Tibet is key to understanding the large-scale dynamics of continental collisions, yet a substantial part of the mantle beneath Tibet has been probed only with relatively low-resolution techniques. We know enough to say that on average seismic wavespeeds in the vast western part of the plateau are unusually high, but we lack sufficient information to discriminate among a large number of plausible scenarios for what is causing these anomalies and what role they might play in the dynamics of mountain building. For example, are these high wavespeed (i.e., relatively cold) masses indigenous to the Tibetan lithosphere, sinking beneath the plateau and drawing adjacent mantle material into it, or do they represent the leading edge a subducting Indian plate that provides a distant push from the side? The primary objective of this project is to map the distribution of these high wavespeed masses in the upper mantle beneath Tibet in order to distinguish among these competing hypotheses. This mapping will be done by analyzing a seismological dataset gathered by deploying a multi-stage, quasi-linear array of broadband stations across the western plateau that combines long period (2-3 year) recording of lower frequency arrivals with shorter period (1 year) staged recording of higher frequency arrivals. A combination of traditional and recently developed techniques, based mostly on variations of arrival time tomography and receiver function analysis, will be used in the processing. This project will be carried out by 3 PIs and 2 or more graduate and undergraduate students in the US in collaboration with a substantial number of coworkers (scientists, technicians, students) from China, mostly from the Chinese Seismology Bureau in Beijing but also from the Tibetan Bureau of Seismology in Lhasa. Additionally, colleagues from the National Geophysical Research Institute in Hyderabad will extend our coverage across the border by carrying out a simultaneous deployment in India.
To first order, the high (~ 5 km) elevations of the Tibetan plateau are a consequence of the collision between the Indian and Eurasian plates that commenced some 50 Ma ago. However, the relative importance of various factors such as lithospheric underplating or delamination, the strength of the lower crust and upper mantle beneath different parts of the plateau, and the role of pre-existing structures in the deformation and evolution of the plateau, remain controversial. A first order boundary condition on models of plateau formation is the extent to which the Indian lithosphere underthrusts Tibet. In the eastern part of the plateau, results for a number of investigations suggests that the Indian lithosphere underplates nearly as far at the northern edge of the plateau near the Kunlun Shan, and that low wavespeed upper mantle wavespeeds once thought to be evidence of delamination are apparently the result of heating caused by decay of radioactive elements in the thickened crust in this part of the plateau. The status of the lithosphere beneath western Tibet has been much more enigmatic, with sometimes completely contradictory results coming from seismic imaging with different data sets (primarily body wave vs surface wave imaging). Even less is known about the status of the lower crust in an area which is reputed to have the thickest crust on the planet (at about 90 km, although like many inferences from this part of the world remains controversial), and the level of microseismic activity. Much of the uncertainty about how this part of the plateau behaves is a result of its remoteness, and the integrative nature of most imaging algorithms. Hence, a primary objective of our project was to install a network of seismic stations in the area to sample the region as directly as possible. Our main activity over the course of the investigation was to collect as much data as possible, and with that in mind we installed a network of 31 autonomous broad band seismograph stations in the area where such measurements have never been done before, between longitudes of 78 and 83 degrees East on the Tibetan plateau. We carried out a number of field campaigns, bringing in equipment from the US, deploying it in collaboration with China Earthquake Administration (an agency combining research and public safety duties), and retrieving data during periodic return visits. Our project collected a total of four years of data, a longest such data collection effort on the Tibetan plateau to date. Upon retrieval data were quality controlled, and archived in a public-access database operated by the Incorporated Research in Seismology. In addition to the field work necessary to maintain the observing equipment, and the technical work necessary to prepare data for archiving and future analysis, we carried out a number of research projects that utilized data already collected in and close to our region of study, and also early returns from our own observing system. All studies already published dealt with observations that reflect the tendency of rocks within the Earth to acquire systematic texture when deformed. We showed that there is indeed clear evidence for such texture beneath our study area, both within the crust and in the underlying mantle regions, and that we have the means to separate signatures of these different textures. We also showed that along the northern limit of the Tibetan plateau there is a marked correspondence between the senses of deformation in the crust and the mantle, implying that they are rigidly or viscously coupled. All these results add to the understanding of how the Tibetan plateau deforms internally, and what may be driving that deformation. We also investigated bulk properties of the rocks making up the crust of the plateau and found them remarkably uniform – a stark departure from results of similar studies in the eastern half of Tibet. The Rensselear team consisted primarily of the PI and a graduate student, but we worked (and continue to work) closely with our collaborator at Rutgers, where we shared support for a postdoc. We also engaged several undergraduate students in to help with preliminary data processing tasks. The dataset created in the course of the project will be further used in studies of the Tibetan plateau.
