Changes in paleo-erosion rates, and interactions between erosion, deposition and deformation in the Issyk Kul basin, Kyrgyzstan
Final Report Abstract
The Issyk-Kul Basin (Kyrgyzstan), situated in the central Tian Shan Mountains, hosts the largest and deepest mountain lake in Central Asia. Erosion of the surrounding Terskey and Kungey ranges led to the accumulation of up to 4 km of sediment in the adjacent depression. This Neogene geological record provides valuable information about the complex interactions between tectonic deformation, global and regional climate in Central Asia. Creation of the basin from regional shortening and uplift likely initiated around the Oligocene-Miocene, yet precise age control is sparse. To better understand the timing of these processes, we obtained magnetostratigraphic age constraints on fossil-poor, fluvio-lacustrine sediments exposed south of Lake Issyk-Kul that agree well with previous age constraints of the equivalent strata outside the Issyk-Kul Basin. Two 500–650 m thick sections comprised mainly of Chu Group sediments were dated at 6.3–2.8 Ma and 7.0–2.4 Ma (late Miocene to early Pleistocene). Together with reinterpreted magnetostratigraphic constraints from underlying strata, we find that syn-tectonic deposition commenced at ∼22 Ma with average sedimentation rates <10 cm/ka. Sedimentation rates increased to 10–30 cm/ka at 7 Ma, concurrent with accelerated uplift in the Terskey Range to the south. A deformation event in one section (Kaji-Say) between 5 and 3 Ma together with concurrent shifts of depositional centers throughout the basin signal the onset of substantial uplift of the Kungey Range to the north at ∼5 Ma. We also investigated differences in timing of regional environmental change and corresponding global climate change to identify the impact of changing relief and elevation of regional topography. We presents facies analyses and Neogene oxygen and carbon isotopic records from the magnetostratigraphically-dated terrestrial sedimentary sections on the southern side of the intermontane Issyk Kul basin and 26Al/10Be isochron burial ages from the southern and eastern sides of the basin. The δ18O and δ13C data show a positive ~2‰ shift in values between ~8 and 7 Ma and a change from a negative to a positive trend. We attribute this change to the upwind growth of the Kyrgyz, Kungey and Trans Ili (Zaili) ranges, which diverted the westerlies, thereby changing the Issyk Kul basin from a windward to a leeward position, enhancing aridification, and establishing the modern-day spring and summer precipitation regime within the basin. The transition from sandstones of the Chu sedimentary group to conglomerates of the Sharpyl Dak group, marking a change from fluvial-alluvial deposits to a proximal alluvial fan, is dated at 2.6- 2.8 Ma by 26Al/10Be isochron burial dating on the southern side of the basin, driven tectonically or by Northern Hemisphere glaciation. Two 4-5 Ma 26Al/10Be isochron burial ages constrain the onset of Sharpyl Dak deposition on the eastern side of the basin; southward paleocurrent directions there suggest the eastward growth of the Kungey range in the Pliocene. This uplift and deformation transformed the Issyk-Kul area into a closed basin that facilitated the formation of the modern deep lake. Increased subsidence on the southern side of the basin and local tectonically-induced river system reorganization led to the commencement of lake formation at ~5 Ma, followed by a ~2 Ma local depositional hiatus. This study concludes that the late Miocene - Pliocene northward growth of Tian Shan significantly altered environmental conditions within the range, preventing the moisture-bearing westerlies from reaching the intermontane Issyk Kul basin and promoted the lake formation and expansion.
Publications
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2021. Miocene to early Pleistocene depositional history and tectonic evolution of the Issyk-Kul Basin, central Tian Shan: Geochemistry, Geophysics, Geosystems, v. 22(4), e2020GC009556
Roud, S. C., Wack, M. R., Gilder, S. A., Kudriavtseva, A., & Sobel, E. R.
