Paleozoic-cenozoic evolution of the East Gobi Fault Zone, southern Mongolia: A protracted record of intracontinental deformation and basin evolution, with implications for tectonics in Eurasia.

摘要

The East Gobi Fault Zone in southeastern Mongolia and associated the East Gobi Basin, is an ideal laboratory for delineating polyphase deformation and basin evolution in an intracontinental setting. Structural interpretation of proprietary 2-D seismic reflection data and correlation to subsurface interpretations reveals six distinct phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin-fill of the East Gobi Basin. These phases include arc accretion and arc-continent collision in the late Paleozoic, Early Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style compression, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous compression with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene left-lateral strike slip faulting across the fault zone. The five postamalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the East Gobi Basin.;Permian sedimentary sequences exposed in southern Mongolia record the final closure of the Paleo-Asian ocean and suturing of the North China Block and the southern margin of the Altaids along the Tien Shan-Yin Shan suture. Geologic mapping, stratigraphic data, sandstone provenance and U-Pb zircon geochronology results suggest that exposures of sedimentary strata at two localities in southern Mongolia, Bulgan Uul and Nomgon, were once part of the same closing ocean basin. Paleocurrent measurements and provenance data collected from both study locations document southeastern transport directions for sediment derived from extinct Carboniferous and Devonian arcs of the southern Altaids. The results are consistent with models for remnant ocean basins, and indicate diachronous west-to-east closure of the Paleo-Asian ocean in the Late Permian.;Analysis of proposed piercing points across the EGFZ, includes new stratigraphic, structural and geochronologic data, allowing for the characterization for two periods of eft-lateral displacement across the EGFZ; a total of ~250-300 km of left-lateral displacement. Approximately 150-200 km of slip occurred along a major shear zone during the Early Triassic (∼210-225 Ma), based on restoration of Carboniferous intrusive suites and Permian remnant ocean basin deposits. A second, lesser-known period of ∼90-100 km of slip occurred in the Late Oligocene, based on offset of Early Cretaceous units and cross-cutting relationships. Cenozoic movement along the EGFZ may have coincided with initiation of the Altyn Tagh Fault in China via accommodation features in the Alxa region. Magnitudes of Cenozoic offset across the EGFZ are significantly less than estimated for the Altyn Tagh Fault, which likely reflects distributed deformation in heterogeneous crust of the southern Altaids.