"Roof of the Earth" Offers Clues About How Our Planet Was Shaped

Eos Vol. 77, No. 39, October 1, 1996, pp. 383-385. © 1996 American Geophysical Union. Permission is hereby granted to journalists to use this material so long as credit is given, and to teachers to use this material in classrooms.
The Tibetan Plateau and its surrounding mountains­the Himalaya on the south, the Kun Lun on the north, and the Pamir and Karakoram on the west­comprise the largest, loftiest, and youngest highland on the Earth (Figure 1). All of the world's peaks higher than 7000 m (with the lone exception of Anchomuma in the Bolivian Andes) as well as the largest concentration of alpine glaciers are found in this vast region that geographers have dubbed "the Roof of the World."

Figure 1. A simplified map of south-central Asia, showing the Himalaya and Tibet, which have uplifted to form the highest and largest topography on Earth. As mountains uplift, the eroded rocks are deposited in the foreland basins in the north of the Indian Peninsula as well as in the marine basins in the Indian Ocean (the Indus and Bengal fans). The Himalaya and Tibet were formed by the head-on collision of the Indian plate with Asia during the Cenozoic. Once the "forbidden lands" in the world, the high mountains and plateaus of south-central Asia have increasingly drawn the attention of scientists. This field of geo-science is now an international endeavor, and scientists hope it will lead to clues about some of the major tectonic processes shaping the Earth.

The Himalaya-Tibet region is virtually the "water tower" of Asia: it supplies freshwater for more than one-fifth of the world's population, and it accounts for a quarter of the global sedimentation budget. Since 1985, a series of international workshops have been held to discuss the latest geoscientific research in the Himalaya-Karakoram-Tibet region. The eleventh workshop was held in Flagstaff, Ariz., from April 29 to May 1, 1996. It was the first to be held in North America. Nearly 120 people from 12 different countries attended.

The workshops are a timely response to the growing interest in the Himalaya-Karakoram-Tibet region among the international geophysical community. The Himalaya and Tibet region is a mountain-plateau system that was formed when the Indian and Asian continents collided during Cenozoic time. It is a "natural laboratory" for scientists studying the geophysical and geological processes involved in collisional tectonics as well as the effects of high mountains and mountain-building processes on climate and the environment.

In the first special session, speakers discussed geodynamic models of the Himalaya and Tibet. Since India collided with Asia about 55 m.y.a., India has moved nearly 3000 km northward. One challenge for geophysicists is to understand the mechanisms that have accommodated this "missing continental crust." Continental crust may have been simply shortened by folding and thrusting, and deformation may have been distributed throughout the Himalaya as well as all of Tibet (the so-called "basement reactivation" model). On the other hand, southeast Asia's Indochina block may have been extruded southeastward out of the way of an Indian indenter and thus opened space for northward Indian motion (the "extrusion or escape tectonics"). Perhaps the Indian crust subducted beneath the entire Tibetan Plateau (the whole-scale "continental subduction" model). Or has a combination of these mechanisms accommodated the space and strain of the India-Asia collision?

The challenge has been to quantify these mechanisms and their relative dominance in space and through time. Linked with these mechanisms are several other fundamental phenomena, which scientists would like to understand, such as the history and patterns of uplift of the Himalaya and Tibet, the deep structure of continental crust in the India-Asia collision zone, and the interactions between crust and mantle in a rapidly evolving orogenic system. Speakers at the workshop reported new data and ideas on these topics. Nevertheless, to paint clear, detailed pictures of the tectonic evolution and associated processes of the Himalaya and Tibet, more geophysical, geochemical, petrological, geochronological, and structural data are needed from various sectors of the Himalaya and Tibet.

Some of the exciting results constraining the tectonic models come from deep seismic reflection profiles. A collaborative U.S.-Chinese project called International Deep Profiling of Tibet and the Himalaya (INDEPTH) carried out seismic reflection studies in southern Tibet over the past decade and discovered among other things, a major north-dipping detachment fault at midcrustal levels that is called the Main Himalayan Thrust. An important aspect of the INDEPTH project is the integration of deep structural data with the knowledge of surface geology of the Himalaya and Tibet. The INDEPTH researchers at the workshop presented their latest results, which have detected fluids at midcrustal levels beneath the Tibetan Plateau. This stimulated debate on the origin and nature of these fluids.

The Himalaya are being formed by tectonic forces and sculptured by powerful denudation processes. Understanding the interplay of these tectonic and geomorphologic processes requires the interaction of geoscientists from various disciplines. The second special session was on the neotectonics and Quaternary geology of the Himalaya and Tibet. Topics covered in this session represented some cutting-edge areas of Himalayan research in neotectonics and Quaternary geology, such as the determination of present-day uplift rates of the Himalaya (both from geophysical modeling and geodetic measurements); monitoring seismic activity and its implications for a "Big One," which is overdue in the central parts of the Himalaya. Geomorphic analysis of mountain terrains by space-based imagery techniques was also discussed, and the role of tectonic activities in the geomorphic development of river systems was examined. Other researchers focused on determining exposure rates of rocks by cosmogenic nuclides, results gained by mapping and monitoring the dynamics of valley glaciers, and studies of loess and late Cenozoic glaciation record in the Himalaya and Tibet. A new development in Himalayan geophysics has been the application of the Global Positioning System to quantify the ongoing convergence between India and Asia.

Although the Himalaya-Tibet region is fundamentally a product of crustal shortening, the east-west extensional grabens in Tibet suggest that the plateau spread due to gravitational force. Furthermore, over the past decade, discovery of a basement-cover detachment fault­the so-called South Tibetan Detachment­in the Himalaya has drawn widespread interest among geologists. The South Tibetan Detachment, as mapped in several areas of the Himalaya, essentially separates the Paleozoic sedimentary formations of the Tibetan Himalaya to the north from the high-grade metamorphic rocks of the Greater Himalaya to the south. Interestingly, this north-dipping detachment fault is parallel to the east-west strike of the Himalaya, and the extant geochronological data and structural analysis indicate temporal and spatial links between the tectonic compression and crustal extension.

Another interesting feature of the South Tibetan Detachment is that this normal fault and shear zone can be traced on top of high summits, such as Mount Everest, at the border between the Higher Himalaya and the Tethys Himalaya, indicating gravitational response of the rocks to Himalayan uplift and height. Some researchers at the workshop also presented detailed maps of small-scale extensional structures within the Higher Himalayan metamorphic core and normal faults in the foothills of the Himalaya. The existence of these two contrasting tectonic regimes­compressional and extensional­in a single orogenic system that is also quite young and still active provides crucial clues to the response of continental crust to tectonic stress and uplift. The third special session discussed Himalaya-Tibetan examples of extensional tectonics in a compressional orogenic system. This area of research is expected to attract more attention in the coming years.

The afternoon sessions of the workshop covered regional topics: western Himalaya and the Karakoram, Sub-Himalaya (foreland sediments), the Main Central Thrust zone, the Higher Himalayan Crystalline terrain, and Tibet and Trans-Himalaya. A total of 101 presentations were made at the workshop: 71 oral and 30 poster. Forty-nine percent of the presentations were on the western Himalaya, indicative of the scarcity of recent research done on the eastern parts of the Himalaya and on the Karakoram-Pamir-Hindu Kush mountains.

Each oral presentation was 20 minutes long, and an hour-long open discussion coordinated by two discussants was held at the end of each session. This permitted more comments, questions, and discussion of session topics. Financial support for the workshop came from registration fees and a grant from the Continental Dynamics Program of the U.S. National Science Foundation. The grant mainly supported travel assistance for several researchers from China, India, Nepal, and Pakistan and students from Europe. Selected, refereed papers resulting from the workshop will be edited by the organizers and published as Geological Society of America Special Paper volume. The 12th workshop will be held in Rome in April 1997, on the occasion of the 100th anniversary of birth of Ardito Desio, a pioneering geologist whose research focused on the Karakoram mountains.

The 11th Himalaya-Karakoram-Tibet workshop successfully brought together many researchers from various countries and specialties to discuss the "biggest tectonic puzzle of Cenozoic Earth." It was also a positive sign of the increasing involvement of American geoscientific community in the Himalaya-Tibetan studies. Over 60% percent of the participants came from the United States.­Rasoul Sorkhabi, Department of Geology, Arizona State University, Tempe; Allison Macfarlane, Department of Geography and Earth Systems Science, George Mason University, Fairfax, Va.; and Jay Quade, Department of Geosciences, University of Arizona, Tucson

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