Influence of Carboniferous structures on Tertiary tectonism at St. Jonsfjorden and Bellsund, western Svalbard

The Tertiary fold-and-thrust belt on Spitsbergen can be divided into a western portion with basement-involved thrust stacks and an eastern thin-skinned portion within Upper Palaeozoic and Mesozoic platform cover strata. At both St. Jonsfjorden and Bellsund, Carboniferous strata structurally higher in the thrust stack are thicker and have a more complete stratigraphic succession that those in lower thrust sheets. These differences, along with observed structures, indicate that the basement-involved thrust stacks transported and telescoped a zone of Carboniferous structures responsible for a thicker Carboniferous basin to the west. This zone may be the eastern margin of the St. Jonsfjorden trough of Gjelberg & Steel. The spatial coincidence between the two sets of structure suggests that the Carboniferous basin geometry controlled the large-scale Tertiary structural architecture by localizing thrust stack development. At St Jonsfjorden portions of Carboniferous fault surfaces were reactivated as Tertiary thrust ramps. In both areas a Middle Carboniferous angular unconformity played a role in Tertiary tectonism. Higher stratigraphic ascent of ramps into the platform cover before bending into a flat (bypassing potential lower thrust flats often utilized elsewhere, and concentration of thrust ramps (decreasing spacing) are two mechanisms for localization of the thrust stack. -Authors