3.5.5. West Greenland: Late Neogene uplift along a rifted margin 50 My after rifting
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| Fig. 63. Geological map of Nuussuaq-Disko area, Western Greenland. For location see red rectangle on inset map. Cross-section through Nuussuaq with projected stratigraphy of the Gro-3 well, showing position of planation surfaces. Post-Paleocene uplift by at least 1200 m is evident from the occurrence of marine Paleocene sediments at this elevation (Piasecki et al., 1992). Uplift of the planation surfaces probably occurred during the Late Neogene (Japsen et al., 2006). Basalts: V: Vaigat Formation (c. 61 Ma; Storey et al., 1998), M: Maligât Formation (c. 60 Ma; Storey et al., 1998) and Ka: Kanisut Member (c. 53 Ma; Storey et al., 1998). Eocene sediments: T: Talerua Member (c. 39 Ma; Schmidt et al., 2005) K-Q: Kuugannguaq-Qunnilik fault. CBFS: Cretaceous Boundary Fault System (c. 65 Ma) (modified after Japsen et al., 2006). |
In Scandinavia, the timing and extent of uplift movements are difficult to determine because the uplifted area consists almost exclusively of ancient metamorphic rocks. In West Greenland however, the preserved Mesozoic–Cenozoic sedimentary and volcanic record renders this a key area for studying the uplift of passive continental margins and thus may provide a model for the Cenozoic development of the NW European margin. In West Greenland, the 2 km high mountains on Nuussuaq and Disko expose a Cretaceous–Eocene sedimentary and volcanic sequence that reveals two phases of extension and rift formation during the Early Cretaceous (Chalmers and Pulvertaft, 2001) and latest Maastrichtian–early Paleocene (Chalmers et al., 1999; Fig. 63). These mountains also contain a detailed record of an uplift episode that occurred during the mid-Paleocene, probably a response to impingement of the Iceland plume on the lithosphere (Dam et al., 1998), immediately prior to rapid km-scale subsidence and deposition of Late Paleocene and Eocene sediments and volcanic rocks in the Nuussuaq Basin (Japsen et al., 2005). Subsidence and infilling of this basin was paralleled by sea-floor spreading in the Labrador Sea (Chalmers et al., 1999; Chalmers and Pulvertaft, 2001). Neogene uplift (Japsen et al., 2005; Bonow et al., 2006a) has brought Paleocene marine sediments to 1200 m above MSL (Piasecki et al., 1992) whilst in present-day offshore areas time equivalent deposits were buried below 3 km of sediments. Farther south, the rift is located offshore, parallel to the coast (Chalmers and Pulvertaft, 2001) whilst the mountainous hinterland is composed of Precambrian basement.
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| Fig. 64. Photograph showing a plateau that is being upheld by Precambrian basement and that has been elevated to about 1 km above M.S.L. For location see red spot in inset map in Fig. 63. A similar erosion surface cuts across the Paleocene-Eocene basalts on Disko and Nuussuaq (Fig. 63). This surface was formed by erosion during Oligocene-Miocene times and was uplifted during two Late Neogene episodes (Japsen et al., 2006). |
Remnants of a high plateau have been identified on Nuussuaq and Disko and in the highlands south of Disko Bugt (Fig. 64; Bonow et al., 2006a; 2006b). Bonow et al. (2006a) interpreted the plateau on Nuussuaq and Disko as an erosional surface, mainly formed by a fluvial system that was graded close to base level, and that was subsequently uplifted to its present elevation. It extends over 150 km east–west, has a low relative relief, is broken along faults, is tilted westwards in the west and eastwards in the east, and has a maximum elevation of c. 2 km in central Nuussuaq and Disko. This erosional surface cuts across Precambrian basement rocks and Paleocene–Eocene lavas, constraining its age as being substantially younger than the last rifting event of the Nuussuaq Basin, which occurred during the Late Maastrichtian and Danian.
In the highlands south of Disko Bugt, a similar plateau can be traced to almost 2 km above MSL (Bonow et al., 2006b). Its surface is tilted towards Disko Bugt, where it cuts off a more steeply inclined etch surface that occurs at lower elevation and has a distinct hilly relief. This hilly relief emerges as an inclined surface from the Cretaceous sediments in Disko Bugt (Bonow, 2005) and is interpreted as a stripped Late Mesozoic etch surface. This surface is cut off towards the south by a less inclined planation surface that thus must be younger and consequently of Cenozoic age.
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| Fig. 65. Fission-track ages from Gro-3 and Gane-1 boreholes plotted against depth. Continuous line: variation of stratigraphic age with depth. Dashed lines: predicted patterns of fission-track age from the Default Thermal History based on the preserved sedimentary section for apatites containing 0.0–0.1, 0.4–0.5, 0.9–1.0 and 1.5–1.6 wt% Cl. The Default Thermal History is obtained by combining the burial history derived from the preserved sedimentary section with the present-day geothermal gradient. The fission-track ages decrease with depth and are much less than the values predicted from the Default Thermal History. This shows that the sampled sedimentary units have been much hotter in the past (after Japsen et al., 2005). |
Borehole samples down to 3 km depth are available from the Gro-3 well on Nuussuaq (Fig. 65). Apatite fission-track analysis (AFTA) data and vitrinite-reflectance data reveal that these samples cooled from maximum paleotemperatures between 40 and 30 Ma followed by two further cooling episodes beginning during 11–10 Ma and 7–2 Ma (Japsen et al., 2005). When the first cooling episode began, the samples from the Gro-3 borehole were buried 1500–2000 m deeper than at present, with the paleogeothermal gradient being 40–48°C/km.
Bonow et al. (2006a) suggested that development of the erosion surface on Nuussuaq and Disko was triggered by an uplift and erosion event starting between 40 and 30 Ma. This erosional surface was fully developed prior to an uplift event starting between 11 and 10 Ma, which caused the incision of valleys. This generation of valleys graded to the new base level and formed a lower erosion surface, at the most 1 km below the summit erosion surface, and thus indicates the magnitude of uplift. Formation of this valley generation was shortly interrupted by a third uplift event also with a magnitude of 1 km, lifting the landscape to near its present position. The subsequent erosion was probably both of a fluvial and glacial nature. Correlation with the fission-track record suggests that this uplift event started between 7 and 2 Ma.
The present-day high mountains of West Greenland were thus not uplifted during the Paleogene, but are erosional remnants of an Oligocene–Miocene planation surface that was offset by reactivated faults, resulting in megablocks that were tilted and uplifted to their present-day elevation during two late Neogene phases (Japsen et al., 2006) These late Neogene uplift phases postdate rifting by about 50 million years, and sea-floor spreading west of Greenland by about 30 million years.