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Tectonic geomorphology and numerical surface-process modeling in the Pamir

Subject Area Palaeontology
Term from 2009 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 140678784
 
Final Report Year 2015

Final Report Abstract

The interplay between topographic, tectonic, and climatic factors has fundamental relevance for understanding the mechanisms of mountain evolution and their susceptibility to changes. The CRP 3 combined geomorphometric analyses with geochronological techniques to determine the distribution and rates of surface processes in the Pamir. Positioned in a transitional setting between the Westerlies and the Indian Summer Monsoon, the Pamir provides ideal conditions to explore surface processes with respect to variable climatic conditions, and to compare such results with those found in other actively deforming high mountains. In this context, this we addressed four main issues - (1) the distribution of tectonic and climatic factors in the Pamir, and their effects on geomorphometry; (2) the challenge of accurate sediment dating for precise process rates in high mountains; (3) the variability of fluvial incision and its implications for the evolution of the Pamir river network; and (4) which factors control erosion, and how erosion rates relate to local lowering of base levels. We first developed TecDEM, a MATLAB toolbox used in conjunction with global DEMs for the extraction of tectonic geomorphologic information. We then discussed how the collinearity of the major tectonic structures, mountain ranges, and the river network exert long-term control on flow orientation and local base levels. We developed a transparent, reproducible analysis routine in three data processing templates using the R package `Luminescence'. The challenges posed by properties of high mountain materials were addressed. Multiple grain and single grain techniques applied to quartz, K-feldspar and plagioclase allowed us to identify prominent events in sediment deposition without interference from bleaching and sediment mixing, or signal loss due to anomalous fading. We applied optically stimulated luminescence (OSL) methods to quantify the variations in fluvial incision along the Panj River. Paleo-glaciations during MIS 2 and MIS 1/2 may have triggered the deposition of terrace sediments, but the rate of incision is primarily consistent with terrace location, rather than time of formation. Where the Panj cuts across the Shakhdara Dome in the southern Pamir, high incision rates of 7-10 mm/yr indicate intense river adjustment. Lower incision rates of 2-4 mm/yr are consistent with sections of the Panj parallel to southern dome boundaries, where the river profile suggests local base levels. To the north-east, the Panj incision reflects transient conditions. These data highlight the structural control on sudden base level drop due to successive river captures, while climatic factors as well as rock erodibility and drainage architecture are of secondary importance. We generated a map of cenozoic and active structures for the entire Pamir. We combined this information with seismological data. We explain the resulting deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik-Afghan depression, where it causes thin-skinned shortening of basin sediments above an evaporitic decollement. We complemented the indications from geomorphometry and fluvial incision with cosmogenic nuclide (CN)-based basin-wide erosion rates. Results suggest a rapid average topographic evolution in the Pamir. However, the pace of erosion at the Pamir Plateau shows a strong contrast to the Pamir margins. High erosion rates of 0.55-1.43 mm/yr integrate over millennial scale conditions at the western Pamir margin, whereas lower rates of 0.05-0.17 mm/yr at the Pamir Plateau also integrate effects of the MIS 1/2 deglaciation. The correlation of erosion with steep slopes (R2 of 0.82) defines the precondition for high erosion in the Pamir. The influence of precipitation only becomes evident in multiple linear regression analyses, explaining erosion as function of slope and precipitation (R2 of 0.93). The almost tenfold discrepancy between fluvial incision rates along the Panj River and basin-wide erosion rates reflects the transience of the landscape with the Panj incising faster than hillslopes adjust. The rate of adjustment increases where the Westerlies supply moisture during winter. This suggests that an efficient sediment transport relates to seasonal peak discharge during the melting season. The methods and results described, highlight the dominance of tectonic structures in controlling surface processes. In contrast to the southern escarpment of the Himalayas where the Indian Summer Monsoon provides intensive rainfalls, precipitation in the Pamir is limited and hence, work as a restricting factor for hillslope adjustment to fluvial incision. Major reorganizations of the Pamir river network highlight river captures as an important trigger of high surface response rates due to the sudden drop in base levels.

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