Early Paleozoic orogenic collapse, tectonic stability, and late Paleozoic continental rifting revealed through thermochronology of K‐feldspars, southern Norway

Abstract

Southern Norway was subjected to contractional orogenesis and subsequent extensional collapse during the early to middle Paleozoic when Baltica collided with Laurentia, resulting in the Caledonian Orogen. The largely unknown post‐Caledonian Paleozoic thermal history of the crust in southern Norway has been assessed through application of multiple diffusion domain thermal modeling of K‐feldspar 40Ar/39Ar data. Samples were collected along a 250 km E‐W traverse from the internides of the orogen along the western coast near Bergen through a series of middle to upper crustal nappes to the east. The K‐feldspar thermal models, which complement published mica and hornblende 40Ar/39AR data, are consistent with rapid cooling at the end of the Caledonian orogenic cycle, when Early Devonian (∼400 Ma) extensional collapse resulted in exhumation through mainly midcrustal conditions. Following extensional collapse, a period of relative tectonic and thermal stability may have prevailed until ∼300 Ma. The K‐feldspar modeling suggests that southern Norway entered a new phase of relatively rapid cooling (a few °C/Ma) in the Permo‐Carboniferous and that the increase in cooling rate was diachronous across the traverse. The onset of this cooling phase occurs at about the time of the earliest faulting and magmatism in the Oslo Graben to the southeast. The increase in cooling rate may also indirectly date the onset of the first important rift phase and associated sedimentation in the North Sea graben system. The distribution of Permian and younger fission track ages in both southern continental Norway and its marginal basin sediments is consistent with the Permo‐Carboniferous rapid cooling recorded in the K‐feldspars. The simplest explanation for the late Paleozoic increase in cooling rate is that rifting around the margin of southern Norway has resulted in a decrease in the base level and an increase in the rate of erosional denudation, resulting in essentially uniform exhumation of ∼2–4 km along the traverse.