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  • pg 1
                           TECTONIC EVOLUTION OF THE
                             EAST COAST OF CANADA
                                                                       Keith Louden
                          Department of Oceanography, Dalhousie University, Halifax, Nova Scotia

                                                                                Figure 2. Plate tectonic reconstructions during the opening of the North Atlantic
Figure 1. Map of Eastern Canada with locations of Nova Scotian, Newfoundland    Ocean at 180 Ma, 130 Ma, 80 Ma, and 50 Ma. Dashed lines give locations of
and Labrador continental margin segments. Dashed lines give locations of        selected sea-floor spreading anomalies. NFZ=Newfoundland Fracture Zone;
Appalachian Front (AF) and Grenville Front (GF), which divide the continental   AFZ=Azores Fracture Zone; GFZ=Charlie-Gibbs Fracture Zone; BTJ=Biscay
regions into three major geological provinces.                                  Triple Junction; DS=Davis Strait (from Coffin et al., 1992).

INTRODUCTION                                                                    improved our understanding of the fundamental processes of
                                                                                lithospheric extension and continental rifting that have formed
   The East Coast of Canada is generally divided into three                     these margins and their hydrocarbon resources.
regions: the Nova Scotian margin in the south, the Newfoundland
margin in the centre and east, and the Labrador margin in the                       It is the purpose of this brief paper to summarize some of these
north (Figure 1). These margins formed during the past 200 million              findings. I will use recently available maps of total sediment thick-
years as the supercontinent of Pangea rifted apart, first as North              nesses (Oakey and Stark, 1995) and marine gravity anomalies from
America separated from Africa and then as it separated from                     satellite altimetry (Sandwell and Smith, 1997) to define the various
Europe and Greenland (Figure 2). These episodes of rifting thinned              basins, and some examples of regional seismic profiles to illustrate
and heated the continental crust and lithosphere, which then                    structures of both sediment and crust. As clearly indicated by the
subsided to form a complex set of marginal basins. Large amounts                maps and profiles, the complex set of sedimentary basins and their
of sediment have since accumulated in these basins and formed                   underlying basement structures that form these continental
sources and traps for hydrocarbon deposits. Exploration activity to             margins extend over a very wide transitional region. A large part
find and exploit these resources, primarily from seismic profiles               of the thick sediment deposits exist in the deeper water slope and
and boreholes of the past 30 years, has resulted in the present                 rise basins within this transition zone. New offshore exploration
production of oil off Newfoundland and gas off Nova Scotia.                     activity is now focussing on these deep water basins. If significant
Exploration activities of both commercial and scientific pursuits
have also yielded a wealth of basic information that has greatly
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accumulations of gas and oil are found, it could fundamentally           evidenced by a wide-spread volcanic pulse known as the CAMP
shift the future emphasis of the Canadian oil and gas industry from      event at 200 Ma (Marzoli, 1999) and the presence of rift successions
west to east. My hope is that results from these new endeavors,          encountered in marginal basins (e.g. Hiscott et al., 1990; Olsen,
both commercial and scientific, may also continue to play an             1997). Rifting continued in the Late Jurassic to Early Cretaceous, as
important role in improving our basic understanding of how these         evidenced by basaltic volcanism in basement drill cores of the
transitional regions form.                                               Newfoundland and Labrador margins (e.g. Pre-Piper et al., 1994;
                                                                         Balkwill et al., 1990).
                                                                             The extended duration of rifting during most of the Cretaceous
    Plate tectonic reconstructions of the North Atlantic region are      (~130 to 60 Ma) progressed further north into the Arctic over a
constrained primarily by identifications of marine magnetic anom-        broad and diffuse region but did not succeed in forming much
alies and major fracture zones that formed during the creation of        ocean crust north of Davis Strait. This period ended with the
ocean crust (Figure 2; Coffin et al., 1992). Such reconstructions can    arrival of a major pulse of volcanism at 60 Ma associated with the
be used to determine the ages and pre-drift positions of margin          Icelandic plume (White et al., 1987). Shortly thereafter, the final
conjugates (i.e. continental sections that were once adjacent to each    stage of rifting that separated Greenland and Europe at 57 Ma
other before subsequent creation of intervening ocean crust). This       (Larsen and Saunders, 1998) was of relatively short duration. Thus
is important for determining the complete pattern of rifting by          it seems that the initial and final rifting stages of the North Atlantic
juxtaposition of crustal sections across both margin pairs. Of           margins were linked to two major pulses of volcanism at 200 and
course, as the age and complexity of subsequent plate motions            60 Ma, while during the intervening period less volcanism was
increases so will the uncertainty of the reconstructed positions.        associated with rifting.

   As defined by the reconstructions, the separation of North            SCOTIAN MARGIN
American and Eurasia formed the North Atlantic margins in five
stages, beginning in the south and progressing to the north:                 Rifting on the Scotian margin occurred in the Late Triassic to
                                                                         Early Jurassic (~230-190 Ma), when red beds, evaporites and
• North America separated from Africa to form the Scotian margin         dolomites formed in fault-controlled half-grabens (e.g. Jansa and
  sometime before chron M29 (160 Ma).                                    Wade, 1975; Welsink et al., 1989; Wade and McLean, 1990).
• North America separated from Iberia to form the Southern               Basement subsidence continued in three main post-rift periods
  Newfoundland margin sometime before chron M3 (125 Ma).                 during the Jurassic, Cretaceous and Teritary, which may be related
• North America separated from Europe to form the Northern               to subsequent rifting events on the Grand Banks and major reori-
  Newfoundland margin sometime after chron M0 (120 Ma).                  entation of the plates as described in the previous section. The
• North America separated from Greenland to form the Labrador            result of this subsidence was to create a number of major sedimen-
  margin sometime before chron 31 (70 Ma).                               tary sub-basins as shown in the total sediment thickness map of
• A final stage of rifting separated Greenland from Europe begin-        Figure 3a. The Cobequid and Chedabucto faults (Co-F and Ch-F)
  ning shortly before chron 24 (55 Ma).                                  are the contact between the Meguma Terrane (to the south) and
                                                                         Avalon Terrane (to the north), which formed during the Paleozoic
    The timing and duration of continental rifting is not very specif-   Appalachian orogen. This fault defines the boundary between the
ically constrained by the reconstructions themselves, since the          late Paleozoic Sydney and Magdalen basins to the north and the
identification of sea-floor spreading anomalies only dates the post-     Mesozoic Fundy and Orpheus basins to the south. The major sedi-
rift formation of ocean crust. Often the first clear marine magnetic     mentary depocenters, however, are located further offshore in the
anomalies are located rather far seaward from the margin, due            Sable, Abenaki and Laurentian sub-basins in the east and the
either to the presence of rather weak anomalies of uncertain origin      Shelburne and other sub-basins to the west.
closer to the margin (southern Newfoundland and Labrador
margins) or to the lack of magnetic reversals (Scotian and northern          Most studies have previously been undertaken in the Sable
Newfoundland margins) during the Jurassic and Cretaceous                 basin leading to the discovery of significant gas reserves. The
Normal Polarities (~210-160 Ma and 118-83 Ma, respectively).             following description is summarized from Welsink et al. (1989) and
More specific dates for rifting would come from exposures on land        Wade and McLean (1990). The sandstone reservoirs are located
and/or drilling of syn-rift sedimentary sequences. Other estimates       within shallow marine to deltaic sediments and are probably
can be made by extrapolating the rates of sea-floor spreading to the     sourced from the Late Jurassic to Early Cretaceous prodelta to
margin or by dating of sedimentary sequences or rocks on land.           pelagic shales of the Verrill Canyon formation. The majority of gas
                                                                         is trapped in rollover anticlines associated with listric faulting.
    Such dates suggest that rifting of the older margins may have        Maturation of the source rock was accomplished by increased
occurred over an extended period before the formation of ocean           post-rift subsidence during the Late Jurassic to Early Cretaceous.
crust and may have affected adjacent margin segments. Initial            Supracrustal faults becoming younger seaward act as migration
rifting started as early as the Late Triassic to Early Jurassic, as      pathways between the source and reservoir as well as forming the

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Figure 3. Maps of the Nova Scotian margin showing (a) total sediment thickness and (b) free-air gravity. Sedimentary basins are identified (AB=Abenake; LB=Laurentian).
White dashed line locates seaward edge of Jurassic carbonate bank. Cobequid (Co-F) and Chedabucto (Ch-F) faults separate Meguma Terrane (south) from Avalon Terrane
(north). Offshore wells (white circles) and selected regional reflection seismic (red) and refraction seismic (black) profiles are shown. EMCA=East Coast Magnetic Anomaly.

structural traps. Other, more minor occurrences of both gas and oil                      Lithospheric thermo-mechanical modelling (e.g. Keen and
are associated with Early Cretaceous clastic sequences (Missisauga                       Beaumont, 1990) has suggested that these differences can be
and Logan Canyon) and are related to the edge of the Late Jurassic                       explained as a response to differing patterns of crustal and lithos-
carbonate bank (Figure 3a) or salt diapirs. Thus, hydrocarbons in                        pheric thinning. For the Sable basin model, the region of increasing
the Sable basin are inherently associated with particular drainage                       crustal thinning from continent to ocean was 200-300 km wide and
patterns and the existence of post-rift subsidence and faulting.                         coiincident with the region of increasing lithospheric thinning.
                                                                                         This led to a wide region of both initial (syn-rift) and thermal (post-
    Further offshore, large thicknesses of sediment also occur                           rift) subsidence that was further deepened by sediment loading.
beneath the lower continental slope and rise of the Sable and                            For the LaHave platform model, the crustal thinning was more
Shelburne basins (Figs. 3a and 4). Recent exploration efforts have                       abrupt (100 km wide) and lithopsheric thinning started further
focussed on these deepwater basins using 2-D and 3-D seismic                             landward. This created a landward zone of thermal uplift and a
profiles in preparation for future drilling. It is expected that reser-                  rather abrupt (<50 km wide) seaward transition zone of increasing
voirs for these deepwater prospects will be associated with                              initial subsidence. This led to erosion of the platform uplift and
Cretaceous and Early Tertiary channels, turbidites and fan deposits,                     syn-rift sediment fill further seaward followed by a wider region of
trapped by the steep walls of salt diapirs (Hogg, 2000), such as the                     post-rift subsidence. Results from these subsidence models were
ones shown in Figure 4. This Salt Diapiric Province extends along                        broadly consistent with data from wells and earlier seismic
the margin southwest of seismic profile 89-1 (Figure 3a). The location                   profiles. However, there was little or no control of the deeper
of the salt previously has been used to mark the offshore boundary                       crustal geometry from existing reflection or refraction profiles. In
between the rifted continental crust and post-rift formation of                          addition, more recent 2-D coverage of the gravity field from satel-
oceanic crust. In seismic profiles (Figure 4), continental basement is                   lite altimetry (Sandwell and Smith, 1997) suggests that the Sable
imaged out to the start of the salt diapirs, but beneath the salt the                    sub-basin is rather unique in its gravity signature.
basement is not clear. Beyond the salt, basement is at first flat and
then rifted by listric faulting (Salisbury and Keen, 1993); but neither                     It is possible that the different patterns of sediment deposition
of these structures is typical of oceanic basement.                                      and extension along the margin may be related to variations in
                                                                                         volcanism during rifting. The North Mountain basalts were
    West of the Sable basin, the edge of the Jurassic carbonate bank                     emplaced at 200 Ma south of the Cobequid Fault along the
follows the present shelf edge. In this region (Shelburne basin), the                    southern margin of the Fundy basin during its rifting phase (Olsen,
greatest sediment thicknesses occur on the present continental                           1997). They form the northern-most exposures of the CAMP
slope and rise as opposed to the outer shelf as for the Scotian and                      basalts which were erupted at the same time and over an extensive
Laurentian basins to the east. Gravity anomalies are also quite                          region of the proto-North Atlantic margin (Marzoli, 1999).
different between the western and eastern regions (Figure 3b).                                                                                      Continued on Page 40

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Figure 4. Seismic reflection profile LE 88-1A and location of coincident (Shubenacadie) and adjacent (Acadia) wells (Keen et al., 1991). Seismic horizons identified are Pliocene
(L); Au/A* (Oligocene and Top Cretaceous); Early Cretaceous (β); Top Jurassic (J); and Late Jurassic (J1, J2). Basement crustal types are defined by characteristic changes in
reflection pattern.
Evidence for another enhanced period of volcanism during final                              Flemish Pass (FPB), and Orphan basins extend beneath the slope
stages of breakup comes from the presence of the East Coast                                 and rise to the east and north. The tectonic history is marked by
Magnetic Anomaly (ECMA), which begins to the south of Sable                                 four periods of subsidence and two main periods of regional uplift
Island (Figure 3). Along the southern part of the Shelburne basin,                          (e.g. Enachescu, 1987; Tankard and Welsink, 1989; Grant and
seismic profiles 89-3/4/5 show that the ECMA is coincident with a                           McAlpine, 1990; Sinclair, 1995). Initial rifting with extension in the
zone of seaward-dipping reflectors in the basement (Keen and                                NW-SE direction began in the Late Triassic to Early Jurassic with
Potter, 1995). These reflectors are similar to ones found along the                         deposition of red beds and evaporites in similar sequences to those
volcanic margins of East Greenland, and Northwest Europe where                              on the Scotian margin. A period of reduced subsidence in the
scientific drilling has sampled sub-aerial or shallow marine basalt                         Early-Middle Jurassic was followed by a second phase of rifting in
flows (e.g. Eldholm et al., 2000). Unfortunately the basement                               the Late Jurassic to Early Cretaceous. The direction of extension
beneath the ECMA along the most of the southern Scotian margin                              rotated to NE-SW causing oblique slip and local transpression on
is masked by salt (Figure 4) and lies seaward of all but a few of the                       the earlier faults. These sequences are truncated by a major set of
seismic profiles.                                                                           mid-Cretaceous unconformities (Barremian to Albian) related to
                                                                                            basin inversion and regional uplift. This was followed in the Late
    In order to improve our understanding of variations in crustal                          Cretaceous by further post-rift subsidence on the Banks and exten-
architecture during rifting of the Scotian margin, a series of three                        sion in Orphan Basin. A base Tertiary unconformity, particularly
long refraction/wide-angle reflection profiles were shot in summer                          prominent in the northern region, post-dates rifting.
2001 (Figure 3b) as part of the Canadian MARIPROBE program
(Louden and Hall, 2000). It is hoped that these new data will help                              The mid-Cretaceous unconformities are related to breakup of
resolve the nature of basement transitions and crustal thickness vari-                      the Grand Banks first from Iberia and then from the Rockall
ations along the three profiles. In addition, further control on sedi-                      margin, when the mid-ocean rift between North America and
ment and basement velocities will allow better depth migration of                           Africa finally propagated to the north. A major volcanic pulse off
the existing MCS reflection profiles and constraints on density in                          the Tail of the Banks formed the “J-anomaly” basement ridge and
gravity models. This may prove particularly useful in regions where                         magnetic anomaly (Tucholke and Ludwig, 1982), which also is
salt structures complicate traditional MCS imaging techniques.                              observed off the southern Iberian margin. This may be related to
                                                                                            mid-Cretaceous volcanism that has been sampled in several wells
NEWFOUNDLAND MARGIN                                                                         (Pre-Piper et al., 1994), but which was previously attributed to
                                                                                            rifting and transform motion. Thus there are two primary candi-
  The Grand Banks of Newfoundland (Figure 5) contains a                                     dates for causing the Cretaceous uplift and inversion: (i) a response
complex series of basins including the Whale (WB), Horseshoe                                to in-plane compressional forces created by varying rates of exten-
(HB), Jeanne d’Arc (JdA), and Carson-Bonnition (CB). The South                              sion and rotation of the axis of extension from NW to NE (Karner
Whale (SWB) and Laurentian (LB) basins on its south-eastern                                 et al., 1993); or (ii) a response to added buoyancy created by
margin connect into the Scotian margin to the south. The Salar (SB),                        volcanic underplating of the margin, in a similar manner as

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ARTICLE                         Cont’d
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                                                                                        are located in Late Jurassic and Early Cretaceous shallow marine
                                                                                        and fluvial sandstones deposited during the second rift and post-
                                                                                        rift phases. Late Jurassic shales of the Egret member contain a
                                                                                        marine-rich source that matured during subsequent burial within
                                                                                        the Late Cretaceous and Tertiary. Traps were formed by the mid-
                                                                                        Cretaceous from rollover anticlines (e.g. Hibernia structure) and
                                                                                        rotated fault blocks, and they were largely preserved during the
                                                                                        subsequent Avalon uplift and erosion.

                                                                                           Most exploration has concentrated on the Jeanne d’Arc basin
                                                                                        and other shallow water basins on the Banks. Only a few wells
                                                                                        have been drilled in deeper water. However, the sediment distri-
                                                                                        bution map (Figure 5a) shows that significant thicknesses exist
                                                                                        beneath most areas of the rise and slope bordering the Banks (e.g.
                                                                                        South Whale, Salar, Carson-Bonnition, Flemish Pass and Orphan
                                                                                        basins). Recently, additional seismic exploration has been under-
                                                                                        taken in these deeper water basins to further assess its economic
                                                                                        potential. Previous analysis of the deeper offshore regions were
                                                                                        made using a few regional seismic profiles collected in the mid-
                                                                                        1980’s (e.g. Keen and de Voogd, 1988; Tucholke et al., 1989). A more
                                                                                        recent set of regional profiles (Figure 5) now extends this coverage
                                                                                        across the northern Newfoundland basin. The deeper water part of
                                                                                        the LE85-4 profile is shown in Figure 6 (J. Hall and S. Deemer,
                                                                                        personal communication, 2001) and a short section of the recent
                                                                                        Ewing2000-3 profile (Louden and Lau, 2002) across the Carson
                                                                                        basin in Figure 7. A series of tilted basement fault blocks is
                                                                                        observed up to 100 km seaward from the shelf break. The first is a
                                                                                        large block possibly with some salt cover that divides the offshore
                                                                                        side of the Carson-Bonnition basin into shallower and deeper
                                                                                        water sections. The deep-water region of thicker sediment and
                                                                                        complex basement structure may hold the best potential for hydro-
                                                                                        carbons (Enachescu, 1992).

                                                                                            Further seaward of the faulted basement, a 100-km wide region
                                                                                        exists where a prominent reflector (U) masks the underlying base-
                                                                                        ment. This reflector appears to terminate against a u series of
                                                                                        elevated basement highs. Above the U-reflector, the A* reflector
                                                                                        defines the Tertiary transition between flat-lying and bottom
                                                                                        current dominated depositional sequences. It is not certain if the U-
Figure 5. Maps of the Newfoundland margin showing (a) total sediment thickness
and (b) free-air gravity. Sedimentary basins are identified (CB=Carson-Bonnition;
                                                                                        reflector is related to the Avalon unconformity of the southern
FPB=Flemish Pass; HB=Horseshoe; JdA=Jeanne d’Arc; LB=Laurentian;                        Banks and/or the Early Cretaceous ß-reflector observed off the
SB=Salar; SWB=South Whale; WB=Whale). Collector magnetic anomaly (CA)                   Scotian margin (Figure 4). The nature of the relatively flat-lying
separates Meguma Terrane (south) from Avalon Terrane (north). Offshore wells            basement within this transitional region is also uncertain. Recent
(white circles) and selected regional reflection seismic (red) and refraction seismic   drilling and seismic results indicate the presence of a wide zone of
(black) profiles are shown.
                                                                                        serpentinized peridotite basement in a conjugate setting beneath
                                                                                        the Iberia margin (Louden and Lau, 2002). A similar model was
proposed to explain uplift and cyclic deposition of submarine fans
                                                                                        previously proposed by Enachescu (1992) for the Newfoundland
in the North Sea (White and Lovell, 1997). The nature of the base
                                                                                        basin. Possible drilling targets to resolve these issues have been
Tertiary unconformity, however, is still unclear.
                                                                                        selected along profile Ewing 2000-2 in the northern part of the
                                                                                        basin (Figure 8). A drilling leg of the Ocean Drilling Program is
   The complex rifting and subsidence history mentioned above
                                                                                        scheduled for this work in July-Sept 2003.
has led to a combination of stratigraphy, structure and timing
conducive to hydrocarbon generation and entrapment (Bell and
                                                                                           To the northwest of Flemish Cap, a very wide zone of thick sedi-
Campbell, 1990). So far, however, significant discoveries have only
                                                                                        ment exists within Orphan basin. This region experienced rifting
been located within several fields (e.g. Hibernia, Terra Nova,
                                                                                        episodes that may have extended into the Late Cretaceous. Most of
Whiterose) of the northern Jeanne d’Arc basin. Primary reservoirs

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Figure 6. Seismic reflection profile LE 85-4 (Keen and de Voogd, 1988), remigrated and coherency filtered by J. Hall and S. Deemer (personal communication, 2001). Seismic hori-
zons identified are Au/A* (Oligocene and Top Cretaceous) and U (Tucholke et al., 1989). Basement crustal types are defined by characteristic changes in reflection pattern.

Figure 7. Seismic reflection profile Ewing 2000-3 across the outer Carson-Bonnition basin showing basement ridge with possible salt that separates basin into inner
(shallow) from outer (deep water) parts. Seismic horizons identified are Au/A* and U, after Tucholke et al. (1989). Note that these horizons pinch out and terminate against
basement and cannot be traced into shallower water.

the basin is underlain by highly thinned continental crust but its                          LABRADOR MARGIN
deep water has precluded much drilling activity. The gravity highs
associated with the shelf edge (Figure 5b) shows a significant                                 The Labrador Sea is a northwestward extension of the North
difference from gravity lows associated with most of the other                              Atlantic Ocean, from the Charlie-Gibbs fracture zone in the south
basins. This has been modeled by replacing the lower crust with                             to Davis Strait in the north (Figure 2), which separates southern
mantle, suggesting the presence of a failed rift that was abandoned                         Greenland from Labrador. Rifting and breakup of these margins
when continental breakup shifted further to the northeast (Chian                            began during the Early Cretaceous (~125 Ma) and ended during
et al., 2001). A very thick sequence of Tertiary sediment in the                            the Late Cretaceous (~85 Ma) based on borehole data (Balkwill
deeper water regions of Orphan basin indicates a predominance of                            1990). Volcanics of Cretaceous and early Tertiary age onlap the rift
post-rift rather than syn-rift subsidence (Keen and Dehler, 1993).
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Figure 8. Seismic reflection profile and location of proposed Ocean Drilling Program drilling sites in the Newfoundland basin (Tucholke et al., 2002). Seismic horizons Au
and U are identified as per Figure 7. For general location map see the Ocean Drilling Program web site (http://www-odp.tamu.edu/publications/ tnotes/fy03/210ab.html)

structures and synrift sediments. In the region of Davis Strait, a                            During subsidence of the Labrador margin, terrigenous source
final period of intense volcanism in the Paleocene (~60 Ma) is asso-                      rocks within the Upper Cretaceous Bjarni Formation and Upper
ciated with the North Atlantic Magmatic Province (Gill et al., 1999).                     Cretaceous to Paleocene Markland Formation matured primarily
Unlike the Newfoundland and Nova Scotia margins to the south,                             to form gas. Of the 31 wells drilled on the Labrador margin during
the pre-existing continental crust varies substantially in its ages                       the 1970’s and early 1980’s, there were six hydrocarbon discoveries
and crustal properties: from the Paleozoic Appalachian Province in                        of which the largest was the Bjarni gas pool (Bell and Campbell,
the south, through the Late Proterozoic Grenville Province to the                         1990). Hydrocarbon reservoirs for these discoveries are formed in
Early Proterozoic Makkovik Province, and finally the Archean                              structural traps of Lower and Upper Cretaceous fluvial sandstone
Nain Province (Figure 9). A recent review of geophysical properties                       overlying basement horst blocks.
of these crustal units, based on results from the Lithoprobe
ECSOOT program, is given by Hall et al. (2002).                                               Obviously, there is much less recent seismic coverage of the
                                                                                          Labrador margin than for the Newfoundland and Nova Scotian
    Following rifting, subsequent seafloor spreading in the                               margins. However, because of the limited width of the Labrador
Labrador Sea is documented by magnetic lineations (Roest and                              Sea and relatively simple seafloor spreading history, a single
Srivastava, 1989), beginning first in the south during the Late                           regional profile was shot that spans the complete width of the
Cretaceous (~70-80 Ma), and then propagating to the north and                             basin and its conjugate margins (Keen et al., 1994). In addition,
ending in the Late Eocene (~40 Ma) when seafloor spreading                                several separate but coordinated refraction profiles were shot
ceased. A major change in spreading occurred at ~55 Ma when                               along and across the same transect. Combination of these data has
rifting began separating Greenland from Europe. During its syn-                           allowed a complete depth section to be made from seafloor to
rift and post-rift period, an immense set of oval-shaped sedimen-                         mantle across the entire basin (Chian et al., 1995; Louden et al.,
tary basins separated by crustal arches formed along the deeply                           1996). The section across the Labrador margin is shown in
subsided crust of the Labrador shelf (Figure 9). Following the                            Figure 10. Of particular note is the interpretation of a wide zone of
initial coarse-grained syn-rift deposits, there was a short period of                     thinned continental crust beneath the outer shelf and slope, which
sediment starvation followed by a large amount of clastic sediment                        contrasts with previous interpretations of oceanic crust (e.g.
influx during the Late Cretaceous and Tertiary. This led to a major                       Balkwill et al., 1990). Further seaward, a zone of high velocity
seaward progradation of sediment over the rift-age grabens and                            lower crust, interpreted as partially serpentinized mantle, sepa-
ridges. As the basement continued to subside, successive Tertiary                         rates the zones of thinned continental crust (landward) and oceanic
sediment horizons downlap and thicken seaward as the shelf                                crust (seaward). Basement above the zone of serpentinized mantle
attained its present position. In comparison, the Southwest                               is relatively flat, in contrast with the faulted basement to either
Greenland shelf is narrow and has experienced little or no subsi-                         side. A prominent sub-basement reflector marks the top of the
dence south of 63°N (Rolle, 1985). Thermal models of borehole                             higher velocities of the serpentinized mantle. This sub-horizontal
data from the Labrador margin were the first to include a greater                         horizon contrasts to the dipping crustal reflectivity to either side.
amount of lithospheric versus crustal stretching (Royden and                              Based on this profile and a similar one across the Southwest
Keen, 1980) in order to explain its larger post-rift versus syn-rift                      Greenland margin, a balanced crustal reconstruction of the two
subsidence history.                                                                       conjugate margins at the point of breakup is shown in Figure 11
                                                                                          (Chian et al., 1995). This indicates that a highly asymmetric pattern

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Figure 9. Maps of the Labrador margin showing (a) total sediment thickness and (b) free-air gravity. Sedimentary basins and continental terranes are identified along with
selected regional seismic reflection (red) and refraciton profiles (black), land seismic stations (white crosses), and offshore wells (white circles). GF=Grenville Front;
MF=Makkovik Front; SAB=St. Anthony Basin.
and absence of significant amounts of mantle melt must have                              CONCLUSIONS
resulted late during the rifting process, in contrast to predictions
from pure-shear models (Louden and Chian, 1999). It would                                Several basic relationships are common to these margins:
certainly be interesting to know if this asymmetry is a common                           • Rifting activity persists for a substantial period of time before
feature of these margins. A subsequent refraction profile 92-5 (Hall                       breakup and subsequent seafloor spreading. For the Nova
et al., 2002) indicates a more abrupt initial thinning of the conti-                       Scotian margin this period was as great as 40-50 Ma, for the
nental crust further to the north (Figure 9), but it does not sample                       Newfoundland margin 30-40 Ma and for Labrador 40-65 Ma.
the complete transition into the oceanic basin.                                            Pulses of volcanic activity during rifting may occur, possibly
                                                                                           resulting in platform uplift as a result of localized underplating

                                                                                                                                                   Continued on Page 47
46   CSEG Recorder        February, 2002
                                                                                                                                 ARTICLE                         Cont’d
Continued from Page 46

Figure 10. Depth section for seismic profile TLS90-1 across the Labrador margin with seismic velocities (in colour) from refraction profiles. Wells and basement crustal types
and boundaries as identified. U=breakup unconformity. (from Louden and Chian, 1999).

                                                                                           • A zone of transitional basement ~150 km wide exists seaward of
                                                                                             the stretched continental crust and landward of the first normal
                                                                                             oceanic crust. This zone is associated with characteristic changes
                                                                                             in basement morphology and depth across the transition zone,
                                                                                             with the deepest, flat-lying basement on the landward side and
                                                                                             elevated basement highs on the seaward side. One possibility is
                                                                                             that this zone is composed primarily of serpentinized mantle
                                                                                             with only minor amounts of crustal melt (Louden and Chian,
                                                                                             1999). The existence of this transition zone is most likely a conse-
                                                                                             quence of very slow rates or multiple periods of extension.
                                                                                           • Additional crustal refraction profiles recently undertaken off the
                                                                                             Newfoundland and Scotian margins will help to show whether
                                                                                             the crustal variations previously observed off Labrador are
                                                                                             common to these other margins. Connection of these profiles
                                                                                             with similar profiles across their margin conjugates will help
                                                                                             demonstrate whether the high degree of breakup asymmetry
                                                                                             observed for the Labrador-Greenland transect is a common
                                                                                             feature of the other margin segments. This would indicate
                                                                                             whether such asymmetry is a fundamental outcome of slow
                                                                                             rates of lithospheric extension.

Figure 11. Possible scenario for asymmetric crustal breakup of Labrador-
                                                                                               Regardless of how many seismic profiles and models we make,
Greenland continental block based on balanced crustal cross-sections from velocity
models. Crustal sections removed during reconstruction (yellow and red) are                however, eventually we need to drill and core at a few locations to
assumed to have formed following breakup by serpentinization of mantle (from               determine what is really there. This is true for basement objectives
Chian et al., 1995).                                                                       as well as for sediment sequences. New scientific drilling in the
                                                                                           Newfoundland basin by the Ocean Drilling Program if successful
  and/or thinning of the lithosphere, but these pulses seem to be
                                                                                           will help to resolve some fundamental questions about its forma-
  localized rather than regional in extent. Thus the margins are
                                                                                           tion. But additional drilling through sequences on the slope and
  predominantly non-volcanic.
                                                                                           rise will also be needed in order to fully understand the nature of
• The spatial extent of primary rift activity eventually leading to
                                                                                           other major structures. Perhaps with a continued combination of
  breakup on the southern margin extends laterally to the adjacent
                                                                                           both scientific and commercial activities, as have previously
  margin to the north. Thus the Late Triassic to Early Jurassic
                                                                                           resulted in such a wealth of both knowledge and resources, these
  rifting on the Scotian margin also affected the Grand Banks and
                                                                                           future goals can be accomplished.
  the Late Jurassic to Early Cretaceous rifting on the Grand Banks
  also affected the Labrador margin.
                                                                                                                                                      Continued on Page 48

                                                                                                                                    February, 2002   CSEG Recorder         47
ARTICLE                                Cont’d
Continued from Page 47

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