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.
Despite being the world’s type locality for active continent-continent collision, and being the highest, most scenic, and most well-known mountain range, the Himalayan-Tibetan system remains one of the more incompletely mapped, and thus least understood, orogenic belts. The primary reason for this is that access to many Himalayan countries for the purpose of scientific research has only come in the last 20-30 years. This is particularly true for the eastern quarter of the Himalayan range, which occupies, from west to east, the state of Sikkim, India, the kingdom of Bhutan, and the state of Arunachal Pradesh, India. Via unprecedented access to the Kingdom of Bhutan through a strong collaboration with Bhutan’s Department of Geology and Mines, we have had the opportunity to collect map data, including rock type, identification and location of structures (faults and folds) and orientations of rocks and structures, over virtually the entire country. Thus, a major outcome of our research is a new geologic map of Bhutan. This map was created by integrating our mapping of the frontal, unexplored portion of the Bhutan Himalayas with both new mapping and existing maps of northern Bhutan and surrounding regions. We have shared the geologic map and our findings with the Department of Geology and Mines in Bhutan. To create the geologic map, we defined the original stratigraphy of the northern Indian margin (pre-collision with Asia) in Bhutan through bedrock mapping and age determination of rocks using the youngest U-Pb age of minerals (zircons). Our data indicates that the pre-Himalayan northern Indian margin was complicated but continuous. The map pattern of rocks and structures allow us to predict what a cross section, or vertical slice though the crust should look like. These cross sections show how faults identified at the surface project into the crust. By estimating the orientation of faults in the subsurface, we can predict the amount of displacement on each fault. Our study shows that the Indian crust has shortened 350-550 km as the Himalayas were built. This amount of shortening is the same or less than that documented in the central Himalayas in Nepal. By tracking the rates at which rocks cool in the Himalayas we have been able to link cooling histories of rock samples to sequentially restored fault motion allowing us to estimate rates of fault motion through time. Specifically we used Th-Pb ages on chemically zoned monazite to determine the time over which a major fault, the Main Central Fault, of the Bhutan Himalaya was active. We showed that this fault moved 230 km over ~6 million years from 21 to 16 Ma. The resulting rate of 40 mm/yr is almost as fast as the plate tectonic convergence rate between India and Asia over this time period (50-60 mm/yr). Because the long term rate of shortening for the region is 17-22 mm/yr, times of fast convergence must be balanced by periods of slower than modern convergence. The most significant of these is over the last 10 Myr, where the rates of shortening were half that recorded by modern global positioning (GPS) data. Integrating these data sets have allowed us to determine the rate and tempo of shortening in the Bhutan Himalayas and allow us to compare that to modern rates of shortening in the region (GPS), paleoseismicity and long-term rates of shortening across the entire Himalayan front.
