Direct 40Ar/39Ar K‐feldspar dating of Late Permian–Early Triassic brittle faulting in Northern Norway

Abstract

While the offshore post‐Caledonian extensional history of the north Norwegian passive margin is well constrained, the tectonic relationship between onshore and offshore regions is less clear because of limited age constraints on the timing of rifting onshore. ⁴⁰Ar/³⁹Ar dating of K‐feldspar from hydrothermally altered fault rocks in a Precambrian gneiss complex in northern Norway was used to study the timing of extensional faulting onshore. In addition, ⁴⁰Ar/³⁹Ar dating of K‐feldspar from the host rock provided insight into the regional rock cooling history prior to brittle deformation. Results indicated a dominant Late Permian–Early Triassic (~265–244 Ma) faulting event and found no evidence for later reactivation, which has been documented offshore. The region cooled to below the closure temperature for ⁴⁰Ar/³⁹Ar K‐feldspar in the Carboniferous to Early Permian, prior to the main brittle faulting event. ⁴⁰Ar/³⁹Ar dating of fault zone K‐feldspar products provided a means to date brittle faulting events.