Rifting of Collapsed Orogens: Successive Incision of Continental Crust in the Proximal Margin Offshore Norway

Abstract

AbstractStructures related to orogenic “collapse” may provide an important template for reactivation during later rifting. In the proximal rifted margin offshore Norway, the tectono‐sedimentary template related to collapse of the Caledonian mountain belt became widely eroded but inherited metamorphic core complexes and detachment faults evolved further in Late Paleozoic and Earliest Mesozoic time and became associated with very significant crustal thinning. The earliest rifting mode was dominated by structures that mimic the Devonian post‐Caledonian template and comprised a suite of Late Paleozoic to Early Triassic metamorphic core complexes, detachment faults, and supradetachment basins. The deep structure indicates necking of brittle over remnant ductile crust, and we document how the main decoupling level utilized by the earliest rifting structures were cut by a suite of steeper, more deeply incising set of seaward‐ and landward‐dipping Triassic faults. The latter exploited deep shear zones related to the previous intracrustal necking, enabling changes in polarity over short distances. The change from a core complex mode of extension to one involving more deeply incising faults took place prior to deposition of the Late Triassic‐Early Jurassic interrift succession and pre‐dated the main phase of lithospheric‐scale necking associated with Jurassic‐Cretaceous extension. The latter phase of large‐magnitude faulting commenced with reactivation of seaward‐dipping, deeply incised faults inherited from early rifting as well as structures related to the phase of Devonian post‐orogenic transtension. The current study demonstrates that great structural complexity and significant magnitudes of extension may be present in proximal margins, in particular those developed along collapsed orogens.