Conjugate strike-slip faulting along the Bangong-Nujiang suture zone accommodates coeval east-west extension and north-south shortening in the interior of the Tibetan Plateau

Abstract

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.