Late Cenozoic Exhumation of the Terskey Range, Kyrgyz Tien Shan

The glaciated, granitic Terskey range and the associated foreland basin within the Kyrgyz Tien Shan is an ideal region to examine whether Plio-Pleistocene climate change has led to enhanced erosion. Three ca. 1000m vertical profiles were collected in the hanging wall of the main, north-vergent thrust, spaced 25 km apart along strike. AFT analysis have been conducted from the westernmost-transect; all profiles have been analyzed using apatite (U-Th-Sm)/He analysis. AFT and Helium data from the Barskoon gorge profile define parallel trends on the age-elevation plot with apparent exhumation rates of ca. 0.08 km/Myr. The uppermost AFT sample resided for a long period within the partial annealing zone (PAZ); hence, the onset of rapid exhumation defined by the base of the exhumed PAZ appears to be 31±5 Ma. This is earlier than expected from the known regional geology. An alternative interpretation is that the next lowest sample has also been partially reset, such that the onset of exhumation is later and the initial exhumation is more rapid. Helium data from the Kichikyzulsu and Turgenaksu profiles define linear trends on age-elevation plots with apparent exhumation rates of 0.1-0.25 km/Myr and 0.2 km/Myr, respectively. Much older ages at high elevations along the former profile suggest that the base of the Helium partial retention zone (PRZ) has been sampled; the onset of exhumation is roughly constrained to be between 10 and 20 Ma. The onset of rapid exhumation at the latter profile can only be constrained as prior to 11 Ma. Combining the apparent exhumation rate with the age of the youngest samples from each transect provides an estimate of the amount of exhumation since that sample cooled through the PRZ. For the 3 profiles, this implies 1 to 1.2 km, 0.7 to 1.8 km, and 1.5 km of exhumation, assuming that the exhumation rate remained constant. The observed small magnitude and slow rate of exhumation suggests that there has been only limited advection and hence limited perturbation of isotherms. However, calculated apparent geothermal gradients are quite high. One possibility is that isotherms are not horizontal and evenly spaced due to structural rotation or topographic perturbation. An alternate interpretation, at least for the Kichikyzulsu and Turgenaksu profiles, is that exhumation rates have increased since the samples cooled through the PRZ due to either climatically enhanced erosion or increased tectonism accompanied by enhanced erosion. The latter explanations suggest that the basal samples could have been exhumed from more typical depths of 2-3 km since passing through the PRZ. Apparent exhumation rates reported herein are lower than those from the structurally similar Kyrgyz Range. The highest rates from that range, 0.3 to 1.5 km/Myr, are associated with the removal of 1.5 km of sediment that formerly overlay the range. Possible reasons for the discrepancy include a slower shortening rate, a thinner sedimentary cover, or less efficient erosion. Thermochronologic and geologic data show that the Kunngey and Zaili ranges, on the north side of Issyk Kul, commenced cooling during the middle-late Miocene. Uplift of these ranges could have created the present orographic barrier prior to the Pliocene. This could explain less efficient Pliocene erosion of the Terskey range.