INTERACTION OF BASEMENT-INVOLVED AND THIN-SKINNED TECTONISM IN THE TERTIARY FOLD-THRUST BELT OF CENTRAL SPITSBERGEN, SVALBARD

The Tertiary fold-thrust belt in Oscar II Land, central Spitsbergen, c onsists of three major zones of distinct structural style: (1) a weste rn basement-involved fold-thrust complex, (2) a central zone of thin-s kinned fold-thrust units above a decollement in Permian evaporites, an d (3) an eastern zone characterized by a frontal duplex system in the fold-thrust belt, bounded eastward by steep, basement-rooted reverse f aults (Billefjorden and Lomfjorden fault zones) beneath subhorizontal platform strata. Offshore seismic data from Isfjorden (Statoil) confir m the threefold zonation and document thick-skinned and thin-skinned s tructural interactions in both the fold-thrust belt and the foreland s ection. An admissible cross section yields about 45%, or 20 km, of sho rtening in Oscar II Land. Deeper parts of the seismic profiles show fa ult-bounded Devonian (central and east) and Carboniferous (west) basin s. The structural grain of the Tertiary fold-thrust belt partly coinci des with the margin-bounding normal faults of these basins, suggesting that preexisting structures and stratigraphy controlled the Tertiary fold-thrust belt development. A kinematic evolution of the fold-thrust belt is invoked: (1) north-northeast-directed, bedding-parallel short ening, (2) major west-southwest-east-northeast shortening, with in-seq uence foreland fold-thrust propagation, (3) basement-involved, west-so uthwest-east-northeast uplift in the eastern foreland zone, (4) eastwa rd out-of-sequence propagation of thrusts, and (5) west-east extension in the hinterland. Our regional structural compilation map and synthe sis of the central Spitsbergen transect advocates structural variation and linked basement-involved thrusting in the hinterland and thin-ski nned/thick-skinned reactivation and out-of-sequence thrusting in the e ast (foreland), and is new compared with previous work of the region. The synthesis also raises several important new structural play concep ts for investigating hydrocarbon prospects in Spitsbergen and adjacent regions; for example, inverted Carboniferous basins, and traps produc ed by Tertiary thin- and thick-skinned contraction and reactivation st ructures.