Paleomagnetic Study of Indo-Asian Convergence and the Central Asian Shallow-Inclination Problem Using 33-45 Ma Volcanic Rocks Newly Discovered in Northern Tibet

PI将对藏北羌塘中部新发现的火山岩进行古地磁研究，在那里最近获得了熔岩流、岩脉和火山锥上的Ar-Ar日期。作为所建议的活动的智力价值的一部分，他们寻求以下两个主要问题的答案：(1)首先是对第三纪岩石的大量高质量古地磁研究得出的令人不安的低古纬度的一致模式，这些研究地点从中国东部一直延伸到吉尔吉斯斯坦，向西3000公里。在中国西部和吉尔吉斯斯坦，这些古地磁纬度与欧亚大陆古地磁基准极预测的纬度之间的差异平均为18度，这意味着第三纪相对于西伯利亚的北移要比地质估计的中间山带缩短要大得多。在我们评估构造、地磁和岩石磁的差异有多大之前，必须先测试这些主要是沉积的古地磁数据的可靠性。(2)第二个问题是西藏地区印亚融合和陆块增生的历史。关于这一主题的现有证据有各种各样的解释，导致对西藏境内和西藏与欧亚大陆之间的地体增生的时间和几何形状的争论。特别是对拉萨、西羌塘、塔里木和准噶尔地块在晚白垩世和早第三世古纬度上的明显困境，以及东羌塘地块和西羌塘地块之间的陆块加积史等两个重要构造演化问题的解答。要回答这些问题，需要可靠的古地磁数据。这种方法是由这些辩论构成的。为了避免我们被常常影响沉积岩的虚假倾斜变浅所误导的风险，PI将在任何有希望的剖面上集中采样火山岩。他们还将对与火山活动和褶皱试验有关的红层进行有代表性的取样。通过这种方式，他们将能够评估红层的磁保真度。本文提出的古地磁程序的成功完成，将首次为直接涉及中亚新生代倾斜浅化争论的藏北新生代火山岩和红层提供可靠的古地磁结果。新的古地磁结果也将提供极位和古纬度来约束印亚辐合速率的计算。来自中国三所大学的中国同事在古生物学、地质年代学、岩石学、岩石磁学、构造和野外地质学方面的专业知识，为这项研究提供了必要的支持，并有力地证明了成功的可能性。PI预计提议的活动将产生以下更广泛的影响：(1)结果将在国家会议上提出，向美国国家科学基金会报告，并在国际同行评审期刊上发表；(2)部分研究成果也将被引入本科和研究生课堂；(3)研究生和本科生研究人员将直接参与现场和实验室工作；(4)关于印度洋-亚洲辐合范围和青藏高原隆升时间的研究结果有助于更好地理解青藏高原对全球气候的影响。

ZhaoThe PI's will carry on a paleomagnetic research program on newly discovered volcanic rocks in central Qiangtang of northern Tibet, where Ar-Ar dates on lava flows, dykes, and volcanic cones have recently been obtained. As part of the intellectual merit of the proposed activity, they seek answers to the following two leading questions: (1) The first is the consistent pattern of disturbingly low paleolatitudes derived from a large number of high-quality paleomagnetic studies of Tertiary rocks from sites that reach all the way from eastern China to Kyrgyzstan, 3,000 km to the west. The difference between these paleolatitudes and those expected from paleomagnetic reference poles for Eurasia averages 18 degrees in western China and Kyrgystan, implying northward displacements relative to Siberia in the Tertiary that are much larger than geological estimates of shortening across intervening mountain belts. The reliability of these mainly sedimentary paleomagnetic data must be tested before we can evaluate how much of the discrepancy is tectonic, geomagnetic, and rock magnetic. (2) The second problem is the history of Indo-Asian convergence and terrane accretion in Tibet. The existing evidence concerning this subject is interpreted in a wide variety of ways, resulting in controversy over the timing and geometry of terrane accretion both within Tibet and between Tibet and Eurasia. In particular, the PI's seek answers to the solution of two important tectonic problems in the tectonic evolution of the region: namely, the apparent dilemma in paleolatitudes for Lhasa, west Qiangtang, Tarim, and Junggar blocks in late Cretaceous and early Tertiary, and terrane accretion history between east and west Qiangtang block.Reliable paleomagnetic data are needed to answer these questions. The approach is framed by these debates. The PI's will concentrate their sampling in volcanic rocks wherever promising sections are available, to avoid the risk of our being misled by the spurious inclination shallowing that often affects sedimentary rocks. They will also make a representative sampling of red beds that are associated with the volcanics and with fold tests. In this way they will be able to assess the magnetic fidelity of redbeds. Successful completion of the paleomagnetic program proposed here will, for the first time, produce reliable paleomagnetic results from Cenozoic volcanic rocks and red beds in northern Tibet, which are directly involved in the debate over Cenozoic inclination shallowing in central Asia. The new paleomagnetic results will also provide pole positions and paleolatitudes to constrain the rate calculation for Indo-Asian convergence. The expertise in paleontology, geochronology, petrology, rock magnetism, and structural and field geology of Chinese colleagues, who hail from three universities in China, ensures the necessary support for this research and argues strongly for the chances of success. The PI's anticipate the following broader impacts of the proposed activity: (1) results will be presented at national meetings, reported to the NSF, and published in international peer- reviewed journals; (2) some of the results of the research will also be introduced into both undergraduate and graduate classrooms; (3) graduate and undergraduate student researchers will be directly involved in both field and laboratory work; and (4) findings on the extent of Indo- Asian convergence and the timing of Tibetan uplift may help lead to a better understanding of the effects of the Tibetan Plateau on global climate.