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Tan-Lu fault zone; apatite fission-track thermochronology; fault-slip analysis; Late Cretaceous-Cenozoic tectonics; Dabie Shan
Abstract:
Apatite fission-track (AFT) and structural data outline the Late Cretaceous - Cenozoic history of the southern Tan-Lu fault zone (TLFZ), one of Asia's major faults, the Triassic-Jurassic Dabie orogen, Earth's largest track of ultrahigh-pressure rock exposure, and its foreland, the Yangtze foreland fold-thrust belt. The fission-track analyses utilized the independent (phi- ), Z- and xi-methods for age determination, which yielded within error identical ages. Ages from Triassic-Jurassic syn- orogenic foreland sediments are younger than their depositional age and thus were reset. A group of ages records rapid cooling following shallow emplacement of granitoids of the widespread latest Jurassic-Early Cretaceous "Yanshanian" magmatism. Most ages are 90 to 55 Ma and document cooling following reheating at 110-90 Ma, the time when the basement units of the Dabie Shan were last at >200 degreesC. This cooling coincides with rifting marked by the Late Cretaceous-Eocene red-bed deposition in eastern China. During this period, the Dabie basements units exhumed in the footwall of the Tan-Lu fault with the Qianshan basin in the hanging wall; the associated stress field is transtensional (NW-trending principal extension direction). The youngest fission-track ages and temperature-time path modeling point to enhanced cooling in the footwall of the Tan-Lu and associated faults at 45 +/- 10 Ma. The related stress field is transtensional, with the principal extension direction changing trend from NW to W. It may be the far-field expression of the India-Asia collision superposed on the back-arc extension setting in eastern China. A regional unconformity at similar to 25 Ma marks an upper bound for the inversion of the Late Cretaceous-Eocene rift structures. During the Neogene, further subsidence in the eastern China basins was accommodated by sub- horizontal NE - SW extension, and followed by the presently active NW-SE extension. The Tan-Lu fault along the eastern edge of the Dabie Shan had normal and then sinistral-transpressive motion during the Late Cretaceous - Eocene. Its motion changed during the Neogene from sinistral transtensive to normal and then to its present dextral transtensive activity. (C) 2002 Elsevier Science B.V. All rights reserved.
