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率在时间尺度上从10,000 -10,000,000年，以及这些速率随时间的变化。研究区域的特点是在现有的大地测量网中有两种截然不同的构造变形风格。在塔里木盆地北部，北向滑脱褶皱变形了9 km的新生代盆地充填体。在较北的较近位置，南向逆冲断层切割了第四纪砾岩，并沿着非常低角度（可能在地表）的分离带着它们的基岩悬壁向南移动了15公里。在这一地区收集的实地数据允许检验下列假设：天山南部的大地应变速率与塔里木盆地北部最南端拆离褶皱第四纪的缩短速率相匹配。2. 尽管天山边缘晚新生代变形的样式和辐合度都发生了显著变化，但自大约5世纪以来，缩短率一直保持稳定。拆离褶皱虽然北倾，但越北的拆离褶皱形成时间越早。如果第一个假设是正确的，它表明大地测量缩短可能是由类似于小喜马拉雅地区的低角度分离的大地震（大约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：天山南缘的地质填图，同构造和前构造年代学，通过磁地层学对上新生代地层进行年代学，对变形的晚第四纪河流阶地进行测量和定年，以及对反射地震剖面进行分析。最终产品将提供：在收缩的褶皱冲断带中进行地质速率与大地速率的测试；天山西南部的变形模式、样式和速率的重建；了解为什么这里会形成不同的变形模式；一种新的陆内造山传播变形模型以及对地震危害的估计。

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.