Signature of cryptic sedimentary basin inversion revealed by shale compaction data in the Irish Sea, western British Isles

Abstract

Inversion is one of the most common modes of intraplate deformation and a primary mechanism of sedimentary basin exhumation. The key criterion for identification of basin inversion is the contractional reactivation of extensional faults. Reverse displacement associated with inversion is focused along the upper segments of faults, so the evidence for inversion may be removed in deeply exhumed basins. We believe this is the case in the Irish Sea, western British Isles. Previous authors have attributed the severe exhumation of this basin to plume-related uplift and have ruled out inversion as a cause of uplift. We present a method for identifying tectonic inversion in a basin where exhumation has removed evidence of reverse displacement on the upper segments of faults. We utilize compaction-based mapping of erosion patterns recorded by anomalously high sonic velocities in Upper Triassic shales. We show that the amount of section removed during exhumation in the Irish Sea varies between 1.3 and 3.3 km and that exhumation patterns are markedly heterogeneous. Neither the heterogeneity of exhumation nor its focusing over hanging wall blocks of major faults is consistent with solely plume-related uplift, and we believe that our results record cryptic basin inversion that is superimposed on epeirogenic exhumation (some of which may be plume related). The direct evidence for this inversion has been removed because erosion has proceeded beneath the null lines of reactivated faults, and we propose erosion of reactivated fault segments as one of the major reasons why shortening is frequently underestimated in inverted basins.