Contribution of Strike-Slip Faults to the Growth of Basement-Cored Uplifts: Case Study of the Kungey-Zailiskey Ala-Too, Tian Shan, Kyrgyzstan and Kazakhstan

Examination of Cenozoic basement-involved orogens shows an intimate association of strike-slip and reverse faulting both at their margins and within the cores of interior ranges. Based on these observations, we hypothesize that strike-slip faults are among the first structures to propagate into foreland basins during growth of basement-involved orogens. This mode of orogenic growth defies the mechanical prediction that shortening should produce low-angle thrust faults but better explains the propensity of basement-involved uplifts to be bounded by high-angle reverse faults oriented obliquely with respect to maximum principal stress direction within the orogen. This faulting pattern could arise due to a decrease of horizontal stress within intact foreland lithosphere or because strike-slip faults are otherwise energetically favored during initial fault propagation. To test if and how propagation of a basement-cored uplift nucleates around a strike-slip fault we investigated the Kungey-Zailiskey Ala-Too, the northernmost range in the tectonically active central Tian Shan. This range consists of two, 200-km-long ridges with peaks exceeding 4 km elevation. The active, ENE-striking sinistral Kemin-Chilik fault separates the Kungey Range on the south from the Zailiskey range on the north. Juxtaposition of contrasting Paleozoic basement terranes indicates that the Kemin-Chilik fault is an inherited structure re-activated during Cenozoic orogenesis. Neogene stratigraphic evidence records a sedimentary wedge derived from the northward-propagating Tian Shan. This wedge is overlain by an upward-coarsening sedimentary section that records initiation and growth of the Kungey Range adjacent to the Kemin-Chilik fault. Deformed Neogene sedimentary units and Quaternary alluvial deposits record overall outward range growth but with varying styles and rates of faulting. For example, patterns of active and dormant reverse faults and stratigraphic relationships document east to west propagation of uplift of the Kungey Range. Faulting activity is highest in the western Kungey Range where sinistral faults bound it on both sides. Reverse faults and active folds with lower rates of slip bound the central Kungey Range, and faults bounding the eastern Kungey Range lack evidence of Late Quaternary activity. Based on these observations we conclude the propagation and growth of the Kungey-Zailiskey Ala-Too nucleated along the Kemin-Chilik fault, consistent with our hypothesis. Future efforts will further document propagation directions of portions of the Kungey-Zailiskey Ala-Too and test if variability in uplift rate and style could be associated with the propagation of the Kemin-Chilik fault.