Active Faulting Along the Ulakhan Fault, Seimchan-Buyunda Basin, Northeast Russia

The Seimchan-Buyunda basin is a topographically flat, 30 x 100 km region that is located along the eastern margin of the Ulakhan fault in northeast Russia. The Ulakhan fault roughly parallels the accretionary boundary between the Kolyma-Omolon superterrane and Siberian craton and delineates the modern plate boundary between the Okhotsk block and the North American plate proper. The subsidence history of the Seimchan basin and scale of seismic activity of the Ulakhan fault remain widely speculative due largely to a lack of detail study in this region. However, preliminary evidence suggests that active subsidence in the basin is being driven by modern offsets along the proto-Ulakhan fault. Our recent field investigations along the Ulakhan fault near the southern edge of the Seimchan basin reveal sag features, pressure ridges, troughs, and a scarp sections that delineates the trace of the fault over ~10 km, across an alluvial fan. These features can be traced within the basin on satellite images to the northwest and southeast of our survey and show left-lateral strike-slip motion associated with the fault. Previous studies support recent offset of up to 24 km as evidenced by river offsets observed northwest of the basin in the Chersky Range. Recent uplift adjacent to the Ulakhan fault and subsequent subsidence within the Seimchan basin may have been recorded by the down cutting of the Kolyma and Indigirka Rivers that have left a series of preserved meander terrace plateaus along the southern and northern margins of the basin. If one accounts for the present location of abandoned incised meanders on the north side of the basin, a 24 km offset is also apparent. A tectonic reconstruction of the area for a time prior to this offset using presumed fault motions shows that the Ulakhan fault system may be responsible for the development of the Seimchan-Buyunda basin as a large pull-apart structure. Seismicity along the Ulakhan fault has been poorly documented due to a lack of local seismic stations. However, preliminary field deployment with short-period instruments indicates a considerably higher level of seismicity than previously expected. Given the location of the Ulakhan fault along the suture boundary, new evidence for left- lateral offset, and recent seismicity, the Ulakhan clearly represented a major locus of displacement in the Pliocene that continues to take up considerable strain at the edge of the North American plate.