KINEMATIC ANALYSIS OF DEFORMATION ALONG THE BILLEFJORDEN FAULT ZONE OF SPITSBERGEN, SVALBARD

The Billefjorden Fault Zone is a long-lived tectonic lineament of Spitsbergen. The basic history of this fault zone has previously been documented, but the role of strike-slip motion is still disputed. The origin of this weak zone is attributed to Caledonian shearing and is debated to be a terrane boundary. During the Devonian, basement structures were reactivated with reverse motion across many strands of the fault. Extensional to transtensional reactivation in the Early to Middle Carboniferous developed a half-graben known as the Billefjorden Trough. Later, the basin migrated eastward, and in the Late Carboniferous the platform deposits subsided. Tertiary reactivation consisted of a period of convergence followed by transtension.

Multiple deformation events along the Billefjorden Fault Zone are reviewed using structural analysis of fabrics adjacent to basin bounding faults, namely the Balliobreen and Odellfjellet. Foliations, fold axes, and other lineations were mapped and measured in Devonian through Middle Carboniferous sediments. These sediments, comprised of carbonates, evaporites, clastics and some conglomerates, display strong foliations indicating dominant dip-slip motion across the Balliobreen and Odellfjellet Faults, as well as a strike-slip component. The sense of strike-slip motion varies along strike as a function of position relative to the relay zone between these two faults. Initial analysis indicates dominant dip-slip motion with a dextral strike-slip component along the Odellfjellet Fault, north of the relay zone. Further south of the relay zone, along the Balliobreen Fault, dip-slip motion also dominates but the overall strike-slip component appears sinistral despite anomalous bedding orientations. Dextral strike-slip motion north of the relay zone is consistent with the Tertiary reactivation, whereas the sinistral, strike-slip motion recognized south of the relay zone could be explained by Early Carboniferous transtension, localized deformation decoupled from later Tertiary movement, or may be more complex. This part of the model is currently being evaluated.