STRUCTURAL GEOLOGY OF THE DEREN SEISMIC ZONE AND Bïrgèd FAULT, SOUTH-CENTRAL MONGOLIA

The Deren Seismic Zone is located between 46° 10’ and 46° 20’ N, 106° and 108° E. Historic seismicity of this region shows a 250 km x 50 km zone of earthquakes near and extending beyond the Deren Fault. There are at least three faults within the Deren Seismic Zone, including the: Deren Fault, West Deren Fault, and Bїrgèd Fault. Based on this study, it appears that these three faults have different levels of activity. This study focused on the West Deren Fault and the Bїrgèd Fault. Satellite images and field mapping along these faults indicate several structural relationships. The Bїrgèd Fault has a strike of ~N36ºE and a surface expression ~30 km in length. It is truncated to the northeast by the younger, E-W striking West Deren Fault and does not appear to continue north of this point. The West Deren Fault loses its surface expression at 106º 3’ E, 3 km west of this intersection. However, the satellite images show the West Deren Fault may continue ~10 km. The lithologic units along the Bїrgèd Fault and West Deren Fault primarily consist of resistant medium- to coarse-grained metasandstone, as well as recessive moderately metamorphosed siltstone. There is a discordance of bedding across the Bїrgèd Fault, which bifurcates toward the southwest. These units are broadly folded into gently plunging antiforms and synforms trending N63ºE and S63ºW. The wavelengths of these folds range from 3-7 kilometers. Data suggests folding occurred before the formation of the Bїrgèd Fault, and before the large Mesozoic(?) granitic intrusion that is within Baga Gadzrїn Chuluu Park. Joints with similar orientations occur in both the intrusive and metasedimentary rocks. These joints have a spacing of 1cm–1m and are oriented N60ºE 70ºSE, N10ºE 30ºNW, and E-W 70ºS. Suggested future work on the Bїrgèd Fault includes determining its dip and lateral extent.