Structural evolution and sequence of thrusting in the Tethyan fold-thrust belt and Indus-Yalu suture zone, southwest Tibet

Abstract

Regional mapping of a north-south traverse from the India-Nepal-China border junction to Mount Kailas in southwest Tibet—combined with previously published geochronologic and stratigraphic data—is the basis for an incremental restoration of the Tethyan fold-thrust belt and deformation along the Indus-Yalu suture zone. From north to south, the major structural features are (1) the Indus-Yalu suture zone, composed of five south-dipping thrust faults involving rocks interpreted to represent parts of the former Indian passive margin and Asian active margin, (2) the Tethyan fold-thrust belt, composed of a dominantly north-dipping system of imbricate thrust faults involving Precambrian through Upper Cretaceous strata, and (3) the Kiogar-Jungbwa thrust sheet. A linelength cross-section reconstruction indicates a minimum of 176 km of north-south horizontal shortening partitioned by the Tethyan fold-thrust belt (112 km) and IndusYalu suture zone (64 km). Sequential restoration of the cross section shows that the locus of shortening prior to the late Oligocene occurred significantly south (possibly .60 km) of the Indus-Yalu suture zone within the Tethyan fold-thrust belt and that a significant amount of unsubducted oceanic lithosphere was present south of the suture in southwest Tibet until that time. An implication of this result is that postcollision (Oligocene/Miocene) high-K, calcalkaline magmatism may be explained by melting due to active subduction of oceanic crust beneath the Kailas magmatic complex until the late Oligocene. A regional profile across the Tibetan-Himalayan orogen from