This research focuses on the South Tibetan fault system, one of the most enigmatic tectonic features of the Himalaya, and one that stirs much controversy in the Tectonics community. The South Tibetan fault system appears to be unique in that its kinematics are those of a low-angle, normal fault system while all other Himalayan fault systems with comparable displacements (many 10s to 100s of kilometers) have the more predictable kinematics of thrust fault systems typically found in mountain belts. The very existence of a feature like the South Tibetan fault system is indicating something fundamental about how the Himalaya, and mountain ranges in general, evolve. The PI and his colleagues work to clarify this message through their research. Unfortunately, in many parts of the Himalaya, the early history of the South Tibetan fault system has been obscured by late-stage deformation. This is not the case in Bhutan, where the PI and others have identified erosional remnants of early faults of the system that have been relatively undisturbed by younger deformation. This proposal describes targeted studies of these remnants. A broad array of techniques will be used to probe the early history of the fault system using these rare exposures. They include geologic mapping and structural analysis; quantitative element-partitioning thermobarometry; 40Ar/39Ar and (U-Th)/He thermochronology; and laser-ablation (U-Th)/Pb geochronology.
Because the Himalaya and Tibetan Plateau are still evolving, studies of this natural laboratory provide critical information regarding the role that mountainous regions play in earth system function. For example, we know that the physiography of the Tibetan Plateau defines the weather patterns of the Asian monsoon climate, and this climate is the enabling mechanism for agriculture that feeds roughly 60% of the people on Earth. By detailing the processes that led to the establishment of the modern-day Himalaya and Tibetan Plateau, studies such as the one proposed here inform more effective strategies for the co-evolution of human societies and mountain landscapes.
The collision of two continental masses results in shortening and thickening of continental crust and the development of dramatic mountain ranges. Most crustal deformation structures in these landscapes are thrust faults and folds. In recent years, scientists have also recognized the existence of large-displacement normal faults in collisional mountain ranges. Their origin is a matter of considerable controversy because such structures are more typically found in regions of extension – like the North American Basin and Range Province – rather than regions of shortening and crustal thickening. Some of the most extraordinary examples of large-displacement normal faults in collisional settings are found in the Himalayan mountain ranges of India, Nepal, southern Tibet, and Bhutan, and are referred to collectively as the South Tibetan fault system. Developed and largely active during the Miocene epoch, this system extends for more that 1500 km near the crest of the Himalaya, but exposures of it in the eastern Himalaya of Bhutan and Sikkim (India) constitute some of the most geologic complex and most poorly understood components. This project was designed to provide better constraints on the evolution of South Tibetan fault structures in the Himalayan foothills and near the Himalayan crest in Bhutan. Through field mapping and the analysis of multispectral satellite imagery, we were able to produce high-quality maps of various normal-fault strands, demonstrating a minimum collective displacement of 80 km on the South Tibetan fault system of Bhutan. While this amount is roughly an order of magnitude less than the displacements estimated on individual thrust fault systems that account for most of the crustal shortening in the Himalaya, it is nonetheless high enough to show that the South Tibetan fault system must have played an important role in the evolution of the Himalaya. Although the geologic community has not yet reached consensus on the exact cause of large-displacement normal faulting in the Himalaya, the fact that this deformation is broadly coeval with the timing of major thrust faulting implies that both contractional and extensional structures may have worked in concert during the evolution of the Himalayan ranges. One intriguing hypothesis is that simultaneous slip on the South Tibetan fault system and structurally lower Main Central thrust system may have enabled the southward ‘expulsion’ of a channel of high-grade metamorphic rocks from beneath the high Himalaya and adjacent southern Tibet toward India. If this is true, the net effect would have been to lower crustal thicknesses in the Himalaya, thereby moderating the excess gravitational potential energy of the very thick crust beneath the southern Tibetan Plateau. Our findings are consistent with such hypotheses, but not uniquely so. Additional detailed studies on the timing and displacement magnitudes on various strands of the South Tibetan fault system and similar features in other mountain ranges should help us better define the range of possible causes for large-displacement normal faults in collisional settings. The research enabled by this grant is informing the design of a major educational exhibit on the evolution of the Himalaya in the new Gallery of Scientific Exploration at Arizona State University’s main campus in Tempe, Arizona.
