Rates of river incision across the main tectonic units of the Pamir identified using optically stimulated luminescence dating of fluvial terraces

Abstract

Calculated incision rates along the Panj, the main river of the Pamir, are used to investigate any influence by tectonics or climate on the architecture of the river. The depositional ages of Panj river terraces were calculated using optically stimulated luminescence (OSL) dating of terrace sand. Fluvial incision rates were generated by integrating the terrace depositional ages with accurate kinematic GPS measurements of terrace heights above the modern Panj. We investigated 16 terraces along the Panj at the western Pamir margin and one terrace from the Vakhsh River to the north of the Pamir. The results reveal brief periods of fluvial deposition over the past 26 kyr. The oldest Panj terrace depositional ages coincide with early MIS 2 and MIS 2/1 glaciations on the Pamir Plateau. Younger terrace ages have no apparent link with glacial cycles. Terraces with varying heights above the modern Panj at different localities yielded similar depositional ages. This suggests that local conditions have determined fluvial incision rates. Combining all of the terrace measurements, the average incision rate of the Panj over the last 26 kyr has been ∼5.6 mm/yr. A high mean incision rate of ∼7.3 mm/yr was calculated from terraces where the Panj has cut a steep-sided valley through the Shakhdara Dome. Significantly lower incision rates (∼2 -3 mm/yr) were calculated from terraces where the Panj flows along the southern boundaries of the Shakhdara and Yazgulom domes. At those localities, graded segments of the Panj river profile and increased valley widths are indicative of local base levels. Downstream of the Yazgulom Dome, river incision rates are generally lower (∼4 -5 mm/yr) than the Panj average. However, there is one exception where higher incision rates (∼6 mm/yr) were calculated upstream of the Darvaz Fault Zone, a major tectonic feature that forms the western boundary of the Pamir. The Vakhsh river terrace to the north of the Pamir yielded a lower incision rate (∼3 mm/yr) compared to the Panj average. Variation in incision rates along the Panj does not correspond to changes in rock type or river catchment area. Instead, incision rates appear to have been primarily influenced by river capture across the southern and central metamorphic domes of the Pamir. Wherever the Panj cuts these domes it displays a convex river profile. The combination of localized river profile convexity and changes in incision rates across the Pamir domes indicates that the dome boundaries have been active recently.

Sensitivity analysis and implications for surface processes from a hydrological modelling approach in the Gunt catchment, high Pamir Mountains

“…Only the north-western margin differs from this pattern. There, higher Westerlies-derived winter precipitation and increased summer precipitation in the area of the Fedchenko Glacier allow for a certain adjustment of erosion to the roughly 3-fold faster incision along the Panj River (Fuchs et al, 2014a). The latter resembles processes described in the western Himalayas.…”

Remote Sensing-Based Assessment of the Variability of Winter and Summer Precipitation in the Pamirs and Their Effects on Hydrology and Hazards Using Harmonic Time Series Analysis

“…Precise information is rare about the actual precipitation distribution due to the lack of a dense gauging network. Different studies comfort the assumption that the region receives variable amounts of precipitation with a decreasing W-E trend [1,8,33]. We hence divide the greater Pamir region (Figure 1) into seven subregions bounded by notable orographic barriers and name them based on dominant mountain systems.…”

Research on fluvial archives: from diversity to multidisciplinarity

“…Such numerical ages are of fundamental importance in the study of past fluvial environments, since they are essential for assessing the role of various drivers such as climate or human societies (see below), as well as the rates of change. They can also be used, for example, to estimate rates of fluvial incision, providing new insight on the relation between fluvial systems and tectonic activity or epeirogenic uplift (Brocard et al, 2003;Cordier et al, 2006;Fuchs et al, 2014;Westaway & Bridgland, 2014). Beyond this, numerical ages represent a major tool to develop regional or global comparison between rivers, allowing the recognition of general trends in their evolution, but also specific behaviours that can be linked either to intrinsic forcing, or to local conditions (Vandenberghe, 2008;Cordier et al, 2014a).…”