General map of central and southern Tibet

The Bangong suture zone is a key location in the central

Tibet Tibet (; ''Böd''; ) is a region in East Asia, covering much of the Tibetan Plateau and spanning about . It is the traditional homeland of the Tibetan people. Also resident on the plateau are some other ethnic groups such as Monpa, Taman ...

conjugate fault zone. Approximately 1,200 km long, the suture trends in an east–west orientation. Located in central Tibet between the

Lhasa Lhasa (; Lhasa dialect: ; bo, text=ལྷ་ས, translation=Place of Gods) is the urban center of the prefecture-level Lhasa City and the administrative capital of Tibet Autonomous Region in Southwest China. The inner urban area of Lhas ...

(southern block) and Qiangtang (northern block) terranes, it is a discontinuous belt of

ophiolites An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found i ...

and

mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically cons ...

that is 10–20 km wide, up to 50 km wide in places. The northern part of the fault zone consists of northeast striking

sinistral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...

strike-slip faults while the southern part consists of northwest striking right lateral strike-slip faults. These conjugate faults to the north and south of the Bangong intersect with each other along the Bangong-Nujiang suture zone.

Description

The Bangong-Nujiang Suture is a ~1200 km long east-west trending zone that separates the

and Qiangtang terranes. It can be divided into three parts:

Bangong Lake Pangong Tso or Pangong Lake (; ; hi, text=पैंगोंग झील) is an endorheic lake spanning eastern Ladakh and West Tibet situated at an elevation of . It is long and divided into five sublakes, called ''Pangong Tso'', ''Tso N ...

-Gertse (western sector), Dongqiao-

Amdo Amdo ( �am˥˥.to˥˥ ) is one of the three traditional Tibetan regions, the others being U-Tsang in the west and Kham in the east. Ngari (including former Guge kingdom) in the north-west was incorporated into Ü-Tsang. Amdo is also the ...

(middle sector), and Dingqing- Nujiang (eastern sector). During the Middle to Late

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, northward subduction of the Meso-Tethys Ocean between the Lhasa and Qiangtang terranes ceased, and during the Early Cretaceous, the Lhasa terrane began underthrusting beneath the Qiangtang terrane. Traces of the Meso-Tethys Ocean are left as fragments of obducted ophiolites within

serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''se ...

-matrix

scattered along the BNS.

Collision and suture development

The geology of the suture includes

marine shale and conglomeratic strata, melange and ophiolites and volcanic rocks from multiple pulses of magmatism. Each of these lithologies can be tied to specific terranes, either island arcs or microcontinents, that were gathered in front of the Indian subcontinent as it drifted northward during the Mesozoic. During the Jurassic-Cretaceous collision of the Lhasa and Qiangtang terranes, the ancient Tethys ocean closed, creating the Bangong suture zone. Oceanic lithosphere (the Meso-Tethys) was consumed during this collision and subducted under the Qiangtang terrane. This led to

obduction Obduction is a geological process whereby denser oceanic crust (and even upper mantle) is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge ...

of ophiolites on the northern margin of the Lhasa terrane This period of obduction is generally accepted to mark the end of oceanic subduction beneath southern Qiangtang and the onset of Lhasa-Qiangtang collision. An important feature of the Bangong suture is the Amdo basement. This exposure of pre-Mesozoic crystalline basement is ~100 km long and ~50 km wide. Geology of the Amdo records Mesozoic metamorphism, magmatism, and exhumation and is composed of orthogneisses and metasediments which are intruded by undeformed granitoids.

Cenozoic reactivation

Suturing of microcontinents was followed by the continued northward drift of the Indian subcontinent, colliding with

Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago ...

during the Cenozoic, about 45-55 million years ago. Since the India-Eurasia collision, the convergence rate with Eurasia is predicted to have slowed by more than 40% between 20 and 10 Ma due to crustal thickening. The high Tibetan plateau resisted further crustal thickening leading to the slowing of convergence and subsequent migration of crustal shortening to the flanks of the plateau. The closure of the Neo-Tethys Ocean occurred at this time, as the southern edge of Eurasia (marked by the Lhasa terrane), collided with India. The penetration of the India into Eurasia reactivated the suture zone (which is located in the middle of the Tibetan Plateau), causing northward movement of both

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s and strike-slip faults. Strike-slip faults were responsible for moving mostly undeformed continental blocks eastward, away from the main convergent zone. Cross section depicting the tectonic evolution of the Bangong suture zone

Cross section depicting the tectonic evolution of the Bangong suture zone

Implications of the Bangong suture

Classical interpretations of

plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

indicates that deformation from the Eurasian-Indian collision should be concentrated along the subduction zone. The Tibetan system does not act in this way, however, with significant deformation occurring along the north and north-east flanks of the

Tibetan plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the ...

. To solve this problem, two end-member models were proposed: a "soft Tibet" model and micro-plate tectonics. According to the "soft Tibet" model, the lithosphere behaves as a thin viscous sheet to accommodate broadly distributed shortening of both crust and lithospheric mantle. Micro-plate tectonics suggests that each terrane acts on its own, according to its own boundaries, and the sutures between them (including the Bangong suture between the Lhasa and Qiangtang) are reactivated in the Cenozoic.

End member model predictions

Each of the two models makes a different prediction for reactivation along the Bangong suture. The "soft Tibet" model suggests that a series of small multiple faults along the suture zone would occur, due to the

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

nature of the lithosphere. Based on the micro-plate tectonics model, large strike-slip faults with significant displacement should be present. Crustal extrusion (in the form of sinistral strike-slip faults) should also be present and would be caused by oblique subduction at the edges of the suture zone. 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 Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...

of the

Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the ...

. Research to identify features in the field that would satisfy either of these hypotheses is ongoing.

References