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.
Viewed from space (or in Google Earth) the Tibetan plateau appears as a broad region of high, and nearly uniform, elevation, that has no analogs on the surface of the Earth. The mechanisms responsible for the formation, and long-term maintenance, of the plateau remain the subject of inquiry. By understanding them we seek to learn how the continents of the Earth behave when motions of tectonic plates cause two of them (in case of Tibet, India and Eurasia) to collide. The enormous, high, and relatively flat surface of the Tibetan Plateau invites explanations in terms of uniform properties and similar mechanisms. The progressively better knowledge about the Earth structure within and beneath the plateau has, however, identified very significant differences in deep structure. Our project was designed to acquire first-order observations in the western Tibet, and area that is much less studied then the more accessible and politically less fraught eastern part. These new data (records of seismic waves radiated from distant earthquakes that provide information about interior structure of the Earth) would be necessary to bring the level of knowledge about the western part of Tibet to that available in the east – a necessary condition for meaningful comparison between them. Our main activity involved deployment and operation of a network of 31 autonomous seismographs (devices sensitive to ground vibrations) 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. Besides 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. At Rutgers the project team included a post-doctoral researcher, a graduate student and three undergraduate assistants. It served as a vehicle to hone professional skills for the postdoc, and a training platform for students. The dataset created in the course of the project will be further used in studies of the Tibetan plateau. The experience gained while operating in a remote part of China is valuable for planning and executing future research efforts there.
