General map of central and southern Tibet

The Bangong suture zone is a key location in the central

Tibet Tibet (; ''Böd''; ), or Greater Tibet, is a region in the western part of East Asia, covering much of the Tibetan Plateau and spanning about . It is the homeland of the Tibetan people. Also resident on the plateau are other ethnic groups s ...

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

Lhasa Lhasa, officially the Chengguan District of Lhasa City, is the inner urban district of Lhasa (city), Lhasa City, Tibet Autonomous Region, Southwestern China. Lhasa is the second most populous urban area on the Tibetan Plateau after Xining ...

(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 (Earth), upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is ...

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 con ...

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 terrane The Qiantang terrane is one of three main west-east-trending terranes of the Tibetan Plateau. During the Triassic, a southward-directed subduction along its northern margin resulted in the Jin-Shajing suture, the limit between it and the Songpa ...

s. It can be divided into three parts: Bangong Lake-Gertse (western sector), Dongqiao-

Amdo Amdo ( �am˥˥.to˥˥ zh , c = 安多 , p = Ānduō ), also known as Domey (), is one of the three traditional Tibetan regions. It encompasses a large area from the Machu (Yellow River) to the Drichu (Yangtze). Amdo is mostly coterminous wi ...

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

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

, northward

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

of the Meso-Tethys Ocean between the Lhasa and Qiangtang terranes ceased, and during the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

, 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 metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

-matrix

scattered along the BNS.

Collision and suture development

The geology of the suture includes

marine

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

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

s, 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 a continental area on Earth, comprising all of Europe and Asia. According to some geographers, Physical geography, physiographically, Eurasia is a single supercontinent. The concept of Europe and Asia as distinct continents d ...

during the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

, 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. I ...

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 (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

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 Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...

. 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 A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

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 refers to the ability of a material to sustain significant plastic deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic deformation, which is reversi ...

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 of the

Tibetan Plateau The Tibetan Plateau, also known as the Qinghai–Tibet Plateau or Qingzang Plateau, is a vast elevated plateau located at the intersection of Central Asia, Central, South Asia, South, and East Asia. Geographically, it is located to the north of H ...

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

References