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Comments on ‘New insight into the structure of the Nuussuaq Basin, central West Greenland’ from Chalmers, Pulvertaft, Marcussen and Pedersen (Marine and Petroleum Geology, 1999, 16, 197–234)
Marine and Petroleum Geology 18 (2001) 947±952
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Comments on `New insight into the structure of the Nuussuaq Basin, central West Greenland' from Chalmers, Pulvertaft, Marcussen and Pedersen (Marine and Petroleum Geology, 1999, 16, 197±234) L. Geoffroy*, J.P. GeÂlard, R. AõÈte, G. CattaneÂo Laboratoire de GeÂodynamique des Rifts et des Marges Passives, UFR Sciences. Universite du Maine EA 3669, Avenue O. Messiaen, 72085 Le Mans Cedex 09, France Received 28 February 2001; accepted 27 June 2001
We aim at discussing the views of Chalmers, Pulvertaft, Marcussen and Pedersen (1999) on the structure of the Nuussuaq Basin, West Greenland. This relatively late comment follows a recent ®eld trip to this area which persuaded us that some of the points developed by Chalmers, Pulvertaft, Marcussen and Pedersen (1999) could lead to confusion in the tectonic and geodynamic understanding of the entire region. Central±west Greenland is an area of crucial interest for both academic research and the oil industry. Between latitudes 698N and 728N (Fig. 1 in Chalmers, Pulvertaft, Marcussen & Pedersen, 1999), we ®nd one of the few places in the world where we can study the onshore transition in time and space between a rifted sedimentary basin and the proximal part of a volcanic-type margin (Geoffroy, GeÂlard, Lepvrier, & Olivier, 1998; Geoffroy, Skuce, Angelier, GeÂlard, Lepvrier, & Olivier, 1999; Geoffroy et al., 2001). In addition, the West-Greenland Mesozoic basins are potential targets for active oil exploration (Christiansen, Bates, Dam, Marcussen, & Pulvertaft, 1996). In particular, the Nuussuaq sedimentary basin comprises Early Cretaceous to Middle Palaeocene deltaic to pro-deltaic formations, with a transition from ¯uviatile-type sedimentation in the south to marine facies in the north (Henderson, Schiener, Risum, Croxton, & Andersen, 1980). These deposits are overlain by basalts, which are ¯at-lying inland and seaward-dipping along the coast, marking a conspicuous seaward-dipping ¯exure (see Fig. 1) whose structure and tectonic development are debated in Geoffroy et al. (2001). The lower basaltic formations are dated as Paleocene, whereas part of the upper basalts are Eocene and appear to accompany the lithospheric break-up between Canada and Greenland. * Corresponding author. Tel: 133-02-4383-3522; fax: 133-02-43833795. E-mail address: [email protected] (L. Geoffroy).
In their contribution to Marine and Petroleum Geology, Chalmers et al. (1999) investigated the structure of the sedimentary basin and the overlying basalts in the Disko± Nuussuaq area using available seismic-re¯ection, gravity, magnetic and onshore data. They present a map of faults that controlled and accompanied the Late Mesozoic and Paleocene sedimentation. Most of these faults would be of N-type, dipping oceanward and striking ca. NS and WNW± ESE. Chalmers et al. (1999) also investigated the structure of the basalts overlying the sedimentary formations. The Itilli Fault is a ®rst-order structure of the area, located west of the Nuussuaq peninsula. Chalmers et al. (1999) claim that the Itilli Fault is a relatively recent left-lateral strike-slip fault of Tertiary age related to the Ungava transform, that connects the Labrador Sea to Baf®n Bay. They state on p. 222 that this fault ªwas a splay from the Ungava strike-slip system and, as such, was only active after sea¯oor spreading startedº. Concerning the Tertiary magmatic evolution, they conclude that extension coeval with basalt formation was mainly carried out by oceanward-dipping normal faults which are located both offshore and onshore (their Figs. 1 and 18). It may be inferred from Chalmers et al.'s analysis that both the location and geometry of the Tertiary extension in the West-Greenland area is similar to the pattern associated with the formation of the Mesozoic sedimentary basin. Finally, Chalmers et al. (1999) present a regional synthesis where they place the main faults of the studied area in the geodynamic setting of separation between Greenland and Canada (their Fig. 18). We cannot contest the usefulness of any attempt at de®ning the complex structure of the West-Greenland area. However, Chalmers et al. (1999) failed to determine any trend of extension or slip on the faults, choosing to ignore a recent quantitative study of more than 800 faults with measured slip-displacements in the Tertiary succession of the same area, as well as structural observations in the lava pile (Geoffroy, GeÂlard, Lepvrier & Olivier, 1998).
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(01)00034-4
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L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
Fig. 1. Proposed 3D lava elevation model at Disko outlining the sharp transition between unstrained inland basalts and coastal ¯exured basalts. This ®gure is based mainly on the interpretation of GGU geological maps of the area. The ®gure shows the surface boundary of the base of the Maligat formation. We assume that the thickness of the Maligat basalts is everywhere constant. Taking a much more realistic seaward thickening of the basalts Ð although not yet quanti®able Ð would considerably increase the intensity of the coastal ¯exure itself.
However, if we consider these other data as well as the results of further investigations (Geoffroy, Skuce, Angelier, GeÂlard, Lepvrier & Olivier, 1999, Geoffroy et al., 2001), most of the assumptions or conclusions made by Chalmers et al. on the Tertiary structure and dynamics appear fundamentally ¯awed and require extensive comment. In the following, we restrict our criticisms to three particular points in Chalmers et al. (1999): (1) the geophysical method, (2) the Tertiary structure and evolution of the area, (3) the interpretation of the Itilli Fault. Finally, we present the key characteristics and importance of the area, points which do not come out in Chalmers et al's study (1999). (1) Chalmers et al. (1999) base most of their structural interpretations on the inversion of mixed offshore and onshore gravity data. They freely admit that their general crustal-scale modelling was `dif®cult and ambiguous'. We agree with them on these two points. Their gravity model fails in three respects, leading to dangerously equivocal results. Firstly, they take a homogeneous continental crust as reference. Although they recognise that the presence of magma plumbing in the crust could modify their conclusions, they do not consider this effect in their modelling. However, strong across-strike and along-strike density gradients clearly exist in the transitional (?) crust underlying the basaltic formations, greatly modifying the proposed models. In most eroded basements of plume-related areas, especially along break-up axes, the continental crust is crosscut by up to 50% in volume of cumulated dykes and hypovolcanic intrusions (see, for example Myers, 1981). Secondly, high-intensity sub-circular gravity anomalies are a frequent feature in the North-Atlantic area, linked to the emplacement of the Thulean plume head below the
continental lithosphere. These anomalies are usually coincident with central intrusive complexes, even when covered by more recent lavas or sediments (e.g. Bott & Tuson, 1973; McQuillin, Bacon, & Binns, 1975; Evans, Abraham, & Hitchen, 1989). Chalmers et al. (1999) do not consider this hypothesis at all in their modelling, although circular anomalies with maxima up to 80 mgals exist in their studied area (i.e. west of Nuussuaq, see their Fig. 12). Finally, and generally speaking, the intense extrusion of Mg- and Fe-rich magmas during the break-up of Greenland and Canada was probably associated with signi®cant `underplating' at the Moho. In addition, there were possible lateral and alongaxis variations in the thickness of the underplated material (e.g. Schlindwein & Jokat, 1999). This magma underplating may explain part of the uplift of the area during the Paleogene (see, for example, White & Lovell, 1997). Whether taken together or separately, these credible hypotheses cast doubt on the models and cross-sections proposed by Chalmers et al. (1999). (2) From the tectonic point of view, Chalmers et al. (1999) make no distinction between extensional strain in the inland area with horizontal basalts and the coastal area with the seaward-dipping ¯exure. On the contrary, they merely draw attention to the few seaward-dipping faults that can be observed inland cross-cutting the horizontal basaltic pile (their Fig. 4). Only these faults are considered as marking the Tertiary extension in West Greenland during the separation from NAM during the Paleogene (their Fig. 18). In addition, they make no real distinction between the geometry of the Mesozoic and the Tertiary tectonic extension (see cross-section on their Fig. 15). We strongly disagree with this way of presenting the Tertiary structure of this area. The coastal ¯exure at Disko (see Fig. 1) and
L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
949
Fig. 2. (A) Interpretative cross-section west of Disko. On this cross-section, fault throws are calculated by assuming that the thickness of basalts is constant. However, ®eld observations favour a seaward increase in thickness of the basalts. (B and C) Interpreted cross-sections west of Nuussuaq, respectively, assuming, west of the Itilli Fault, a constant thickness (B) or a seaward increase in thickness (C) of the lavas. For all cross-sections the offshore attitude of faults in lava is speculative and relate to structures observed onshore.
Nuussuaq is not as well developed as further north in Ubekendt±Ejland or Svartenhuk (see Geoffroy et al., 2001). Despite this, structural observations along the West-Greenland ¯exure clearly demonstrate that the seaward coastal ¯exure is (1) syn-tectonic, (2) partly synvolcanic, (3) coeval with NAM-Gr plate break-up, and (4) associated with signi®cant horizontal stretching (Geoffroy et al., 1998, 1999, 2001). The overall structure of the observed tectonic ¯exure everywhere resembles a rollover anticline associated with normal faults that are almost all continentward-dipping (see Geoffroy et al., 1999, Fig. 5), especially when the degree of seaward-¯exuring is high. Larsen & Pulvertaft (2000) contest the importance of the syn-volcanic part of the ¯exuring, and their analysis leaves out some of the major points of the structure (such as lowangle extensional faulting). Nevertheless, these authors have recently drawn a cross-section in the Svartenhuk
area (their Plate I) that can be readily interpreted in the same way as Geoffroy et al. (1999). Although we could not calculate the crustal stretching factor b with suf®cient accuracy in all areas, it may exceed a value of two in the coastal area due to penetrative continentward-dipping faulting on all scales in the basalts (e.g. Fig. 2A). This is similar to what is observed at the level of the East-Greenland coastal ¯exure (see Nielsen & Brooks, 1981). In any case, the value of b is proportional to the amount of ¯exuring of the margin (some seaward-dipping faults also exist in the proximal part of the ¯exure). In contrast, the ®nite b factor in the inland area is negligible, less than 1.01 on average (taking into account dykes and normal faults), and decreases signi®cantly from the bottom to the top of the lava pile. This contrasts strongly with the views of Chalmers et al. (1999) who only draw attention to the normal faults that crosscut the ¯at-lying horizontal basalts inland. With the possible
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L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
Fig. 3. Two stages in evolution of rifting in the Nuussuaq basin. This ®gure is schematic and scales are not respected. Stage A is mainly Mesozoic, associated with classical syn-sedimentary rift-zone tectonics characterised by continent-ward dipping tilted blocks. Stage B is mainly Paleogene, partly syn-volcanic, and associated with (1) the seaward shift of crustal stretching, and (2) the development of tectonically driven coastal ¯exures.
exception of the Itilli Fault (see below), most of the inland Tertiary faults recognised by Chalmers et al. (1999) represent moderate reactivation of the major underlying Mesozoic structures during the earliest Cenozoic. Most of these faults are capped over by the Paleocene±Eocene Maligat basaltic formations whose extrusion accompanies most of the coastal ¯exure during the Eocene. We illustrate this last point by the cross-sections through Northern Disko and Nuussuaq (see Fig. 2), which are based on a combination of published geological maps and our own ®eld observations. (3) Although the Itilli Fault is the major structure of their study area, Chalmers et al. (1999) offer an interpretation that
we consider obscure and erroneous. First of all, the Itilli Fault cannot be easily explained as a post-C24 splay of the ªUngava transform faultº during the Eocene. Indeed, on kinematic grounds, it is dif®cult not to regard the Ungava area as a simple bend of the Labrador±Baf®n opening axis during the Late Mesozoic and the Palaeocene (Geoffroy et al., 2001). This area of oblique opening, whose trend was probably inherited, was later reactivated during the Eocene due to N±S opening at the Davis Strait, then becoming a transpressive brittle±ductile domain. Secondly, the post-C24 opening axes in Baf®n Bay and the Labrador Sea were located to the west and southwest of the Disko±Svartenhuk area. Therefore,
L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
contrary to Chalmers et al., the eventual splays of transform faults belonging to the opening system were evidently inactive during the Tertiary (i.e. during probable oceanic accretion) in the considered area. Thirdly, the fault-slip measurements and fault geometry in the Paleocene±Eocene basalts west of the Itilli Fault (Geoffroy et al., 2001, Fig. 7) clearly indicate a dominant N-type stress regime (and not S-type) in Nuussuaq during the transpressive strike-slip activity on the Itilli Fault postulated by Chalmers et al. Therefore, it is quite dif®cult to accept that the Itilli Fault could be the only newly formed strike-slip fault initiated under this putative S-type regional stress regime. Lastly, Chalmers et al. (1999) propose a Tertiary syn-volcanic dip-slip component of up to 8 km for the Itilli Fault, a hypothesis dif®cult to reconcile with their views on the transpressive nature of this major discontinuity. Adopting Chalmers et al's (1999) hypothesis for the Itilli Fault, the component of extensional oblique-slip on this structure should be negligible (even nil, considering the text and ®gures), let us say less than 108. Taking 108 as a maximum, and assuming a ®nite vertical throw of 8 km, we obtain a horizontal displacement of more than 45 km along this fault, which seems absurd in view of the regional geology. For all these reasons, it is much more likely that the Itilli Fault represents an older inherited structure reactivated several times during the Mesozoic and Cenozoic. Its major component was dip-slip during the Tertiary. However, we cannot rule out the existence of a small component of extensional sinistral shear on the inherited Itilli Fault during the Eocene. This is because of a small deviation of the extension in WNuussuaq, trending ca. WNW±ESE, i.e. away from the perpendicular to the fault trend (for quantitative data see Geoffroy et al., 2001). It should also be noted that the general interpretation of the NW±SE cross-section in Fig. 15 of Chalmers et al. (1999) is quite surprising. Following Chalmers et al. (1999), the westward-dipping strike-slip Itilli Fault bounds a major eastward-dipping and volcanic®lled extensional crustal block to the east. This block contains up to 8±10 km of syn-tectonic lavas whose thickness increases towards the fault. This con¯icts with the average westward and northwestward dip of the lavas west of the fault. Entirely different interpretations are possible, such as those proposed in Fig. 2B and C. These assume either a constant thickness (Fig. 2B) or a more probable westward increase in the thickness of the lavas (Fig. 2C). Geoffroy et al. (1998, 1999), as well as, more extensively, Geoffroy et al. (2001) have outlined the clear analogy in structure and development between the West Greenland seaward coastal ¯exure (see Fig. 1) and the East Greenland coastal ¯exure (e.g. Nielsen & Brooks, 1981). This latter represents the most internal part (emergent continental domain) of the North-Atlantic volcanic margin. However, (1) there remain doubts on the existence of seaward-dipping re¯ectors in the offshore South-Baf®n area, and (2) the WGreenland area is complicated by the presence of large inherited structures (such as the Itilli Fault and, more gener-
951
ally, the Ungava bend). Nonetheless, this area clearly represents a volcanic-type rift or passive margin that formed during NAM±Greenland plate separation probably above a mantle plume. As such, the W-Greenland ¯exure (see Fig. 1) deserves particular attention because it likely re¯ects syn-magmatic mechanisms in the most internal part of the continental domain during break-up (like the E-Greenland coastal ¯exure, see Karson & Brook, 1999). Note that, at a VPM, the internal seaward-dipping volcanic prism (associated at depth with a continental to transitional-type crust) passes seaward to offshore-located SDRS prism(s) or true oceanic crust (sometimes through an area of more or less tabular basalts). This pattern has been highlighted by several authors and might well represent a rule rather than an exception (e.g. Barton & White, 1997; Tard, Masse, Walgenwik & Gruneisen, 1997; Bauer et al., 2000; Geoffroy, 2001). In West Greenland, we can observe the evolution in time and space from a sedimentary basin (mostly predating any mantle plume decompression) to a volcanic-type passive margin. Although we cannot develop this point further here, the W-Greenland area shows one of the major features of such an evolution, i.e. a change in both location and tectonic style of the continental extension associated with the weakening of the lithosphere by a probable mantle plume. The extension began between Baf®n Island and Greenland during the Mesozoic and Early Paleocene in a broad area with a classical pattern of tilted blocks associated with dominantly seaward-dipping N-type faults (see Fig. 3A). This extension shifted `oceanward' in the Late Paleocene and Eocene, during the extrusion of basalts coeval with the de®nitive break-up between Canada and Baf®n Island, as demonstrated by (1) the capping over of most of the Mesozoic faults by basalts and (2) by the concentration of extensional strain along the coastal area (see Fig. 3B). The style of extension itself then changes dramatically, with tectonically driven coastal seaward-¯exures accommodated by continentward-dipping faults (see Fig. 3B). None of these aspects are debated in Chalmers et al. (1999). References Barton, A. J., & White, R. S. (1997). Volcanism on the Rockall continental margin. Journal of the Geological Society, London, 154, 531±536. Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K., Schulze, A., Trumbull, R. B., & Weber, K. (2000). Deep structure of the Namibia continental margin as derived from integrated geophysical studies. Joumal of Geophysical Research, 105, 25829±25853. Bott, M. H. P., & Tuson, J. (1973). Deep structure beneath the Tertiary Volcanic regions of Skye, Mull and Ardnarmurchan. Nature, 242, 114± 116. Christiansen, F. G., Bates, K. J., Dam, G., Marcussen, C., & Pulvertaft, T. C. R. (1996). Continued geophysical and petroleum geological activities in West Greenland in 1995 and the start of onshore exploration. Bulletin Gronlands Geologiske Undersogelse, 172, 15±21. Chalmers, J. A., Pulvertaft, T. C. R., Marcussen, C., & Pedersen, A. K. (1999). New insight into the structure of the Nuussuaq Basin central West Greenland. Marine and Petroleum Geology, 16, 197±224.
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Evans, D., Abraham, D. A., & Hitchen, K. (1989). The Geikie igneous center, west of Lews, its structure and in¯uence on Tertiary geology. Scottish Journal of Geology, 25, 339±352. Geoffroy, L., Callot, J. P., Scaillet, S., Skuce, A., GeÂlard, J. P., Ravilly, M., Angelier, J., Bonin, B., Cayet, C., Perrot, K., & Lepvrier, C. (2001). The SE-Baf®n volcanic margin and the NAM-Greenland plate separation. Tectonics, 20, 566±584. Geoffroy, L., GeÂlard, J. P., Lepvrier, C., & Olivier, P. (1998). The coastal ¯exure of Disko (West Greenland), onshore expression of the `oblique re¯ectors'. Journal of the Geological Society, London, 155, 463±473. Geoffroy, L., Skuce, A. G., Angelier, J., GeÂlard, J. P., Lepvrier, C., & Olivier, P. (1999). Discussion on the coastal ¯exure of Disko (West Greenland), onshore expression of the `oblique re¯ectors': reply. Journal of the Geological Society, London, 165, 1051±1055. Henderson, G., Schiener, E., Risum, J. B., Croxton, C. A., & Andersen, B. B. (1980). Geology of the North Atlantic Borderlands. In J. W. Kerr & A. J. Fergusson, Memoir of the Canadian Society of Petroleum Geologists, 7, (pp. 399±429). Karson, J. A., & Brooks, K. (1999). Structural and magmatic segmentation of the Tertiary East Greenland volcanic rifted margin. In P. Ryan & C. MacNiocaill, J.F. Dewey Volume on Continental Tectonics (pp. 313± 318). London: Geological Society Special Publication.
Larsen, J. G., & Pulvertaft, T. C. R. (2000). The structure of the Cretaceous±Paleogene sedimentary±volcanic area of Svartenhuk Halvo, central West Greenland. Geology of Greenland Survey Bulletin, 188, 1±39. McQuillin, R., Bacon, M., & Binns, P. H. (1975). The Blackstones Tertiary igneous complex. Scottish Journal of Geology, 3, 179±191. Myers, J. S. (1981). Structure of the coastal dyke swarm and associated plutonic intrusions of East-Greenland. Earth and Planetary Sciences Letters, 46, 407±418. Nielsen, T. F. D., & Brooks, C. K. (1981). The East Greenland rifted continental margin: an examination of the coastal ¯exure. Journal of the Geological Society, London, 138, 559±568. Schlindwein, V., & Jokat, W. (1999). Structure and evolution of the continental crust of the east Greenland from integrated geophysical studies. Journal of Geophysical Research, 104, 15227±15245. Tard, F., Masse, P., Walgenwik, F., & Gruneisen, P. (1991). The volcanic passive margin in the vicinity of Aden, Yemen. Bulletin Centres Recherche et d'Exploration Production Elf-Aquitaine, 15, 1±9. White, R. S., & Lovell, B. (1997). Measuring the pulse of a plume from sedimentary records. Nature, 387, 888±891.
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Comments on `New insight into the structure of the Nuussuaq Basin, central West Greenland' from Chalmers, Pulvertaft, Marcussen and Pedersen (Marine and Petroleum Geology, 1999, 16, 197±234) L. Geoffroy*, J.P. GeÂlard, R. AõÈte, G. CattaneÂo Laboratoire de GeÂodynamique des Rifts et des Marges Passives, UFR Sciences. Universite du Maine EA 3669, Avenue O. Messiaen, 72085 Le Mans Cedex 09, France Received 28 February 2001; accepted 27 June 2001
We aim at discussing the views of Chalmers, Pulvertaft, Marcussen and Pedersen (1999) on the structure of the Nuussuaq Basin, West Greenland. This relatively late comment follows a recent ®eld trip to this area which persuaded us that some of the points developed by Chalmers, Pulvertaft, Marcussen and Pedersen (1999) could lead to confusion in the tectonic and geodynamic understanding of the entire region. Central±west Greenland is an area of crucial interest for both academic research and the oil industry. Between latitudes 698N and 728N (Fig. 1 in Chalmers, Pulvertaft, Marcussen & Pedersen, 1999), we ®nd one of the few places in the world where we can study the onshore transition in time and space between a rifted sedimentary basin and the proximal part of a volcanic-type margin (Geoffroy, GeÂlard, Lepvrier, & Olivier, 1998; Geoffroy, Skuce, Angelier, GeÂlard, Lepvrier, & Olivier, 1999; Geoffroy et al., 2001). In addition, the West-Greenland Mesozoic basins are potential targets for active oil exploration (Christiansen, Bates, Dam, Marcussen, & Pulvertaft, 1996). In particular, the Nuussuaq sedimentary basin comprises Early Cretaceous to Middle Palaeocene deltaic to pro-deltaic formations, with a transition from ¯uviatile-type sedimentation in the south to marine facies in the north (Henderson, Schiener, Risum, Croxton, & Andersen, 1980). These deposits are overlain by basalts, which are ¯at-lying inland and seaward-dipping along the coast, marking a conspicuous seaward-dipping ¯exure (see Fig. 1) whose structure and tectonic development are debated in Geoffroy et al. (2001). The lower basaltic formations are dated as Paleocene, whereas part of the upper basalts are Eocene and appear to accompany the lithospheric break-up between Canada and Greenland. * Corresponding author. Tel: 133-02-4383-3522; fax: 133-02-43833795. E-mail address: [email protected] (L. Geoffroy).
In their contribution to Marine and Petroleum Geology, Chalmers et al. (1999) investigated the structure of the sedimentary basin and the overlying basalts in the Disko± Nuussuaq area using available seismic-re¯ection, gravity, magnetic and onshore data. They present a map of faults that controlled and accompanied the Late Mesozoic and Paleocene sedimentation. Most of these faults would be of N-type, dipping oceanward and striking ca. NS and WNW± ESE. Chalmers et al. (1999) also investigated the structure of the basalts overlying the sedimentary formations. The Itilli Fault is a ®rst-order structure of the area, located west of the Nuussuaq peninsula. Chalmers et al. (1999) claim that the Itilli Fault is a relatively recent left-lateral strike-slip fault of Tertiary age related to the Ungava transform, that connects the Labrador Sea to Baf®n Bay. They state on p. 222 that this fault ªwas a splay from the Ungava strike-slip system and, as such, was only active after sea¯oor spreading startedº. Concerning the Tertiary magmatic evolution, they conclude that extension coeval with basalt formation was mainly carried out by oceanward-dipping normal faults which are located both offshore and onshore (their Figs. 1 and 18). It may be inferred from Chalmers et al.'s analysis that both the location and geometry of the Tertiary extension in the West-Greenland area is similar to the pattern associated with the formation of the Mesozoic sedimentary basin. Finally, Chalmers et al. (1999) present a regional synthesis where they place the main faults of the studied area in the geodynamic setting of separation between Greenland and Canada (their Fig. 18). We cannot contest the usefulness of any attempt at de®ning the complex structure of the West-Greenland area. However, Chalmers et al. (1999) failed to determine any trend of extension or slip on the faults, choosing to ignore a recent quantitative study of more than 800 faults with measured slip-displacements in the Tertiary succession of the same area, as well as structural observations in the lava pile (Geoffroy, GeÂlard, Lepvrier & Olivier, 1998).
0264-8172/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0264-817 2(01)00034-4
948
L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
Fig. 1. Proposed 3D lava elevation model at Disko outlining the sharp transition between unstrained inland basalts and coastal ¯exured basalts. This ®gure is based mainly on the interpretation of GGU geological maps of the area. The ®gure shows the surface boundary of the base of the Maligat formation. We assume that the thickness of the Maligat basalts is everywhere constant. Taking a much more realistic seaward thickening of the basalts Ð although not yet quanti®able Ð would considerably increase the intensity of the coastal ¯exure itself.
However, if we consider these other data as well as the results of further investigations (Geoffroy, Skuce, Angelier, GeÂlard, Lepvrier & Olivier, 1999, Geoffroy et al., 2001), most of the assumptions or conclusions made by Chalmers et al. on the Tertiary structure and dynamics appear fundamentally ¯awed and require extensive comment. In the following, we restrict our criticisms to three particular points in Chalmers et al. (1999): (1) the geophysical method, (2) the Tertiary structure and evolution of the area, (3) the interpretation of the Itilli Fault. Finally, we present the key characteristics and importance of the area, points which do not come out in Chalmers et al's study (1999). (1) Chalmers et al. (1999) base most of their structural interpretations on the inversion of mixed offshore and onshore gravity data. They freely admit that their general crustal-scale modelling was `dif®cult and ambiguous'. We agree with them on these two points. Their gravity model fails in three respects, leading to dangerously equivocal results. Firstly, they take a homogeneous continental crust as reference. Although they recognise that the presence of magma plumbing in the crust could modify their conclusions, they do not consider this effect in their modelling. However, strong across-strike and along-strike density gradients clearly exist in the transitional (?) crust underlying the basaltic formations, greatly modifying the proposed models. In most eroded basements of plume-related areas, especially along break-up axes, the continental crust is crosscut by up to 50% in volume of cumulated dykes and hypovolcanic intrusions (see, for example Myers, 1981). Secondly, high-intensity sub-circular gravity anomalies are a frequent feature in the North-Atlantic area, linked to the emplacement of the Thulean plume head below the
continental lithosphere. These anomalies are usually coincident with central intrusive complexes, even when covered by more recent lavas or sediments (e.g. Bott & Tuson, 1973; McQuillin, Bacon, & Binns, 1975; Evans, Abraham, & Hitchen, 1989). Chalmers et al. (1999) do not consider this hypothesis at all in their modelling, although circular anomalies with maxima up to 80 mgals exist in their studied area (i.e. west of Nuussuaq, see their Fig. 12). Finally, and generally speaking, the intense extrusion of Mg- and Fe-rich magmas during the break-up of Greenland and Canada was probably associated with signi®cant `underplating' at the Moho. In addition, there were possible lateral and alongaxis variations in the thickness of the underplated material (e.g. Schlindwein & Jokat, 1999). This magma underplating may explain part of the uplift of the area during the Paleogene (see, for example, White & Lovell, 1997). Whether taken together or separately, these credible hypotheses cast doubt on the models and cross-sections proposed by Chalmers et al. (1999). (2) From the tectonic point of view, Chalmers et al. (1999) make no distinction between extensional strain in the inland area with horizontal basalts and the coastal area with the seaward-dipping ¯exure. On the contrary, they merely draw attention to the few seaward-dipping faults that can be observed inland cross-cutting the horizontal basaltic pile (their Fig. 4). Only these faults are considered as marking the Tertiary extension in West Greenland during the separation from NAM during the Paleogene (their Fig. 18). In addition, they make no real distinction between the geometry of the Mesozoic and the Tertiary tectonic extension (see cross-section on their Fig. 15). We strongly disagree with this way of presenting the Tertiary structure of this area. The coastal ¯exure at Disko (see Fig. 1) and
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Fig. 2. (A) Interpretative cross-section west of Disko. On this cross-section, fault throws are calculated by assuming that the thickness of basalts is constant. However, ®eld observations favour a seaward increase in thickness of the basalts. (B and C) Interpreted cross-sections west of Nuussuaq, respectively, assuming, west of the Itilli Fault, a constant thickness (B) or a seaward increase in thickness (C) of the lavas. For all cross-sections the offshore attitude of faults in lava is speculative and relate to structures observed onshore.
Nuussuaq is not as well developed as further north in Ubekendt±Ejland or Svartenhuk (see Geoffroy et al., 2001). Despite this, structural observations along the West-Greenland ¯exure clearly demonstrate that the seaward coastal ¯exure is (1) syn-tectonic, (2) partly synvolcanic, (3) coeval with NAM-Gr plate break-up, and (4) associated with signi®cant horizontal stretching (Geoffroy et al., 1998, 1999, 2001). The overall structure of the observed tectonic ¯exure everywhere resembles a rollover anticline associated with normal faults that are almost all continentward-dipping (see Geoffroy et al., 1999, Fig. 5), especially when the degree of seaward-¯exuring is high. Larsen & Pulvertaft (2000) contest the importance of the syn-volcanic part of the ¯exuring, and their analysis leaves out some of the major points of the structure (such as lowangle extensional faulting). Nevertheless, these authors have recently drawn a cross-section in the Svartenhuk
area (their Plate I) that can be readily interpreted in the same way as Geoffroy et al. (1999). Although we could not calculate the crustal stretching factor b with suf®cient accuracy in all areas, it may exceed a value of two in the coastal area due to penetrative continentward-dipping faulting on all scales in the basalts (e.g. Fig. 2A). This is similar to what is observed at the level of the East-Greenland coastal ¯exure (see Nielsen & Brooks, 1981). In any case, the value of b is proportional to the amount of ¯exuring of the margin (some seaward-dipping faults also exist in the proximal part of the ¯exure). In contrast, the ®nite b factor in the inland area is negligible, less than 1.01 on average (taking into account dykes and normal faults), and decreases signi®cantly from the bottom to the top of the lava pile. This contrasts strongly with the views of Chalmers et al. (1999) who only draw attention to the normal faults that crosscut the ¯at-lying horizontal basalts inland. With the possible
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L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
Fig. 3. Two stages in evolution of rifting in the Nuussuaq basin. This ®gure is schematic and scales are not respected. Stage A is mainly Mesozoic, associated with classical syn-sedimentary rift-zone tectonics characterised by continent-ward dipping tilted blocks. Stage B is mainly Paleogene, partly syn-volcanic, and associated with (1) the seaward shift of crustal stretching, and (2) the development of tectonically driven coastal ¯exures.
exception of the Itilli Fault (see below), most of the inland Tertiary faults recognised by Chalmers et al. (1999) represent moderate reactivation of the major underlying Mesozoic structures during the earliest Cenozoic. Most of these faults are capped over by the Paleocene±Eocene Maligat basaltic formations whose extrusion accompanies most of the coastal ¯exure during the Eocene. We illustrate this last point by the cross-sections through Northern Disko and Nuussuaq (see Fig. 2), which are based on a combination of published geological maps and our own ®eld observations. (3) Although the Itilli Fault is the major structure of their study area, Chalmers et al. (1999) offer an interpretation that
we consider obscure and erroneous. First of all, the Itilli Fault cannot be easily explained as a post-C24 splay of the ªUngava transform faultº during the Eocene. Indeed, on kinematic grounds, it is dif®cult not to regard the Ungava area as a simple bend of the Labrador±Baf®n opening axis during the Late Mesozoic and the Palaeocene (Geoffroy et al., 2001). This area of oblique opening, whose trend was probably inherited, was later reactivated during the Eocene due to N±S opening at the Davis Strait, then becoming a transpressive brittle±ductile domain. Secondly, the post-C24 opening axes in Baf®n Bay and the Labrador Sea were located to the west and southwest of the Disko±Svartenhuk area. Therefore,
L. Geoffroy et al. / Marine and Petroleum Geology 18 (2001) 947±952
contrary to Chalmers et al., the eventual splays of transform faults belonging to the opening system were evidently inactive during the Tertiary (i.e. during probable oceanic accretion) in the considered area. Thirdly, the fault-slip measurements and fault geometry in the Paleocene±Eocene basalts west of the Itilli Fault (Geoffroy et al., 2001, Fig. 7) clearly indicate a dominant N-type stress regime (and not S-type) in Nuussuaq during the transpressive strike-slip activity on the Itilli Fault postulated by Chalmers et al. Therefore, it is quite dif®cult to accept that the Itilli Fault could be the only newly formed strike-slip fault initiated under this putative S-type regional stress regime. Lastly, Chalmers et al. (1999) propose a Tertiary syn-volcanic dip-slip component of up to 8 km for the Itilli Fault, a hypothesis dif®cult to reconcile with their views on the transpressive nature of this major discontinuity. Adopting Chalmers et al's (1999) hypothesis for the Itilli Fault, the component of extensional oblique-slip on this structure should be negligible (even nil, considering the text and ®gures), let us say less than 108. Taking 108 as a maximum, and assuming a ®nite vertical throw of 8 km, we obtain a horizontal displacement of more than 45 km along this fault, which seems absurd in view of the regional geology. For all these reasons, it is much more likely that the Itilli Fault represents an older inherited structure reactivated several times during the Mesozoic and Cenozoic. Its major component was dip-slip during the Tertiary. However, we cannot rule out the existence of a small component of extensional sinistral shear on the inherited Itilli Fault during the Eocene. This is because of a small deviation of the extension in WNuussuaq, trending ca. WNW±ESE, i.e. away from the perpendicular to the fault trend (for quantitative data see Geoffroy et al., 2001). It should also be noted that the general interpretation of the NW±SE cross-section in Fig. 15 of Chalmers et al. (1999) is quite surprising. Following Chalmers et al. (1999), the westward-dipping strike-slip Itilli Fault bounds a major eastward-dipping and volcanic®lled extensional crustal block to the east. This block contains up to 8±10 km of syn-tectonic lavas whose thickness increases towards the fault. This con¯icts with the average westward and northwestward dip of the lavas west of the fault. Entirely different interpretations are possible, such as those proposed in Fig. 2B and C. These assume either a constant thickness (Fig. 2B) or a more probable westward increase in the thickness of the lavas (Fig. 2C). Geoffroy et al. (1998, 1999), as well as, more extensively, Geoffroy et al. (2001) have outlined the clear analogy in structure and development between the West Greenland seaward coastal ¯exure (see Fig. 1) and the East Greenland coastal ¯exure (e.g. Nielsen & Brooks, 1981). This latter represents the most internal part (emergent continental domain) of the North-Atlantic volcanic margin. However, (1) there remain doubts on the existence of seaward-dipping re¯ectors in the offshore South-Baf®n area, and (2) the WGreenland area is complicated by the presence of large inherited structures (such as the Itilli Fault and, more gener-
951
ally, the Ungava bend). Nonetheless, this area clearly represents a volcanic-type rift or passive margin that formed during NAM±Greenland plate separation probably above a mantle plume. As such, the W-Greenland ¯exure (see Fig. 1) deserves particular attention because it likely re¯ects syn-magmatic mechanisms in the most internal part of the continental domain during break-up (like the E-Greenland coastal ¯exure, see Karson & Brook, 1999). Note that, at a VPM, the internal seaward-dipping volcanic prism (associated at depth with a continental to transitional-type crust) passes seaward to offshore-located SDRS prism(s) or true oceanic crust (sometimes through an area of more or less tabular basalts). This pattern has been highlighted by several authors and might well represent a rule rather than an exception (e.g. Barton & White, 1997; Tard, Masse, Walgenwik & Gruneisen, 1997; Bauer et al., 2000; Geoffroy, 2001). In West Greenland, we can observe the evolution in time and space from a sedimentary basin (mostly predating any mantle plume decompression) to a volcanic-type passive margin. Although we cannot develop this point further here, the W-Greenland area shows one of the major features of such an evolution, i.e. a change in both location and tectonic style of the continental extension associated with the weakening of the lithosphere by a probable mantle plume. The extension began between Baf®n Island and Greenland during the Mesozoic and Early Paleocene in a broad area with a classical pattern of tilted blocks associated with dominantly seaward-dipping N-type faults (see Fig. 3A). This extension shifted `oceanward' in the Late Paleocene and Eocene, during the extrusion of basalts coeval with the de®nitive break-up between Canada and Baf®n Island, as demonstrated by (1) the capping over of most of the Mesozoic faults by basalts and (2) by the concentration of extensional strain along the coastal area (see Fig. 3B). The style of extension itself then changes dramatically, with tectonically driven coastal seaward-¯exures accommodated by continentward-dipping faults (see Fig. 3B). None of these aspects are debated in Chalmers et al. (1999). References Barton, A. J., & White, R. S. (1997). Volcanism on the Rockall continental margin. Journal of the Geological Society, London, 154, 531±536. Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K., Schulze, A., Trumbull, R. B., & Weber, K. (2000). Deep structure of the Namibia continental margin as derived from integrated geophysical studies. Joumal of Geophysical Research, 105, 25829±25853. Bott, M. H. P., & Tuson, J. (1973). Deep structure beneath the Tertiary Volcanic regions of Skye, Mull and Ardnarmurchan. Nature, 242, 114± 116. Christiansen, F. G., Bates, K. J., Dam, G., Marcussen, C., & Pulvertaft, T. C. R. (1996). Continued geophysical and petroleum geological activities in West Greenland in 1995 and the start of onshore exploration. Bulletin Gronlands Geologiske Undersogelse, 172, 15±21. Chalmers, J. A., Pulvertaft, T. C. R., Marcussen, C., & Pedersen, A. K. (1999). New insight into the structure of the Nuussuaq Basin central West Greenland. Marine and Petroleum Geology, 16, 197±224.
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