MESOZOIC EXTENSION VERSUS TERTIARY COMPRESSION ALONG THE BILLEFJORDENFAULT ZONE SOUTH OF ISFJORDEN, CENTRAL SPITSBERGEN

The Billefjorden Fault Zone displays a complex tectonic history going back into Precambrian times. Observed thickness variations within Meso zoic sequences across the fault zone have previously been related to M esozoic extensional movements along the fault zone. Recent structural, stratigraphical and sedimentological investigations suggest that the thickness variation observed across the fault zone is due to Tertiary compressional tectonics, and that Mesozoic extensional movements are m inor or nonexistent. Tertiary deformation is characterized by a combin ed thin-skinned/thick-skinned structural style. Thin-skinned thrusting is due to the development of decollement zones in the Jurassic/Cretac eous Janusfjellet Subgroup and in the Triassic Sassendalen Group. Thic k-skinned faulting is related to reverse faulting along a steep, east- dipping fault rooted in the pre-Mesozoic basement. Interacting thin-sk inned thrusting and thick-skinned faulting has resulted in out-of-sequ ence thrusting, and is responsible for the thinning of Mesozoic sequen ces across the fault zone.