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. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.
Introduction
With an average elevation of just above 5,000 m, the Tibetan Plateau is the largest elevated region on Earth. Explaining how such a large area (2.5 million km2) can have such high elevations has perplexed geologists for some time. It is known that significant tectonic activity took place before the Indo-Asian collision as terranes were being accreted onto the Eurasian plate during the Late Jurassic-Early Cretaceous, but the extent of deformation and the influence these earlier tectonic events had on the subsequent evolution of the Tibetan Plateau is poorly understood. In search of clues, geologists have looked to the high pressure terranes outcropping along suture zones to find answers. The Bangong-Nujiang suture, in particular, features extensive high pressure terranes throughout much of its length.
Development of high pressure terranes
There are multiple processes that can lead to the development of high pressure terranes. First, upper crustal rocks have to be carried to great depths, nearing the mantle boundary. This could be accomplished by continental margin subduction, microcontinent subduction, sediment subduction, intracontinental subduction, subduction erosion, or foundering of a crustal root. After burial at depth, these continental rocks can then return to the surface through:
eduction - the process where a slab of continental crust is subducted due to being attached to a denser subducting oceanic plate, and at some point, the downward slab pull force exceeds the strength of the slab, thus causing necking and detachment to occur, and the positive buoyancy of the continental slab leads to its exhumation.
microplate rotation - exhumation of continental crust through the process of the subducting plate reversing in motion and rotating due to changing boundary conditions.
crustal stacking - extrusion of weak buoyant material due to the detachment of two layers, a weak buoyant layer atop a stronger negatively buoyant layer, once buoyancy forces exceed slab pull.
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.
channel flow - exhumation of continental material through a confined channel. The material undergoes circulation caused by tractions at the base of the channel and through the relative buoyancy of the material within the confined channel.
transmantle diapirs - Diapiric ascent of material derived from subduction erosion.
Each of these separate geodynamic processes for formation and exhumation of high pressure terranes leave certain structural, petrological, and chronological fingerprints. 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. The size of the high pressure terrane is inversely proportional to the speed of exhumation, and these reflect the stage of continental collision. Along the BNS, these terranes are variable in size, thus would have had differences in the timing of exhumation.
Bangong-Nujiang Suture Zone
The Bangong-Nujiang Suture is a ~1200 km long east-west trending zone that separates the Lhasa and Qiangtang terranes. It can be divided into three parts: Bangong Lake-Gertse (western sector), Dongqiao-Amdo (middle sector), and Dingqing-Nujiang (eastern sector). During the Middle to Late Jurassic, 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-matrix mélange scattered along the BNS.
Types of high pressure rocks associated with suture zones
Ophiolites
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. A typical ophiolitic suite contains peridotite and harzburgite, layered gabbro, sheeted dykes, pillow basalts, and pelagic sediments.