Meso‐Cenozoic morphotectonic evolution of southern Norway: Neogene domal uplift inferred from apatite fission track thermochronology

Abstract

Apatite fission track (AFT) thermochronology of Precambrian and Paleozoic basement samples from southern Norway reveals a post-Paleozoic exhumation history, related to offshore Mesozoic and Cenozoic extensional basin development. The data indicate two major phases of rapid exhumation. A first Mesozoic phase started in the Triassic (-220 Ma) in the east and south of the study area and migrated to the west where Jurassic (-160 Ma) ages of exhumation predominate. A second event is indicated by thermal history modeling of AFT ages and track length distributions. It is inferred to be Neogene in age, initiated at about 30 Ma, and it produced a domal pattern of AFT isochrons which follow present-day topographic elevation. Youngest AFT ages (-100 Ma) are encountered at sealevel in the inner fjords near the areas of highest topography; ages increase radially outward to the mountain peaks and the coastlines. Forward modeling of age-elevation patterns suggests that Mesozoic geothermal gradients were 10-15øC/km higher than the present value of 20øC/km. During the Triassic and Jurassic, a total of 1.3-3.5 km of overburden was removed from the study area, assuming a 30øC/km geothermal gradient for that period. We attribute this to rift margin erosion as a result of erosional base level lowering and flank uplift, as evidenced by thick continental clastic sequences deposited in Triassic-Jurassic half grabens in the North Sea basins. We propose that 1.5-2.5 km of Neogene exhumation were a result of late stage domal uplift. This is supported by basinward dipping pre-Neogene strata in the basins surrounding southern Norway and the infill of a 1-to 2-km-thick Neogene sediment wedge containing various internal unconformities. Domal uplift probably started in the Late Oligocene, may have been amplified in the Pliocene, and was overprinted by Plio-Pleistocene glacial erosion. Maximum Neogene tectonic uplift is estimated at approximately 1-1.5 km, radially decreasing outward to a value <500 m near the shoreline. Neogene domal uplift is coincident with Oligocene and Pliocene plate reorganizations in the North Atlantic; similar Neogene domes are found around the Norwegian-Greenland Sea (i.e., Svalbard and the Barents Sea, northern Norway, east Greenland), suggesting a regional tectonic cause. The onset of Neogene uplift postdates major volcanism and continental breakup by-25 m.y. and predates Plio-Pleistocene glaciations. Its origin is possibly a combina-Copyright 1995 by the American Geophysical Union. Paper number 95TC00088. 0278-7407/95/95TC-00088510.00 tion of induced mantle convection, resulting in thermal erosion of the lithosphere, and the operation of intraplate stresses. Introduction It has become apparent in recent years that rifted margins may record significant postrift (late stage) uplift events, in addition to better understood synrift flank uplift [e.g., Cloetingh and Kooi, 1992; Sales, 1992]. For instance, Cloetingh et al. [1990; 1992] stress the importance of accelerated postrift subsidence, in conjunction with margin upl...

Post-Caledonian brittle deformation in the Bergen area, West Norway: results from K–Ar illite fault gouge dating

“…Fine-grained K-feldspar has a potentially lower closure temperature (350-150°C; Lovera et al, 1989) than illite/muscovite (425-250°C; Harrison et al, 2009;Duvall et al, 2011) and could therefore be reset at lower temperatures, yielding younger ages. However, this requires fault activity at temperatures exceeding the closure temperature of K-feldspar; the temperatures indicated by the high KI values of the sample (100-200°C) are barely high enough and hostrock temperatures in the Late Jurassic were well below the closure temperature of K-feldspar (Rohrman et al, 1995;Ksienzyk et al, 2014). Alternatively, synkinematic authigenesis of K-feldspar has been documented in faults in southern Norway (Torgersen et al, 2015) and might also have occurred here.…”

“…Thermal modelling based on apatite fission track and apatite (U-Th)-He ages suggests that cooling rates remained relatively high (2-3°C/Ma) throughout the Triassic and Early Jurassic, followed by much slower cooling (<1°C/Ma) in the Late Jurassic and Cretaceous (Rohrman et al, 1995;Leighton, 2007;Ksienzyk et al, 2014). The Triassic-Early Jurassic cooling is generally interpreted as continued rift flank erosion following Permo-Triassic North Sea rifting.…”

“…Without stratigraphic markers, it can be difficult to determine the amount and even the sense of offset, let alone the time of fault activity. Thermochronological studies help to reconstruct cooling histories of basement rocks and reveal periods of uplift and erosion or sedimentary burial (Andriessen & Bos, 1986;Rohrman et al, 1995;Leighton, 2007;Johannessen et al, 2013;Ksienzyk et al, 2014). Several of these studies (Leighton, 2007;Johannessen et al, 2013;Ksienzyk et al, 2014) show that the distribution of fission track and (U-Th)-He ages is strongly tectonically controlled and fault-bound blocks have distinct cooling histories from their neighbours, highlighting the significance of active fault tectonics throughout the Mesozoic.…”

Filling the North Sea Basin: Cenozoic sediment sources and river styles

“…Ancient exhumed surfaces occur on the flanks of the domal uplift. According to Rohrman et al (1995) southwestern Norway was uplifted by as much as to 2 km during Neogene times. This uplift may be a continuation of updoming initiated in the Paleogene, associated with the opening of the North-east Atlantic Ocean, taking surfaces to levels nowadays at 1200 m, although it might have been initiated earlier in the Late Eocene-Early Oligocene.…”

Erosion and rheology controls on synrift and postrift evolution: Verifying old and new ideas using a fully coupled numerical model

“…The former produce and laterally redistribute normal loads of 50 to 500 MPa; that is of the same order as the tectonic forces (5-22 km of sediments over 50-200 km horizontal distances, e.g., Pannonian basin, Albert rift, Baikal rift, Dnieper-Donetz basin [e.g., ure lb and Burov and Cloetingh [1997]). These effects are actually observed [e.g., Ebinger et al, 1989Ebinger et al, , 1999 and, naturally, should influence subsidence phases inferred from tectonic geomorphology and fission track age/length patterns [Rohrman et al, 1995].…”