Thick-skinned thrusting in the northern Tien Shan foreland, Kazakhstan: structural inheritance and polyphase deformation

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.

Late-Paleozoic emplacement and Meso-Cenozoic reactivation of the southern Kazakhstan granitoid basement

“…Remnants of these flat erosional surfaces can be found in the Tien Shan and Altai De Grave and Van den haute, 2002;Sobel et al, 2006 and references therein). In the IBB, sedimentation resumed during the late Eocene-Oligocene, and became widespread since the early Miocene (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000. During the late Eocene-early Miocene the deposits were mainly fluvio-lacustrine in origin and reflect distal, low-energy sedimentary environments (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000.…”

“…In the IBB, sedimentation resumed during the late Eocene-Oligocene, and became widespread since the early Miocene (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000. During the late Eocene-early Miocene the deposits were mainly fluvio-lacustrine in origin and reflect distal, low-energy sedimentary environments (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000. Comparable to the IBB, the adjacent Chu Basin contains ± 6 km of Cenozoic sediments, resting on Cretaceous deposits (Bullen et al, 2001 and references therein).…”

“…Wang et al, 2009;Wang et al, 2001). Likewise, in the IBB, sedimentation became more widespread since the Neogene (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000. During the middle Miocene, a large lake/playa system existed in the center of the IBB (Kober et al, 2013).…”

“…Up to then, sedimentation in the IBB was accompanied by local normal faulting. These normal faults became reactivated as reverse or strike-slip faults since the middle-late Miocene (Kober et al, 2013). Since the Plio-Pleistocene, coarser detrital sediments are deposited in the IBB (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000.…”

“…These normal faults became reactivated as reverse or strike-slip faults since the middle-late Miocene (Kober et al, 2013). Since the Plio-Pleistocene, coarser detrital sediments are deposited in the IBB (Kober et al, 2013;Lucas et al, 1997Lucas et al, , 2000. Our AFT results are in agreement with these findings and confirm that more shortening and deformation took place in the IBB since the Miocene.…”