INTRAPLATE DEFORMATION PHASE I OF THE EAST GOBI FAULT ZONE, MONGOLIA: EARLY MESOZOIC SINISTRAL SHEAR

The East Gobi Fault Zone (EGFZ) is a major tectonic feature of SE Mongolia defined by a structural corridor of NE-striking faults. Paleozoic rocks in this region record collision and accretion of volcanic arcs, marine basins and fragments of continental crust. Metamorphic tectonites in the EGFZ have traditionally been mapped as Precambrian basement due to relatively high metamorphic grade and/or ductile strain. However, combined structural and basin analysis studies of the EGFZ reveal that the metamorphic tectonites belong to a ductile shear zone in which deformation postdates Late Paleozoic arc accretion and continental amalgamation, and predates Late Jurassic-Early Cretaceous rift basin formation.

We have documented the shear zone over a distance of 250 km along strike within the EGFZ. The shear zone comprises a suite of synkinematic intrusions, amphibolite-facies gneisses and greenschist-facies mylonites. Fabrics are dominated by S-L tectonites with steeply-dipping NE-striking foliations and subhorizontal WSW-plunging stretching lineations. Overall, the exposed structural levels of the regional shear zone appear to deepen to the NE. In areas that correspond to higher structural levels, variably metamorphosed Paleozoic sedimentary sequences are caught up in discrete zones of mylonitic deformation.

Kinematic indicators such as S-C fabrics, asymmetric boudinage, and sigma and delta-type objects have been observed at mesoscopic and microscopic scales and indicate sinistral shear sense. Lithic fragments and feldspar porphyroclasts that display magmatic zoning are present in several samples, supporting the inference that Paleozoic arc and sedimentary sequences are protoliths for at least some of the metamorphic tectonites exposed at deeper structural levels. In addition to one previously published Ar/Ar weighted mean age of 209 +/- 2 Ma on biotite from mylonite, regional overlap relations also suggest a Late Triassic to Late Jurassic age of deformation.

At least three subsequent deformation events have exploited the shear zone: Late Jurassic-Early Cretaceous rifting, mid-Late Cretaceous transpression, and early Cenozoic sinistral strike-slip faulting. Thus the EGFZ records a long history of intraplate deformation likely associated with the growth and amalgamation of Asia.