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.

Crustal–lithospheric structure and continental extrusion of Tibet

“…These are reflected along a slight relative eastwest topographic low by NE-SW left-lateral strike-slip faults that lie just north of the Bangong-Nujiang suture in the Qiangtang terrane and are also kinematically linked to the north with further north-south-oriented normal faults (Yin et al 1999). This pattern of faulting indicates a 250 km by 1500 km east-west-trending zone of conjugate strike-slip faulting across central Tibet that simultaneously accommodates north-south contraction and east-west extension (Taylor et al 2003). By mapping three of the fault systems, Taylor et al (2003) found average offsets in Tertiary thrusts and Palaeozoic-Mesozoic lithological units of only 12 km.…”

“…This pattern of faulting indicates a 250 km by 1500 km east-west-trending zone of conjugate strike-slip faulting across central Tibet that simultaneously accommodates north-south contraction and east-west extension (Taylor et al 2003). By mapping three of the fault systems, Taylor et al (2003) found average offsets in Tertiary thrusts and Palaeozoic-Mesozoic lithological units of only 12 km. This supports a model of coeval east-west extension and north-south contraction on multiple faults in a distributed pattern rather than the Qiangtang terrane behaving as a rigid block translating eastward, the latter requiring larger offsets on these faults.…”

“…This is similar to estimates from Armijo et al (1988) of several kilometres of offset with slip rates of 1-4 mm a À1 for faults in southern Tibet. In a mirror image to the connection of strike-slip faults to the southern Tibet rifts, the rifts of northern Tibet are connected at their southern end to left-lateral strike slip faults (Taylor et al 2003). This forms a set of conjugate strike-slip faults that accommodates the coeval north-south shortening and east-west extension of the plateau (Taylor et al 2003;Taylor & Peltzer 2006).…”

“…In a mirror image to the connection of strike-slip faults to the southern Tibet rifts, the rifts of northern Tibet are connected at their southern end to left-lateral strike slip faults (Taylor et al 2003). This forms a set of conjugate strike-slip faults that accommodates the coeval north-south shortening and east-west extension of the plateau (Taylor et al 2003;Taylor & Peltzer 2006). McCaffrey & Nabelek (1998) suggested that the driving force for the extension seen in southern Tibet was basal shear from the oblique sliding of the Indian plate beneath the plateau.…”

“…The right-lateral SE-NW strike-slip faults lie largely south of the Bangong-Nujiang suture in the Lhasa terrane and are terminated in the south by north-south normal faults (Taylor et al 2003; Fig. 1).…”