Sequential reactivation of the Billefjorden Fault Zone during evolvement of the West Spitsbergen Fold and Thrust Belt, Svalbard.
Authors: Smyrak-Sikora Aleksandra , Braathen Alvar , Osmundsen Per Terje , Senger Kim 
Published: March 2024

The Paleogene West Spitsbergen Fold-and-Thrust Belt, WSFTB, is present in Svalbard (Norwegian High Arctic archipelago; 74-81&#176;N, 15-35&#176;E). The WSFTB&#8217;s contraction reflects the Eurekan Orogeny and transpressional continental breakup along a transform fault zone, followed by opening of a seaway between the North Atlantic and Arctic oceans. The overall timing of the contraction is not well constrained, and many authors link it to the development of a deep and narrow foreland basin system filled with Uppermost Paleocene and Eocene to Oligocene(?) deposits, while the Lower Paleocene deposits are considered to be deposited in a regional subsidence prior to Eurekan deformation. The WSFTB divides into a thick-skinned, basement-involved fold-thrust complex in the west, passing into a central zone of thin-skinned, fold-thrust units with associated detachments developed in Permian evaporites and Mesozoic organic-rich mudstones, separated by a prominent duplex system that reflects a thrust-ramp transferring movements from Permian to Mesozoic detachments eastwards. Stress transfer also reactivated the steep, basement rooted Billefjorden and Lomfjorden fault zones farther east, showing up to 200 m of reverse stratigraphic offsets. These fault zones are long-lived structural elements exposing multiple reactivation events since the Late Palaeozoic.Our study targets the Paleogene reactivation and especially the sequence of deformation of the Billefjorden Fault Zone, based in detailed interpretations of onshore seismic lines, and field mapping supported by acquired and interpreted mountain-scale digital outcrop models. Results suggest initial shortening by reactivation of deep-rooted extensional faults, seen as reverse offsets of stratigraphic units and formation of fault-propagation monoclines as anticlines-synclines pairs. Subsequent deformation involved the regional-scale (tens of km long) decollement zones hosted by the Mesozoic mudstones. West of the Billefjorden Fault Zone, the decollement is seen in the Jurassic Agardhfjellet Formation. Crossing the Billefjorden Fault Zone, this decollement is seen truncating the reverse fault (digital outcrop data) and fault propagation fold (seismic data) and displacing them 2 km to the east, where the decollement continuous in the uplifted mid-Triassic mudstones of the Botneheia Formation. This sequence of deformation, with the reactivated deep-rooted faults truncated by the decollement, contradicts previously published models that advocate for reactivation of deep-rooted faults taking place late during the deformation, synchronised with an uplift of thick-skinned basement-involved thrusts.The timing of the deep-rooted faults reactivation is poorly constrained. In this study, we hypothesise that it can be as early as the early to middle Paleocene, corresponding to phase of regional compression at 61 Ma indicated, by the recent studies, for the Lower Paleocene Firkanten Formation. This contraction could also be determined from the westward thickening and locally NE and E sourcing deposits of the Firkanten Formation and overlying Basilica and Grumantbyen formations, which indicate an existing topographic high or a foreland bulge as suggested by some of the authors. The timing of the second stage is consistent with the main shortening phase in the Eocene.