High pressure metamorphic terranes along the Bangong-Nujiang Suture Zone

High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied.

[2] In search of clues, geologists have looked to the high pressure terranes outcropping along suture zones to find answers.

[3] slab rollback - if the subducting oceanic lithosphere rolls back at a faster rate than plate convergence, extension occurs, allowing for buoyant continental crust detach and exhume to the surface.

[3] Each of these separate geodynamic processes for formation and exhumation of high pressure terranes leave certain structural, petrological, and chronological fingerprints.

[3] For example, slab rollback predicts, structurally, a microcontinent with thrust faulting at the base, petrologically, it is associated with back-arc spreading, and chronologically, subduction to exhumation may take approximately 15 million years with a monotonic down-dip gradient in ages.

Ophiolites are fragments of oceanic crust as well as upper mantle material that become tectonically emplaced onto continents during orogenic events, and their occurrence is generally along suture zones.

[8] A typical ophiolitic suite contains peridotite and harzburgite, layered gabbro, sheeted dykes, pillow basalts, and pelagic sediments.

Serpentinites are hydrated (15-16 wt.% H2O) ultramafic rocks that are composed of predominately serpentine, a weak and buoyant mineral with a broad P-T stability field, and are generally associated with subduction zones.

[9] The formation of serpentinites is caused by the release of fluids from subducting hydrated oceanic slabs as they become heated with depth to a maximum temperature of 650-700 °C.

[12] The Dong Tso ophiolitic suite includes metaperidotites and harzburgites, serpentinites, isotropic and layered gabbros, sheeted dykes, pillow basalts, and minor amounts of chert.

[13] Once part of the ancient Tethys Ocean, the Amdo massif formed during the Permian-Triassic as a microcontinent as the Qiangtang and Lhasa terranes rifted apart.

[14] Eclogites of Triassic age have been discovered within the Basu massif, and their geochemistry shows two different types of clinopyroxenes that are interpreted to reflect extremely fast exhumation of these rocks.

[15] In this model, continental lithosphere is presumed to be more ductile, and growth of the Tibetan Plateau would be caused by continuous crustal thickening due to the convergence of the Indian and Eurasian plates.

[16] Oblique subduction and large-scale sinistral strike-slip faults leading to eastward extrusion of lithospheric material would be responsible for the growth of Tibet to the east.

General map of central Tibet. High pressure terranes along the Bangong-Nujiang suture zone are highlighted in red: Bangong Lake-Gertse (western sector), Dongqiao-Amdo (middle sector), and Dingqing-Nujiang.
Generalized ophiolite sequence. Ophiolites are fragments of oceanic crust that appear sporadically along the BNS.
Metamorphic Facies Diagram. Amphibolite-facies, greenschist-facies, and eclogite-facies metamorphism have been observed throughout central Tibet.
Cross section depicting the tectonic evolution of the Bangong-Nujiang suture zone. 1. Slab rollback of the oceanic crust between the Lhasa and Amdo terranes causes an Early Jurassic oceanic back-arc basin to form between the Amdo and Qiangtang terranes. 2. During the Early-Middle Jurassic, subduction of oceanic crust beneath the Amdo terrane continues. 3. The back-arc basin closes and ophiolitic obduction occurs. The Lhasa and Qiangtang terranes collide in the Early Cretaceous, forming the Bangong-Nujiang suture.