Tectono-stratigraphic evolution and crustal architecture of the Orphan Basin during North Atlantic rifting
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The Orphan Basin is located in the deep offshore of the Newfoundland margin, and it is bounded by the continental shelf to the west, the Grand Banks to the south, and the continental blocks of Orphan Knoll and Flemish Cap to the east. The Orphan Basin formed in Mesozoic time during the opening of the North Atlantic Ocean between eastern Canada and western Iberia–Europe. This work, based on well data and regional seismic reflection profiles across the basin, indicates that the continental crust was affected by several extensional episodes between the Jurassic and the Early Cretaceous, separated by events of uplift and erosion. The preserved tectono-stratigraphic sequences in the basin reveal that deformation initiated in the eastern part of the Orphan Basin in the Jurassic and spread towards the west in the Early Cretaceous, resulting in numerous rift structures filled with a Jurassic–Lower Cretaceous syn-rift succession and overlain by thick Upper Cretaceous to Cenozoic post-rift sediments. The seismic data show an extremely thinned crust (4–16 km thick) underneath the eastern and western parts of the Orphan Basin, forming two sub-basins separated by a wide structural high with a relatively thick crust (17 km thick). Quantifying the crustal architecture in the basin highlights the large discrepancy between brittle extension localized in the upper crust and the overall crustal thinning. This suggests that continental deformation in the Orphan Basin involved, in addition to the documented Jurassic and Early Cretaceous rifting, an earlier brittle rift phase which is unidentifiable in seismic data and a depth-dependent thinning of the crust driven by localized lower crust ductile flow.
KeywordsPassive margins Rift basins Tectonics Sedimentation
Although several contributions have examined the large-scale structural and crustal architecture of the Orphan Basin (e.g. Enachescu et al. 2004, 2005; Welford et al. 2012; Watremez et al. 2015; Lau et al. 2015), only few tried to quantify its Mesozoic syn-rift history and the driving tectonic processes (Dafoe et al. 2015; Gouiza et al. 2015). In this contribution, (1) we examine the stratigraphic and tectonic evolution of the Orphan Basin using exploration wells and regional seismic lines, (2) we investigate its structural and crustal architectures to quantify amounts of crustal extension versus crustal thinning during rifting, and (3) we propose tectonic processes that can explain its tectono-stratigraphic evolution.
Data and methods
2D seismic reflection data
Five deep marine multichannel seismic reflection lines (labelled A to E; Fig. 2) were acquired by TGS in the Orphan Basin in 2002. Lines A and B extend NW–SE over 617.9 and 677.54 km, respectively. Lines C and D run NE–SW over 593.07 and 369.36 km, respectively. Line E is oriented WNW–ESE and is 518.75 km long. All lines were recorded to a maximum two-way time (TWT) of 12 s and were provided to us (courtesy of TGS) in time-migrated SEG-Y format.
Mesozoic sedimentary architecture in the Orphan Basin from well and 2D seismic reflection data
The Cenozoic succession, characterized in the well logs by the Cenozoic unconformity at its base, is extremely thick on the continental shelf (3000–4000 m), but thins towards the east in the deep basin (1500 m) and towards the southeast in the Flemish Pass (600–1400 m). The Cenozoic succession is formed by the Banquereau Formation and consists mainly of shales, mudstones, and thin intercalations of carbonates. On seismic data, this unit is characterized by continuous high-amplitude reflections alternating with very weak amplitude reflections (Figs. 5, 6).
The Upper Cretaceous succession is defined by the Cenozoic unconformity at the top and the Aptian–Albian unconformity at the base. The latter unconformity was initially interpreted as base Cenomanian (Enachescu et al. 2005), but new palynological analyses yielded a more reliable age at the Aptian–Albian boundary (Dafoe et al. 2015). The Upper Cretaceous succession is drilled in the continental shelf (650 m thick in Baie Verte J-57) and the deep Orphan Basin where it becomes significantly thin towards the east (10 m thick in Great Barasway F-66). It mainly consists of the Cenomanian–Campanian Dawson Canyon Formation and contains predominantly medium- to coarse-grained siliciclastic sediments (siltstones and sandstones), mudstones, and marls in the south (Sheridan J-87). The Upper Cretaceous unit shows two different seismic signatures on seismic profiles. It is mainly characterized by continuous low-amplitude and low-frequency reflections in the eastern part of the basin and overlain by continuous high-amplitude and high-frequency reflections in the western part of the basin (Figs. 5, 6). This sequence roughly corresponds to the breakup sequence recognized in Northwest Iberia by Soares et al. (2012).
The Lower Cretaceous succession is defined by an Aptian–Albian unconformity at the top and the Tithonian unconformity at the base. It is found only in the East Orphan Basin (Great Barasway F-66) and the Flemish Pass wells (Baccalieu I-78 and Gabriel C-60). It includes mainly the Nautilus Formation and the Whiterose Formation, formed by shale-dominated series with occasional siltstone and sandstone interbeds. This unit has a recognizable seismic signature along the profiles consisting of discontinuous reflections with medium to low amplitudes and high to medium frequencies (Figs. 5, 6).
The Jurassic succession, capped by the Tithonian unconformity, occurs only in the Great Barasway F-66 and the Baccalieu I-78 wells in the East Orphan Basin and Flemish Pass, respectively. It includes the Tithonian Jeanne d’Arc Formation and the Callovian–Kimmeridgian Rankin Formation, which consist of shales, siltstones, sandstones, and minor limestone interbeds. The base of this succession was never drilled in this area, and the age of the first Mesozoic sediments deposited in the Orphan Basin, in relation to the Atlantic rifting, has yet to be determined. However, well data in the Jeanne d’Arc Basin to the south indicate the presence of Upper Triassic clastics lying unconformably over the Palaeozoic basement (McAlpine 1990). This configuration is similar to West Iberia (Alves et al. 2003), West France (Roberts and Bally 2012), and West Ireland (Murphy and Ainsworth 1991). In this work, the Jurassic succession includes the sediments between the Tithonian unconformity and the Palaeozoic basement, which may or may not include Upper Triassic deposits. The Jurassic unit is very thin and scattered in the western part of the basin where it is represented by discontinuous high-amplitude reflections. It is much thicker in the eastern part of the basin and characterized by medium- to very-low-amplitude reflections (Figs. 5, 6). The major uncertainty in our seismic interpretation is the Jurassic deposits drilled only in the East Orphan Basin and for which there is no direct evidence in the west.
The oldest succession, encountered in wells drilled on structural highs, is the pre-Mesozoic basement probably of Palaeozoic to Neoproterozoic age. It comprises low-grade metamorphosed sediments and igneous granitic rocks. The basement is characterized in seismic data by chaotic and discontinuous reflections with medium to high amplitudes (Figs. 5, 6).
Structural and stratigraphic architecture imaged by the 2D seismic data
The Jurassic sediments are found mainly in the eastern domain of the basin to the east of the horst structure, where they show large thickness variations related to east-dipping normal faults. They show growth strata and thickening towards the faults (Fig. 6a–c), a character indicating a syn-tectonic deposition during crustal stretching. The Jurassic deposits get much thinner and scattered as we move to the west of the horst structure, and are absent on the continental shelf. They are characterized by sub-parallel reflections with no evidence of syn-depositional tectonics in the western part of the Orphan Basin (Figs. 6a, d, 11). The Jurassic succession is capped by an erosional unconformity (Tithonian unconformity) across the basin illustrated on the seismic data by truncated reflections (Fig. 6).
The Lower Cretaceous sediments unconformably overlay the Jurassic and show more lateral continuity across the deep basin and the continental shelf (Figs. 6, 10). Their thickness is relatively constant across the continental shelf, but shows substantial variations in the deeper domains of the basin where the normal faulting appears to have controlled sedimentation. This is particularly the case in the western part of the deep basin where the Lower Cretaceous package is characterized by significant thickening inside the half-graben and strata growth patterns towards the faults (Figs. 6d, 11), a character yet again similar to West Iberia’s Lusitanian Basin (Alves et al. 2003, 2006, 2009) and offshore Ireland (Shannon et al. 2001). East of the horst structure, the Lower Cretaceous succession is also affected by syn-rift normal faults but displays smaller offsets than in the west. The topmost seismic reflections of this sedimentary unit exhibit truncation patterns related to the Aptian–Albian erosional unconformity (Figs. 6, 10).
The Upper Cretaceous succession seals most of the syn-rift structures in the eastern part of the basin (Figs. 6b, c, 10), where it is characterized by sub-parallel reflections onlapping the underlying Aptian–Albian unconformity. It is much thinner in the western part of the basin (Figs. 6d, 11) between the continental shelf and the horst structure where the west-dipping faults were still active and the tilted block ridges remained emerged during Late Cretaceous time.
The Cenozoic succession has a very consistent character across the entire Orphan Basin. Its base shows reflections sub-parallel to the underlying Upper Cretaceous with no evidence of erosional or angular unconformity. This succession is extremely thick above the edge of the continental shelf and thins considerably as we move to the east of the basin (Figs. 6, 10). Its internal architecture is well imaged along the different seismic profiles but will not be discussed in detail in this work as the main focus is on the syn-rift evolution.
Crustal structure of the Orphan Basin
Moho and crustal thickness along seismic line E
The crustal architecture of the Orphan Basin, illustrated on seismic line E (Fig. 12) crossing the Orphan Basin in a WNW–ESE direction, clearly shows the existence of four distinct crustal domains. The continental shelf domain to the west is characterized by the thickest post-rift succession (up to 7000 m), a very thin syn-rift (<2000 m), and no syn-rift faulting affecting the basement. The crust is 28 km thick in the west where the Moho is 30 km deep, but thins progressively and considerably towards the east where the Moho is 15 km deep and the crust is only 7 km thick. The western sub-basin domain displays a thick post-rift succession in the west (<5500 m), which is much thinner in the east (<2500 m), and isolated syn-rift depocentres in the half-graben filled mainly by Lower Cretaceous sediments (<3000 m thick). The crust is affected by a series of west-dipping faults, which bound the half-graben and sole out at a mid-crustal detachment ~14 to 15 km deep (Fig. 12). The crust is 9 km thick in the west and 16 km thick in the east. The crust thickens as the Moho deepens towards the east but thins locally under the half-graben where it may be only 5 km thick. The eastern sub-basin domain is separated from the western sub-basin by a 150-km-wide structural high, showing very reduced syn- and post-rift successions (<3000 m thick) and a 17-km-thick crust with a 21- to 22-km-deep Moho. The eastern sub-basin is covered by a 2.5- to 4-km-thick post-rift succession, overlying the Jurassic–Lower Cretaceous syn-rift sediments (<5000 m thick), which fill the half-graben. This domain is affected by major east-dipping listric normal faults soling out at the Moho level. The crust is 12 to 15 km thick in the west but only 4 km thick in the east. The Moho underneath the sub-basin shallows rapidly towards the east, reaches a minimum depth of 12 km in the centre, and remains about 14 km deep in the east (Fig. 12).
2D gravity model along Line E
We performed 2D forward gravity modelling along line E, using the GM-SYS profile modelling module in Geosoft Oasis Montaj software, to assess the consistency of our interpretation of the Moho and the resulting crustal structure (Fig. 14). We use the satellite altimetry gravity anomalies (Fig. 14a) (Sandwell and Smith 2009), which can be downloaded from Scripps Institution of Oceanography (https://scripps.ucsd.edu/). The free-air gravity anomalies in the Orphan Basin are overall negative and surrounded by strong gravity highs over the continental shelf, the Flemish Cap, and the Orphan Knoll (Fig. 14a). A narrow N–S elongated gravity high is also shown within the Orphan Basin and corresponds to the structural high domain, which is documented along the seismic lines between the eastern and western sub-basins.
Reconstruction of the tectono-stratigraphic evolution of the Orphan Basin
To reconstruct the tectonic evolution of the Orphan Basin and quantify the amount of crustal stretching during Mesozoic rifting, we restore crustal section E (Fig. 12) using the MOVE® software (http://www.mve.com/). The 2D restoration algorithm assumes two fundamental rules which are bed length conservation and bed area conservation. The former is based on the assumption that a given sedimentary bed keeps the same length before and after deformation. The latter rule assumes that deformation does not change the cross-sectional area of sedimentary units. The kinematic restoration is based on the relationship between fault geometry and deformation patterns in the hanging walls of the normal faults, as documented along the seismic profile (Figs. 6, 10). Therefore, a simple shear model is used for most of the faults and the shear angle is defined based on the hanging-wall geometries (i.e. rollover structures, sedimentary growth, and flexural slip). Most of the normal faults show an antithetic shear angle ranging between 90 and 60°. During the restoration, decompaction is applied to the different lithostratigraphic successions using surface porosities and rates of decay with depth according to Sclater and Christie (1980). Decompaction requires palaeowater depth to be constrained along the profile. We assume that the basin remained at a very shallow position near sea level during the entire syn-rift period (i.e. from Jurassic to Late Cretaceous times). This is supported by the terrigenous nature of the Upper Jurassic and Lower Cretaceous sediments and by the emerged position of the basement highs (i.e. horsts and tilted block ridges) during that time and until the Late Cretaceous. We also assume that the basin started to deepen in the Palaeocene and that the water depth increased linearly with time until it reached the present-day bathymetry. Local Airy isostasy is used to account for the isostatic response to sedimentary unloading during decompaction. We adopt average values for sediment (2300 kg/m3), crust (2800 kg/m3), and mantle (3340 kg/m3) densities, which are inferred from the density model described above.
Syn-rift restoration was carried out in two steps. The first step consisted of removing the Lower Cretaceous succession, decompacting the Jurassic sediments, and restoring the normal faults to the end of Jurassic time (Fig. 17c). This step (Fig. 17c) substantially affected the western sub-basin domain where Early Cretaceous deformation is more pronounced. Extension is distributed along west-dipping normal faults, and tilting of the faulted blocks is compensated at the mid-crustal level. The obtained profile (Fig. 17c) shows a wide and continuous Jurassic basin in the east and isolated Jurassic troughs in the crustal high and western sub-basin domains. In contrast to the Jurassic sediments in the eastern sub-basin, which show syn-depositional tectonic structures (Fig. 10), the scattered Jurassic deposits to the west along the profile show no evidence of fault-controlled deposition (Fig. 11). Therefore, we assume that any (modest?) strata deposited in the western sub-basin early during the Jurassic syn-rift phase were eroded sometime before the initiation of the second rifting phase in the Early Cretaceous (Fig. 17d). The second syn-rift restoration step (Fig. 17e) involved removing the Jurassic succession and restoring the basement rocks. It mainly affected the eastern sub-basin, which was the locus of crustal stretching during the Jurassic.
The restored pre-rift profile in Fig. 17e displays the crustal architecture prior to rifting, assuming that the Jurassic to Early Cretaceous extension was entirely accommodated by brittle deformation along the faults. It shows that the total extension is on the order of 110 km; 50 km of extension occurred during the Jurassic and was primarily accommodated by the east-dipping normal faults in the eastern sub-basin domain (Fig. 17c), while 60 km of extension is related to the Early Cretaceous rifting phase accommodated predominantly by the west-dipping normal faults (Fig. 17b). The restored profile (Fig. 17e) shows a 25- to 28-km-thick crust beneath the western part of the shelf, the western sub-basin and the crustal high domains, and a 10- to 12-km-thick crust beneath the eastern portion of the continental shelf and the eastern sub-basin domains. This is reflected in the topography of the restored mid-crust boundary (Fig. 17e), which displays two highs in the continental shelf domain and in the east sub-basin domain.
Similarly, seismic data and kinematic restoration of highly attenuated crustal domains found on the Iberian rifted margin (Manatschal et al. 2001; Manatschal 2004) indicate that the pre-faulting crustal thickness does not exceed 12 km. The authors argue that the pre-faulting stage does not correspond to a pre-rift stage, for which an equilibrated crust (i.e. 30 km thick) is assumed.
Tectono-stratigraphic evolution of the Orphan Basin during rifting
Data presented in this work indicate that rifting in the Orphan Basin occurred in at least two phases. The initial Jurassic rifting phase affected mainly the eastern part of the basin where continental extension is documented by east-dipping faults and thick Jurassic sediments, preserved in the hanging wall of the half-graben (Figs. 6, 10). Seismic reflections within the Jurassic succession show growth structures (Fig. 6), an indication that the normal faulting was coeval with, and controlled, sedimentary deposition. The Jurassic succession in the West Orphan Basin is substantially thinner and discontinuous with no evidence of syn-depositional tectonics (Figs. 6, 11). The major uncertainty in our seismic interpretation relates to the Jurassic deposits drilled only in the East Orphan Basin and for which there is no direct evidence in the west. However, the Deep-Sea Drilling Project (DSDP) Site 111, drilled on the Orphan Knoll, intersected coarse sandstone and shales at the base of the well that were determined to be Middle Jurassic (Bajocian) in age and above the Palaeozoic basement (Laughton et al. 1972). The compositions of these Middle Jurassic clastics indicate they were deposited in a coastal plain environment (Laughton et al. 1972), which we believe extended over most of the West Orphan Basin and may be part of the continental shelf.
The second rifting phase took place in the Early Cretaceous. During this phase, deformation spread towards the west of the Orphan Basin and resulted in half-graben bounded by west-dipping faults and filled by thick Lower Cretaceous deposits (Figs. 6, 11). In the east, some of the Jurassic east-dipping normal faults were reactivated and new sedimentary depocentres were developed (Fig. 6). Two-phase rifting in the Orphan Basin was already mentioned in previous work by Enachescu et al. (2005) who proposed an initial NW–SE extension in the Jurassic localized in the eastern part of the basin, and a late E-W extension which mainly affected the West Orphan Basin, but can also be documented in the east, during the Early Cretaceous. Seismic and well data indicate that rifting in the Orphan Basin was interrupted by two erosional events corresponding to the Tithonian and the Aptian–Albian truncation unconformities. The resulting angular unconformities between Jurassic and Lower Cretaceous strata (Tithonian unconformity), and Lower Cretaceous and Upper Cretaceous strata (Albian–Aptian unconformity) suggest that the basin was first uplifted then eroded. However, the duration of these uplift/erosion events remains difficult to constrain.
Late Cretaceous time marks the beginning of the post-rift stage as lithospheric break-up of the Newfoundland–Iberia/Europe rift occurred east of the Orphan Basin and the Grand Banks. Mantle lithosphere rocks were exhumed before the initiation of sea-floor spreading which occurred near the Aptian–Albian boundary (Tucholke et al. 2007). In the Orphan Basin, the Upper Cretaceous succession sealed most of the syn-rift structures but did not cover the structural highs (i.e. horsts and tilted block ridges), which remained above sea level at that time (Figs. 6, 10, 11). Some of the west-dipping faults in the western part of the basin appear to have been reactivated during the Late Cretaceous (Figs. 6, 10, 11), probably in relation to the ongoing rifting to the north between Labrador and Greenland. It is only in the Palaeocene that the Orphan Basin evolves in a clearly post-rift setting, with a strong subsidence resulting in deposition of a thick Cenozoic succession over the rift structures.
Crustal extension and thinning in the Orphan Basin
The Moho and basement top in our interpretation of seismic line E (Fig. 12) indicate substantial variations in crustal thickness beneath the Orphan Basin. The crust is revealed to be significantly thin beneath the east and west sub-basins, and much thicker beneath the westernmost part of the continental shelf and the structural high separating the two sub-basins. The gravity modelling in this paper supports our interpretation of the seismic Moho as it shows that the observed free-air gravity anomalies can be explained by assuming a mid-crust boundary separating a low-density upper crust from a high-density lower crust (Fig. 14b). The obtained mid-crust boundary is consistent with the syn-rift structures imaged on the seismic data (Fig. 16). The syn-rift faults appear to be deeply rooted in the crust and sole out at the mid-crust boundary in West Orphan Basin and reach the Moho discontinuity in the East Orphan Basin where the crust is much thinner.
Our structural restoration (Fig. 17) shows that the observed brittle extension during rifting can explain only 60 % of the total thinning in the Orphan crust (i.e. in 2D along line E), assuming an initial pre-rift crustal thickness of 28 km—the maximum thickness observed beneath the continental shelf. The remaining crustal thinning is unlikely to be localized further offshore and should occur within the Orphan Basin because the crustal blocks surrounding the basin appear to be relatively undeformed—i.e. the Orphan Knoll and, especially, the Flemish Cap.
The discrepancy between recorded brittle extension and crustal thinning in the Orphan Basin is in accordance with the results of subsidence analysis. The latter shows that there is a large syn-rift subsidence deficit when we consider the amount of crustal thinning observed across the basin (Dafoe et al. 2015; Gouiza et al. 2015). According to Couiza et al. (2015), the syn-rift succession should be at least 1.5–2.5 km thicker in the Orphan Basin. This may suggest the existence of, in addition to the preserved brittle extension, another extension phase that took place early during the rift process (Early Jurassic or even Late Triassic?) which is not easily recognizable on seismic data. Reston (2007, 2009) attributes the extension/thinning discrepancy to unrecognized polyphase faulting which is very difficult to identify in seismic data due to limitations in seismic resolution with depth and structural overprinting. However, fault geometries, syn-rift stratigraphic architecture, and basement structures imaged in the examined seismic data do not provide solid evidence that polyphase rifting occurred in the Orphan Basin. Furthermore, Reston (2007) reckons that polyphase faulting is expected to occur in rifted margins where extension exceeds 100 % rather than in rift basins where crust is often less stretched (only 25 % of crustal extension in the Orphan Basin, along Line E). Alternatively, brittle extension in the Orphan Basin could be only partially preserved due to important crustal flexure, uplift, and erosion (Burov and Cloetingh 1997; Burov and Poliakov 2001). Well and seismic data presented above clearly indicate sub-aerial exposure and truncation of the syn-rift successions and tilted blocks (Enachescu et al. 2004; Dafoe et al. 2015). The existence of discontinuous pockets of Jurassic sediments in the West Orphan Basin might even be the remnants of a more extensive Jurassic or even Late Triassic (McAlpine 1990; Driscoll et al. 1995; Lau et al. 2006; Van Avendonk et al. 2009) syn-rift basin that have been eroded during the ensuing rift phase.
Another source of uncertainty in quantifying the pre-rift crust is inheritance. The Mesozoic North Atlantic rift formed on remnants of the Palaeozoic Caledonian–Appalachian Orogeny, where the closure of the Iapetus Ocean led to continental collision, crustal and probably lithospheric thickening (imbrication of multiple terranes), crustal uplift, and erosion (Dewey and Kidd 1974; Williams 1984, 1995; McKerrow et al. 2000). The existence of four distinct crustal domains (namely the East Orphan sub-basin, the structural high, the West Orphan sub-basin, and the continental shelf), showing different deformation styles and migration of rifting, suggests that lateral heterogeneities—variations in thickness and/or rheology inherited from the pre-Mesozoic Caledonian–Appalachian Orogeny—may have played a crucial role in the tectonic evolution of the Orphan Basin during, and following rifting (Smith and Mosley 1993; Vauchez et al. 1998; Huismans and Beaumont 2007; Gouiza et al. 2015; Chenin et al. 2015).
Conjugate Irish margin
Palaeoreconstructions of the North Atlantic Ocean to a time prior to lithospheric break-up and oceanic accretion (Chron M0; Fig. 1) show that the East and West Orphan sub-basins were juxtaposed against the Porcupine and the Rockall Basins, respectively, which are presently located on the conjugate Irish margin (Knott et al. 1993; Louden et al. 2004). These three basins probably evolved as one large extensional system during the Mesozoic rifting (Lundin and Doré 2011; Lau et al. 2015) and should exhibit comparable stratigraphic, structural, and crustal syn-rift evolution.
Seismic data from the Porcupine and Rockall Basins indicate the presence of thick Late Palaeozoic to Cenozoic sedimentary successions (up to 10 km) overlying faulted blocks caused by extensive crustal stretching and suggest a multiphase Mesozoic syn-rift history (Mackenzie et al. 2002; Morewood et al. 2005; O’Reilly et al. 2006), similar to our observations from the Orphan Basin. The Porcupine Basin, where the syn-rift succession is well defined, shows three rift episodes of Permo-Triassic, Late Jurassic, and mid-Cretaceous age which were separated by periods of uplift and erosion (Morewood et al. 2005). The major episode of brittle extension is believed to be Jurassic, which is consistent with westward migration of the loci of rifting described in the Orphan Basin. Like in the Orphan Basin, the deep structures of the Porcupine and Rockall Basins are characterized by thick crust (25–30 km) beneath the basin-bounding highs and highly thinned crust below the basins (2–10 km) (Morewood et al. 2005; O’Reilly et al. 2006). The Moho beneath the two basins shows a very pronounced asymmetry that is less evident in the Orphan Basin, but we do observe a lateral offset between areas of main upper crustal faulting and areas of extreme lower crustal thinning, especially in the western sub-basin (Fig. 12). The major difference between the two conjugate margins is revealed by the various velocity models which show strong evidence of low-velocity serpentinized mantle beneath the highly thinned crust in the Porcupine and Rockall Basins, but not in the Orphan Basin (Chian et al. 2001; Morewood et al. 2005; O’Reilly et al. 2006; Watremez et al. 2015; Lau et al. 2015).
The Orphan Basin, located offshore of the Newfoundland rifted margin, is a broad deepwater rift basin, characterized by thick Mesozoic to Cenozoic sedimentary successions which overlie a highly thinned continental crust.
The deep seismic reflection data used in this work clearly demonstrate the existence of strong brittle deformation coupled with a high degree of crustal thinning. Sedimentary successions, preserved in the Orphan Basin, suggests a two-phase rifting during the Jurassic and the Early Cretaceous, respectively, separated by periods of uplift and erosion. Upper crustal extension initiated in the east sub-basin in the Jurassic and spread to the west sub-basin in the Early Cretaceous. The latter experienced a late phase of deformation that reactivated the syn-rift structures and that was likely related to the contemporaneous rifting to the north in the Labrador Sea.
However, the interpreted structural and crustal architectures in the Orphan Basin suggest that the observed brittle extension does not fully match the quantified crustal thinning. An Early Jurassic, or even Late Triassic, brittle rift phase and localized thinning by ductile flow in the lower crust are proposed as additional processes that could have contributed to continental deformation in the basin.
This work is a contribution to the project of Plate Reconstruction of the North Atlantic between Ireland and Canada, part of the work programme of the North Atlantic Petroleum Systems Assessment initiated by the Petroleum Infrastructure Programme in Ireland and Memorial University of Newfoundland. The research is funded by Nalcor Energy, Husky Energy and the Natural Sciences and Engineering Research Council of Canada. Seismic data are shown courtesy of TGS. Midland Valley Exploration provided the academic licence for the Move software, and Schlumberger provided the licence for the Petrel software. We are grateful to Garry Karner, Harm Van Avendonk, Tiago M. Alves, and Tim Minshull for reviewing earlier versions of the manuscript and for their constructive comments.
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