Collaborative Research: Investigation of Syncollisional Extension and Basin Development in the High Himalaya

来自亚利桑那大学和休斯顿大学的一组科学家正在研究喜马拉雅隆起的扎达盆地地区从收缩到伸展的转变的本质。扎达盆地是喜马拉雅地区最大的晚第三纪盆地，其时代大致限定在晚中新世至更新世。由于萨特莱吉河的切割，盆地填充物及其下面的基底引人注目地暴露出来，萨特莱吉河是一条向南流动的跨喜马拉雅河流。该盆地目前位于喜马拉雅冲断带腹地，海拔约3500米，发育于区域性广泛的早中新世至中中新世冲断体系之上。该逆冲系统位于卡拉斯盆地两侧，该盆地被认为发育于中第三纪，但年代仍不确定。扎达盆地和凯拉斯盆地共同记录了该地区从高海拔到相对低海拔的崩塌，以及从侵蚀剥蚀到厚沉积的转变，仅用了几百万年。多学科研究利用构造分析、热气压测定、氩-氩热年代学、盆地分析和氧同位素古高程测定等方法，对：(1)曲苏姆滑脱、藏南滑脱和大逆冲构造的几何、运动学和滑动强度进行了研究；(2)克拉斯逆冲盆地和扎达盆地物源及沉降史；(3)扎达盆地古高程史；(4)主要断裂掘出岩石中记录的压力-温度-时间路径；(5)主要结构的运行深度。研究结果用于确定：(1)喜马拉雅地区高海拔伸展的开始时间；(2)扎达盆地内外伸展运动调节模式；(3)弧垂直和弧平行延伸速率；(4)扎达盆地伸展开始时的高程、古高程和沉积古环境对伸展的响应；(5)扎达盆地古水系与现代萨特莱吉河源头融合的时间。通过对这些问题的回答，对主要造山带由弧-正收缩向弧-平行伸展过渡的三种理论进行了检验：(1)伸展发生在地壳过厚的地区；(2)伸展是由于弧形逆冲前缘向外扩张所致；(3)伸展是由中地壳挤压引起的。该项目涉及本科生和研究生的研究，与中国科学家的合作，以及在美国和中国的教育和推广工作。

A team of scientists from the University of Arizona and the University of Houston are investigating the nature of the transition from contraction to extension in the Zada basin area of the Himalayan uplift. The Zada basin is the largest late Tertiary basin in the Himalaya and is broadly constrained to be Late Miocene to Pleistocene in age. The basin fill and its underlying basement are spectacularly exposed owing to incision by the Sutlej River, a major southward-flowing trans-Himalayan drainage. The basin currently lies at approximately 3500 m elevation in the hinterland of the Himalayan thrust belt and developed on top of a regionally extensive Early to Middle Miocene thrust system. The thrust system flanks a second major basin, the Kailas basin, which is considered to have developed during mid-Tertiary time, but remains poorly dated. Together, the Zada and Kailas basins archive the collapse of this region from high to relatively low elevation and the transition from erosional denudation to thick sediment accumulation in only a few million years. The multidisciplinary study is using structural analysis, thermobarometry, argon-argon thermochronology, basin analysis, and oxygen isotope paleoaltimetry to place constraints on: (1) geometry, kinematics, and magnitude of slip of the Qusum detachment, South Tibetan Detachment, and Great Counter thrusts; (2) provenance and subsidence history of the Kailas thrust basin and Zada basin; (3) paleoelevation history of the Zada basin; (4) pressure-temperature-time paths recorded in the rocks exhumed by the major faults; and (5) the depth to which the major structures operated. Results are being used to determine the: (1) timing of onset of high-elevation extension in this part of the Himalaya; (2) mode of kinematic accommodation of extension in and around the Zada basin; (3) rates of arc-perpendicular and arc-parallel extension; (4) elevation at which extension began, and the evolution of paleoelevation and paleoenviornments of deposition in the Zada basin in response to ongoing extension; and (5) date at which the paleodrainage pattern in Zada basin become integrated into the headwaters of the modern Sutlej River. By answering these questions, three theories to explain the transition from arc-normal contraction and to arc-parallel extension in major orogenic belts are being tested: (1) extension occurs in regions of overthickened crust; (2) extension is due to outward expansion of the arc-shaped thrust front; or (3) extension is caused by extrusion of the middle crust. The project involves undergraduate and graduate students in the research, collaboration with Chinese scientists, and education and outreach efforts in both the United States and China.