Late–/postglacial age and tectonic origin of the Nordmannvikdalen Fault, northern Norway

The Nordmannvikdalen Fault (NF) represents one of the two observed postglacial faults in Norway. The two faults constitute the northernmost part of the Lapland province of postglacial faults, occurring in large tracts of northern Sweden and northern Finland. The 1.3 km-long, NW– SE-trending NF is thought to be a normal fault with scarp height increasing from less than 0.50 m in the NW to c. 1.50 m in the SE. A tectonic origin for the Nordmannvikdalen Fault, which seems to be aseismic today, has recently been questioned and alternative causes as either gravitational collapse or overburden creep have been suggested. We carried out three 3–5 m-deep trenches and two ground penetrating radar (GPR) profiles in September 2017 to study the fault at depth. The trenching reveals deformation structures within the lodgement till. The faulting led to cracking of the ground, forming a vertical wedge-shaped crevice, with a width similar to previously recorded large ice wedges and ice wedge casts (fossil ice wedges) in polygonal pattern ground in Arctic areas. The width increases with increasing scarp height, i.e., the vertical displacement. The crevice was filled with sediment, snow and water freezing to ice, with subsequent infilling, during melting seasons, of more debris from the side walls of the host material and cryoturbated and soliflucted soil. The Nordmannvikdalen Fault appears, from the trenching, to have been formed in one single seismic event. The new GPR data show bedrock reflectors dipping approximately 38–45° towards the NE, below the NF scarp. The average angle of the terrain slope between the Nordmannvikdalen Fault scarp and the valley floor is 14°, and the altitude difference between the fault scarp and the Nordmannvikdalen valley floor is approximately 200 m. We find no reason to downgrade the fault to ’very unlikely to be neotectonics’.