Trenching and 14C dating of the Stuoragurra fault complex in Finnmark,Northern Norway – with some accompanying data included
The Stuoragurra Fault Complex (SFC) constitutes the Norwegian part of the larger Lapland province of postglacial faults in northern Fennoscandia. The 90 km long SFC consists of three separate fault systems; the Fitnajohka Fault System in the southwest, the Máze Fault System in the central area and the Iešjávri Fault System to the northeast. The distance between the fault systems is 7–12 km. The faults dip at an angle of 30–75° to the SE and can be traced on reflection seismic data to a depth of c. 500 m. Here we present data from trenching of different sections of the fault complex. The trenching reveals deformed overburden in all 8 sites, and inclusions of peat and organic bearing soil in the deformed and partly overrun loose deposits on the footwall in all but one site. Radiocarbon dating of organic matter located in buried and severely deformed sediment horizons indicates late Holocene ages for the (final) formation of the different fault segments, more specifically that the Máze, Fitnajohka and Iešjávri (Guovziljohka) Faults formed during earthquakes younger than 600 years, younger than 1,300 years and younger than 4,000 years BP, respectively. The youngest age is at the Masi (Mazé) site, where plant macrofossil data from the buried sediments suggest an early to late Holocene vegetation cover. The reverse displacement of c. 9 m and fault system lengths of 14 and 21 km of the two southernmost fault systems indicate a moment magnitude of c. 7 on Richter’s scale if just one rupture event is associated with each of these systems. The fault rupture with length and height of fault scarps, and injections and throw-out of angular boulders and wedges of fault breccia reaching up to 15–20 m away from the fault scarp give the most distinct expressions of the associated earthquake magnitude with the SFC. A total of c. 60 landslides, some of these possibly earthquake-induced, has been recorded along the SF. Initial breakage or fracturing of bedrock with a potential to lead to larger rock avalanches are also recorded at a few places in or close to the fault zone.