DEVONIAN KINEMATICS ALONG THE LONG-LIVED BILLEFJORDEN FAULT ZONE, SPITSBERGEN, NORWAY

The Billefjorden Fault Zone (BFZ) is a long-lived lineament known to have moved in Silurian, Devonian, Carboniferous and Tertiary times. Dispute in the literature as to the nature of the ‘Devonian’ Svalbardian phase characterizes the kinematics as either pure contractional or as including a large sinistral component. Mapping of the footwall Devonian section adjacent to the BFZ in the Mimerdalen area near Pyramiden indicates the following. 1) The Mimerdalen syncline is a major structure with a locally overturned limb, which plunges gently NE and is distinctly oblique to the N-S Munindalen fault. 2) The upper Munindalen fault, a splay of the BFZ, dips >60° E, obliquely truncates the Mimerdalen syncline, and has significantly more cleaved and deformed rocks with a cleavage-flat relationship in the hanging wall. 3) In orientation the upper Munindalen fault is similar to the major BFZ Balliolbreen fault 1.5 km to the E, both with contractional kinematics. 4) Seismic lines indicate 4 sec of twt throw on the BFZ. 5) An angular unconformity exists at the base of the Late Famennian Plantekløfta Mbr. of the Mimerdalen Formation. 6) The Planteklofta Mbr. strata, conglomeratic with primarily Devonian sandstone clasts, is partially involved in the Mimerdalen syncline, and is cut by the Munindalen faults. These field relationships suggest pre or syn Fammennian oblique fold development in an orientation consistent with sinistral motion along the BFZ, followed by significant BFZ orthogonal contractional movement. Whether this pattern reflects the evolution of regional crustal block motions or local evolution of strain partitioning along a reactivated lineament is debatable.