MAJOR MESOZOIC CONTRACTIONAL AND EXTENSIONAL EVENTS IN SOUTH GOBI AREA, CHINA AND MONGOLIA

Field studies along the China-Mongolia border in the South Gobi area, traditionally regarded as part of a late Paleozoic orogenic belt, have revealed two major Mesozoic tectonic events: a major south-directed thrust of Jurassic age and the younger Yagan-Onch Hayrhan metamorphic core complex. In the earlier event Meso-Proterozoic dolomitic limestone was emplaced over Paleozoic to Lower and Middle(?) Jurassic rocks with a minimum displacement of 180 km. The core complex developed across the southern part of the thrust sheet, where a regional extensional detachment fault truncates the thrust surface. Kinematic indicators in its footwall mylonitic shear zone conclusively demonstrate that higher structural levels were displaced SSE-ward relative to deeper levels. The core complex is one of the largest in eastern Asia, with an along strike length (ENE) of ca 90 km. Three biotite samples in syn-extensional footwall mylonitic rocks on the Mongolian (Onch-Hayrhan) side of the border yield 40Ar/39Ar ages from 129 to 126 Ma and appear to constrain late stages of ductile extensional shear (Webb et al., 1999).

An earlier history of footwall magmatic events on the China (Yagan) side of the border is revealed by samples recently collected and dated by us. Zircons from a late extensional pluton in the footwall of the detachment fault give an Early Cretaceous U/Pb age of 135 +/- 1 Ma. A mylonitic granite below the detachment fault yields a U/Pb age of 228 +/- 7 Ma. Whole rock Rb-Sr ages from this tectonite are 161 +/- 10 Ma and possibly represent early higher temperature extensional deformation. Perhaps the most surprising information comes from the dating of a footwall augen gneiss. Its U/Pb zircon age of 916 +/- 16 Ma reveals that the metamorphic core complex has evolved within a previously unrecognized Precambrian terrane. Given the regional uniqueness of this young Proterozoic age, the possibility exists that this terrane was a microcontinental component of the Paleozoic orogenic collage that developed along the northern border of the Archean North China craton.