Continental collision

Get in the spirit, this video actually touches on overthrusting and subduction, melt generation and collision:

Recent GPS studies show significant crustal shortening in the southern 100 km across the frontal thrust (estimated elastic strain corresponding to a slip deficit of about 14 mm/yr; Banerjee and Burgmann 2002). Additional convergence is accommodated by the strike slip faults along the edges of the Tibetan Plateau, including at least 11 mm/yr on the Karakoram Fault.

Below: Hodges (2000) summary map of the geology of the west-central Himalaya. The rocks are roughly grouped as follows, from north to south:

Orange batholith rocks correspond to arcs active just before and during the final closure of the Tethys and early stages of Indian Collision. They lie on or near the former edge of the Eurasian Plate during early collision.

Pink/tan belt is a Cretaceous-aged island arc with batholith complex (pink: Kohistan and Ladakh Batholiths) and older basement/country rock (tan) into which the arc intruded. This was the last terrane to accrete to Eurasia before India collided.

Bright green with black spots: Accretionary complexes (green) with ophiolites (black). Some are in place along the suture zone (ITSZ in the SE corner of the map) while other outcrops are fragments of klippe which were thrust over the Tibetan Zone marine seds (aqua)- for example KO (center of map).

Also shown in green are basins filled with molasse (molasse = sedimentary detritus of a tectonic uplift, usually coarse grained, can be marine or continental in depositional environment). In some areas, the transition from marine to continental sedimentation is preserved in these basins (24-17 Ma).

The aqua belt is the Tibetan Zone, the continental margin sediments deposited on the northern margin of India from the Cambrian to the Eocene. The section is thought to be nearly complete. The section records periods of Paleozoic and Mesozoic tectonism in India, and Permian rift basalts associated with Gondwana rifting. Regional unconformities within the Cretaceous section are associated with increasing tectonic influence before the Tethys closure. Terrestrial seds appear in Eocene marking collision.

Yellow: North Himalayan Gneiss Domes. These are high-grade equivalents of Tibetan zone sediments, uplifted on low angle normal faults. Some are cored by young granites (18-10 Ma), implying dome formation during Miocene.


Dark gray: The Greater Himalayan Zone - high grade metasedimentary and meta-igneous rocks, with associated plutons (red). Separated from Tibetan Zone by a giant extensional detachment (Southern Tibetan Fault System). Amphibolite to migmatite meta seds and volcanics with a weird granitic layer continuous near the top of the section for 100s km along strike. NB: some of the eclogitic rocks show evidence for subduction and exhumation to >100 km depth. The plutons (red) are thought to be derived from the migmatites in the metamorphic sequence.


Light yellow: Lesser Himalayan zone - low greenschist to low amphibolte metasediments, classic fold-thrust belt, retrodeformable/balanced cross sections have been done in several places.

Light gray: Lesser Himalayan Crystalline - discontinuous belt of medium-grade metasediments, granites and granitic gneisses within the LHZ.


Light yellow - Subhimalayan zone - modern molasse.

Cretaceous (to 75 Ma) south-vergent folding and faulting associated with ophiolite emplacement At 54-50 Ma, sedimentation shifted from marine to subaerial along the suture zone. Direct dating of the deformation associated with the suture is not conclusive but seems to young to the east.
High P metamorphic rocks in the GHZ give metamorphic ages ~ 50 Ma, if these are the leading edge of the Indian continent this represents date soon after subduction of the leading edge
Story is complicated because exhumation has not been explained, but biotite cooling geochron suggest T<300 by about 30 Ma, so uplift soon after subduction, perhaps facilitated by doming.
Cross section balancing shows 100-200 km shortening in the Tibetan zone since 50 Ma.
Miocene - initiation of Main Central Thrust (still active)
Last 20 My - activity on both So. Tibetan Detachment and the Main Central Thrust - basically exhuming the Greater Himalaya.

This 1981 paper by Nicolas et al. documents a structural transect across one of the best exposed ophiolites of the Indus-Tsangpo Suture Zone: the Xigaze Ophiolite. The major ophiolitic lithologies are well represented in stratigraphic order, and the opihiolite is overlain by turbidite forearc sequence. The base is sheared over Mesozoic turbidites/flysch affiliated with the Indian sub- continent. These turbidite sequences include huge slide blocks, some up to km wide, of Tethyan (Mesozoic) carbonates and shallow water marine rocks (Hodges, 2000).

And here's a video showing the large scale deformation during collision. Key things to note: the development of curved to N-S strike slip fault through the Syntaxes on both sides of the collision zone, also - the block uplift of the Tibetan Plateau and foreland advancement of the Himalayan fold-thrust belt.

Sandbox models have been used to investigate fault geometry and strain partitioning in orogenic wedges. These models have the disadvantage of only incorporating one type of rheology (frictional) but they have the advantage in that the strength of materials and of basal contacts can be varied, and material can be removed from the top in between deformation steps to simulate erosion. Here, Malavieille (2010) summarizes a number of sandbox experiments to show the structural developments of different types of wedges.

The internet provides many videos of such experiments.

Here is a singly vergent wedge:

Video at the bottom of the page shows a sandbox experiment of a single-sided wedge with steepening of older structures (folds and faults) as the wedge builds outward toward the foreland. Sad Haaq, Purdue University

Here is a doubly vergent wedge:

Here is a wedge with erosion removing the uplifted material - note the significant difference in fault geometry!!

Models based on the physics of critical sand wedges are sometimes used (at least on the broad scale) to describe even the viscous parts of orogenic belts. Gervais et al. (2011) compared 3 models for how to get lower crustal rocks (dark gray) into outcrop in the orogenic wedge, in between lower-grade rocks. The top model (A) is based on the sandbox experiments of Malavieille (above). The center model (B) utilizes a low-viscosity, melt-rich rheology in lower-crustal rocks so they flow up the detachment. The third model (C) invokes buoyant uplift and doming during extension.

Channel-flow models for melt-present lower crustal exhumation through dipping shear channels have been applied to other orogens, including the Canadian Cordillera:


Banerjee and Burgmann (2002) GRL

Gervais et al (2010) J. Struct. Geol.

Gervais et al. (2011) Lithosphere

Hodges (2000) GSA Bulletin

Malavieille (2010) GSA Today

Nicolas et al. (1981), Nature