Separation of Palaeogene and Neogene uplift on Nuussuaq, West Greenland

Journal of the Geological Society, Mar 2005 by Japsen, Peter, Green, Paul F, Chalmers, James A

Abstract:

The geological record exposed on Nuussuaq, central West Greenland, shows that uplift in the Palaeocene, probably caused by impact of the Iceland plume head, was followed by kilometre-scale subsidence. Analysis of apatite fission-track and vitrinite-reflectance data from borehole samples down to 3 km depth reveals that the samples cooled from maximum palaeotemperatures between 40 and 30 Ma followed by two further cooling episodes beginning in the intervals 11-10 and 7-2 Ma. When the first cooling episode began, the samples from the neighbouring Gro-3 and Gane-1 boreholes were buried 1500-2000 m deeper than at the present day, and the palaeogeothermal gradient was 40-48 �C km^sup -1^. It is not clear whether this cooling involved exhumation or if it was due solely to reduction in heat flow and a drop in surface temperature. The two later episodes definitely involved exhumation because by then the palaeogeothermal gradient had declined to a value close to the assumed present value of 30 �C km^sup -1^, which agrees with estimates from offshore wells. The most recent cooling episode corresponds to the incision of the present-day relief (c. 1100 m) below the summits around the two boreholes. We conclude that the present-day high mountains of West Greenland were not uplifted during the Palaeogene, but are erosional remnants of a landmass uplifted during the Neogene.

Keywords: West Greenland, Cenozoic, uplift, exhumation, fission-track dating.

Studies of the Cenozoic uplift and erosion of the passive continental margins around the North Atlantic commonly show evidence for multiple episodes of uplift (see Japsen & Chalmers 2000; Dor� et al. 2002). Scandinavia appears to have been uplifted at least twice, in the Palaeogene (e.g. Faleide et al. 2002; Lidmar-Bergstr�m & Naslund 2002; Nielsen et al. 2002) and again in the late Neogene (Riis & Fjeldskaar 1992; Rohrman et al. 1995; Faleide et al. 2002; Japsen et al. 2002; LidmarBergstr�m & N�slund 2002). Many studies of the British Isles have focused on Palaeogene uplift (e.g. Green 1986; White & Lovell 1997; Jones et al. 2002) but there is ample evidence that uplift and erosion also affected the British Isles during the late Cenozoic (e.g. George 1966; Green 1989, 2004; Japsen 1997; Green et al. 2001b; Stoker 2002). Whereas Palaeogene uplift has been related to the impact of the Iceland plume during the Palaeocene and the opening of the North Atlantic during the Eocene, the causes of late Cenozoic vertical movements in the region remain obscure.

The difficulty in resolving both the timing and magnitude of the Cenozoic phases of vertical movements is probably the most critical obstacle in recognizing their nature and hence in understanding their causes. Such movements are frequently ignored because they are manifested by removal of sedimentary cover and have led to exhumation of rocks that were formed long before the onset of erosion, giving the impression that no cover was ever deposited. For example, for many years the present-day mountains in both Scotland and Scandinavia were thought of as being the worn stumps of the much higher mountain range formed during the Caledonian orogeny c. 400 Ma ago instead of being recognized as blocks of crust uplifted to their present elevations during the Cenozoic (e.g. Rohrman et al. 1995; Thomson et al. 1999b). Indeed, the lack of a generally accepted hypothesis to explain the presence of mountains in Norway has been termed 'the Scandinavian enigma'.

West Greenland has also been subjected to uplift during the Cenozoic, in a similar way to NW Europe (Mathiesen 1998; Chalmers 2000). One difference is that the exposed Nuussuaq Basin in central West Greenland contains a detailed record of early Palaeogene uplift and erosion followed by rapid subsidence and infilling by volcanic rocks and sediments (Figs. 1 and 2) (e.g. Pedersen et al. 1993, 2002b; Dam et al. 1998; Chalmers et al. 1999). This record allows separation of the effects of Palaeocene-Eocene uplift or subsidence, evident in the sedimentary and volcanic succession, from later events that lifted Palaeocene marine sediments to their present heights of over a kilometre above sea level (Fig. 3) (Piasecki el al. 1992). However, the precise timing of uplift remains uncertain.

In this study, we have used new apatite fission-track analyses (AFTA�; see Green et al. 2002) and vitrinite-reflectance (VR) data from two boreholes through lower Palaeocene and Mesozoic sediments on Nuussuaq, combined with geological and topographical constraints, to show that kilometre-scale uplift and erosion occurred during the late Neogene, probably in two discrete phases. We think this may be the first time that it has been possible to separate the effects of Palaeogene and Neogene uplift so clearly because the effects of the Palaeogene phase can be studied in the preserved succession that is now exposed as a result of the Neogene uplift.

Palaeogene uplift and subsidence in central West Greenland

Nuussuaq Basin onshore