TIMING AND NATURE OF RECENT MOVEMENT ALONG THE EGIIN DAVAA FAULT IN CENTRAL MONGOLIA, AND THEIR TECTONIC IMPLICATIONS

The Egiin Davaa fault (EDF, 47.15°N, 99.835°E) is a ~85 km long normal fault in the Hangay Mountains of central Mongolia, an area of doming and extension within the larger tectonic context of the Indian-Eurasian collision. A relatively young scarp (1-4 m high) occurs along the 52 km southwest segment of the fault and was the subject of this study. The scarp is assumed to have been formed in one event. Khil'ko et al. (1985) report that the last rupture on the EDF likely occurred in 1570 AD based upon local legends and interpreted scarp morphology. However, radiocarbon dating of colluvial sediments from two paleoseismic trenches constrain the age of the most recent rupture to between 7,369 ± 35 and 4,916 ± 38 calibrated years BP. Furthermore, diffusion modeling of scarp profiles supports the radiocarbon age of the last earthquake rupture and discredits a more recent event age based upon the earthquake legend. Slickenlines indicate that right-lateral strike-slip is equal to dip-slip. Mean total displacement on the active scarp is 7 m. Based on this displacement and the surface rupture length, the most recent earthquake on the EDF has a moment magnitude (Mw) of 7.3 ± 0.2. Last glacial maximum recessional moraines along the fault have been offset by about 8 m vertically (~17 m total displacement). Cosmogenic surface exposure ages of moraine-top boulders between 17 and 14 ka, suggest a slip rate for the EDF of approximately 1 mm/yr. If the most recent event was a characteristic earthquake, then the faulted moraine likely preserves two events and the recurrence interval for the fault is ~10 ka. The direction of maximum extension of the EDF is 021º. This is nearly parallel with the direction of compression induced by the Indian-Eurasian collision, indicating that the strain on the EDF is not consistent with the expected strain from the Indian-Eurasian collision. Extension of the EDF is consistent, however, with previous studies in the Hangay Mountains suggesting that extension in all directions is caused by buoyant uplift resulting from processes distinct from the Indian-Eurasian collision.