Late Triassic sinistral shear in the East Gobi Fault Zone, Mongolia

Abstract

Tectonic studies of the East Gobi Fault Zone in southeastern Mongolia reveal multiple, distinct intracontinental deformation events postdating late Paleozoic arc accretion and continental amalgamation. Metamorphic tectonites of the Tsagan Subarga and Tavan Har blocks, previously mapped as Precambrian basement, comprise a sinistral shear zone dominated by steeply-dipping, northeast-striking foliations. Field observations and petrographic analyses indicate that the protoliths of the metamorphic tectonites are Paleozoic arc volcanic and sedimentary sequences. 40Ar/ 39Ar step-heating analyses of minerals from pre-, syn-, and late- to post-kinematic lithologies bracket the timing of ductile sinistral shear as Late Triassic. The main phase of distributed deformation associated with cooling through amphibolite–upper greenschist-facies conditions occurred ca. 225 Ma and shear zone activity waned ca. 210 Ma. Cooling rates inferred from the 40Ar/ 39Ar data are on the order of 40–20 °C Myr − 1 ; apparent differences for the two basement blocks may reflect subsequent differential uplift during Late Jurassic–Cretaceous rifting. Relatively rapid Late Triassic cooling suggests a transtensional component to the deformation and is coincident with core complex formation in northern China. Late Triassic intraplate deformation in southeastern Mongolia is likely the result of far field forces associated with collision between Mongolian arcs and the Siberian craton (i.e. closure of the Mongol-Okhotsk ocean) and/or collisions associated with closure of the Paleotethys. The ductile shear zone has been documented over 250 km along strike and has been modified by subsequent brittle deformation events in the Mesozoic and Cenozoic.