The Himalaya?Tibet orogenic system has been the subject of extensive research for decades, and continues to be a fruitful laboratory for understanding orogenic processes and continental evolution. Geological and geophysical studies have provided important insights into the structure and dynamics of the modern system, but imparted less information about the orogen?s early development, and the characteristics of the colliding continents that may affect subsequent evolution. The goal of this project is to investigate how crustal and lithospheric thickness changes in southern Tibet in order to establish the ?boundary conditions? for the India-Eurasia collision (~50 My ago) and how lithosphereic thickness changes during and after collision. As pointed out by the PIs, lithospheric thickness defines the thermal state and rheology of the crust and lithosphere, which are important boundary conditions and properties necessary for understanding the nature of crustal and lithospheric deformation during collision.

The PIs will integrate geochemistry, geochronology, paleoelevation, seismology, tectonics, and modeling to understand the pre-collision structure and evolution of the leading edge of the southern Asian continental margin ? the Lhasa block ? and its role in controlling the development of the broader orogen. They seek to answer two sets of related questions:

? How has the crustal thickness of the Lhasa block evolved from collision to the present? ? How have the elevation and denudation histories changed through time? ? What role does metamorphism play in altering crustal rheology and density and as a consequence facilitate surface uplift and erosion? ? What is the contribution of lithospheric deformation to thickening of the region?

This set of questions is relevant because of the way crustal thickness and surface elevation can influence the mechanics of an orogen.

The second set of related questions addresses the later evolution of the Lhasa block:

? What is the origin of post-collisional magmas? ? To what extent has melting contributed to crustal weakening and flow? ? How have magmatic and aqueous fluids and lateral heterogeneity in their distribution influenced evolution and deformation of the Lhasa block? ? Why are elevations in the Lhasa block relatively uniform despite considerable variations in geology, exhumation, and deep structure?

The importance of these questions derives from the control that rheology and its spatial variations play in determining how orogens deform at large and small scales.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Leonard E. Johnson
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Lehigh University
United States
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