Tectonics of the Northern Tien Shan in Kazakhstan: New Fission-Track Ages and Open Questions

The Tien Shan mountains of central Asia form the world´s largest and most active intraplate orogen. The Tien Shan lies in the distal northern foreland of the India-Eurasia collision zone, separated from the Himalaya-Tibet orogen by the nearly undeformed Tarim basin which acts as an extension of the Indian indenter. It began to rise c. 20 Ma ago and today accommodates c. 20 mm/yr of the N-S India-Asia convergence. The Tien Shan is partitioned along-strike into a western, central and eastern segment that differ markedly in their topography and structure. The western segment comprises a series of reverse-fault bounded ranges around 4000 m high separated by intervening valleys. The central segment is a more compact thick-skinned thrust belt with only one large intramontane basin and peak elevations around 7000 m. The eastern segment consists of a string of thrust-related, large anticlinal uplifts with important left-lateral strike-slip components. The western and central segments trend ENE while the eastern one trends ESE. A complex zone of displacement transfer links the central and eastern segments. A conspicuous feature of the western and central Tien Shan is a set of NW- trending, almost evenly spaced right-lateral strike-slip faults that obliquely cross-cut the mountain range and die out in its northern foreland. A more subdued conjugate set of NNE-trending left-lateral faults is also present, but not expressed as continuous fault zones. However, seismic activity highlights some of these hidden basement faults. Associated earthquakes, topography and GPS data suggest that the oblique faults are transpressional, causing only limited orogen-parallel stretching or lateral escape. The southern margin of the western and central Tien Shan is fringed by thin-skinned thrust belts, whereas the northern margin is underlain by relatively steep basement thrusts. In the western segment this border fault system is remarkably stationary since c. 11 Ma as revealed by apatite fission-track (AFT) dating, and faults within the range have remained active. We report new AFT data from the central segment suggesting that the border fault system there may have propagated northward during the same time span. AFT ages around 10 Ma from the interior region also show that the topographically highest part is more deeply eroded than most of the Tien Shan where AFT ages indicate Mesozoic cooling and limited exhumation during the Cenozoic orogeny. Several aspects of intracontinental mountain building in the Tien Shan remain poorly understood, including the key question why the Tien Shan is mechanically weaker than its surroundings. Neither the reactivation of older orogenic structures nor a thin lithospheric mantle provide a convincing explanation for the entire chain. Another problem is crustal volume balance, because tectonic shortening is apparently insufficient to account for a thick crustal root. Seismic tomography has in fact been interpreted as showing no thick low-density crust even beneath high topography, but this in conflict with gravity data.