山前挠曲盆地是大陆上最常见的盆地之一,在我国西部和青藏高原及其周边广泛发育,其最大特点是盆地基底在相邻山地冲压下会产生显著挠曲下沉和冲断褶皱,并严格控制了盆地构造和地层与沉积相的发育,是开展新生代尤其第四纪盆地地层以及地层记录的气候变化研究的关键区域.然而该类盆地地层和沉积相变化很大,在开展地层长期气候变化和地层对比研究时容易产生严重的偏差.本文以临夏盆地的精细研究为例,概括出山前挠曲拗陷盆地的构造地层和沉积相时空演化模型,以及它们如何控制地层的对比和古气候记录的产生,以期为我国新生代古气候研究中遇到的大量地层年代争论和古气候记录矛盾的解决提供借鉴.%Intracontinental flexural(foreland) basin is one of the most common occurred basins in the continent.It is widely distributed in and around the Tibetan Plateau.Its striking features are the remarkable flexural depression and thrust-fold of basin marginal basement and sediments due to loading of nearby mountains from push,which strictly control the evolution of the basin tectonics and the development of the stratigraphy and sedimentary facies.The sediments in such basin are now frequently used as archives for detailed paleoclimatic and sedimentary environmental reconstruction,fossil and stratigraphic correlation,and tectonic evolution and uplift of basin and orogen.However,sedimentologic characteristics vary considerably in time-space with the evolution of flexural basin,apt to cause misinterpretation of climatic change and stratigraphic correlation.Especially for those who are used to study climatic changes using homogeneous sediments,such as eolian dust deposits,or on more or less stable sediments in down-faulting basins with less lateral sedimentary facies variation,it is more at risks to make serious mistakes.Perhaps this is one of the major reasons why there are so many serious contradictions in age determination of stratigraphy,paleontological vs.paleomagnetic stratigraphy correlation,sedimentary facies and climatic records and changes in basins in Western China.Based on high resolution fossil mammal and magnetostratigraphic constraints and sedimentary facies analysis,here we take the Linxia Basin at the front of the NE Tibetan Plateau as a case to demonstrate and figure out a model how sedimentology and stratigraphy vary temporospatially with the evolution of such flexural basin,which at first order strictly control the proxy climatic records,thus climatic change history.We wish our research could shed light on making clear many of the differences or contradictions in basin stratigraphy and climatic studies in Western China.The results show that the typical intracontinental foreland Linxia Basin experienced two phases,the early subsidence and the late thrust-fold,of flexural deformation exerted by the West Qinling (Mts.) and NE Tibetan Plateau to the south.The early subsidence phase manifests as the basin being mainly subjected to flexural depression mostly in response and balance with pushing and loading mountains,during which the uplift and erosion of the loading mountains are generally in balance with supplying and infilling of mostly fine sediments to flexural basin,forming remarkable sediment wedge in the foredeep.In the Linxia Basin,this commenced latest at the beginning of the Late Oligocene(26.5 Ma).During such tectonosedimentary environment,a big river-shallow lake system develops along the foredeep and parallels the pushing and loading mountains.The late thrust-fold phase is characterized with the loading mountains begin to uplift rapidly and thrust onto the basin,causing strong mountain erosion,thrust-fold belt propagation and basin overfilling.This forced the mountains-parallel river-lake system to turn to the mountains-perpendicular alluvial-braided river system,and finally to an outflow system.Concurrent are rapid rotation of the basin,occurrence of growth strata and late unconformities,widespread expansion of boulder conglomerates,great decreasing and increasing sedimentation rates above and before the hanging wall of the faultfold system and new supplementary provenance from the thrust-fold system.In the Linxia Basin,this phase began at ca.8 Ma.This demonstrates that in stable climate,same lithologic units such as distinct lacustrine sediments and alluvial conglomerates will migrate basinwards with the foredeep moving into basin,causing a great diachroneity and often misleads to recognize the same lithologic unit in space as one unit in time.
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