MICROSTRUCTURAL EVIDENCE FOR STRAIN-SOFTENING ALONG THE MAIN CENTRAL THRUST ZONE IN THE DARJEELING-SIKKIM HIMALAYA

The Main Central thrust (MCT) system is one of several major fault zones that extend the length of the Himalayan fold-thrust belt. In many parts of the Himalaya, the MCT has two recognizable imbricate thrusts, e.g. the Vaikrita and Munsiari thrusts in the Kumaon-Garwhal Himalaya (Valdiya 1980). In the Darjeeling-Sikkim Himalaya, the two faults are described simply as MCT1 and MCT2. The MCT1 is closer to the hinterland (north) and formed before the MCT2.

In northwest Sikkim, the MCT1 fault zone places Greater Himalayan granulite facies ortho- and para-gneisses (Kanchenjunga gneiss) over Lesser Himalayan amphibolite facies Lingtse ortho- and Paro para-gneisses. The fault zone is approximately 800 meters thick and, based on balanced cross sections, accommodates upward of 100 kilometers of movement. The large magnitude of displacement along the MCT1 indicates its importance as a dominant thrust in the Himalayan fold-thrust belt, and an understanding of its fault zone processes is essential to deciphering overall Himalayan tectonic evolution.

The large displacement along a relatively thin fault zone suggests some form of strain softening along the zone. Microstructural analyses of samples from the MCT1 fault zone, collected along roadcuts in NW Sikkim, reveal that the zone is made up of high-grade mylonites and ultramylonites, suggesting large amounts of shearing. The presence of sheath folds in some paragneisses and evidence for top to the south shearing in the form of s-porphyroclasts and mica fish also support this conclusion. Two different protoliths have been mylonitized, forming a quartz-biotite-garnet-sillimanite-kyanite mylonite from paragneiss and a quartz-plagioclase-kyanite mylonite from orthogneiss.

Grain size analyses show that there is notable grain size reduction across the meta-sedimentary protolith, reaching a steady state recrystallized grain size that suggests deformation dominated by dislocation creep. However, grain size reduction is not sufficient to trigger strain softening by diffusion-controlled processes; instead, crystallographic preferred orientation of recrystallized grains suggests geometric strain softening.