The Effect of North Sea and North Atlantic Rifting on Onshore SW NO: Evidence from Low-temperature Thermochronology

Anna Ksienzyk

In the absence of an onshore sedimentary record, dating post-Caledonian faults in southwestern NO is difficult. While a large number of faults and fractures with a variety of orientations and characteristics cut basement rocks and Caledonian nappes alike, the ages of formation and possibly reactivation of these structures are very poorly constrained. While some may be related to the later stages of orogenic collapse, others are onshore expressions of the continental rifting that resulted in the opening of the North Sea and ultimately the North Atlantic. In order to capture these latest stages of the Caledonian cycle, orogenic collapse and continental breakup, we employ a combination of low-temperature thermochronological techniques, such as apatite and zircon fission track and (U-Th)/He analyses and K/Ar illite dating of fault gouge samples. While apatite fission track analyses of samples from southwestern NO yield Permian-Jurassic, but mostly early-mid Jurassic ages, single grain apatite (U-Th)/He ages range from Triassic to Cretaceous. Thermal history modelling based on these data indicates relatively constant cooling rates of ca. 2 °C/Ma throughout Permian-early Jurassic times and slower cooling rates of < 1 °C/Ma since the middle Jurassic. Both fission track and (U-Th)/He ages are offset across faults, indicating significant vertical movements of crustal blocks during the Mesozoic. Additionally, fault-bounded blocks along a profile sampled for (U-Th)/He dating show distinctly different cooling histories from their neighbours. These findings are substantiated by K/Ar ages of illite from fault gouges, which define fault activity in early Carboniferous, Permian, late Triassic-early Jurassic and Cretaceous-earliest Palaeogene times, respectively. The regional significance of these events and the applicability of the method are highlighted by good correlations with other dating methods and independent geological constraints, such as palaeomagnetic and Ar/Ar dating of fault breccias and periods of dyke intrusion. The combined data document therefore an extended history of fault tectonics from Palaeozoic to early Cenozoic times. Extensional deformation was clearly not confined to the offshore rift systems but also affected onshore areas. The documentation of significant fault activity in the Permian may reflect similar activity in the North Sea rift, thus documenting the initiation of this rift already in the Permian, i.e. simultaneous with the formation of the Oslo Rift.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013