Thermochronologic Records of Intraplate Deformation in the Northern East Gobi Fault Zone, Mongolia

Tavan Har and Tsagan Subarga comprise faulted basement blocks within in the northern East Gobi Fault Zone that record a complex history of polyphase Triassic to Quaternary intraplate deformation. Part of the Central Asia Orogenic Belt, or Altaids, the early Mesozoic history includes partial melting and metamorphism of principally Carboniferous arc/forearc protoliths and development of a northeast-striking Middle-Late Triassic ductile sinistral shear zone. Based on U/Pb zircon and 40Ar/39Ar dating, deformation at amphibolite-facies conditions, large scale isoclinal folding, and emplacement of syntectonic intrusions occurred ca. 225 Ma and were followed by relatively rapid cooling. Ductile deformation and cooling rates waned by ~210 Ma. The integrated thermal history (including apatite fission track data and (U-Th)/He ages) indicates a second phase of rapid cooling during the late Early-Middle Jurassic, followed by either moderate to slow cooling or reheating during the Late Jurassic-Early Cretaceous. The later phases of cooling and/or reheating are linked to Middle Jurassic thrusting and Late Jurassic-Early Cretaceous rifting, respectively. While no distinct structures at Tavan Har or Tsagan Subarga related to Middle Jurassic thrusting have been identified in outcrop, seismic reflection profiles from the East Gobi Basin reveal folded and faulted reflectors corresponding to an Early Jurassic pre-rift foreland basin megasequence and reactivation of its non-conformity with Paleozoic basement. Late Jurassic-Early Cretaceous rifting is well documented regionally and rift basin bounding faults appear to reactivate NE-striking fabrics of the Late Triassic shear zone. The 40Ar/39Ar feldspar data suggest that exhumation associated with rifting was more pronounced at Tavan Har compared to Tsagan Subarga. Syn-rift deposits locally onlap the mylonitic foliation of the Late Triassic shear zone, and in other localities contacts between the metamorphic basement and syn-rift sediments are faulted. Detrital zircon U/Pb age probability spectra from rift strata record major Late Jurassic-Cretaceous peaks and minor Carboniferous, Permian, and Triassic peaks. The present distribution of basement apatite fission track and (U-Th)/He ages is, in part, a consequence of subsequent Late Cretaceous-Cenozoic strike-slip deformation. Other structures may contribute to subtle differences in exhumation level, particularly for faults active during Late Jurassic-Early Cretaceous rifting or middle Cretaceous basin inversion, but post Early Cretaceous exhumation is likely negligible. In general, thermochronological age constraints for deformation at Tavan Har correlate well with the history of Asian orogenesis and plate margin processes suggesting they are the primary driving forces of intraplate deformation within the East Gobi Fault Zone.