Extension, hyperextension and mantle exhumation offshore Norway: A discussion based on 6 crustal transects

Abstract

In spite of its classification as `magmatic´, the rifted margin off Mid Norway evolved as a hyperextended margin in Late Jurassic and Cretaceous time, with a pre-breakup structural evolution that resembles that of magma-poor margins. Because the distal margin was not drilled to crystalline basement, considerable uncertainty exists with respect to the distribution of lithologies at depth. In this contribution, we discuss the implications of various scenarios for deep-seated lithologies and evaluate the possibilities for mantle exhumation in the deep margin offshore Norway. A suite of Jurassic– Cretaceous, very large-magnitude, extensional faults that exhumed successively deeper structural levels seawards are well imaged in recent long-offset seismic reflection data. Based on these key profiles, we document the structural configuration that created the extremely sharply tapered south Møre margin as well as subhorizontal detachment faults in the distal Vøring margin. The latter were associated with rotated extensional allochthon and supradetachment basins and contributed to the exhumation of very deep levels including rocks from the lower crust and/or upper mantle. In the outer margin, truncation and excision of this system by a new set of incising detachment faults occurred later in the Cretaceous and the earliest Cenozoic. We discuss the relationships between these detachment systems under the distal and outer margins and positive structural features such as the Rån and Gjallar ridges. The interpretation of basement lithologies at depth in the distal margin is ambiguous due to overlap in physical properties. Based on a process-oriented evaluation of the different interpretation scenarios, we favour a model where mantle windows were exhumed in the footwalls of some of the master faults in the Cretaceous.