Abstract

The normalization of overthickened orogenic crust after a continent-continent collision typically involves several different processes, including erosion, plate divergence, gravity-driven collapse of the orogenic wedge, and viscous flow of the lower crust. As a contribution to this discussion, we here utilize reprocessed deep seismic data to image the Scandinavian part of the major Caledonian continent-continent collision zone where we identify a double set of extensional shear zones: one in the upper to middle crust (Hardangerfjord shear zone) and a major oppositely dipping Moho-offsetting shear zone in the lower crust and upper mantle. The latter shear zone may have a true displacement close to 50 km and appears to offset the Moho vertically by ∼10 km, extending downward beyond the 50 km depth limit of the seismic data. While the upper structure offsets the basal Caledonian thrust, the Moho shear zone is more difficult to date. However, they are kinematically consistent and located at the transition zone between thick- and thin-skinned post-collisional tectonics, and we suggest that they may represent a crustal “pinch” or zipper-like structure that separated viscously flowing Caledonian lower crust from cooler rigid basement representative for the rest of the Baltic Shield. Later extension created the North Sea rift, but the general rift axis and the associated rift-related thinning developed in a narrower zone that runs oblique to the Devonian trend, hence the two generations of extension and thinning can be distinguished at the crustal scale.

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