Geologic Versus Geodetic Rates of Convergence in the Southwestern Tien Shan, China

在天山南部的研究正在确定地质应变梯度、实际模式和长期变形速率，时间尺度从10，000年到10，000，000年不等，以及这些速率随时间的变化。该研究区的特点是在现有的大地测量网络内有两种截然不同的构造变形类型。在北方塔里木盆地，北向拆离褶皱目前变形的新生代盆地充填9公里。在更靠北的位置，向南的逆冲断层切割了据称是第四纪砾岩的岩层，并将其基岩上盘沿沿着极低角度（可能在地表）的断层带向南搬运了15 km。在该地区收集的实地数据允许测试以下假设：1。天山南部的大地测量应变率与最南端拆离褶皱（塔里木盆地北方）的第四纪缩短率相匹配。2.尽管天山边缘晚新生代变形的样式和聚散度都发生了显著变化，但自约5百万年以来缩短速率一直保持稳定。即使拆离褶皱是北向的，更北的形成最早。如果第一个假设是真的，它表明大地缩短可能是由类似于小喜马拉雅山的低角度拆离断层上的大地震（约8级）所适应的。构造和年代学数据的收集主要采用四种方法：天山南缘地质填图、同构造和前构造年代学、通过磁性地层学确定的上新生代地层、变形的晚第四纪河流阶地的测量和定年以及反射地震剖面分析。最终产品将提供：在收缩褶皱和冲断带的地质与大地测量率的测试;模式，风格和变形率的重建，在西南天山;为什么对比变形模式在这里发展的见解;一个新的模型，在陆内造山运动传播变形;和地震灾害的估计。

Research in the southern Tien Shan is defining geologic strain gradients, actual patterns and rates of long-term deformation at time scales ranging from 10,000 -10,000,000 yr, and changes in those rates through time. The research area is characterized by two contrasting styles of structural deformation within an existing geodetic network. In the northern Tarim basin, north-vergent detachment folds presently deform the 9 km of Cenozoic basin fill. In a more proximal position farther north, south-vergent thrust faults cut purported Quaternary conglomerates and have carried their bedrock hangingwalls 15 km southward along very low-angle (perhaps at the surface) detachments. Field data being gathered in this region permit tests of the following hypotheses:1. Geodetic strain rates in the southern Tien Shan are matched by Quaternary rates of shortening in the southernmost detachment folds (northern Tarim basin). 2. Despite striking changes in both style and vergence of Late Cenozoic deform-ation across the Tien Shan margin, rates of shortening since approximately 5 My have remained steady.3. Even though the detachment folds are north-vergent, the more northerly ones formed earliest.If the first hypothesis is true, it suggests that geodetic shortening may be accommodated by large (approximately magnitude 8) earthquakes on low-angle detachments analogous to those in the Lesser Himalaya. Structural and chronologic data are being collected using four primary approaches: geologic mapping across the southern margin of the Tien Shan, chronologies of syn- and pre-tectonic, Upper Cenozoic strata via magnetostratigraphy, surveying and dating of deformed Late Quaternary fluvial terraces, and analysis of reflection seismic profiles. The end product will provide: a test of geologic versus geodetic rates in a contractional fold-and-thrust belt; a reconstruction of the pattern, style, and rates of deformation across the SW Tien Shan; insights into why contrasting deformation patterns developed here; a new model for propagating deformation during intracontinental mountain building; and an estimate of earthquake hazards.