[1] A >500-km-long east-west trending metamorphic belt in the Qiangtang terrane of central Tibet consists of tectonic melange that occurs in the footwalls of Late Triassic -Early Jurassic domal low-angle normal faults. The melange is comprised of a strongly deformed matrix of metasedimentary and mafic schists that encloses lesser-deformed blocks of metabasites, Carboniferous-Triassic metasedimentary rocks, and early Paleozoic gneiss. Both the blocks and melange matrix exhibit greenschist, epidoteblueschist, and locally, epidote-amphibolite facies mineral assemblages. Thermobarometry reveals that the metamorphic belt experienced pressures of >10 kbar. Maximum equilibration temperatures for mafic schists in the melange matrix decrease from east to west, from $660°C near Shuang Hu (33°N, 89°E), $500°C near Rongma (33°N, 87°E), to $425°C near Gangma Co (34°N, 84°E). Equilibration at consistently high pressures over a large range of temperatures is compatible with metamorphism of Qiangtang melange within a low-angle subduction zone beneath a continental margin. Coupled structural, thermobarometric, and 40 Ar/ 39 Ar studies suggest that Qiangtang melange was exhumed in an intracontinental setting from depths of >35 km to upper crustal levels in <12 Myr by Late TriassicEarly Jurassic crustal-scale normal faulting. Detrital zircons from metasandstones within the melange matrix yield U-Pb ion-microprobe ages that range from early Paleozoic to Early Archean, and could have been sourced from terranes to the north of the Jinsha suture. Our results support a model in which Qiangtang melange was underthrust $200 km beneath the Qiangtang terrane during early Mesozoic flat-slab southward subduction of Paleo-Tethyan oceanic lithosphere along the Jinsha suture. This model predicts that significant portions of the central Tibetan continental mantle lithosphere were removed during early Mesozoic low-angle oceanic subduction and that the present-day central Tibetan deeper crust includes large volumes of underthrust early Mesozoic melange.

Geologic investigations of how the Tibetan plateau is currently deforming have focused primarily on its boundary faults. Consequently, how the interior of the plateau deforms remains poorly understood. To fill this gap in knowledge, we conducted field mapping, analysis of remote sensing and digital topographic data, and reinterpretation of existing geologic maps in central Tibet. This study reveals a 200–300 km wide and 1500–1800 km long east trending zone conjugate strike‐slip faults across central Tibet. The central Tibet conjugate fault zone is comprised of northeast striking left‐slip faults north of the Bangong‐Nujiang suture and northwest striking right‐slip faults south of the suture zone. These strike‐slip faults are kinematically linked with north trending Tibetan rifts located north and south of the conjugate fault systems. Without exception, all conjugate faults intersect or merge toward one another along the Bangong‐Nujiang suture zone. Motion on these faults accommodates coeval east‐west extension and north‐south contraction. To determine the fault kinematics and the magnitude of fault slip, we investigated three conjugate fault sets in the central Tibet fault zone. These include from east to west, the Dong Co, Bue Co, and Aishi Co conjugate fault systems, which are adjacent to the Bangong‐Nujiang suture zone and separated by a distance of 400 and 70 km, respectively. The average magnitude of fault motion on individual strike‐slip faults is ∼12 km as determined by offsets of Tertiary thrusts and Paleozoic‐Mesozoic lithologic units. The conjugate fault configuration requires ∼12 km of north‐south contraction across the 200–300 km fault zone since its initiation. Because the conjugate strike‐slip faults are kinematically linked with the north trending Tibetan rifts which initiated between 14 and 8 Ma, our estimated magnitude of north‐south contraction implies a contraction rate of ∼1–2 mm/yr across central Tibet. The relatively closely spaced (<150 km) basins may result from a series of conjugate strike‐slip fault systems in the interior of Tibet. These structures likely formed by eastward spreading of the Tibetan crust via distributed eastward extrusion of small (<150 km wide) wedge‐shaped crustal blocks that leave a space at their trailing end.

We have compiled the distribution of active faults and folds in the HimalayanTibetan orogen and its immediate surrounding regions into a web-based digital map. The main product of this study is a compilation of active structures that came from those documented in the literature and from our own interpretations based on satellite images and digital topographic data. Our digital tectonic map allows a comparison between the distribution and kinematics of active faults with the distribution and focal mechanisms of earthquakes. The active tectonic map is also compared with the contemporary velocity fi eld, obtained by global positioning system studies, that allows a better assessment of partitioning of decadal strain-rate fi elds across individual active structures that may have taken tens of thousands of years to a million years to develop. The active tectonic map provides a basis to evaluate whether the syncollisional late Cenozoic volcanism in Tibet was spatially related to the distribution and development of the active faults in the same area. These comparisons lead to the following fi ndings: (1) Tibetan earthquakes >M5 correlate well with mappable surface faults; (2) the short-term strain-rate fi eld correlates well with the known kinematics of the active faults and their geologic slip rates; and (3) Tibetan Neogene-Quaternary volcanism is controlled by major strike-slip faults along the plateau margins but has no clear relationship with active faults in the plateau interior. Although not explored in this study, our digital tectonic map and the distribution of Cenozoic volcanism in Tibet can also be used to correlate surface geology with geophysical properties such as seismic velocity variations and shear wave-splitting data across the Himalaya and Tibet.

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