An integrated metamorphic and isotopic study of crustal extrusion along the Main Central Thrust, Sikkim Himalaya

Mottram, Catherine Mary (2014). An integrated metamorphic and isotopic study of crustal extrusion along the Main Central Thrust, Sikkim Himalaya. PhD thesis The Open University.



Mountains form where the Earth's plates collide; during this upheaval rocks are deformed by massive forces. The Himalayan orogen represents the ideal natural laboratory to decode the record of the deformational processes encrypted in the rocks. In the eastern region of Sikkim, a unique series of 'time windows' are exposed by doming of a major ductile fault, revealing the inner workings of one of the major mountain-building structures that accommodated the India-Asia collision.

The temporal and thermal evolution of the complex zone of deformation associated with this structure, the Main Central Thrust (MCT), was investigated using a combination of whole rock geochemistry (εNd), geochronology (U-Pb and 40Ar/39Ar), accessory and major phase geochemistry, and pressure-temperature modelling.

The results demonstrate that: (1) isotope geochemistry can distinguish rock packages that have been juxtaposed over many hundreds of kilometres in complex ductile shear zones; (2) during prolonged ductile deformation of the MCT zone from ~21-9 Ma there was progressively downwards-penetrating deformation and accretion of colder original footwall material to the hotter hanging wall; (3) the associated zone of inverted Barrovian metamorphism documents a sequence of 'paleo-thrusts' that evolved as the thrust-zone deformed rocks at successively lower pressure and temperature conditions (<500 to >650°C and 8 to 10 kbar); (4) during the Miocene thrusting progressed at a rate of ~10 mm yr -1, followed by moderately rapid cooling at a rate of ~50-70°C Ma -1. These findings are consistent with a tectonic model where rocks were accreted to a partially-molten mid-crustal channel of ductilely deforming material along the MCT.

This study provides new insight into how deformation is accommodated along major thrust faults during mountain building and has implications for how geological tools such as linked geochronology-geochemistry and P-T modelling are used to aid the interpretation of rock deformation in the cores of evolving mountain belts.

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