西域砾岩的地层属性与成因:进展与展望

Stratigraphic attribute and origin of the Xiyu conglomerates in NW China:progress and prospect

西域砾岩是我国西北造山带(如天山、昆仑山、祁连山)山前发育的一套晚新生代冲-洪积相沉积,其记录了晚新生代以来的山体隆升、气候变化等信息,是研究盆山耦合、环境演变的良好载体。西域砾岩自首次命名以来,吸引了众多地学研究人员开展了系统的研究工作,特别是在西域砾岩的岩性特征、沉积过程、地层年代、成因等方面取得了显著进展。本文通过系统梳理前人成果,结合自己研究取得的认识,总结了目前关于西域砾岩的研究现状和进展,并提出了今后的研究展望。为更好地揭示西域砾岩所记录的环境演化信息,需要进一步开展西域砾岩的物源研究,如锆石U-Pb年龄谱分析、重矿物分析等方面的工作。

Background,aim,and scope The Cenozoic terrestrial sedimentary sequence in the foreland basin play an indispensable role in the inversion of the tectonic deformation induced by the Indo-Asian collision.Predecessors have done a variety of research on the Cenozoic terrestrial sedimentary sequences including Xiyu conglomerate in Tian Shan,Qilian Shan and Kunlun Shan through magnetostratigraphy and field observation of growing strata.Xiyu conglomerate is a set of alluvial dark gray to brown gravel which deposit during late Cenozoic.Its genesis is still is in dispute.Most researchers consider that the Xiyu conglomerate has been controlled by tectonics,while others believe it triggered by climate changes at^2.6 Ma.Consequently,in this paper we aimed at making an overview of the lithology,sedimentary processes,stratigraphic ages and genesis of the Xiyu conglomerates.Materials and methods Review and prospect are made based on the domestic and international investigations of lithostratigraphy and magnetostratigraphic in the past two decades.Results Xiyu conglomerate is a prograding diachronous gravel wedge,whose thickness gradually thins toward the front of the mountain.On the basis of detailed magnetostratigraphic investigations,the basal age of the Xiyu conglomerates in the piedmonts of the Tian Shan,the Qilian Shan and the Kunlun Shan ranged from 15 Ma to<1 Ma.The relationship between diachronism of the Xiyu conglomerate deposits sequences and the progradation of the depositional system is verified through the identification of growing strata and the study on fault and fold migration.Discussion Variations in age clearly indicate that the Xiyu conglomerate in northwest China should only be considered as a lithostratigraphic unit that lacks temporal implications beyond the local area.Therefore,Xiyu conglomerates cannot be considered as a product of specific tectonic events or climate events due to its diachronism.Since a specific tectonic movement cannot reveal the temporal and spatial variation of Xiyu conglomerate in the foreland basin,the interpretation should establish in a reliable,three-dimensional,timecontrolled sedimentary framework.Combining the spatial-temporal coupling of flexural basin and adjacent orogen,the spatial-temporal evolution of the sedimentary facies of the Xiyu conglomerate should be derived from the uplifted mountain,while climate changes only regulate its deposition.Conclusions The Xiyu conglomerate is a lithostratigraphic unit deposited during Late Cenozoic,which is mainly caused by the tectonic movement instead of climate changes.Recommendations and perspectives Further investigation in the future work should pay more attention to the provenance analysis of this lithostratigraphic unit,such as zircon U-Pb age spectrum analysis and heavy mineral analysis.Moreover,through the provenance analysis of Xiyu conglomerate,we can invert the characteristics and tectonic setting of the of the source area of the sedimentary basin and reproduce the spatial-temporal coupling between the sedimentary basin and the adjacent orogenic belt in the northwestern China.