Inverted Metamorphism in the Sikkim-Darjeellng, Himalaya, India

Inverted metamorphic sequences from chlorite zone phyllites grading progressively upwards into staurolitc/kyanite zone schists and gneisses with associated granitic bodies occur intermittently in the Lower Himalaya from Kumaon to Sikkim. In Sikkim-Darjeeling area, this progressive metamorphism is broadly late-kinematic with respect to the polyphase fabric elements in the metasedimentaries and is broadly coeval with the emplacement of metasomatic/anatectic granitic masses, which occur as low to moderately dipping sheets within both greenschist and amphibolite facies metasediments and carry I-type enclaves. Locally, strong, gravity induced polarity has concentrated metasomatic transformations along the hanging wall sections of the granite.

This 'inverted' metamorphism, as well as positive gravity anomaly gradients and shallow focus seismicity in this sector, are correlative with differential vertical displacements, which have exposed the naturally inverted metamorphic sequences along footwall sections of moderately dipping intrusive masses. LANDSAT and air photo interpretations of various types of linears and lineaments with supporting indications from the location of hot springs, lakes, water falls, abrupt changes in river gradient, truncation of marker bands and erraticity in terrace distribution, suggest that the area has been witnessing differential uplift along a number of regional and local sub-vertical fault surfaces including a number of geofaults, some of which are transverse to the axis of the range in a crypto-aulacogen style. This tectonic regime appears to be an integral part of postorogenic processes in Sikkim-Kumaon Himalaya possibly extending eastwards up to Arunachal Pradesh.