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Restraining bends, transpressional deformation and basement controls on development |
Department of Geology, University of Leicester, Leicester Road, Leicester, LE1 7RH, UK (e-mail: wdc2{at}le.ac.uk)
Restraining bend mountain ranges are fundamental orogenic elements in the Altai, Gobi Altai and eastern Tien Shan. In this paper, 12 separate restraining bends are reviewed to identify common structural and topographic characteristics. The 12 restraining bends occur in one of three different tectonic settings: (1) strike-slip fault termination zones; (2) at a major strike-slip fault bend where the individual strike-slip fault can be traced continuously from one end of the range to the other; and (3) where two separate strike-slip fault segments converge and overlap. Fault maps of the 12 separate bends reveal that they are all flower or half-flower structures in cross-section, but there is considerable architectural diversity and all have unique individual topographic, structural and dimensional characteristics. Many factors account for the architectural diversity of the restraining bend mountains, especially stepover width, total amounts of strike-slip displacement, reactivation of older structures, tectonic setting, and the angular relation between fault trace and maximum horizontal stress. The stepover sense for regionally important strike-slip faults is controlled by pre-existing basement heterogeneities and is dominantly contractional. Therefore, releasing bends and transtensional basins are largely absent. Throughout the region there is a continuum of mountain range types, from purely contractional ridges to isolated restraining bends along strike-slip-dominated zones. Nucleation, topographic uplift, along- and across-strike growth of the bend, and restraining bend coalescence with adjacent ranges appears to be an important mountain-building process in the Altai, Gobi Altai and eastern Tien Shan; similar processes are likely in other intracontinental transpressional orogens.
