The 2,400 km long Himalayan fold and thrust belt extends from the Nanga Parbat massif in northern Pakistan to the Namche Barwa syntaxis in eastern Tibet. There are only a handful of estimates that quantify the magnitude and geometry of shortening along this fold and thrust belt with no such estimates in the eastern portion of the range, a region characterized by fundamental changes in the geomorphic, geometric, and lithologic expression of the belt. In particular, the lack of information on the magnitude, style, and timing of shortening in the Lower Himalayan Series, a thick succession of early to middle Proterozoic-Paleozoic greenschist facies to unmetamorphosed metasedimentary rocks within the belt, inhibits understanding of the development of the Himalayan orogenic belt. The goal of this project is to determine the geometry, amount of shortening, and kinematic history of deformation in the Lower Himalayan Series as exposed in the Kingdom of Bhutan. Toward this end, a team of scientists from Princeton University and Bhutan are mapping the Lesser Himalayan Sequence from the Main Central Thrust to the Main Frontal Thrust. These field relationships and new U-Pb ages of detrital zircons are used to develop a regional stratigraphic framework. Balanced crustal-scale structural cross-sections are restored sequentially when combined with new thermochronologic and geochronologic ages in order to elucidate regional cooling patterns and to constrain timing of fault motion. These results allow evaluation of how deformation is partitioned in space and time, prediction of minimum shortening estimates, determination of minimum lateral and vertical erosion estimates compatible with both the balanced section and thermochronometers, and in the end, comparison of the timing and magnitude of deformation and exhumation between the Lesser and Greater Himalayan Series.
The Himalayan mountain range is a consequence of the ongoing collision between India and Asia. During collision, the sedimentary cover that had blanketed northern India for the last 1.8 billion years became detached from the underlying basement. This deformed sedimentary cover, and the structures that developed within it, record the deformation history of the collision. Although it is one of the world's most famous mountain ranges, only a handful of estimates quantify this deformation and there are no such estimates for the eastern portion of the mountain belt. This study of the deformed rocks in the Kingdom of Bhutan will provide new estimates for this poorly understood portion of the range and, when compared to estimates for other portions of the Himalayan Range, will lead to a better understanding of the development of collisional mountain belts. The project fosters scientific collaboration between scientists from Bhutan and the United States and helps to build scientific infrastructure in Bhutan by training of Bhutanese students and an improved knowledge of the geology of the country, which in turn is important for understanding earthquake hazards and natural resources.
