Geology of the Bangong suture Detailed

General map of central and southern Tibet depicting the Bangong suture (located between the BSZ and Qiangtang terrane) and Bangong suture zone (BSZ) and surroundingterranes. Also featured is the Amdo basement. Map is modified from Guynn et al., 2011.^[1]

The Bangong Nujiang suture zone (BNSZ) is located in central Tibet between the Lhasa (southern block) and Qiangtang (northern block) terranes. It is approximately 1200 km long and trends in an east-west orientation.^[2] The Bangong suture is 10–20 km wide and up to 50 km wide.^[3] In general, the suture is composed of a discontinuous belt of ophiolites and melange.^[1] Ophiolites are due to closing of the Bangong Ocean through subduction and subsequent obduction.^[3] The basic geology of the suture includes: Jurassic marine shale and conglomeratic strata, melange and ophiolites (as previously mentioned), and volcanic rocks.^[4] Volcanic rocks are due to multiple phases of magmatism. The Bangong suture is a key location in the central Tibet conjugate fault zone. The north part of the fault zone consists of northeast striking left lateral strike-slip faults while the south part consists of northwest striking right lateral strike-slip faults.^[5] All of these conjugate faults to the north and south of the Bangong "intersect or merge toward one another along the Bangong-Nujiang suture zone".^[5] Understanding the evolution and structure of these faults as well as other boundary faults (faults that surround the Tibetan plateau) is important to constraining the formation and deformation of the Tibetan Plateau.During the Mesozoic, India drifted northward towards Eurasia leading to an assembly of microcontinents and island arcs.^[6] The ancient Tethys oceanic realm inAsia separated these terranes, or microcontinents from each other. .^[1] The ancient Tethys oceanic realm is subdivided into three regions: the Paleo-Tethys, Meso-Tethys, and Neo-Tethys; the Meso-Tethys forms the oceanic division between the Lhasa and Qiangtang terranes. .^[1] The Meso-Tethys represents the oceaniclithosphere that separated Lhasa and Qiangtang and was later consumed upon subduction under Qiangtang. .^[1] Prior to Cenozoic Indo-Asian collision was the Jurassic-Cretaceous collision of the Lhasa and Qiangtang terranes.^[7] This collision formed the Bangong suture zone. Subduction of the southern Lhasa terrane beneath the northern Qiangtang terrane led to the closing of the Bangong Ocean.^[7] This led to obduction of ophiolites onto the northern bounding side of the Lhasa terrane.^[7][edit]

Collision and suture development

[edit]Cenozoic reactivation

This suturing of microcontinents was followed by the collision of India into Eurasia during the Cenozoic at ~45-55 Ma.^[8] This collision is marked by the closure of the Neo-Tethys Ocean which separated India from the southern edge of Eurasia (the Lhasa terrane).^[1] The penetration of India into Eurasia caused the northward reactivation of sutures, including the reactivation of the Bangong suture.^[9] This collision caused reactivation in the form of thrust and strike-slip faulting. The strike-slip fault in the Bangong suture zone is one of several large-scale strike-slip faults that move mostly undeformed continental blocks eastward away from the main convergent zone.^[5]

Cross section depicting the tectonic evolution of the Bangong suture zone. 1. An oceanic backarc basin forms separating the Amdo basement from the Qiangtang terrane. Slab rollback is a possible explanation for this extension. 2. Continued subduction of oceanic crust in the early-middle Jurassic. Closure of the oceanic back arc basin causes ophiolite obduction and a period of metamorphism in the Amdo basement. 3. In the early Cretaceous, the Lhasa and Qiangtang terranes collide creating the Bangong suture. A foreland basin also forms at this time. Cross section is modified from Guynn et al., 2006.^[7]

[edit]Implications of the Bangong suture

According to classic plate tectonic theory, deformation from the collision of India into Eurasia would be concentrated along the subduction zone. Tibetan tectonics acts contrary to this theory with significant deformation occurring along the north and north-east flanks of the Tibetan plateau. Two proposed end member models explain this unique deformation: "soft Tibet" and micro plate tectonics.^[10][9] Based on "soft Tibet", the lithosphere would thicken as a thin viscous sheet to accommodate convergence with broadly distributed shortening of both crust and mantle.^[10] Based on micro plate tectonics the sutures (including the Bangong suture) formed during the assembly of terranes (i.e., the Lhasa and Qiangtang) are reactivated in the Cenozoic.^[9]

[edit]End member model predictions

The two end member models make different predictions for reactivation along the Bangong suture. According to "soft Tibet", a series of small multiple faults would be evident along the suture zone due to the ductile composition of thelithosphere.^[10] Based on the microplate tectonics model, large strike-slip faults with significant displacement would dominate due to brittle deformation.^[9] These large strike-slip faults, caused by oblique subduction, would cause crustalextrusion in the form of left lateral strike-slip faults.^[9]