How many sutures in the southern Central Asian Orogenic Belt:Insights from East Xinjiang-West Gansu(NW China)?

How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction systems of the southern Siberian active margin from those of the northern Tarim;and （2） a major Permian suture zone that separates the Tianshan-Beishan from the northern active margin of the Tarim Craton.These new observations and interpretations have broad implications for the architecture and crustal growth of central Asia and other ancient orogens as well.

Fisson-track constrains on superposed folding in the Beishan orogenic belt, southernmost Altaids

The Hongyanjing inter-arc basin, is located at the central part of Beishan Orogenic College （BOC）, Gansu Province, northwest China. Thick sequences of Permian sediments were strongly folded, forming extremely spectacular superposed folds. To better understand the thermal history of Hongyanjing interarc basin and to potentially constrain the timing of deformation, apatite fission track thermochronology method was applied on two superposed folds in the Hongyanjing Basin. Samples from the basin, yield central AFT ages ranging from - 206 to 118 Ma. AFT peak ages were largely consistent between samples and can divided into three groups： 245, 204-170 and 112-131 Ma. Subsequent thermal history modeling of the samples from the Hongyanjing Basin can be summarized as follows： （1） thermal reheating by sedimentary burial at - 260 to -220 Ma; （2） major cooling from -220 to -180 Ma; （3） an episode of very slow subsequent cooling from -180 to 65 Ma （-80 ℃） to present-day outcrop temperatures. Sediments in the Hongyanjing Basin were folded forming F1 fold during the early to late Triassic （-240--220 Ma）, by regional stress, and at the time that the adjacent Xingxingxia shear zone started to become active. It is further suggested that the F2 folding occurred at -225-219 Ma. The deformation age of F2 should he extended to 180 Ma based on our thermal history modeling for the Hongyanjing Basin, which show a rapid exhumation and cooling at the late Triassic to early Jurassic （-220-- 180 Ma）. In our interpretations, the F1 folding is therefore thought to he related to the final closure of the Paleo-Asian Ocean, while the F2 folding occurred at - 225-180 Ma associated with a major pulse of orogenesis in the BOC.