Paleoseismic trenching, slip-rates and kinematics of a high-angle rift-transform junction, Húsavík, Iceland

Abstract:

We present results of recent fieldwork related to the Húsavík-Flatey Fault (HFF), a ~100 km dextral fault on the southern boundary of the Tjörnes Fracture Zone in North Iceland through which the Kolbinskey spreading ridge is connected with Iceland's Northern Volcanic Zone (NVZ). High-angle rift-transform junctions are rarely exposed on land, and tectonic studies of them are relatively rare (e.g. Mouslopoulou, et. al., 2007) – accordingly, the HFF provides an excellent field laboratory for the study of strike-slip terminations and oblique tectonics in general. The 20 km long onshore section of the fault is deflected by ~15° driving transtensional deformation and these segments display complex kinematics as the ~6.8 mm/yr of dextral motion, estimated from GPS measurements (Metzger, et al., 2012) interacts with the ~20 mm/yr extension of the Northern Rift Zone. Structural mapping of the on land section suggests a pattern of oblique deformation which gradually transforms a significant portion of the strike-slip into dip-slip motion as the fault moves closer to the NVZ. Although bedrock exposures are rare, due to glacial and tectonic erosion, the “Arctic desert” environment preserves Holocene-age deformation signals in geomorphology and soil profiles. Deflected drainages and soft-sediment deformations provide insight on kinematics and offset tephras and lavas provide Quaternary to mid-Holocene vertical slip-rate estimates on some structures in the fault zone. Preliminary results of paleoseismic trenching in a small basin on the western on land segment show evidence of multiple earthquakes, in a sequence that extends at least to the earliest Holocene. The established tephrachronology in the region is primarily used for chronology and we will perform Geotek MSCL on the trench walls. These records will also be correlated with sediment cores taken from Botsvatn (Lake), located only 1.5 km from the trench site.