TOWARD A SOLUTION FOR THE HIMALAYAN PUZZLE - MECHANISM OF INVERTED METAMORPHISM CONSTRAINED BY THE SIWALIK SEDIMENTARY RECORD

Since the mid-19th century, inverted sequence ('hot-side-up') of metam orphic rocks in the Himalaya has formed a controversial subject, which has drawn an increasing attention in recent years, Models to explain this phenomenon are either 'thermal' (a heating event in the middle to upper parts of the crust directly responsible for the inverted metamo rphism) or 'structural' (tectonic disruption of a normal metamorphic s equence), Data on the heavy-mineral analysis of the Siwalik sedimentar y rocks derived from the Himalaya are consistent with 'structural' mod els. Metamorphic index minerals preserved in the Siwaliks record succe ssive denudation of lower- to higher-grade metamorphic rocks of the Hi malaya preserved in the Lower through Upper Siwalik sediments, suggest ing differential denudation of metamorphic tomes in the Himalaya over the past 18 million years, We argue that this pattern of denudation an d deposition, in conjunction with other geological and theoretical con siderations, constrains a plausible 'structural' model for the inverte d metamorphism in the Himalaya, according to which the apparent invers ion of metamorphic sequence was a product of differential uplift and d enudation of metamorphic zones probably related to imbricate thrusting and internal ductile shearing of the metamorphic rocks in a collision al tectonic setting.