DIFFERENTIAL STRAIN RATE AS A MECHANISM FOR THE FORMATION OF DETACHMENT FAULT CORRUGATIONS AND THE EXHUMATION OF UHP ROCKS: A CASE STUDY FROM WESTERN NORWAY

Transport-parallel corrugations on normal-sense detachment faults are described in extensional tectonic settings at the range through orogen scale. The mechanisms by which these corrugations form have implications for the original shape of the detachment fault, pre-existing basement structures, the formation and architecture of supra-detachment basins, and the overall tectonic setting of the orogenic event. Western Norway provides an ideal setting to investigate the formation of detachment fault corrugations. There, the normal-sense Nordfjord Sogn Detachment Zone--thought to be one of the primary structures responsible for exhuming the high- and ultrahigh-pressure rocks of the Western Gneiss Complex--is corrugated by a series of extension-parallel folds that deform footwall rocks, hanging wall rocks, and muscovite cooling chrontours.

Here, we suggest that the largest of these corrugations represent transfer faults/shear zones between regions of differential exhumation localized by the density-driven ascent of the ultrahigh-pressure northern Western Gneiss Complex. The viability of this hypothesis was tested through a series of 2-D thermal models. Subducted crust with an initially cold geotherm was allowed to exhume via a combination of pure shear thinning and simple shear removal of upper crust along a broad detachment zone, followed by conductive relaxation. The models indicate that even modest differences in exhumation rate (<20%) create fold amplitudes up to 7 km and produce 2–5 km offsets of the 400 degree isotherm (muscovite closure to Ar). These results are consistent with outcrop patterns and 10–15 Myr differences in muscovite cooling ages observed in Western Norway. This work has important implications for the geology of Western Norway suggesting that the corrugations on the Nordfjord Sogn Detachment Zone may be the result of differential exhumation rather than orogen-scale constriction and transtension, and indicates that transfer faults/shear zones should be considered a viable mechanism for corrugated detachments in extensional settings worldwide.