Structural and geomorphic origin of anomalous topographic culminations in the Chinese Pamir: Muztagh Ata and Kongur Shan
Final Report Abstract
The peaks Muztagh Ata and Kongur Shan reach nearly 8,000 meters elevation, and tower above the surrounding landscape. They are flanked by exceptionally large glaciers which have carved deep valleys into their sides. They sit in the footwall (lower part) of the Kongur Shan Extensional Fault System (KES) and they are cored by domes of high-grade metamorphic rock exhumed from deep within the Earth’s crust. The correlation of topography, erosion and exhumation initially led us to hypothesize coupling between tectonics and climate in this tectonically complex region of western China. We undertook a comprehensive study, surveying and fault scarps, dating fluvial terraces and glacial moraines, calculating slip rate across these faults, measuring and modeling the cooling rate of rocks along the length of the range, and analyzing the morphology of the landscape, glaciers and rivers. The first major goal of our study was to determine when, how fast, and with which spatial pattern rocks were brought to the surface in the footwall of the KES, and to place this deformation in a regional tectonic context. Building on previous work and integrating new zircon (U-Th)/He, apatite fission track and 40Ar/39Ar data with 1-D thermal modeling, we generate new temporal and spatial constraints on the tectonic evolution of the KES and neighboring basement domes. We elucidate a significant temporal and spatial variation during the evolution of this fault system. Our findings have important implications for deciphering the growth of the eastern Pamir during ongoing Indian-Asian convergence. Over the last ~5 Ma, the northeast Pamir are deforming in a transtensional system and appear to be decoupled from transpressional deformation in central and western Pamir. Our new data support a model of diachronous growth for the Muztagh Ata and Kongur Shan domal massifs. In contrast to Muztagh Ata, published and new thermochronology data indicates that the Kongur Shan massif is characterized by late Miocene to recent rapid cooling and tectonically-driven exhumation of deep-seated rocks in the footwall of KES. Along the range-crossing Yarkand River in the south, we found evidence that elucidates the northward motion of the Pamir Mountains. Our data indicate that this northward motion must have ceased by at least 4-5 Ma. This is much earlier than previously thought, and has led us to rethink tectonic models. We suggest that the slow-down in northward motion reflects acceleration of the Tarim block with respect to the Tien Shan rather than collision of the Pamir with the Tien Shan mountains to the north. Our provenance work also fills in the long-term evolution of the Pamir, suggesting uplift of the Central Pamir around 40 million years ago. Our synopsis of our new and previously published thermochronologic data from the entire Pamir and the Tien Shan to the north shows that the hanging wall of the Main Pamir Thrust experienced relatively minor amounts of late Cenozoic exhumation. Our observations suggest that the Pamir orogen as a whole is an integral part of the overriding plate in a subduction system, while the remnant basin to the north constitutes the downgoing plate, with the bulk of the convergence accommodated by underthrusting. We demonstrate that the observed deformation of the upper and lower plates within the Pamir-Alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback, subduction erosion, subduction accretion, and marginal slab-tear faults. The curvature of the North Pamir is genetically linked to the short width and rollback of the south-dipping Alai slab; northward motion (indentation) of the Pamir is accommodated by crustal processes related to this rollback. Our second major goal was to study the nature of potential climate-tectonic coupling in the Pamir, focusing on the many large and small glaciers. We collected data such as size, aspect, slope, elevation distribution and debris cover for 156 glaciers. By correlating these with the exhumation pattern (determined by mineral cooling ages) and climatic data, we were surprised to discover that fluvial, rather than glacial erosion appears to be correlated to the maximum rock exhumation, and the location of the high peaks is controlled by the presence of strong rock. We also have explored the fascinating ways in which the mountainous topography has shaped regional snowfall patterns, allowing glaciers on the “downwind” side of the range to grow much larger and to erode aggressively toward the “fault” side of the range. As a result glacier erosion appears to have influenced the shape of the fault today. Through our work in the Chinese Pamir, we have produced an important new tectonic, erosional and climatic datasets. We now better understand the tectonic evolution of the Pamir over the last 40 million years and how it has interacted with climate and erosion in the last few million years.
Publications
- 2005. Glacial erosion, deep exhumation and the development of high topography along the Kongur detachment, Pamir Mountains, Western China. Eos Transactions American Geophysical Union, 86(52), Fall Meet. Suppl., Abstract T23C-0576
Schoenbohm, L., Chen, J., Sobel, E., Thiede, R., and Strecker, M.
- 2008. Is Focused Erosion Enhancing Denudation of Domes in the Pamir Mountains? In: Garver, J.I., and Montario, M.J. (eds.), Proceedings from the 11 International Conference on thermochronometry, Anchorage Alaska, Sept. 2008, p. 208-209
Sobel, E.R., Thiede, R.C., Schoenbohm, L., Chen, J., & Sudo, M.
- 2009. Geomorphology of anomalously high glaciated mountains at the northwestern end of Tibet: Muztag Ata and Kongur Shan. Geomorphology, v. 103, p. 227-250
Seong, Y.B., Owen, L.A., Yi, C., Finkel, R.C. and Schoenbohm, L.
- 2009. Syntectonic extension and dome development during NE-Chinese Pamir-formation, decreasing during the Plio-Pleistocene? Eos Transactions American Geophysical Union, v. 90, Fall Meet. Suppl., Abstract T43C-2101
Thiede, R.C., Sobel, E.R., Schoenbohm, L., Chen J., Schildgen, T.F.
- 2010. Erosion and asymmetric relief evolution of the Central Asian Pamir Plateau. International workshop “Deep intracontinental subduction in the Pamir”, May 17-19, 2010, Geo-Forschungs-Zentrum, Potsdam
Thiede, R.C.
- 2011. Late Miocene–Pliocene deceleration of dextral slip between Pamir and Tarim: Implications for Pamir orogenesis. Earth and Planetary Science Letters, v. 304, p. 369-378
Sobel, E.R., Schoenbohm, L., Chen, J., Thiede, R., Stockli, D., Sudo, M. &. Strecker, M.R.
- 2011. Slab Rollback and Subduction Erosion Model for the North Pamir - Alai Intracontinental Subduction Zone. Eos Transactions American Geophysical Union, v. 92, Fall Meet. Suppl. Abstract T51J-05
Sobel, E.R., Schoenbohm, L.M., Chen, J., Thiede, R., Stockli, D., and Sudo, M.
- 2011. Spatial and temporal variation in slip rate along the Kongur Normal Fault, Chinese Pamir. Proceedings for the 26th Himalaya-Karakoram-Tibet Workshop, Canmore, Alberta, Canada, July 12-14, 2011
Schoenbohm, L.M., Chen, J., Yuan, Z.-D., Kirby, B., Sobel, E.R., and Owen, L.A.
- 2012. Cenozoic evolution of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China). Journal of Asian Earth Sciences, v. 44, p. 136-148
Bershaw, J., Garzione, C., Schoenbohm, L., Gehrels, G., and Li, T.
