GEOLOGICAL RECONNAISSANCE OF THE MALANGIYN GOL AND TöHöMIYN NUUR VALLEYS, CENTRAL MONGOLIA: IMPLICATIONS FOR POTENTIALLY ACTIVE FAULTING

Remote sensing and geologic field observations reveal potentially active faulting in the Malangiyn Gol and Töhömiyn Nuur Valleys, approximately 190 km southwest of Ulaanbaatar, Mongolia. The Malangiyn Gol Valley trends N60°W, is approximately 80 km long and 10 km wide and is defined by left-stepping strike-slip faults. The average strike of these faults is N55°W. The strike-slip faults form scarps, sag ponds, and linear valleys and truncate alluvial fans. One of the fault segments curves to the east and changes strike to E-W. The Malangiyn Gol Valley is directly northwest and adjacent to the Töhömiyn Nuur Valley.

The Töhömiyn Nuur Valley is roughly rectangular in shape, covering approximately 2,300 km². The northern edge of the Töhömiyn Nuur Valley is bounded by a set of strike-slip faults that extend north into the Malangiyn Gol Valley. These faults change strike from N55°W to E-W near the town of Buren. The western and southwestern sections of the Töhömiyn Nuur Valley display a set of N-S- and NW-striking normal faults and a set of NW-striking strike-slip faults. The normal faults appear to be older and truncated by the strike-slip faults. Geologic and geomorphic evidence for normal faulting includes triangular facets, fault benches, slickenlines and meter-wide breccia zones. Sets of E-W to E-NE-striking igneous dikes are abundant in the southern part of the valley and impart a fabric to the topography visible in remote sensing images.

In general, historic seismicity in Mongolia occurs along E-W-striking, left-lateral strike-slip faults and NW-striking, right-lateral strike-slip faults. Results of this study indicate more complicated fault kinematics. The faults in this area appear to be late Quaternary in age and may be capable of producing destructive earthquakes. Although the study area is sparely populated, the proximity of these faults to Ulaanbaatar may have important ramifications for seismic hazard assessment.