The crustal assembly of southern Mongolia: New structural, lithological and geochronological data from the Nemegt and Altan ranges

The Gobi Altai region is an ideal setting for studying processes of continental growth and subsequent intracontinental and intraplate deformation, including terrane accretion and dispersal, ophiolite obduction, crustal reactivation and intraplate mountain building. To assess the diverse tectonic evolutionary models of the Gobi Altai and the wider region, more field data and geochronological data are required to constrain the tectonic evolution of individual terranes, and the relationship of adjacent crustal domains to each other throughout time. In this paper, we present new lithological, structural and 40Ar/39Ar age data, which constrain the crustal evolution across a previously unreported late Paleozoic terrane boundary in the Gobi-Altai. and Altan Nuruu are topographically linked mountain ranges that were formed by Miocene-recent uplift at a right-stepping restraining bend along the left-lateral Gobi–Tien Shan Fault System in southern Mongolia. Ordovician–Carboniferous arc rocks and an ophiolite are exposed in the mountain ranges and form a small part of the east–west arcuate Trans-Altai Zone. Field observations of rock types and structures, combined with petrographic data are used to distinguish metamorphosed volcano-sedimentary arc rocks in Altan Nuruu and western Nemegt Nuruu from arc rocks in central and eastern Nemegt Nuruu. These distinct sequences are correlated with the Dzolen and Edrengin terranes in the Trans-Altai Zone along strike to the west. Integration of field data, 40Ar/39Ar age data and published studies are used to describe a polyphase deformation history that includes late Carboniferous ophiolite obduction, mid-Permian to late Triassic shortening and lateral terrane redistribution, Cretaceous rifting and late Cenozoic intraplate mountain building.