Formation of foreland rifts

Abstract

The Lower Permian Rotliegendes troughs in Central Europe are directly comparable to the Upper Cenozoic Basin and Range province in the western United States in most tectonostratigraphic aspects. These intracratonic rifts were formed by extensional tectonics in the foreland area of major Cordilleran-type orogenies beginning 20–25 Ma after mountain building. The main characteristics of the basins include: crustal thinning from 40–45 to 30 km, change from calc-alkaline magmatic arc suites to basaltic or bimodal (basalt-rhyolite) igneous suites, normal to listric block faulting, high heat flow and hydrothermal activity, regional uplift, and immature clastic and evaporite sediments rapidly deposited within the subsiding rifts in a semi-desert environment. The genetic model proposed for these “foreland rifts” assumes that an extensional force under the continental foreland results from mantle material being drawn back towards the subducting plate by viscous drag. This force is counteracted by a compressional force produced at the trench by subducting a young, relatively buoyant plate until subduction is ended, at which time the plate breaks off and continues to sink. Tension in the foreland, possibly produced by the sinking slab, continues for some time later, causing stretching and thinning of the continental crust, extensional tectonics and higher regional heat flow and uplift. The decrease in overburden pressure and higher heat flow induces partial melting in the lower lithosphere and bimodal volcanism. Hotter asthenosphere rising up behind the trailing edge of the sinking plate causes thermal expansion and further uplift. The heat flow anomaly rapidly decays by hydrothermal activity within 15–20 Ma after last volcanism. Block faulting and rift basin infilling follow the main period of volcanism and continue until the sinking slab is decoupled from the overriding plate and no longer influences the continental foreland.