Gneiss domes in southern Tibet: A record of contraction, extension, and middle crustal flow

Several mechanisms have been proposed for the formation of gneiss domes, and each mechanism provides insight into middle crustal processes and regional tectonics. We combined geologic mapping, structural geology, metamorphic petrology, geochronology, and thermochronology studies to characterize middle crustal processes and gneiss dome formation in the gneiss domes of southern Tibet, and to understand the tectonic role of these gneiss domes during the India-Asia collision. Our results indicate that middle crustal rocks in the gneiss domes record history of late Eocene to late Miocene NS-contraction, followed by peak metamorphism reaching migmatite grade and vertical thinning and NS-horizontal stretching. Exhumation and formation of the domal geometry occurred as a consequence of subsequent thrust faulting and erosion. Contractional deformation is easily reconciled with the NS-collision between India and Asia, but based on strain compatibility arguments, middle crustal ductile extension  must have been accommodated by its southward extrusion.