Has late Cenozoic climate change lead to enhanced erosion in the Kyrgyz and Chinese Tien Shan?
Final Report Abstract
Intra-continental mountain belts typically form as a result of tectonic forces associated with distant plate collisions. In general, each mountain belt has a distinctive morphology and orogenic evolution that is highly dependent on the unique distribution and geometries of inherited structures and other crustal weaknesses. This project investigated the complex and irregular Cenozoic orogenic evolution of the Central Kyrgyz Tien Shan in Central Asia, which is presently one of the most active intra-continental mountain belts in the world. This work involved combining a broad array of datasets, including thermochronologic, magnetostratigraphic, sediment provenance and stable isotope data, to identify and date various changes in tectonic deformation, climate and surface processes. Many of these changes are linked and can ultimately be related to regional-scale processes that altered the orogenic evolution of the Central Kyrgyz Tien Shan. The Central Kyrgyz Tien Shan contains a sub-parallel series of structures that were reactivated in the late Cenozoic in response to the distant India-Eurasia collision. Over time, slip on the various reactivated structures created the characteristic succession of mountain ranges and intermontane basins. New quantitative constraints on the exhumation histories of several mountain ranges have been obtained by using low temperature thermochronological data from 95 samples (zircon (U-Th)/He, apatite fission track and (U-Th)/He). Timetemperature histories derived by modeling the thermochronologic data of individual samples identify at least two stages of Cenozoic cooling in most of the region’s mountain ranges: (1) initially low cooling rates (<1°C/Myr) during the tectonic quiescent period and (2) increased cooling in the late Cenozoic, which occurred diachronously and with variable magnitude in different ranges. This second cooling stage is interpreted to represent increased erosion caused by active deformation, and in many of the sampled mountain ranges, provides the first available constraints on the timing of late Cenozoic deformation. Although thermal modeling suggests that cooling rates increased in the last 5 Ma, the magnitude of this erosion was too small to be precisely dated using thermochronology. Therefore, the chronologic constraints are unfortunately insufficiently precise to prove a link between enhanced erosion and climate change. New constraints on the timing of deformation have also been derived from the sedimentary record of the intermontane Issyk Kul basin, where new magnetostratigraphic data from two sedimentary sections suggests that deposition of the first Cenozoic syntectonic sediments commenced at ~26 Ma. Zircon U-Pb provenance data, paleocurrent and conglomerate clast analysis reveals that these sediments were sourced from the Terskey Range to the south of the basin, suggesting that the onset of the late Cenozoic deformation occurred >26 Ma in that particular range. These new constraints are used to infer the spatiotemporal distribution of deformation in a transect through the Central Kyrgyz Tien Shan and determine the order in which mountain ranges started deforming. These data suggest that deformation began in a few widely-spaced mountain ranges in the late Oligocene and early Miocene. Typically, these earlier mountain ranges are bounded on at least one side by a reactivated structure, which probably corresponds to the frictionally weakest and most suitably orientated inherited structures for accommodating the roughly north-south directed horizontal crustal shortening of the late Cenozoic. Moreover, tectonically-induced rock uplift in the Terskey Range, following the reactivation of the bounding structure before 26 Ma, likely caused significant surface uplift across the range, which in turn lead to enhanced orographic precipitation. These wetter conditions have been inferred from stable isotope data collected in the two magnetostratigraphically-dated sections in the Issyk Kul basin. Subsequently, in the late Miocene (~12‒5 Ma), more mountain ranges and inherited structures appear to have started actively deforming. Importantly, the onset of deformation at these locations in the late Miocene coincides with an increase in exhumation of ranges that had started deforming earlier in the late Oligocene‒early Miocene.
Publications
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2008, Late Cenozoic Enhanced Exhumation of the Terskey Range, Kyrgyz Tien Shan, AGU Fall meeting: Eos, Trans. AGU 89 (53): San Francisco
Sobel, E.R., Macaulay, E.A., Mikolaichuk, A., and Kohn, B.P.
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2010, Exhumation and deformation history of the Kyrgyz Tien Shan, 12th International Conference on Thermochronology, Thermo2010, Glasgow, 18-20 August, 2010, p. 211
Macaulay, E., Sobel, E. R., Mikolaichuk, A. and Kohn, B.
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2011, Linking exhumation and deformation in the Eastern Teresky Range in the Kyrgyz Tien Shan, AGU Fall meeting:, Volume Eos, Trans. AGU: San Francisco
Macaulay, E., Sobel, E.R., Mikolaichuk, A., Kohn, B., and Stuart, F.
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2011, Unravelling the deformation and erosional history of the Kyrgyz Central Tien Shan, EGU General Assembly, Volume 13: Geophysical Research Abstracts, p. EGU2011-1324
Macaulay, E., Sobel, E., Mikolaichuk, A., Kohn, B., and Stuart, F.
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2013, Thermochronologic insight into Late Cenozoic deformation in the basement-cored Terskey Range, Kyrgyz Tien Shan. Tectonics, v. 32, p. 487–500
Macaulay, E.A., Sobel, E.R., Mikolaichuk, A., Landgraf, A., Kohn, B., and Stuart, F.
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Universität Potsdam, 2013: The orogenic evolution of the Central Kyrgyz Tien Shan
Euan A. Macaulay
