Tectonic history of the East Gobi Fault Zone, southeastern Mongolia: An integrated study using structural geology, geochronology, and thermochronology.

摘要

Asia is an ideal natural laboratory to study the processes of continental growth and intracontinental deformation. Throughout the Paleozoic-Cenozoic numerous collisional orogenies between arcs, microcontinents, and cratonic blocks have built the continent, resulting in a complex tectonic setting where former collisional zones are commonly reactivated to accommodate the far-field effects of convergent plate boundary forces.;In southeastern Mongolia, the East Gobi Fault Zone (EGFZ) records a complex tectonic history of Late Paleozoic arc genesis, continent-arc collision, and subsequent Mesozoic-Cenozoic intracontinental deformation, making it an ideal location to study the processes of arc accretion and intracontinental deformation within the Asian subcontinent. This study used multiple methods to better constrain the tectonic history of the EGFZ. U-Pb zircon dating and microstructural analysis revealed that the protoliths to high-grade/high-strain basement in southeastern Mongolia, long suspected to be Precambrian in age, formed during Paleozoic-Mesozoic arc magmatism and continent-arc collision. These results do not support the existence of large swaths of Precambrian basement in southern Mongolia, long cited as evidence for the South Gobi Microcontinent. Instead they emphasize the heterogeneous, highly-deformed nature of the rocks found throughout this region.;Low-temperature thermochronology of exposed basement revealed that most exhumation along the EGFZ occurred during periods of intracontinental deformation during Late Triassic sinistral transtension(?) and late Early-Middle Jurassic shortening. Late Early-Middle Jurassic shortening is associated with the emplacement of large thrust sheets in the southern EGFZ. Reheating is recorded during Late Jurassic-Early Cretaceous extension, likely resulting from increased heat flow, volcanism, and burial by syn-rift sediments.;Analysis of low-temperature brittle structures along the fault zone revealed two phases of Cenozoic deformation. The first phase accommodated ~120 km of sinistral displacement along ENE-NE-striking, left stepping faults. It crosscuts Eocene and Oligocene strata but not Quaternary surface deposits. The second phase is recorded by mainly NW-W striking sinistral, N-NE-striking dextral, and W or E-dipping thrust faults. Where observed, displacements are not greater than a few kilometers. Phase 2 crosscuts Quaternary surface deposits and the kinematics of moderate earthquakes in the region indicate it is still active.