STRUCTURAL GEOLOGY AND REGIONAL TECTONICS OF PALEOZOIC STRATA, SHIN JINST, SOUTHERN MONGOLIA

Phanerozoic rocks of southern Mongolia record a key part of the tectonic growth, amalgamation, and deformation of central Asia. Understanding the deformation of these rocks is crucial to unraveling the geologic history of central Asia and will contribute to our understanding of continental growth and intracontinental deformation. Few structural studies, however, have been conducted in this region. We present here initial results of a structural study of one area within southern Mongolia, near the town of Shin Jinst. We mapped two areas, chosen for their location, excellent exposure, age of formations, and diversity of lithologies.

The stratigraphy within the mapping area is Ordovician to Carboniferous and includes limestone, shale, sandstone, conglomerate, and pyroclastic flow deposits. Beds strike northwest to southeast, typically ranging from 90 to 140°, and are steeply dipping to the northeast to slightly overturned to the southwest.

Left-lateral strike-slip faults are the most prevalent fault type in the area, with offsets ranging from a few centimeters to three kilometers. The majority of these strike east-northeast (50-85°), while two smaller populations strike north-northeast (15-30°) and south-southeast (140-170°). All are vertical to steeply-dipping. There is a smaller population of right-lateral strike-slip faults that strike 135-170° and are vertical to steeply-dipping. Field relations, including repeated section, and numerous folds indicate the presence of thrust faults, only a few of which are well-exposed.

The right-lateral strike-slip faults and thrust faults are typically cut by left-lateral faults. The thrust faults may have developed during contraction associated with collision of tectonic blocks to the south during the late Paleozoic and early to middle Mesozoic. Left-lateral strike-slip faults may be related to much larger, similar faults active in Mongolia and China during the Mesozoic and Cenozoic (Lamb et al., 2001). Alternatively, the strike-slip faults may be tear faults and thus coeval with thrust faulting. These faults are cut by a younger, east-west striking fault that extends for over 70 kilometers.