User:Hongcheng Guo/Main Central Thrust Sandbox

Main Central Thrust
The Main Central Thrust is one of the north-dipping and NW—SE strike thrust faults along the Himalaya. Generally speaking, the Main Central Thrust is the ductile shear zone, along which the Great Himalayan Crystalline complex was placed over the Lesser Himalayan Sequence.

Geologic Background
The Himalayan orogen is the production of the collision between continents of India and Asia. It is structurally dominated  by three north-dipping, fault—bound units stacked by, from north to south, the Southern Tibetan Detachment, the Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust. These units, from south to north, are:

(1)  The Lesser Himalayan Sequence (LHS), mainly low-grade Proterozoic metasediments to unmetamorphosed rocks, which is bound above by the Main Central Thrust.

(2) The Greater Himalayan Crystalline complex (GHC), mainly high-grade gneiss and migmatite, which is bound below and above by the Main Central Thrust and the Southern Tibetan Detachment, respectively

(3) The Tethyan Himalayan Sequence (THS), mainly Proterozoic to Eocene sediments. deformed in an Paleogene fold-thrust belt.

See figure 2 for the structural relationships between rock units and faults.

Kinematic Models
To better understand the position and role of the Main Central Thrust, we should point out the kinematic models of Himalayan Orogen. Generally, there are three kinematic models for the emplacement of the Greater Himalayan Crystalline complex above the Main Central Thrust shown in figure 2.

(1) Wedge extrusion mode

(2) Channel flow-focused denudation involve extrusion of the Greater Himalayan Crystalline complex to the surface between two surface-breaching faults

(3) Tectonic wedging model shows emplacement of Greater Himalayan Crystalline complex with the two faults merging to the south.

The Difficulties in Understanding the Main Central Thrust
Although the general definition of the Main Central Thrust has been given, it is not enough due to the complication and difficulties in defining the Main Central Thrust.

For long, many researchers has defined the Main Central Thrust by different criteria, including by lithology different between the hanging wall and the footwall, by metamorphic grade change from the hangingwall to the footwall, by the different U—Pb detrital zircon ages, by the different Nd isotope compositions, by the different strain, etc. Some of these criteria has also been combined. However, none of these criteria can be reliable if they are used independently. Meanwhile, these criteria cannot all be satisfied neither. The most dominating problems are:

(1) the lithology and stratigraphy have not been completely investigated and understood

(2) the metamorphic grades across the Main Central Thrust shear zone are continuously changing, thus any one particular isograd is not reliable for determining the location of the Main Central Thrust.

(3) one of the most important difficulty in defining the location of the Main Central Thrust is that, in broad sense, the Main Central Thrust is not one exact fault. Instead, it could be a series of thrust faults that active at different time (shown in figure 4).

Definition of the Main Central Thrust from Various Perspectives
Despite the above difficulties in defining the Main Central Thrust, it is better than never to list various definition of the Main Central Thrust by various criteria:

(1) by lithologic criteria, the Main Central Thrust is defined as the boundary between the quartzite and phyllite, which belong to the Lesser Himalayan Sequence, and the orthogneiss biotite-rich schist, which belong to the Greater Himalayan Crystalline complex

(2) by metamorphic isograd, the Main Central Thrust follows the kyanite isograd. Under this criterion, the crystals of kyanite appears upward only within several meters of the lithologic change stated in the above

(3) by the differences in U—Pb detrital zircon ages, 1.87-1.0 Ga zircons have been reported from the Lesser Himalayan Sequence which is bound above by the Main Central Thrust, and 0.8-1.0 Ga zircons have been reported from the Greater Himalayan Sequence which is bound below by the Main Central Thrust

(4) by the Nd isotope composition, different Nd composition (for example, an average Nd value of -21.5 has been reported in the Lesser Himalayan Sequence while an average Nd value of -16 has been reported in the Greater Himalayan Sequence) marks the Main Central Thrust

(5) by strain, the Main Central Thrust is defined as a broad zone which a few kilometers thick. This zone accomodated most of the ductile shear zones and brittle thrust faults between the lowermost part of the Greater Himalayan Crystallin complex and the uppermost part of the Lesser Himalayan Sequence

Prospect
None of the above definitions is exactly and reliable because the Main Central Thrust developed and changes its style not only vertically but also along its strike, and even thought time. To better understand the Main Central Thrust, more research should be done along its strike and though time and its definition should be limited in a thrust fault, but should be a broader fault zone and even should be regards a series of fault developed in different time.