STRIKE-SLIP AND EXHUMATION HISTORY OF THE EAST GOBI FAULT ZONE, SOUTHEASTERN MONGOLIA, WITH EMPHASIS ON THE CENOZOIC ERA

The East Gobi Fault Zone (EGFZ) is a NE-trending structural corridor in southeastern Mongolia that records a polyphase history of Mesozoic–Cenozoic intracontinental deformation and has been featured in tectonic models of far-field deformation resulting from India-Asia collision, including a possible linkage with the Altyn Tagh fault. We present results from field-based studies of the EGFZ that bear on such models.

New work in the southern EGFZ has mapped two generations of faults of inferred Cenozoic age near the Mongolia–China border and, when combined with previous work, suggest a two stage deformation history for the EGFZ during the Cenozoic: 1) NE-striking sinistral faults were identified that are characterized by N-trending maximum shortening axes and E-trending maximum extension axes, constrained by fault-slip inversion. During this stage, the EGFZ was a major sinistral fault zone, likely connected to the Altyn Tagh and Alxa fault zones to the southwest. Compared to the northern EGFZ, sinistral faults are less prominent in the southern EGFZ where inherited preexisting weaknesses in the basement are less suitably oriented for reactivation. 2) Major sinistral faulting is overprinted by younger deformation that is kinematically similar to that found in the Gobi Altai region of Mongolia. N- to NE-striking dextral faults, roughly E- to SE- striking sinistral faults, and E-striking thrust faults cross-cut both Paleozoic basement and unconsolidated sediment and are associated with NE–ENE-trending shortening axes. Though historically relatively quiescent when compared to western Mongolia, the kinematics of this deformation is similar to the focal mechanisms of infrequent earthquakes near the EGFZ.

⁴⁰Ar/³⁹Ar and apatite fission track thermochronology of samples from exposed amphibolite-grade tectonites that record Late Triassic sinistral shear (ca. 225 Ma) in the northern EGFZ suggest a 3 stage cooling history with stages of relatively rapid cooling during (1) Late Triassic sinistral transtension(?), (2) early–Middle Jurassic shortening, and (3) Late Jurassic–Early Cretaceous extension. The lack of a Cenozoic signal in the thermochronology results and the subdued topography of the EGFZ suggest that major Cenozoic sinistral faulting was not associated with a significant vertical-oblique component.