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10,000-10,000,000年，以及这些速率随时间的变化。研究区域的特点是在现有大地测量网络内存在两种截然不同的构造变形样式。在塔里木盆地北部，向北的滑脱褶皱目前使9公里的新生代盆地填充物变形。在更靠北的更近的位置，南向的逆冲断层切割了据称的第四系砾岩，并将它们的基岩悬移壁沿着非常低的角度(可能在地表)向南移动了15公里。在该地区收集的野外资料可以检验以下假设：1.天山南部的大地应变率与最南端滑脱褶皱(塔里木盆地北部)的第四纪缩短速率相匹配。2.尽管天山边缘晚新生代变形的样式和强度都发生了显著变化，但大约5My以来的缩短速度保持稳定。即使拆离褶皱是向北的，但越早形成的拆离褶皱越早形成。如果第一个假设是正确的，它表明大地测量缩短可能被类似于小喜马拉雅的低角度拆离上的大地震(约8级)所适应。利用四种主要方法收集构造和年代学数据：天山南缘地质填图、同构造层和前构造年代学年代学、上新生代磁性地层学年代学、晚第四纪河流阶地变形测量和年代测定以及反射地震剖面分析。最终产品将提供：收缩褶皱冲断带的地质与大地测量速率的测试；重建西南天山的变形模式、样式和变形速率；洞察为什么在这里形成反差的变形模式；陆内造山过程中变形传播的新模型；以及地震危险的估计。

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.