Abstract

The northern front of the Cenozoic Tien Shan mountains in Kazakhstan comprises east- to NE-trending thrust-related basement uplifts. Some of these are open, asymmetric anticlines, whereas others are fault-bounded blocks. Where emergent and exposed, the bounding faults dip steeply at 45–70°. Large-wavelength open folds in the Cenozoic cover also overlie basement structures. The Palaeozoic basement of volcanic, (meta-) sedimentary and granitic rocks contains older structures such as folds, slaty cleavage, faults and dykes. Some Cenozoic faults truncate all earlier structures, just as some Cenozoic folds are independent of the attitudes of underlying stratified basement rocks. The strongest control on the Cenozoic structure is exerted by steep, NW-striking basement faults that induce along-strike segmentation and lateral terminations of some basement ridges. A few of these basement faults had already been reactivated as normal faults during a Cenozoic phase of east–west extension that preceded folding and thrusting. Some normal faults show reactivation as dextral strike-slip faults during the contractional phase, which is still active today. Since the NE to east trend of the main basement ranges has no obvious precursor structures, we interpret the thick-skinned structures to essentially reflect the modern shortening direction, modulated but not dominated by pre-existing basement faults. Variations in local kinematics over time are probably due to strain partitioning in the anisotropic basement, not to changing far-field stresses. The occurrence of steep dip-slip reverse faults apparently unrelated to reactivation presents an unsolved mechanical paradox similar to some low-angle normal faults.