The Tertiary fold-and-thrust belt on Spitsbergen can be divided into a western portion with basement-involved thrust slacks 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 than 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 & Steet. The spatial coincidence between the two sets of structures 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 /bypasssing potential lower thrust flats often utilized elsewhere) and concentration of thrust ramps ( decreasing spacing) are two mechanisms for localization of the thrust stack.
Hormon D. Maher, Jr., Dept. of Geography and Geology, University of Nebraska, Omaha, NE 68182, USA;
Alastair l. Welbon, Dept. of Geological Sciences, Queens University, Kingston, Ontario K7L3N6, Canada.