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[1] The present-day topography of the Tian Shan range is considered to result from crustal shortening related to the ongoing India-Asia collision that started in the early Tertiary. In this study we report evidence for several episodes of localized tectonic activity which occurred prior to that major orogenic event. Apatite fission track analysis and (U-Th)/He dating on apatite and zircon indicate that inherited Paleozoic structures were reactivated in the late Paleozoic-early Mesozoic during a Cimmerian orogenic episode and also in the Late Cretaceous-Paleogene (around 65 60 Ma). These reactivations could have resulted from the accretion of the Kohistan-Dras arc or lithospheric extension in the Siberia-Mongolia zone. Activity resumed in the late Mesozoic prior to the major Tertiary orogenic phase. Finally, the ongoing deformation, which again reactivates inherited tectonic structures, tends to propagate inside the endoreic basins that were preserved in the range, leading to their progressive closure. This study demonstrates the importance of inherited structures in localizing the first increments of the deformation before it propagates into yet undeformed areas. Citation: Jolivet, M., S. Dominguez, J. Charreau, Y. Chen, Y. Li, and Q. Wang (2010), Mesozoic and Cenozoic tectonic history of the central Chinese Tian Shan: Reactivated tectonic structures and active deformation, Tectonics, 29, TC6019, doi: 10.1029/2010TC002712.

The Ordovician-Silurian transition was a critical interval in Earth's history marked by dramatic climatic, oceanic, and biological turnovers. Here we present the chemical index of alteration (CIA) as a proxy of changes in intensity of chemical weathering, and its variations across the Ordovician-Silurian boundaries (Wufeng through Guanyinqiao, to Longmaxi Formations) from Wangjiawan and Nanbazi on the Yangtze block, south China, in order to explore the climatic changes. Our data show that the CIA values of sediments commonly range from 75 to 90 in the Wufeng and Longmaxi Formations, indicating a high degree of chemical weathering and thus a hot and humid climate during deposition. In contrast, a sharp drop in CIA values (most 60-70) in the Guanyinqiao Formation (or Hirnantian) suggests an overall cold and arid climate, interrupted by several intervals of warm climate when deposited. The temporal coincidence of two phases of massive biotic extinctions with the beginning and end of the cold climate epoch, respectively, suggests that the large climatic changes could be one of the main controls on the mass extinctions, although other factors may also have played a role. doi: 10.1130/G30961.1

[1] The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ~1 Ma and ~23.6 Ma, with some uncertainty below ~21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ~16-15 Ma and ~11-10 Ma. Rock magnetic parameters also undergo important changes at ~16-15 Ma and ~11-10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ~16-15 Ma and ~11-10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (

The Yunkai massif is a key region to decipher the tectonic evolution of the south China block since the contained high-grade metamorphic rocks experienced a polyphase deformation. The earliest event ([D.sub.1]) corresponds to a top-to-the-northwest ductile shearing, coeval with amphibolite facies metamorphism, probably developed during a postorogenic synmigmatization extensional event occurred in early Paleozoic time. The main ductile deformation ([D.sub.2]) is top-to-the-northeast shearing, occurred in early Mesozoic time, coeval with amphibolite to greenschist facies metamorphism and associated with the development of northeast verging recumbent folds within the Neoproterozoic-Paleozoic sedimentary cover of the south China block. The tectonic significance of D2 is possibly linked with the subduction of Indochina beneath south China along the Song Ma belt of North Vietnam. Another compressional event ([D.sub.3]) of Late Jurassic to Cretaceous age is indicated by SE verging recumbent folds with NE-SW trending axes developed both in the sedimentary cover and gneissic basement. The northwestward subduction of the Pacific plate or the SE thrusting of the Songpan-Ganzi fold belt upon the Yangtze craton can be considered as the geodynamic causes of this late Mesozoic event. Last, NE SW trending brittle sinistral strike-slip and normal faulting ([D.sub.4]) control the massif uplifting and the subsiding of continental half grabens. This structure, which is widespread in the eastern margin of the Eurasia continent, can be related either to the westward subduction of the Paleo-Pacific plate or to lithosphere removal during the late Mesozoic.