Episodic Cenozoic tectonism and the development of the NW European ‘passive’ continental margin
Introduction
Atlantic-type continental margins are described as passive due to their relative tectonic quiescence, following a syn-rift period of active faulting and magmatism (see Bond and Kominz, 1988). The archetypes are the margins bordering the North Atlantic Ocean, the post-rift successions of which record an overall pattern of decaying tectonic subsidence (e.g. Sleep, 1971, Steckler and Watts, 1978, Keen, 1979, Barton and Wood, 1984) that is consistent with a simple 1D model of cooling from a thermal anomaly introduced by stretching (thinning) of the lithosphere (McKenzie, 1978, Jarvis and McKenzie, 1980). The success of the ‘stretching and cooling’ model in explaining the first-order decay of subsidence with time has in many ways come to shape our idea of how Atlantic-type margins should behave. Thus, it is now standard practice to use the predicted rates of decay of subsidence to fit or even invert backstripped data, on the implicit assumption that the post-rift tectonic history is entirely determined by the amount and rate of syn-rift extension (eg. White, 2003).
However, Atlantic-type continental margins exhibit behaviour that is not explained solely by cooling of the lithosphere (e.g. Keen, 1985, Keen, 1987, Keen and Beaumount, 1990), notably differential vertical movements corresponding to episodes of uplift and/or subsidence. The archetypes for such epeirogenic movements are, again, the North Atlantic margins, where departures from the predicted decay of thermal subsidence were first observed in backstripped wells off eastern Canada (Keen, 1979, Royden and Keen, 1980, Gradstein and Srivastava, 1980) and the eastern US (Heller et al., 1982). The latter authors identified a stepwise pattern of decaying Cretaceous-Cenozoic subsidence, in which phases of slow subsidence or uplift (lasting 20–30 Ma) were separated by three shorter episodes (<10–15 Ma) of rapid subsidence, up to hundreds of metres in amplitude, that together account for 50–95% of the post-rift total (Heller et al., 1982). The subsidence steps have been linked by some authors to the generation of regional unconformities and inferred to coincide with stages in the (poorly dated) uplift of eastern North America (e.g. Poag and Sevon, 1989, Poag, 1992), as have younger Cenozoic subsidence steps observed to the north on the Canadian margin (e.g. Gradstein and Srivastava, 1980, Grant, 1980; see Gradstein et al., 1990). Subsidence steps correlative to those on the Canadian margin have also been recognised on the NE Atlantic margins of Greenland and NW Europe (e.g. Cloetingh et al., 1990, Joy, 1992, Hall and White, 1994, Gradstein et al., 1994), where basinal subsidence was accompanied by the rise of onshore and shallow shelf areas (see Fig. 1) during two main episodes of Cenozoic uplift (see Japsen and Chalmers, 2000, Doré et al., 2002). The episodic character of Cenozoic uplift and subsidence is also reflected in the stratigraphic successions of the NW European margin, which contain regional shelf-to-deep-water unconformities inferred to mark tectonically driven changes in sedimentation and ocean circulation (e.g. Jordt et al., 1995, Stoker, 1997, Jordt et al., 2000, Andersen et al., 2000, Stoker et al., 2002, Stoker et al., 2005a).
Evidence of post-rift epeirogenic movements is also available from other Atlantic-type margins, via studies that have integrated onshore analyses of exhumation with offshore analyses of subsidence and stratigraphic development. One or more post-rift movements, typically involving some combination of uplift and subsidence linked to regional changes in sedimentary architecture, have been recognised across the margins of eastern and western Africa (Summerfield, 1985, Partridge and Maud, 1987, Walgenwitz et al., 1992, Rasmussen, 1996, Lavier et al., 2001) and of eastern South America (Brown et al., 2000, Cobbold et al., 2001) as well as of Arctic Canada and Greenland (Harrison et al., 1999, McNeil et al., 2001). Such studies are consistent with longstanding geomorphological evidence for the Cenozoic epeirogenic uplift of Atlantic-type margins, most of which exhibit broad topographic elevations referred to as greater or lesser escarpments (e.g. Ollier, 1985a, Ollier, 1985b, Cox, 1989, Summerfield, 1991), and provide support for persistent proposals that the major stratigraphic sequence along such margins record some form of tectonic control (e.g. Sloss and Speed, 1974, Bond, 1978, Cloetingh, 1986, Burgess and Gurnis, 1995).
If the occurrence of epeirogenic movements on Atlantic-type continental margins is beyond dispute, the same cannot be said of their causes. A variety of mechanisms have been put forward to account for permanent or transient intra-plate movements (e.g. Keen and Beaumount, 1990, Ollier, 1985b, Summerfield, 1991), most of which have been proposed for, or applied to, the North Atlantic margins (see Doré et al., 2002). In general, mechanisms driven by vertical interactions of the lithosphere with mantle flow may be distinguished from those driven by horizontal motions of the plates. Mantle–lithosphere interactions have been considered in one of two conceptual frameworks, whole mantle plumes versus small-scale (upper mantle) convection (see Anderson, 2001, Foulger, 2002). The problematic causes of epeirogenic movements are of wide interest, as they are central to the unresolved problem of how the mantle interacts with the lithosphere to generate plate motions (Bercovici, 2003).
The interaction of a mantle plume with rifting or rifted lithosphere may generate permanent or transient surface movements of up to km-scale via a variety of possible mechanisms, most formulated in the hypothetical context of a plume beneath the NE Atlantic region (White et al., 1987, White and McKenzie, 1989; cf. Foulger, 2002). Permanent movements have been postulated to take place due to syn-rift magmatic underplating (McKenzie, 1984) or to secondary convection induced by the syn- to early post-rift impingement of a plume head on colder sub-cratonic asthenosphere (Vågnes and Amundsen, 1993, Rohrman and van der Beek, 1996, Nielsen et al., 2002). Transient movements have been suggested to record dynamic topographic responses to regional variations in upwelling flow above a (broad) plume head (e.g. Nadin et al., 1997, White and Lovell, 1997, Jones et al., 2002) or migration of the plates relative to a (narrow) plume axis (e.g. Clift et al., 1995, Clift et al., 1998). In general, the various plume-driven mechanisms are ad hoc solutions, invoking otherwise unexplained changes in the behaviour of a plume that fortuitously acts on the lithosphere (see Foulger, 2002).
Plume models implicitly assume the upper mantle to be isothermal and static (e.g. White and McKenzie, 1989). An alternative view is that the upper mantle (<660 km) is organised from above by cooling, a situation that gives rise to small-scale convective cells controlled by thermal gradients, and so topography, at the base of the lithosphere (e.g. Anderson, 1998, Anderson, 2001, Korenaga and Jordan, 2002). Upper mantle convective flow is predicted to be periodic in space and episodic in time, with important thermo-mechanical and dynamic consequences for the uplift and subsidence history of rifts and continental margins (Keen, 1985, Keen, 1987, Buck, 1986, Boutillier and Keen, 1999). Upper mantle convection cells have been proposed to account for Cenozoic vertical movements of the North Atlantic margins (Keen, 1985, Keen, 1987, Vogt, 1991), including the Norwegian margin (Stuevold et al., 1992, Stuevold and Eldholm, 1996). In addition, the evolution of upper mantle convective flow during ocean opening has been proposed to be the underlying cause of episodic plate reorganisations (Vogt, 1991, Anderson, 1998, King et al., 2002).
Plate reorganisations generate variations in intra-plate stresses that are capable of causing transient flexural deflections across continental (or basin) margins (Cloetingh, 1986, Karner et al., 1993, van Balen et al., 1998), a mechanism that has been proposed to account for epeirogenic uplift and subsidence across Atlantic-type margins (Cloetingh et al., 1990, Cloetingh and Kooi, 1992). The predicted amplitudes of flexural deflection are rather low, however, as noted for the North Atlantic margins (e.g. Rohrman et al., 1995, Japsen and Chalmers, 2000, Nielsen et al., 2002). Tectonic deflections of whatever orgin may be amplified by load redistribution due to erosion and deposition, which can contribute to a history of surface movements, e.g. the retreat of a syn-rift topography (Gilchrist and Summerfield, 1990) or the erosion of basin margins in interaction with sea level variation (Nielsen et al., 2002). However, erosion and deposition result in low rates of deflection (e.g. Nielsen et al., 2002) and cannot themselves generate tectonic movements, despite suggestions that erosion, due to late Cenozoic climate change resulted in ‘illusory’ uplift (Molnar and England, 1990, Peizhen et al., 2001).
The differing mechanisms for intra-plate epeirogenesis have implications for the timing, magnitude and form of post-rift movements on Atlantic-type margins and our current inability to distinguish between them largely reflects our limited understanding of the movements themselves. On the NW European margin, the majority of studies have focused on one method (e.g. onshore uplift versus offshore subsidence) or area (e.g. Fennoscandia vs Britain and Ireland) or episode (e.g. early vs late Cenozoic), so that the overall pattern of post-rift movements remains uncertain (see Doré et al., 2002). Studies that have sought to link analyses of the uplift of onshore and shallow shelf areas with higher resolution records of subsidence and stratigraphy have yielded insights into the succession of Cenozoic epeirogenic movements (e.g. Riis and Feldskaar, 1992, Stuevold et al., 1992, Riis, 1996, Japsen, 1997, Japsen, 1998, Japsen and Chalmers, 2000). However, regional syntheses have been hampered by the lack of a stratigraphic framework to allow an accurate correlation of observed changes along the margin.
The purpose of this paper is to present an integration of onshore and offshore information on Cenozoic epeirogenic movements along the NW European margin, drawing on the results of the STRATAGEM project (Stoker et al., Stoker et al.). The focus is on the characterisation of a succession of epeirogenic movements, of varying character, recognised to be distinct from the effects of compressive tectonism (see Stoker et al., Stoker et al.). We begin by reviewing the syn- to post-rift setting of the North Atlantic and the methods used to characterise different types of post-rift movements. Quantitative analyses of tectonic uplift and subsidence along the NW European margin are then reviewed and correlated to the higher resolution records afforded by offshore stratigraphy, in order to document three episodes of km-scale epeirogenesis. Finally, the information on the inferred succession of epeirogenic movements is used to evaluate models of their causes in relation to plate motions and mantle convection.
Section snippets
Regional setting
The NE Atlantic continental margins are the products of a long history of late Palaeozoic through Mesozoic intracratonic extension that culminated in the Late Cretaceous to early Palaeogene separation of the European and North American plates (e.g. Ziegler, 1988, Doré et al., 1999). The complex physiography of the broad NW European margin (Fig. 1) is in large part a reflection of crustal thickness variations due to continental rifting and related magmatism, but also includes onshore and
Methods—types of tectonic movement and their measurement
Our interest here is to recognise different forms of tectonic movement, in order to describe their variation through time on the NW European margin. We first distinguish the effects of extensional and compressive deformation from two forms of epeirogenic movement (Fig. 2a,b), for which evidence will be presented in the following sections. The temporal and spatial resolutions of the three main techniques that have been used to obtain absolute and relative measures of differential vertical
Absolute measures of uplift and subsidence
Here, we review information available from analyses of uplift and subsidence along the NW European margin, in order to establish what is known about the magnitude and character of Cenozoic tectonic movements. Absolute measures of uplift and subsidence (in two main areas) are summarised schematically in Fig. 3. The timing of uplift is based in part on correlation to stratigraphic records, which provide high-resolution constraints on the timing and character of movements, presented in more detail
Tectono-stratigraphic indicators of relative movements
The absolute measures summarised above are indicative of three episodes of km-scale tectonic movement, comprising coeval uplift and subsidence (i.e. tilting) in the early and late Cenozoic and strongly differential subsidence (i.e. sagging) in the mid-Cenozoic (see Fig. 3). In this section, we show that each of these movements found expression in the stratigraphic record of changes in sedimentation patterns along the NW European margin. The stratigraphic record also contains evidence of two
A succession of Cenozoic epeirogenic episodes
Taken together, the absolute and relative measures of tectonic movement reviewed above show that the NW European margin has experienced three episodes of km-scale epeirogenic movement, in the early, mid- and late Cenozoic, corresponding to a history of tilting, sagging and tilting (Fig. 6). The succession of three episodes has not previously been recognised, but all are recorded in analyses of tectonic uplift and/or subsidence (see Fig. 3) and are inferred to have driven changes in the patterns
Discussion—causes of epeirogenic episodes
Epeirogenic movements on Atlantic-type margins have been explained in three different ways: as transient flexural responses to (unexplained) plate reorganisations, as permanent or transient anomalies attributable to mantle plumes (unconnected to plate motions) or as transient expressions of upper mantle convection (coupled to plate reorganisations). Here we argue that the Cenozoic succession of epeirogenic tilting, sagging and tilting observed on the NW European margin (Fig. 6) cannot be
Conclusions
Integration of absolute measures of tectonic uplift and subsidence with higher resolution stratigraphic indicators of relative movements shows that the NW European ‘passive’ continental margin has experienced two types of epeirogenic movement, each involving vertical tectonic displacements >1 km in amplitude but varying in wavelength: tilting (coeval uplift and subsidence across hundreds of kilometres, rotations of <1°) and sagging (strongly differential subsidence across <100 km, rotations of
Acknowledgements
This work forms part of the EC-supported project STRATAGEM (Stratigraphic Development of the Glaciated European Margin), funded through the 5th Framework Programme (contract number EVK3-CT-1999-00011). The manuscript was improved by the constructive comments of two referees, Erik Lundin and Peter Japsen.
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