INVERTED, TELESCOPED AND SUPERPOSED METAMORPHIC SEQUENCES: PEAK TEMPERATURE STRUCTURE OF THE EASTERN HIMALAYA (Invited Presentation)

In collisional orogens there is typically a marked contrast in tectonic style and metamorphism between upper and lower tectonic levels; these may be coupled or decoupled from each other at different stages of development of the orogen. Similarly, it has been recognized that there is a fundamental contrast in structural style and deformation mechanisms between the deep interior of the orogen and the shallower regions at the orogenic front.

The aim the present paper is to highlight some fundamental observations about the tectonics of the Himalayan orogen, which remains the best-studied location for continent-continent collision orogen, and which contains classic, well-preserved examples of both critical Coulomb wedge and channel flow tectonics in order to demonstrate that:

1. Large, hot collisional orogens are characterized by spatial and temporal superposition of superstructure, infrastructure and orogenic front units that have contrasting metamorphic and deformational histories and styles
2. These transient tectonic domains developed at different times and in different locations within the orogen than where these units currently lie.
3. The present-day apparent metamorphic gradient across an orogen thus displays an integrated record of diachronous and contrasting deformation and metamorphism.

We illustrate these points with peak temperature data collected in the Bhutan Himalaya over the last 20 years. This compilation offers a unique and relatively complete peak metamorphic temperature-time transect from the foothills to the top of the Himalayan orogen. These data help us propose a conceptual model of Himalayan tectonics that is consistent with field observations and with the results of a range of numerical modeling experiments.