THICK-SKINNED THRUSTING IN THE NORTHERN TIEN SHAN FORELAND, KAZAKHSTAN: STRUCTURAL INHERITANCE AND POLYPHASE DEFORMATION

The northern front of the Tien Shan mountains in Kazakhstan comprises an array of thrust-related basement uplifts of varying size and orientation. Many of these uplifts are asymmetric anticlines with long, gently dipping backlimbs, rounded hinges and more steeply dipping, short frontlimbs, suggesting they may overlie listric thrust faults. Where exposed, the bounding reverse faults dip steeply at 45-70°. The pre-Cenozoic basement consists of volcanic, (meta-)sedimentary and granitic rocks. It contains a variety of older structures of different orientation such as folds, slaty cleavage or steeply dipping faults and dykes. The relations of the Cenozoic structures with these older basement fabrics are highly variable. Some young faults truncate all earlier structures, just as some Cenozoic folds appear largely independent of the attitudes of underlying stratified basement rocks. Reactivation of dipping bedding planes as thrust faults is not uncommon but always localized and of small scale. The strongest control on the Cenozoic structure is exerted by steep, NW-trending faults which were reactivated as dextrally transpressive faults and induced along-strike segmentation and lateral terminations of some basement ridges. The same faults were locally reactivated as normal faults during a Cenozoic phase of roughly E-W extension that preceded folding and thrusting. Some of the normal faults show evidence of mild reactivation as strike-slip faults during the contractional phase which is still active today. The geometries of the thick-skinned structures reflect the slightly fanning modern shortening direction modulated by pre-existing basement faults and a Cenozoic phase of extension. Instead of an arcuate thrust belt with continuously changing strike, interfingering structures following two discrete, ENE and ESE structural trends are developed. The ESE trend coincides with a prominent set of steep faults present in the Permo-Carboniferous basement. The ENE trend has no obvious precursor structures on a local scale.