Recent models suggest that the middle crust beneath the high Tibetan Plateau flows southward toward India because of the difference in topography between Tibet and India; the models further suggest that this extruding material emerges at Earths surface in the high-erosion area of the Himalaya. These models make specific predictions about how the flow varies in magnitude and direction with depth. This project uses structural geology and geochronology to test these predictions to see whether the models are correct, and if they are not, how the models might be refined. This is an important test to conduct because the India?Asia collision is Earths largest ongoing continent collision and serves as an important lens for viewing older continental collisions that happened throughout Earths history.
Researchers Jeff Lee of Central Washington University, Bradley Hacker from the University of California Santa Barbara, and their students will investigate the spatial and temporal distribution of kinematics, vorticity, finite strain, and deformation temperature in strongly deformed middle crustal rocks exposed in the North Himalayan gneiss domes, southern Tibet, using structural petrology, finite-strain analysis, electron-backscatter diffraction (EBSD), and metamorphic monazite geochronology. This research is motivated by recently formulated thermal, mechanical channel flow/extrusion models that postulate that the middle crust exposed in the high Himalaya and southern Tibet was a low-viscosity, ductile material, bounded above and below by coeval normal- and thrust-sense shear zones, respectively, that flowed and extruded to the south. Flow within a channel can range from pure Couette flow to Poiseuille flow, or be a combination of the two. The investigators and their students will document the deformation and timing during ductile flow to provide a comprehensive spatial, thermal, and temporal history of deformation and flow in middle crustal rocks in southern Tibet. These studies, combined with similar published and ongoing studies in the high Himalaya, will provide an unprecedented view of middle crustal flow parallel to the transport direction over a distance of 50 to 100 km.
Intellectual Merit The specific objective of this project was to test a thermal-mechanical model whereby middle crustal rocks (rocks that were at depths as great as ~25 km beneath the Earth’s surface and at temperatures as high as ~700°C) exposed in the high Himalaya and southern Tibet were once part of a low-viscosity, ductile material (imagine warm silly putty), bounded above and below by coeval faults, that flowed and extruded to the south. Analyses used to test this model include kinematics (documenting the motion of rock during ductile deformation), documenting the temperatures during deformation, vorticity (documenting the flow patterns during ductile deformation), and geochronology (documenting the age of rocks and timing of deformation). We interpret the kinematic, deformation temperature, and vorticity data on mid-crustal rocks exposed in Mabja and Kangmar domes, southern Tibet as indicating ductile deformation, characterized by southward flow, within a wedge- or slab shaped middle crust, but the patterns of middle-crustal ductile flow define a hybrid flow regime that is more complex than predicted by the model. Geochronology, using the radioisotopic systems Lu-Hf (lutetium-hafnium) and U/Th-Pb (uranium/thorium-lead) from mid-crustal rocks exposed in the Mabja and Kangmar domes, yield Lu-Hf ages of 54-52 Ma (millions of years ago) and of 51-49 Ma and U/Th-Pb ages 28–15 Ma. On the basis of structures and elemental analysis of the rocks, we interpret the Lu-Hf ages as recording growth of the mineral garnet during middle crustal contractional (compression) deformation at ~54.3 Ma, followed by mineral recrystallization during subsequent high-temperature metamorphism (a change in minerals, due to increase in pressure and temperature, in pre-existing rock without melting the rock) and ductile extension (pulling apart). Our Lu-Hf ages are the first to confirm the suggestions made from field data and models that significant crustal thickening and contraction in the Tibetan Himalaya started at ~55 Ma. These processes were broadly synchronous with the collision between a small Tibetan-Himalaya continent and the Eurasian plate, and the development of fold-and-thrust (compression) belt in southern Tibet. Also on the basis of structural and elemental analysis of the rocks, we interpret the U/Th-Pb ages as indicating that regional metamorphism in southern Tibet began by 28 Ma and continued for ~13 million years to 15 Ma. The timing of metamorphism in middle crustal rocks in southern Tibet is the same as observed in middle crustal rocks in the Pamirs, ~1700 km away. The simultaneous beginning and end of metamorphism in the Mabja and Kangmar domes, southern Tibet and in the Pamirs imply that a common Himalayan-Tibetan-Pamir-wide change in plate dynamics, rather than local processes, drove the evolution of these spatially distinct regions. Our kinematic and U/Th-Pb, Ar/Ar (argon/argon), and (U-Th)/He (uranium-thorium/helium) geochronologic investigations in Gianbul Dome, northwest India indicate that the development of Gianbul’s domal morphology initiated during the early stages of extension in the middle crust at ~26 Ma. We propose that, during extension, doming was driven by a positive feedback among melting of rock, the buoyancy of melted rock, and extension. Doming culminated with the final emplacement of melts at ~22 Ma. At this time (~22-21 Ma) ductile extension in these middle crustal rocks transferred to brittle deformation (imagine breaking cool silly putty) by normal (extensional) slip along the Zanskar fault. A second, rapid episode of brittle movement along the Zanskar fault occurred between ~14 and 10 Ma and was possibly followed by third episode of rapid normal faulting between ~9 and 6 Ma. Broader Impacts Broader impacts of this project include: (1) participation of two principle investigators, one from a primarily undergraduate university (Central Washington University, CWU) and the other from a research-I university (University of California, Santa Barbara, UCSB), and collaboration with researchers at other institutions including the US Geological Survey and University of Texas, Austin; (2) involvement of six graduate students from CWU and UCSB; (3) training of one undergraduate student from CWU in laboratory data acquisition and analysis to broaden their educational experience; and (5) continued development of EBSD, a relatively new analytical tool for characterizing kinematics. The interactions among the principle investigators, graduate students, undergraduate student, and collaborators from four different institutions promoted intellectual cross-pollination.
