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Himalayan thrust belt

Himalayan thrust belt is mountain belt stretched between India and Tibet Plateau. The thrust belt is divided by several major fault, from south to north, they are Main Frontal thrust (MFT), Main Boundary thrust (MBT), Main Central thrust (MCT) and South-Tibet detachment (STD) Heim and Gansser (1939). Bounded by these crustal scale faults, the thrust belt is divided into following tectonostratigraphic zones: (from south to north) Sub-himalaya, Lesser Himalaya, Greater Himalaya and Tethyan Himalaya.

Initial Collision time
Research about initial time of intercontinental collision between India and Asia has been conducted from many aspects.

Paleo-geographic reconstruction of thrust belt deformation
Paleo-geographic reconstruction is also known as palinspastic reconstruction. This type of work can be done in 2-dimension or 3-dimension. When the reconstruction work is geometrically constrained, it is called balanced palinspastic reconstruction. A cross section though a research area can be reconstructed (or restored) through this method and the result of the work is called balanced cross section. In this method, paleo-elevation and the exposure time of certain strata can be estimated. In the Himalayan tectonics scenario, balanced palinspastic reconstructions are conducted to geometrically constrain Cenozoic deformation of the Himalayan thrust belt. This method is used to estimate geometry of fault, total shortening absorbed by thrust belt, deformation style and test mountain building model and so on.

Also see, AAPG wiki "Balanced Cross Section".



This section introduces major methods used to solve thrust belt tectonic problem. Methods will be briefly introduced in principle and application examples.

Emplacement of Greater Himalayan Crystalline (GHC)
There are several models proposed to explain how Greater Himalayan Crystalline was placed to its position. The following chart shows major models and theirs supporting evidences.

Hinterland GHC chracteristics
Parrish & Hodges (1996) and DeCelleset al.(2000) showed that High Himalayan Crystalline Series rocks from Nepal, contain detrital zircon populations in metasediments of 0.8– 1.0 Ga, up to 1.7 Ga andc.500 Ma zircons from orthogneisses and granitic plutons, whereas Lesser Himalayan Series rocks from the same area were characterized by >1.6 to 2.6 Ga zircons. The majority of published U–Th–Pb monazite ages from the High Himalayan Crystalline Series metasediments and leucogranites give Tertiary ages (Simpson et al. 2000; Searleet al. 1999 and references therein).

Detrital records that signal exposure of GHC
Sediment eroded from Greater Himalayan Crystalline Complex (GHC) is transported and deposited into foreland basin, modern rivers and offshore fans as detrital record. Through provenance study, exposure time of GHC is estimated. Table below shows several examples of exposure time of GHC.

There are several problem associated with the listed detrital signals of exposure of GHC.
 * Some of provenance signals of THS and GHC are very similar (e.g., Myrow et al., 2003; Ravikant et al., 2011) as Haimanta group units of THS and the GHC hae the same sedimentary protolith.(e.g., Miller et al., 2001; Webb et al., 2011a).
 * Nd and Sr signals are used to detect the exposure time of GHC, but it couldn't distinguish THS and GHC sources (France-Lanord et al.,1993; Najman et al.,2000).
 * Detrital monazite, detrital mica, almandine garnet and staurolie in Dharamsala Formation (White et al., 2001, 2002) are used to detect exposure of GHC, but the detrital minerals could be also sourced from THS as they are well documented in (Chambers et a., 2009; Webb et al., 2001a).

But what is an unequivocal signal that only indicate source from GHC not THS? High-grade metamorphic minerals kyanite and sillimanite are used to discriminate source area of GHC and THS, because these minerals are abundant in the GHC and alomost absent in teh THS(Webb 2013, Vannay and Grasemann, 1988).

Using Foreland Basin Deposit to Constrain Kinematic History of Hinterland
When continent collide with each other ( for example, Indian plate collide with Eurasian plate and subducted beneath it),mountain belt build up. At the same time, parallel to the the mountain belt, as the plate is bent and lots of rocks are eroded, foreland basin is formed. The development of mountain and foreland basin are coupled processes. The sediment layers accumulated in the foreland basin are like book pages that record the development history of the mountain belt. So geologist use foreland basin deposit record to unveil hinterland kinematic history.

Workflow
Geologist can

P-T path (Geothermobarometry)
Mineral assemblages and mineral compostion can be very sensitive indicators of metamorphic Pressure-Temperature (P-T) conditions. Theory of metamorphic phase equilibriais used in calculation of pressure and temperature conditions. Equilibrium constant (Keq) is a function of pressure and temperature." The goal of geothermometry and geobarometry (or geothermobarometry) is to infer the conditions at which a sample equilibrated by measuring the value of the equilibrium constant and using that value to calculate T,P or bother P and T." For example, the garnet-biotite Fe-Mg echange thermometer,Net transfer reaction the GASP geobarometer. Spear, F.S. (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths, 799 p. Mineralogical Society of America, Washington, D. C. example of thermobarometric studies Epard et al., 1995 Vannay and Grasemann, 1998 Vannay et al., 1999 Jain et al., 1999 Walker et al., 1999 Chambers et al., 2009

Instruments XRF to acquire composition map of mineral crystal (can also assess zoning patterns) Electron microprobe (e.g. JEOL 8200 electron microprobe)

Programs AX THERMOCALC 3.21 (Holland and Powell, 1998)

Case study

Major and trace element and REE geochemistry
Major and trace element geochemistry data can be obtained for granitic rocks to characterize their sources, evolution and tectonic settings. "Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics.""Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb−Y−Nb and Rb−Yb−Ta space, particularly on projections of Y−Nb, Yb−Ta, Rb−(Y + Nb) and Rb−(Yb + Ta)." Elemental data can be plotted into discrimination diagram.(reference: Sources and Settings of granitic rocks) Rare Earth Element (REE) data can be plot on Chondrite-normalized rare earth element chart to estimate source setting.

Also see

 * Geology of Nepal