Basement‐cored uplifts bounded by steeply dipping reverse faults are mechanically difficult to explain. Reactivation of strike‐slip faults that aid the formation of new, high‐angle reverse faults in the surrounding crust may provide one origin for these structures. This hypothesis is explored by examining the late Miocene to Quaternary evolution of the Kungey and Zailiskey ranges in the northern Tian Shan. These ranges are cored by the Kemin‐Chilik fault (KCF), an inherited Paleozoic structure with sinistral separation of basement terranes. Range growth in response to northward propagation of the Tian Shan has taken place along a network of steeply dipping reverse and oblique‐slip faults surrounding the KCF. Deformation of a low relief unconformity separating Neogene strata from Paleozoic basement records structural growth in response to fault slip. Deformed river terraces surrounding the ranges are correlated to a well preserved chronosequence in the southern Kungey Range. Cosmogenic10Be dating of this chronosequence combined with offset measurements yields slip rates ranging from 0.07 to 0.37 mm/yr for dip‐slip faults, and 1.1 to 1.5 mm/yr for strike‐slip faults Late Quaternary activity in the Kungey‐Zailiskey ranges is consistent with the longer‐term, outward stepping pattern of range growth. Based on cross sections constrained from the folded unconformity surface, deformed Neogene strata and Quaternary terraces, faults building the Kungey Range are inferred to steepen at depth and emanate from a shear zone co‐located with the reactivated KCF. This geometry is consistent with a slip partitioned system developed by an obliquely slipping reactivated fault at depth.

Key Points:• Channel incision relation to steepness calibrated from tilted bedrock surface • Chlorine-36 exposure age dating shows bedrock surface exhumed at 1 to 2 m/kyr• Low n values, consistent with shear stress (n = 2∕3) satisfy field data Supporting Information:• Table S1 • Table S2 • Readme Abstract We examine the relationship of channel steepness to incision rate from channels eroding into a previously tilted, planar, and progressively exhumed unconformity surface. Channel and unconformity slopes are measured from a suite of channels developed on erosionally resistant Paleozoic limestone exhumed by the removal of Cenozoic sediments from the Baybeiche Range bordering the Naryn basin in the western Tian Shan. The compiled data set, sampling 5 orders of magnitude of upstream drainage area (0.03 to 227 km 2 ), is used to derive the exponent, n, relating channel steepness to channel incision rate and the ratio, K∕V, of the rate constant for channel incision of the resistant substrate, K, to the erosion rate, V, of the cover strata. We show that for a typical value of intrinsic concavity (slope-area exponent, = 0.5), erosion rates that are proportional to specific stream power (n = 1) satisfy the data set. However, valley width data suggest that the intrinsic concavity is higher ( = 0.8) and that the channel incision data can also be fit if erosion is proportional to basal shear stress (n = 2∕3). Our results do not support values of n significantly greater than one. Using 36 Cl exposure age dating of the unconformity surface, we independently demonstrate that the Cenozoic cover strata have been progressively stripped downward from the unconformity surface at a vertical rate of 1 to 2 m/kyr. Using V = 1 m/kyr, we constrain the rate constant, K, to between 6 ± 1 and 9 ± 2 ×10 −4 kyr −1 for incision of resistant limestone bedrock in this field setting.

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