Active tectonics in the central Tien Shan, Kyrgyz Republic
Thompson, Stephen C., 1970-
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Fault slip rates show that late Quaternary contraction is distributed across the Kyrgyz central Tien Shan, and not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north-south shortening on thrust and reverse faults. In a region that spans almost two-thirds of the north-south width of the central Tien Shan, slip rates on eight faults in five basins range from ∼0.1 to ∼3 mm/yr. Fault slip rates come from faulted and folded river terraces and from trench-wall exposures. Radiocarbon, infrared-stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. A major episode of regional river incision occurred ∼13.5--15.7 x 103 cal yr B.P., coincident with abrupt warming of global climate at the end of the last glacial period. The penultimate major river incision occurred ∼141 ka, coincident with the end of the penultimate glacial period. The sum of late Quaternary rates of shortening resembles current rates of north-south shortening measured using Global Positioning System (GPS) geodesy since 1992. This similarity suggests that deformation is concentrated along major fault zones near range-basin margins. Such faults, separated by essentially rigid blocks, accommodate most of the shortening in the upper crust.Paleoseismic data from offset river terraces and trench exposures of faults in the northern foreland and intermontane basins provide evidence ∼2--4 m of displacement during earthquakes, and include a ∼3.5 m displacement from the 1885 M ≈ 6.9 Byelovoda earthquake. The range of displacement is similar to the observed surface displacement after the 1992 Ms = 7.3 Suusamyr earthquake. A preliminary source map of high slip-rate faults across the Kyrgyz central Tien Shan indicates that a 2--4 m event should occur every 60--70 years. This result is generally consistent with the historical record.
- Geology