Unexpected postglacial faulting in passive continental margins: Storfjorden glacial trough, Barents Sea

Abstract

In high latitude continental margins, glacio-seismotectonics becomes particularly relevant during postglacial unloading periods and related isostatic rebound. Based on multibeam bathymetry and parametric profiles, an unexpected active 30 km long NNW-SSE fault is imaged for the first time in the inner Storfjorden glacial trough, at the passive continental margin of the Barents Sea. The 10 km southern tip of the fault was surveyed in detail and it is characterised by a total fault throw of 65 m. The fault forms an asymmetric valley with an upraised block to its east (top at 245 m water depth), a downthrown block to the west (at 310 m water depth), and a most recent fault scarp with 8 m relief at the seafloor valley axis. Recent fault activity is evidenced by its morphological expression on the seafloor, the faulting of the glacial, glaciomarine and marine sediments and streamlined landforms formed during the last glacial-interglacial period. Mass-flow deposits along the upraised block would also indicate recent fault activity. This fault may trigger earthquakes of up to MW 6.2, based on the magnitude/length ratio of the detailed surveyed segment, which is similar in magnitude to those that occurred in the nearby northeastern regions during 2008–2012. In any case, earthquakes may reach up to MW 6.8 if the entire fault length is activated. Rather than the expected normal fault on the passive margin, this fault seems to be reversed and dipping eastwards. The integration of the Storfjorden glacial trough reverse fault with regional structures suggests that the fault activated the southwestward front of a wide, continental crustal block. These results are in accordance with a change in the stresses in the passive continental margin since the rifting stage, from extension to post-rift compression, and suggest that postglacial unloading may have favoured the fault development.