hadrynian.
lower and middle Paleozoic rocks present on the Avalon
Peninsula and elsewhere in the Maritime provinces were
used to compare basement samples obtained offshore.
Submarine
samples have been collected
by Lilly (1965,
19661, Pelletier (1971) and King et al. (1985, 1986) in the
vicinity of the Avalon Peninsula from shoals further offshore and from Flemish Cap. The locations where samples were recovered and described are displayed on the
offshore Newfoundland map (Figure 2). The granodiorite
samples from Flemish Cap (King et al., 1985, 1986) were
compared to the Holyrood granite (McCartney, 1969;
Miller, 1983). based on age and chemical composition, in
an effort to establish the offshore extent of the Avalon
zone. However, King et al. (1985, 1986) could not
demonstrate the similarity of the two granitic bodies.
Thus, the Flemish Cap granodiorite may belong to either
the Avalon zone or to a more easterly African terrane. No
basement rocks were recovered from the only DSDP site
on Orphan Knoll (Site Ill), because the hole was too shallow (Laughton and Berggren. 1972).
Approximately 120 exploratory wells have been drilled
on the Grand Banks. Only the I3 with locations shown in
Figure 2 bottomed in the basement (Jansa and Wade,
1975; Barss et al., 1979; COGLA. 1985). Several conclusions
can be drawn
from
basement
coring
and sampling
EXTENDED
I
?’
I
BASEMENT
BENEATH THE INTRACRATONK
/Gt.sr >vliWFO,w,>,,;I ‘I)
iHK,.F
--~&*’
ion
RlFTED BASINS
51
,-
Fig. 2. Index map of Newfoundland
and Grand Banks showing Appalachian fertanes, main fracture zones, locations (dots) of submarine
cores (from King et al., 1986). numbered wells that sampled basement. Lithoprobe seismic profiles 84-3 and 85-1 to 85-4 (from Keen et
al., 1987b) and selected industry seismic lines used in this paper. Podions 01 seismic lines displayed in later figures are shown by broken
lines (e.g., c-c’). The tectonostructural
framework and occurrence of basement rocks is adapted from Enachescu (1986. 1987) and King et
al. (1985 1986). Shading denotes pre-Mesozoic
basement terranes (Paleozoic and Precambrian). Mesozoic intracratonic and platemar~
gin basins are unpatterned. For the Mesozoic rifted area, the map is at the level of the Aptian-Avalon
unconformity. Wiggly lines, x-x’ and
y-y’, on the Bow&ta
Platform represent
water-bottom
subcrop 01 the Avalon unconformity
and top of metamorphic
basement
(Hadrynian). respectively. J.D. = Jeanne #Arc Basin; F.P. = Flemish Pass Basin: O.K. = Orphan Knoll Basin; C-B = Carson-Bonnition
Basin; S.J.D. = South Jeanne d’Arc Basin: W = Whale Basin; SW. = South Whale Basin: CR = Central Ridge; ER = Eastern Ridge; T.B. =
Tail 01 the Bank; DT = Dominion transfer fault (dashed line); Pz = mainly Paleozoic rocks: PC = mainly Hadrynian rocks: VR = Virgin Rocks;
ES = Eastern Shoals; P = Pellefier core; PA = Panam core. The wells that drilled into basement are: 1 = Jaeger A-49; 2 = Bitiern M-62; 3
= Gannet O-54; 4 = Phalarope P-62; 5 = Razorbill F-54; 6 = Hermine E-94; 7 = Sandpiper 2J-77; 8 = Murre G-67; 9 = Hibernia G-55; 10 =
Linnet E-63; 11 = Cumberland B-55: 12 = Bpnavista C-99; 13 = Blue H-28. Fl is the basin-bounding
fault of tha Mesozoic intracratqnic
basins system. F2 is the bounding fault of the plate-margin Oceanic basin. The Collector (Haworth and Keen, 1979; Keen and Haworth,
1985) and Glooscap faults are interpreted elsewhere as the eastern extension of the Cobequid-Chedabucto
fault, or the Avalon-Meguma
suture zone. The existence 01 the Collector fault is not supported by this data set. The magnetic lineamenf mapped and named Collector is
rather associated with a Cretaceous hot-spot trace.
in the wells. The basement rock types are numerous.
Variable thicknesses of the different tectonostratigraphic
successions of the basement that have been penetrated
may have resulted from folding and faulting followed by
erosion. The basement is situated beneath the prerift
unconformity (pre-Late Triassic), where the basement is
relatively deep, or beneath the Avalon unconformity (preAlbian), where the basement is shallow and the upper
Triassic to lower Aptian section has been removed or was
not deposited.
On the map of tectonic and structural framework of the
Grand Banks (Figure 2) the basement is shaded and locally differentiated into Paleozoic (Pz) and Precambrian
(PC). This division was made on the basis of submarine
samples and well-core descriptions mentioned in the literature, as well as on the integration of geophysical results.
The exploratory wells, located in Figure 2, encountered
basement
generally
in three
situations:
(a) when
drilling
52
M.E. E.NClKSCU
the “drape over baxment hors” type of hydrocarbon
prospect. the cast with Cumberland B-55 (Figure 6): Cb)
when penetrating the upthrown block below basin-bounding faults. the case with llibernia G-55 (Figure 3): and (cl
when piercing downthrown imhricatc blocks ahovc the
basin-bounding fault\. the case with Linnet E-63 (Figure
6) and Murre (i-77 (Figure X). As an example. the intcrprctation 01 line a-a (Figure 3) shows that the Ilihernia
Fig. 3. Location of the Hibernia
G~55 well. Uninterpreted
and interpreted
the Murre fault plane and bottomed,n metamorphicbasement.
G-55 well missed the Jurassic reservoir sand, crossed the
fult plane and bottomed in metamorphic basement.
Geophysical information
In previous papcrs. structural and tectonic clcmcnts of
the harement were interpreted from potential-field data
(Haworth ;md Lefort. 1970: Jacobi and Kristoffersen.
IOXI: Miller ct al., 10X5; Miller. 1987). refraction stud&
seisrmc section a~~‘. with location on
Figure 2~The well crossed
EXTENDED
BASEMENT
BENEATH
(Sheridan and Drake, 1968; Haworth and Keen, 1979)
and shallow high-resolution
seismic (Monahan and
Macnab, 1975; Fader and King, 1981; King et al., 19X5,
1986).
In order to extract additional information on basement
tectonics, this paper mainly utilises industry reflection
data and deep-reflection profiles. More than 200 000 km
of (a) reconnaissance, (b) detailed and (c) 3D seismic
data presently cover the Newfoundland shelf. Four deepreflection profiles, part of the Canadian Lithoprobe East
project (indexed 84-3; 85-1, 85-Z; 85-3; X5-4). were shot
by the Atlantic Geocience Centre and discussed by Keen
et al. (1986, 1987a, b) and de Voogd and Keen (1987).
Their locations are displayed in Figure 2, together with
industry reflection lines used to document the concepts
reported in this paper. Some of the more important questions that stem from the geophysical study of the Grand
Banks basement will be briefly addressed, as they may
apply to other rifted areas.
BASEMENT
REFLWTIONS
The reflection seismic shot by the petroleum industry
has not been previously used for investigation of the
basement. Seismic processing was instead focussed on
the Mesozoic sediments; no attempts to clean the noise
beneath the so-called “acoustic” basement were made.
Moreover, until recently it was believed that, except for a
fortuitous situation, no penetration could be achieved in a
deformed basement complex and thet metamorphic rocks
were seismically transparent. An industry line b-b’
recorded over the Bonavista Platform helps to correct this
misconception (Figure 4).
Unconformities, bedding planes, thrust faults and folding in the Paleozoic and Precambrian rocks can be interpreted from the seismic data if we ignore the random
noise and the severe multiple problem and make use of
the Avalon-terrane regional style of deformation and
faulting. However, as can be noted, the interpretation
below three seconds is quite difficult and only correlations with other lines not shown here allow one to sketch
fault planes and layering.
Marine deep-seismic-reflection profiles from the northern Appalachian belt including the Bonavista Platform,
were first discussed by Keen et al. (1986, 1987a. b) and
de Voogd and Keen (1987). Lithoprobe lines are usually
recorded to 20 s and processed to I6 to IX s. They can
produce intrabasement information from deeper levels of
the crust and even from the upper mantle, regions previously accessible only to refraction seismological methods
or geophysical modelling.
The Lithoprobe line c-c’ in Figure 5 shows a thin
veneer of postrift sediments above the Avalon unconformity. Under the unconformity, the continental crust can
be divided into an upper transparent crust associated with
lower Paleozoic and upper Precambrian rocks and a
lower reflective crust corresponding to older Precambrian
THE
NrKACRATcJNlC
RFTED
53
BASlNS
structures and layered mafic material. The Moho surface
is very well imaged at about IO s, equivalent to appronimately 30 km m depth.
MAJOR
CRUSTAL
FAULTS
The most recent industry data show that the hinge zone
previously defined as a Mesozoic basin border (Grant,
1987) is actually a major listric basement fault which
defines the western limit of the rifted basins and the
area’s master detachment zone (Enachescu, 1986, 1987).
This remarkable megafault (Fl on Figure 2) is a “basinbounding fault”. It accounts for the majority of the basement extension and allows for the creation of the helfgraben-shaped rift basins (Figure 6. line d-d’).
In Figure 7, seismic line e-e’, the basin-bounding fault
is almost vertical through Tertiary sediments, cuts
through the entire upper crust and curves down to a deep
crustal level below the 7 s of recorded data. On the downthrown side of the basin-bounding fault is the Hibernia
structure, and further to the east is the depocentre of the
Jeanne d’Arc Basin infilled by approximately 20 km of
synrift and postrift sediments.
The segment f-f of Lithoprobe line X5-4 shows that the
basement fault flattened very deep in the crust at about IO
s, or 30 km depth, corresponding to the Moho level
(Figures 2 snd X). On the snme line, the Murre G-77 well
intersected the fault plane or drilled into a small synthetic
imbricate block and bottomed in Paleozoic metasedimerits. Accordingly, the fault was named the Murre filult
and is known by this name south of the Hibernia area.
North of Hibemia, the same fault is sometimes referred to
as the Mercury fault.
Basin-bounding faults are less visible on the seismic
data collected on the Avalon uplift and southern Grand
Banks. This is probably due to severe erosion of both
flanks of the faults, up to a level where the fault plane is
at a low angle (5 to 15’). and to the fact that the velocity
contrasts xvxs the fault plane are small (Figure 9, line
E-g’).
BASEMENT RIDGES
A series of intrabasinal ridges were recognired east of
the bounding fault. Most important for rift tectonics are
basement-cored ridges such as the Central Ridge, which
separates the Jeanne d’Arc Basin from the Flemish Pass
Basin (Figure 2). Most of the basement ridges are, in fact,
elongated horsts formed by antithetic and synthetic faults
of the basin-bounding fault (Figure IO).
The Central Ridge (Figure I I, line h-h’) was an intrarift
relatively high area during the initial Late Triassic riftvalley episode and had a median elevated position in the
subsequent evolution of the intracratonic failed rift system (Enachescu, 1987). During the Early Cretaceous, this
ridge reached its maximum elevation due to increased
rotation of its basement core on the basin-bounding fault
ML, l3ACHCSCI~
54
BONAVISTA
@
PLATFORM
o.ol,__l,l,l,l-l~---llll~,,~.,,:
@
SW
NE
,,,,..:,
,..~~~~~~~I:l.~:~.~~~.~,l.::.l..~:~~
:
0.0
1.0
2.0
3.0
4.0
5.0
6.0
6.0
Fig. 4. Uninterpreted and interpreted industry line b-b on the Bonavista Platform showtng top oi the basement (Awlon) unconformity. fop
of Hadrynian (metamorphics)
uncontormity. thrust faults and folding in metamorphosed
rocks~ The upper arrows indicate erosional surf
faces and the lower arrows show fault planes. For this and the following seismic sections the key to symbols is: Z = salt: A = top of metamorphic complex; B = prerift uncomformity: C = Awlon unconformity: D = sub~Tertiary unconformity: J = Jurassic: K = Cretaceous (Lower
= Kl, Middle = KS. Upper = K3): Pz = Paleozoic: PC = Precambrian: Fl, BBF = basnbounding
fault.
I‘S I’LNDFI)
0
- ENE
“ASLMmT
TAIL OF THE BANK
“EN,:AIH
‘THE IS’1 K:,<X4TONK
,<,Fl,X
“ASINS
55
C-64. an indurated succession appronlmately 5 km thick
of Upper Triassic-Upper Jurassic sediments still overlies
Ihc Ccnrral Ridge basement. lnlrahasinal hescment rocks
ima at il st~allmvcr depth (I to ? km) in the south and C~~II
he wmplcd only on the narrow strip separating the South
Jeanne d‘Arc from the Car\on~Bonniti~rn Basin (Figure
21.
0
WSW
TKANSFEK FnLlI,‘rs
1 I :M
I
14 KM
I
30 KM
t
Fig. 5. Short segment c-c’. uninterpreted and interpreted. of
deep seismic profile (Lithoprobe) 85-2, located on the Tail 01the
Bank. The interpreted section shows the Terfiary platform sedi~
merits. the Avaion and sub-Teertiaryunconformities merged CD),
the crust divided into an upper transparent crust and a lower
reflective crust. the Moho reflection and the mantle,
plane. The ridge was cxpnwd iis an island chain or pcninsula and consequently the u,>pcr I:lq’rr Jurs\\ic 1~1Louct
Cretaceous sediments arc missing due to erosion and mrn~
deposition. As demonstralcd hy rhc drilling of Golcond;~
Crash-basin C11ulrswrrc interpreted fro111the seismic
Iinn (Figure II. i-i’) and potential-field data. These have
hccn recently referred to as transfer faults, which are the
corll~~l~~llid cqoivalenl OSthe oceanic transform Saulrs that
offxt midocc;mic ridfcs (Gihhs, I YX4: Enachescu. l YXh.
IYX7: ‘Tankard and Wclsink. IYX7. IYXX). Thcsc fault5
allow for changes in the direction or amounl or slip of Ihc
h;lsin-h(,nnditl~ fault.
Transfu filulls arc rccofni~ed on seismic lincs only
when struclurul style and lithrrlogies \,ary dramatically
acres rhc fault plant. Sciswr-type motion involving both
wlimentxy
infill and harcmcnt typically occurs alw~g
lmhfer klulls. ‘They are most probably ro~~tcdin the xuni
deep crustal dctnchmenc /one as ihc hasin~h(,ulldin;l
faults.
‘The Dominion triin\fcr fault (Figorc 2) i,ff\c~\ the
nupr slrxcluriil iwd Icctonic elelllrnls of the norlhcastcrn
Grand Banks (linachcscu. 10x7). Olhcr transfer fault\ [hilt
ma); c,)m[);lrtnrcnt;llire lhe hxscmcnt on the Gr;md Banhs
arc not :I\ vi\ihlc in the :reophysical data and. thcrcfws.
wcrc tno! inciudcd in the tccwnic frameu~ork (Figure 2).
Etheridfr ct al. (IYXS. 1’9x7) and Twhanl and Wclhink
(lYX7. IYXX: \ugecst a hifhrr density of such lilults fragmenl~nf. rcspcctively, off\horc southea\trrr Australia and
the Grand Banh\ Basin. Seismic data. at least for the area
tn Figure 1. oflrr IIO cvidcnce that the rigid continental
h;ecmcnt undel~lyin~ the (;nlnd Bank\ was dcnrelq faultcd oblique 10 the hnsin marfin\ (5 to 10 km hetwecn
Iransfcr faults). Such dcnsily of transl~r fault\ would crciitc nomerour comprcssi~mal t’e;lturcs in the ,,verlyinp
sedimenls. which is !not the cast Sor the I\lrsirlaic scdi~
mrn!s or the Jeanne d‘Arc Bxsin.
‘The trilnsitioll zone heturcn the c(mtinentaI and OCC~IIic crust was crossed by Lithoprnhe line X5-3 (Figure 13.
scgmen~ ,j-.j’), ‘I’wu conspicuous slanted rcl’lcction zone\.
suggesting ~normal faoltin~ and extension arc observed
thcneath the Aplinn hrcakup unumfnrnlity. A Iowc~.m~~st
peculiar strip of rcflcction\ appears at the theoretical
depth of the oceanic Muhu (dc Vw)~d and Keen. I YX7).
The interprct;ltion of Figure I3 places a [bin wedge 01
continental crust beneath a IhIck hlankct of oce~~nic
hxalt\. with cxtcnsional fault\ in both layers. Basic intrusion\ and has;kltic Iloods arc hclieved 10 travc hccn
cmplaccd and \uhw,ueruly
cxlcndcd ahove an alread)
faul~cd and suh\ided hawnrnt. The strong magnetic sifts
56
ME. EPIACHESCU
JEANNE D’ARC BASIN
BONAVISTA
PLATFORM
CUMBERLAND
RIDGE
LINNET E-63
@SW
.:,
~: ~,..:,.~
Fig. 5. Industry reflection line dud’. across northern Jeanne d‘Arc Basin. showing the basin~bounding
tary basin and the Cumberland Ridge with Cumberland B-55 well location. Linnet E-63 encountered
rotated basement block.
fault (F1). the half~graben sedimenPaleozoic metasediments
in a small
EXTENDED
BASEMENT
RENEAW
.‘HE ,NTRACK,~‘ON,C
57
RlFTED HASlNS
JEANNE D’ARC BASIN
BONAVISTA
PLATFORM
HIBERNIAC-98
HIBERNIAK-14
CENTRAL
ADOLPHUSD-50
RDGE
SOUTH TEMPEST G-88(PR.)
Fig. 7. Seismic line ewe’showing the basin~boundingfault (F1) sfeep in Tertiary and increasingly lisfric at depth, Hibemia OiIfield.Adolphus
diapir. Jeanne #Arc Basin depocentre and the Central Ridge. This seismic line suggests that the basiwbounding fault represents a major
crusfal detachmentof plate-tectonic significance.
nature of this unusual terranc. shown in Figure 14 and in
the magnetic anomaly maps and profile of the Atlantic
slope (Jacobi and Kristoffersen, 19X11. is caused by the
presence of thick (5 to 8 km) basic igneous rocks under a
thin (1 .S km) sedimentary cover and not by a true oceanic
crust.
The boundary between continental and oceanic crust
was placed west of the Figure I3 segment, at the base of
the slope. by Sullivan (19X3) and Keen and de Voogd
(I 98X). The manner of extension and the seismic interval
velocities of the deeper layer, howcvcr. we characteristic
of continental crust, and the true extent of continental
crust must be moved further east. approximately 700 km
front the Newfoundland
coastline. When atlrmptin~
paleocontinental reconstructions based on hathymctric
contours or magnetic lineation\,
unexpected gaps
hetween continents often occur (Vink. 1982: Mason and
Miles, 19841. The interpretation of Figure 13 suggests
that these gaps may be due fo ignoring similar strips of
“lost” contincntel crust, which are obscured by deep sobsidcncc and ii blanket of oceanic basalIs.
The F2 lineament in Figure 2 must bc rcfarded as a
major down-to-the ocean l’ault %ooc and not as the continent&ocean boundary (COB). Beyond F2. on the conlinental slope and rise, lies the transition zone, made of the
easternmost sectors of Mesozoic hasinal areas (e.g.
Carson-Bonnition
Basin), basement ridges. volcanic
mounds, batholiths and atypical terranes such as those
interpreted in Figure 13. The continent-ocean transition
zone is therefore geologically complex and is probably
limited occanward by the mafqetic linealions J(M,,) in the
south and 34 in the north.
NEWFOUNDLAND GEODYNAMIC Molm~s
The geologic section A-A’ (Figure 14) summarixs
the
main tectonic and structural elements of the Grand
Banks. The haernent plntforms. the basin-boondin~ fault.
the hasemenwored Central Ridge, {ht. two infracontincn~
till abandoned rift hasins (Jeanne d‘Arc and Flemish
Pass), Ihe Flemish Cap basement block, and the continrntal-oceanic transition lone underlying a thinned platcmargin hasin, are interprctcd from pocenrial-field maps.
regional seismic lincs and existing
well control
(Enachcscu. IYX7). This particular cross section was chw
sen to describe the area hccause it almost hal;mca wilh a
p factor of I .54. Similar tranxcls 10 the north and south.
perpendicular to the Jcannr d‘Arc Basin axes, do not halmce and conquently
are not shown hue. Intersection of
these hrctions with transfer zones or local transcurrcnt
faults may cxplaio the lack of halancing. In ‘my opinion,
pcrtxt halancing of 3 section in a complcn rifted ,onc
such as offshore Newfoundland is difficult to achieve and
exceptionally rare. There will always he oblique slip ot
certain blocks. rcsultinf in an imbalance oi the hasemcnt
material in any pivcn vertical plant. Only a tcnlafivc
three~dimcnsiotlnl balancing can he successful if 111~.
actual bascmcnt configuration is adequately known.
Variou frodynamic models of continenta stretching
and basin formation have been proposed for this aru during the lart IO years. The extensioni~l models 01
McKen+ (147X). Wcrnicke (IYXI, IYXS) and Wernickc
end Burchfiel (19x2) can be considered as end~mcmhcr
models (Coward. 19X6), Ihe first symmetrical and the
second asymmctricsl. Some authors suggest hyhrid models applicable to an area of increased complenily such as
the C;rand Banks (Keen et al.. 19x1: Keen cr al.. IYX7a.
b). However, there is II pronounced structural asymmetry
in both strike and dip directions. confirmed hy various
geophysical data. that moc( likely supports viwanls of ;i
simple-shear extensional model, such as those published
hy Wcrnicke j IYXS. Figure 3). Beech (14x5. IYXh).
Fig. 8. Segment 1CVof the Lithoprobe line 85~4 intersecting South Jeanne d‘Arc Basin. The basin~bounding
10 5. equivalent to 30 km in depth. This is the depth ascribed to the Moho discontinuity (Keen et al.. 198713).
Coward (1986). Lister ct al. (1986). Allmendinfer et al.
(19X7), and Pinet et ill. (19X7).
The reflector “S” ol de Charpal ct al. (lY7X). which is
probably an upper crustal dctachmrnt on the Galicia
Bank, is missing in the studied area. Thereftrc. Grand
Banks geologic cross-sections and extensional models
can be constrained only hy a Iowcr crustal detachment,
which may die out at the hasc 01 the crust or may he rooted in the upper mantle.
fault flattens at approximately
By studying the conjugate European margin, a group of
French rcsexchers proposed variants of Wcmicke’s continental stretching model l’or the Grand Banks-lbcrian
Shelf (Boillot et al.. IYX7a. 198X) or the Grand BanksGohan Spur (Malod, 19x7) separations.
These mod&
envision a lilhosphere~pcnctrative simple shear and both
result in crustal asymmetry of the opposing margins.
Mantle-derived scrpcntinized peridotitc was recovcrcd at
the base oC the continental slope. both hy ODP leg IO3
60
M.E. EN,\ClKS(‘II
Fig. 10. Simplilied
diagram
showing
partition
01 the downthrown
block by antithetic
and synthetic
laults to the basin-bounding
catagory
of basement-involved
laults (Enachescu,
1987) creates
ing grabenihorsl
structure
of the Grand Banks.
0
basement
fault. This
the allernat-
CENTRAL
PLATFORM
GRABEN
HORST
GRABEN
HORST
RIDGE
0
w
E
0.0
1.0
2.0
3.0
4.0
5.0
5.0
7.0
Fig. 11. Seismic
line h-h over the Central
Ridge. This ridge separates
the main Mesozoic
basins oi the Grand Banks: west of the ridge is
the Jeanne
d’Arc Basin.
while to the east is the Flemish
Pass Basin.
The core 01 the ridge comprises
inferred
Hadrynian
(Upper
Precambrian)
and Paleozoic
rocks. A salt layer folded and probably
laulted lies on top of the basement.
Depth lo basement
in the middle
of the panel is approximately
7 km.
0
NORTH
CENTRAL
NNW
0.0 ~~~:..
: ~.~~:
Fig. 12. Industry seismic
line i-i’ intersecting
style of the deformation
change
dramatically
moved up and into the plane of the illustration.
RIDGE
SOUTH
CENTRAL
RIDGE
@J
SSE
0.0
the Dominion
transfer
fault. The character
of the Cretaceous
and Jurassic
reflections
and the
across this fault zone. The left block moved down and out of the plane. while the right block
The basement
is here deeper than 7 s and. hence. not shown by the seismic
section.
EXTEN,>,I,>“:,S,‘*,,iN’l ,,,is,:\n,
EAST
61
‘ml,: ,x’rK:\~‘,<;\IoN,~‘ K,,T,i,~ “:\S,NS
FLEMISH
CAP RISE
4
3,330m -
5,000m 6
8
11,000m
10
12
14
35.000m
16
Fig. 13. UninWpreted
and interpreted
segment
fhe AptiS” breakup
unconformity.
The proposed
ed basaltic
layer (1). The oceanic
signature
Of these terranes.
rocks
intruded
j-j’ of the Lithoprobe
interpretation
shows
and flooded
line 85~3. Two peculiar
extended
and subsided
an already
laundered
basement
obliwereflectian
zones are visible beneath
~~nf~nentai
basement
(2) beneath
an extendand are respanslble
for the Strong
magnetic
EXTENDED
BASEMENT
BENEATH
Fig.
15. Geodynamic
model
explaining
the formation
of
Newfoundland
(Grand Banks) and Iberia (Galicia
Bank) continental margins,
adapted
after Boillot et al. (1988,
see their Figure
13). The model is a variant
of the Wernecke
(1981) simple-shear
model and assumes
that tectonic
denudation
of the mantle took
place by nonunilorm
stretching
of the continental
crust. A major
crustal detachment
fault underlies
the entire rifted area beneath
the eastern
Newfoundland
shelf and slope. Symbols
as described
by Boillot et al. (1988) are: 1 = lithosphere
subcrustal
peridotite:
2
= upwelling
peridotite
experiencing
paltial
melting
and behaving
as an asthenosphere
bulge; 3 = poorly depleted
asthanospheric
peridotite
further accreted
to the lithosphere;
G = gabbro.
by a submersible
cruise,
west of Galicia
Bank.
This
suggests that mantle rocks were emplaced at the North
Atlantic paleorift axes before the beginning of seafloor
spreading (Aptian Age). Boillot et al. (1987a. b, 1988)
concluded that tectonic denudation of the mantle took
place by nonuniform
stretching
of the pre-Mesozoic
continent. Their slightly modified model is presented here
in Figure 15. In this scenario, the Grand Banks basement
was extended along and is underlain east of the Murre
fault by the western portion of this megafault. Oceanic
crust of the North Atlantic now separates the two halves
of the Mesozoic detachment fault.
and
DISCUSSION
Two intriguing
problems
concerning
the nature of
basement in rifted areas deserve comment. The first
refers to the “inheritance”
hypothesis,
in which riftbounding megafaults are presumed to be reactivations of
older weak zones such as deformation fronts, major thrust
faults, suture zones, subduction zones, etc. {Haworth and
Keen, 1979; Haworth. 1981; Brewer and Smythe, 1984;
Grant et al., 1986b; Tankard and W&ink,
1987, 1988).
This hypothesis
is largely
embraced
and used by
researchers in different regions of the world to explain
rift-related,
normal or strike-slip faults, and it seems at
first very logical (Etheridge
et al., 1984, 1985, 1987;
Masson and Miles,
1984: Grant et al., 1986a. b).
However, data do not always support this assumption
THE IP TRACRATONIC
RIFTED
BASlNS
63
(e.g., Beach, 1985, 1986). For example, the Jeanne d’Arc
at d Whale Basins have bounding faults with crustal-scale
displacements
that, as a general rule, cut across older
Appalachian
trends (Figure 2). The same disregard for
older features is shown by a large number of other major
basement faults that fragment the Grand Banks. It is clear
that in this area, and perhaps elsewhere, “inheritance”
is
not a general rule and extension can create new fault
trends
which
ignore the previous fabric.
Another intriguing observation of this and other studies
(e.g., Meissner, 1986, chapter 6; Pinet et al., 1987) is the
lack of a clear Moho reflector beneath the central zone of
the relatively young rifted basins. Lithoprobe lines show
that the Moho reflection is well imaged beneath platforms
and ridges (Figures 5 and 9) but poorly imaged beneath
the basins (de Voogd and Keen, 1987). On line drawings
of deep reflection profiles, the Moho is sometimes liberally traced above (most frequently) or below the interpolated horizontal position. in a zone where, in fact, there is
a lack of coherent reflections.
Although
smne authors
is due to attenuation
suggest that this lack of coherency
and absorption of seismic waves by the basin infill, or to
scattering of the waves (Keen et al., 1987a) caused by
increased complexity
of the rift, there is no conclusive
proof for the suggestion that seismic energy is less capable of penetrating the basinal areas than equally complex
basement rocks of the Appalachian
orogen. Instead, 1
believe that the seismic data truly reflect a poorly developed Moho beneath the basins. Although speculative, 1
suggest that, beneath basins established
on the downthrown side of deep crustal detachment zones, there is a
zone where the lower crust was fragmented, intruded and
partly assimilated
by mantle material (Figure 9). The
mantle itself is contaminated by crustal constituents, and
thus, the sharp contrast between lower crust and upper
mantle, which is the MohoroviEiC discontinuity,
is locally
destroyed and replaced by B transient transitional
zone.
Only with time will the material in the crust-mantle transition zone be rearranged and the velocity contrast which
marks the Moho discontinuity
be restored. When balancing geologic
cross-sections
in extensional
terranes
(Gibbs, 1983). by rotating back the basement blocks to
their prerift position, there is always some crustal material unaccounted for if the Moho is constrained to be flat. A
portion of the lower crust can only “vanish” in this way
through assimilation
of the roots of the rotated blocks
into the upper mantle. A lower-continental-crustiuppermantle mix is formed, and hence, a gradual increase of
the seismic velocities instead of an abrupt velocity contrast is expected.
CONCI.USIONS
The Grand Banks basement
consists
of variou
Precambrian and Paleozoic successions and has a variable
thickness above the Moho. Significant
intrabasement
reflections from deeper zones can be seen on both indus-
M.E. ENACHESCU
64
try seismic and Lithoprobe lines. Extension of the basement occurred along a major, listric, easterly dipping normal fault, which is designated here as a basin-bounding
fault. This fault is actually the western portion of the
master detachment
between the opposing
lower-plate
margin (Grand Banks) and upper-plate
margin (Iberia
Shelf). Its associated synthetic and antithetic faults created the alternating
half-graben/ridge
structure
of the
Newfoundland
rifted margin. The complexity
of basement faulting is increased by transfer faulting. Extended
and subsided basement is present beneath an oceanic-type
layer further offshore on the continental
slope and rise.
There is a large zone floored by transitional (intermediate) crust and, therefore, the continent-ocean
boundary is
not a sharp contact as often interpreted. The continental
crust is present offshore beyond the lower continental
slope, up to a distance of 700 km from the Newfoundland
coastline (east of Flemish Cap). The chain of magnetic
anomalies interpreted as J(M,) in the south, and 34 in the
north, are the first indications of true oceanic crust.
General claims for the reactivation
of older zones of
weakness as rift-forming
faults and the presence and
position of Moho beneath rift basins need better documentation and theoretical treatment before being accepted
as hard-and-fast rules.
~_~
1987. Tectonic
and btr”Ct”rat
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of the nartheast
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Can. Sot.
Pm Fe”,., Mrm. 12, 117-146.
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