Great Jurassic thrust sheets in Beishan (North Mountains)—Gobi areas of China and southern Mongolia

Jurassic thrust sheets with minimum displacements of 120–180 km have been discovered within the ‘Hercynian-Indosinian’ orogenic belt of the Beishan of China and south Gobi area. The thrusts strike E-W, extend over 1200 km in length, and carried Meso-Proterozoic massive dolomitic limestones over strata ranging from NeoProterozoic (Cryogenian and Terminal Proterozoic) to Lower-Middle Jurassic. Slip-linear plots based on kinematic indicators, such as slickenlines and groove lineations, fiber lineations and ‘drag folds’ adjacent to the fault surface, vergence of folds and imbricated thrusts in the upper plate, indicate northward movement in the Beishan area to the west and southward movement in the South Gobi area to the east. The two major thrust faults, the Beishan thrust and South Gobi thrust, are presumably separated by a major tear fault, the Ruo Shui fault. The major thrust faults were later deformed into a series of E-W antiforms and synforms and the sheets are separated, due to erosion, into a number of klippen mainly located on synforms of the major faults. The Yagan metamorphic core complex, which is a result of an extensional event that postdates the thrust event, yields an40Ar-39Ar plateau age of 155.1 +- 10 Ma, and RbSr isochron age of 153 +- 6.2 Ma. The thrust sheets formed during the late Middle Jurassic, long after the closure of any oceans in the study area previously reported for this region, and are ascribed to a phase of intracontinental deformation. The closing of the Jurassic Tethys or retroarc deformation behind an active continental margin at the southern edge of Asia, prior to the Tethyan collision, or/and the closing of Mongolo-Okhotsk oceans might be responsible for this event.