GEOLOGY AND SEISMIC POTENTIAL OF THE HUSTAIN FAULT, CENTRAL MONGOLIA

The Hustain fault is a 100-km long, northeasterly-striking, active extensional structure which bounds the Tuul River Valley (TRV) and is located within 30-km of Ulaanbaatar, Mongolia. Tectonostratigraphic terrain analysis and microscopic textures of radiolarian jaspers collected adjacent to the fault trace suggest that it originated as a late Paleozoic compressional structure during closure of the Mongol-Okhotsk Ocean. The fault plane was subsequently intruded by a Permo-Triassic (?) granitic pluton and a Cretaceous (?) felsic dike swarm. After a period of geologic quiescence during the early Cenozoic, the fault was reactivated as an extensional structure. The Hustain fault and TRV may represent the same extensional event that formed Baikal Lake, located 400-km due north. Because Tertiary rocks are not exposed in the TRV we could not determine the timing of reactivation.

The trace of the Hustain fault is clearly visible in false-color satellite images and topographic maps due to its large escarpments, truncation of Mesozoic intrusions, and a series of active springs located in the Quaternary alluvium. Topographic analyses of the TRV suggest the fault is listric and roots to the south into a regional low-angle décollemont. A southeast-northwest extensional vector (144º-324º ±5º azimuth), approximately perpendicular to the strike of the fault, was deduced from slickenlines, tensional veins, folds, and mineral lineations. This fault also possesses a significant strike-slip component as evidenced by sinistral offsets of recent spring and rock fall deposits. Based upon optically stimulated luminescence dating methods, one such rock fall dates to 7,750 ±1,500 years ago suggesting the fault has been active during the Holocene. Assuming a seismogenic crustal thickness of 10-km and a 100-km rupture length, this fault is capable of producing magnitude 7.5 earthquakes that could severely impact the inhabitants of Ulaanbaatar.