PALEOSEISMIC DISPLACEMENT MEASUREMENTS FROM LANDFORMS SUBJECTED TO PERIGLACIAL PROCESSES: OBSERVATIONS ALONG THE JARAI GOL FAULT NEAR THE TAMYN AM HILLS, DARHAD DEPRESSION, NORTHERN MONGOLIA

The Jarai Gol rupture (Khil’ko et al. 1985) is located along a west-facing escarpment of the Sayan Mountains, at 51° 25’N, 99° 50’E in the Darhad depression of northern Mongolia. The fault trace is located east of two prominent, latest Pleistocene end moraine complexes known as the Tamyn Am Hills. A reconnaissance-level investigation primarily using morphostratigraphic position revealed that the fault deforms lateral and ground moraines as well as an inset (?Holocene) recessional outwash plain composed of multiple terraces heavily modified by icings (aufeis), but does not deform younger fluvial terraces located near the flood plain of the Jarai Gol (‘gol’=river).

Observations along 6 km of the trace indicate that motion on the ~N10°W striking Jarai Gol fault is oblique normal left-lateral. Motion is determined by tectonomorphology. The trace is composed of west-facing, ~N-S striking en echelon faults that are connected at left-steps by N30-40°W striking faults forming releasing bends. The entire trace contains well-formed troughs, pressure ridges, and depressions, characteristic of lateral motion. Ground moraines have multiple-event scarp heights of ~7.0-7.7 m with a far-field slope of ~18-20° and single-event scarp heights of ~2.3-3.6 m with maximum scarp slopes of ~33-35°. The younger (?Holocene) outwash plain has a single-event scarp with a height of ~3.6 m and maximum scarp slopes of ~40°.

Moraine and outwash plain landforms did not display any lateral offsets. Piercing points on the footwall of the outwash plain, in the form of terrace treads and risers do not cross the fault onto the hanging wall. The surface of the hanging wall is gently sloping and ~3-5 m lower in relief than the footwall. This surface is armored by tightly packed well-rounded cobbles that locally exhibit 1-2 m wide cobbley linear mounds that are ~0.5 m in relief resembling portions of stone polygons or nets, features typical of patterned ground. It appears that continual frost action has obliterated the original surface morphology and stratigraphy of the outwash plain on the hanging wall by heaving and inversely grading cobbles. This phenomenon presents difficulties in performing paleoseismic investigations (e.g. lateral displacement measurements or trenching) in regions that are subjected to periglacial processes such as northern Mongolia.