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 b een discovered within the 'Hercynian-Indosinian' orogenic belt of the Beishan of China and south Gobi area. The thrusts strike E-W, extend o ver 1200 km in length, and carried Meso-Proterozoic massive dolomitic limestones over strata ranging from Neo-Proterozoic (Cryogenian and Te rminal Proterozoic) to Lower-Middle Jurassic. Slip-linear plots based on kinematic indicators, such as slickenlines and groove lineations, f iber lineations and 'drag folds' adjacent to the fault surface. vergen ce of folds and imbricated thrusts in the upper plate, indicate northw ard 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 Bei shan thrust and South Gobi thrust, are presumably separated by a major tear fault, the Ruo Shui fault. The major thrust faults were later de formed 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 Ar-40-Ar-39 metamorphic core complex , which is a result of an extensional event that postdates the thrust event, yields an Ar-40-Ar-39 plateau age of 155.1 +/- 10 Ma, and Rb-Sr isochron age of 153 +/- 6.2 Ma. The thrust sheets formed during the l ate 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 o r retroarc deformation behind an active continental margin at the sout hern edge of Asia, prior to the Tethyan collision, or/and the closing of Mongolo-Okhotsk oceans might be responsible for this event. Copyrig ht (C) 1996 Elsevier Science Ltd