Exhumation of Basement-cored Uplifts: Example of the Kyrgyz Range Quantified with Apatite Fission-track Thermochronology

The Kyrgyz Range, the northernmost portion of the Kyrgyzstan Tien Shan, displays topographic evidence for lateral propagation of surface uplift and exhumation. The highest and most deeply dissected segment lies in the center of the range. To the east, topography and relief decrease, and preserved remnants of a Cretaceous regional erosion surface imply minimal amounts of bedrock exhumation. The timing of exhumation of range segments defines the lateral propagation rate of the range-bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology. New apatite fission-track (AFT) data from three transects from the eastern Kyrgyz Range, combined with published AFT data, demonstrate that the range has propagated over 110 km eastwards over the last 7-11 Myr. Based on the thermal and topographic evolutionary history, we present a model for a time-varying exhumation rate driven by rock uplift and changes in erodability and the time scale of geomorphic adjustment to surface uplift. Easily eroded, Cenozoic sedimentary rocks overlying resistant basement control early, rapid exhumation and slow surface upliftrates. As increasing amounts of resistant basement are exposed, exhumation rates decrease while surface uplift rates are sustained or increase, thereby growing topography. As the range becomes high enough to cause ice accumulation and develop steep river valleys, fluvial and glacial erosion become more powerful and exhumation rates once again increase. Independently determined range-noma1 shortening rates have also varied over time, suggesting a feedback between erosional efficiency and shortening rate

СЕЙСМОТЕКТОНИКА ВНУТРЕННЕГО ТЯНЬ-ШАНЯ: СУУСАМЫРСКАЯ ВПАДИНА И ПРИЛЕГАЮЩИЕ ТЕРРИТОРИИ

The Ms=7.3 Suusamyr earthquake of August 19, 1992 occurred in an area reputedly aseismic. Because it was not expected there, this event attracted worldwide attention of researchers in seismology and seismotectonics, but their results have not been included in the most recent seismic zoning map of Kyrgyzstan. New studies of neotectonic structures and focal mechanisms of earthquakes in the Suusamyr area and adjacent areas give reason to revise the established notions about the seismicity of the region. The seismic hazard in Inner Tienshan appears important and Mmax are comparable to those of the Northern and Southern Tienshan, where numerous destructive events were documented in the XIX and XX centuries. For the southern parts of the study area, along Naryn River, where hydroelectric power stations are planned, the new data should be used.Суусамырское землетрясение (М=7.3) произошло 19 августа 1992 г. в предположительно асей­смичном районе. Поскольку это событие оказалось неожиданным, оно привлекло внимание исследователей в области сейсмологии и сейсмотектоники всего мира. Однако результаты данных исследований не нашли отражения на последней карте сейсмического районирования Кыргызстана. Новые структурные исследования активных разломов и механизмов очагов землетрясений Суусамырской впадины и прилегающих территорий дают основание пересмотреть сложившиеся представления о сейсмичности региона. Сейсмическая опасность во Внутреннем Тянь-Шане (Ммах) представляется сопоставимой с таковыми в Северном и Южном Тянь-Шане, где были зарегистрированы многочисленные разрушительные события XIX и XX века. Полученные данные желательно учитывать на юге изученной территории, где планируется строительство гидроэлектростанций Нарынского каскада

Great earthquakes in low strain rate continental interiors: An example from SE Kazakhstan

The Lepsy fault of the northern Tien Shan, SE Kazakhstan, extends E-W 120 km from the high mountains of the Dzhungarian Ala-tau, a sub-range of the northern Tien Shan, into the low-lying Kazakh platform. It is an example of an active structure that connects a more rapidly deforming mountain region with an apparently stable continental region (SCR), and follows a known Palaeozoic structure. Field-based and satellite observations reveal a ~10 m vertical offset exceptionally preserved along the entire length of the fault. Geomorphic analysis and age control from radiocarbon and optically stimulated luminescence (OSL) dating methods indicate that the scarp formed in the Holocene and was generated by at least two substantial earthquakes. The most recent event, dated to some time after ~400 years BP, is likely to have ruptured the entire 120 km fault length in a Mw 7.5{8.2 earthquake. The Lepsy fault kinematics were characterised using digital elevation models and high-resolution satellite imagery, which indicate that the predominant sense of motion is reverse right-lateral with a fault strike, dip and slip vector azimuth of ~110°, 50°S and 317{343 respectively, which is consistent with predominant N-S shortening related to the India-Eurasia collision. In light of these observations, and because the activity of the Lepsy fault would have been hard to ascertain if it had not ruptured in the recent past, we note that the absence of known active faults within low-relief and low strain-rate continental interiors does not always imply an absence of seismic hazard.We thank NERC for a small grant (NE/G010978/1) awarded to RTW, the Royal Society International Travel Grant, Mike Coward Fund of the Geological Society of London, Percy Sladen Fund of the Linnean Society, The Gilchrist Educational Trust, and the Earth and Space Foundation for their support in funding the field component of this project. GEC's doctoral studentship is funded by the National Environmental Research Council (NERC). RTW is supported by a University Research Fellowship awarded by the Royal Society. We also thank the National Centre for Earth Observation (NCEO), the Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET+), and the NERC-ESRC funded project, Earthquakes without Frontiers (EwF), all of which have contributed to the funding of this project. DigitalGlobe high resolution satellite imagery was obtained from Google Earth (earth.google.co.uk/). Shuttle Radar Topography Mission (SRTM) was obtained from CGIAR-CSI.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/2015JB01192

An early Devonian intra-plate bimodal volcanic suite in the Kyrgyz North Tianshan belt, the central Asian orogenic belt

A suite of bimodal volcanic rocks in the Kyrgyz North Tianshan belt is composed of totally similar to 100-m-thick basalt and basaltic andesite layers of the Karakastek Formation and totally similar to 700-m-thick rhyolite layers of the Kastek Formation, with minor amounts of interlayered felsic to intermediate tuff/ignimbrite. Zircon grains from a rhyolite sample have a weighted mean Pb-206/U-238 age of 416 +/- 8 Ma, which is considered to be the crystallization age of the rhyolite. The zircon grains have delta O-18 ranging from +6.5 to +7.3 parts per thousand. The calculated zircon saturation temperatures (T-Zr) range from 850 to 900 degrees C. Rhyolites have high total alkaline, Zr, Y and Nb contents and high Ga/Al similar to A(1)-type granitoids formed in an antra-plate setting. Mafic volcanic rocks have SiO2 ranging from 49.4 to 55.3 wt% and FeO2/MgO from 1.4 to 2.4, showing a tholeiitic trend. They have negative Nb and Ta anomalies on the primitive mantle-normalized trace element patterns and have epsilon(Nd)(t) of -1.5 to 0. There is a positive correlation between epsilon(Nd)(t) and Delta epsilon(Hf)(t) of the mafic volcanic rocks, indicative of crustal contamination. However, one least evolved basalt sample has Mg# of 57 with low Zr and Hf concentrations, indicating insignificant crustal contamination. Modeling results indicate that the mafic volcanic rocks may have been derived from a dry, garnet-bearing lherzolite mantle source. They have epsilon(Hf)(t) and epsilon(Nd)(t) varying along the "Island arc array", which is interpreted as the mantle source was metasomatized by slab-derived melts rather than fluids. In addition, the rhyolites have epsilon(Nd)(t) of -2.4 to -2.9, much lower than the mafic volcanic rocks, and were thus unlikely produced by the fractionation of basaltic magmas. The rhyolites show Nd-Hf isotope decoupling, which is interpreted as a result of the anataxis of Precambrian lower crust associated with the breakdown of garnet. Therefore, the early Devonian bimodal volcanic suite in the Kyrgyz North Tianshan belt was considered to have formed in an intra-plate setting