JAPSEN, PETER, JAMES A. CHALMERS*, Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark, and Lars Ole Boldreel, University of Copenhagen, Denmark.
Abstract: Late Cenozoic Vertical Movements around the North Atlantic
There appear to have been at least two significant episodes of uplift around the North Atlantic during the Cenozoic, and in many places it is not easy to separate the two. One episode, mostly in the Palaeogene, was probably caused by effects related to emplacement of the Iceland plume.The second episode took place in the late Cenozoic, and comprised uplift of basin margins as well as accelerated subsidence of basin centres.
Cenozoic uplift of Scandinavia and of the British Isles has been suggested since at least the beginning of the 20th century. However, it is only recently being recognised in the literature that a major Neogene tectonic event has affected nearly every continental margin in the area (including western and eastern Greenland) and far into the European craton (e.g., Cloetingh et al. 1990; Jensen et al. 1992; Japsen 1997; Lidmar-Bergström 1998; 1998). Accelerated late Cenozoic subsidence is well known in many offshore areas adjacent to the uplifted land-masses.
We have compiled a map (Fig.1) to show areas of Neogene uplift and areas of accelerated subsidence and/or substantial deposition around the North Atlantic. The map is based on more references than it is possible to cite in this abstract; for details see Japsen & Chalmers submitted. Pre-Cenozoic rocks are generally exposed onshore and the pre-Quaternary sediments offshore are generally of Neogene age. Especially revealing, we believe, are the marginal areas bordering the oceanic basins and the North Sea. In all these areas, Palaeogene sediments or volcanics lie either at outcrop or at subcrop to an unconformity at the base of the Pleistocene, and dip towards the basin centre.All of the Palaeogene outcrop/subcrop patterns that we have examined appear to be caused by structural relationships of the type described by Riis (1996) and Chalmers (in press).
Cross-sections of the uplifted/subsided areas are also revealing, since several of the uplifts appear to be asymmetric. Scandinavia, Scotland and the Faeroe Islands verge towards the west, i.e. towards the Atlantic. Whether the uplifted Greenland and North American margins do so too is not clear from existing data, but the fact that the high mountains in all these areas are not far from the coast may be a clue.The uplifted landmasses are commonly associated with pronounced late Cenozoic depocentres.
A variety of methods that have been used to investigate uplift, erosion and redeposition: studies of maximum burial, fission tracks, geomorphology, sediment supply and of structural relations.These methods each investigate only one aspect of the phenomenon, and a thorough understanding of the processes of uplift and erosion can only be achieved if results from these methods are integrated.
Late Cenozoic Vertical Movements around the North Atlantic
The main mechanisms suggested in the literature for the large-scale, late Cenozoic events are: emplacement of magma in and at the base of the crust leading to isostatic uplift, flow of astenospheric material into active diapirs, isostacy associated with glacial erosion, phase changes in the lithosphere due to pressure relief and regional compression of the lithosphere. A general model must be constrained by observations from all affected areas and must be.based on several methods rather than just one. It must also take into account that the fact that the effects reach well into the craton.
Chalmers, J.A. in press. Evidence for Neogene uplift offshore southern West Greenland and its relation to sea-floor spreading. Global and Planetary Change.
Cloetingh, S., Gradstein, F.M., Kooi, H., Grant,A.C. & Kaminski, M. 1990. Plate reorganization; a cause of rapid late Neogene subsidence and sedimentation around the North Atlantic. Journal of the Geological Society, London, 147, 495-506.
Japsen, P. 1997. Regional Neogene exhumation of Britain and the western North Sea. Journal of the Geological Society, London, 154, 239-247.
Japsen, P. 1998. Regional velocity-depth anomalies, North Sea Chalk: a record of overpressure and Neogene uplift and erosion. AAPG Bulletin, 82, 2031-2074.
Japsen, P. and Chalmers, J.A. submitted. Neogene uplift and tectonics around the North Atlantic: Overview. Submitted to Global and Planetary Change.
Jensen, L.N., Riis, F. & Boyd, R. (eds) 1992. Post-Cretaceous uplift and sedimentation along the western Fennoscandian shield. Norsk Geologisk Tidsskrift, 72.
Lidmar-Bergström, K. in press. Uplift histories revealed by landforms of the Scandinavian domes. Special Publications. Geological Society.
Riis, F. 1996. Quantification
of Cenozoic vertical movements of Scandinavia by correlation of morphological
surfaces with offshore data. Global & Planetary Change, 12, 331-357.
Figure 1. Pre- Quaternary geology around the North Atlantic with indication of areas of Neogene/ Late Cenozoic uplift/ erosion and of accelerated subsidence/ deposition (references given in Japsen & Chalmers, submitted). In the marginal areas bordering the oceanic basins and the North Sea, Palaeogene sediments or volcanics lie either at outcrop or at subcrop to an unconformity at the base of the Pleistocene, and dip towards the basin centre.
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