The origin of Himalayan anatexis and inverted metamorphism: Models and constraints

The key to comprehending the tectonic evolution of the Himalaya is to under stand the relationships between large-scale faulting, anatexis, and inverte d metamorphism. The great number and variety of mechanisms that have been p roposed to explain some or all of these features reflects the fact that fun damental constraints on such models have been slow in coming. Recent develo pments, most notably in geophysical imaging and geochronology, have been ke y to coalescing the results of varied Himalayan investigations into constra ints with which to test proposed evolutionary models. These models fall int o four general types: (1) the inverted metamorphic sequences within the foo twall of the Himalayan thrust and adjacent hanging wall anatexis are spatia lly and temporally related by thrusting; (2) thrusting results from anatexi s; (3) anatexis results from normal faulting; and (4) apparent inverted met amorphism in the footwall of the Himalayan thrust is produced by underplati ng of right-way-up metamorphic sequences. We review a number of models and find that many are inconsistent with available constraints, most notably th e recognition that the exposed crustal melts and inverted metamorphic seque nces not temporally related, The generalization that appears to best explai n the observed distribution of crustal melts and inverted metamorphic seque nces is that, due to specific petrological and tectonic controls, episodic magmatism and out-of-sequence thrusting developed during continuous converg ence juxtaposing allochthonous igneous and metamorphic rocks. This coincide ntal juxtaposition has proven to be something of a red herring, unduly infl uencing attention toward finding a causal relationship between anatexis and inverted metamorphism. (C) 1999 Elsevier Science Ltd. All rights reserved.