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

赵家将对藏北羌塘中部新发现的火山岩进行古地磁研究，最近在那里获得了熔岩流、岩墙和火山锥的Ar-Ar年龄。作为拟议活动的学术价值的一部分，他们寻求以下两个主要问题的答案：(1)第一个是从中国东部一直延伸到吉尔吉斯斯坦以西3,000公里的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贯模式。在中国和吉尔吉斯斯坦西部，这些古纬度与欧亚大陆古地磁参考极点的预期差平均为18度，这意味着第三纪相对于西伯利亚的北移比地质估计的跨越中间山脉的缩短要大得多。这些主要是沉积的古地磁数据的可靠性必须得到检验，然后我们才能评估这些差异中有多少是构造、地磁和岩石磁性的。(2)印亚汇聚与西藏地体聚积的历史问题。关于这一问题的现有证据有各种各样的解释，导致在西藏内部和西藏与欧亚大陆之间关于地体增长的时间和几何问题存在争议。特别是，该区构造演化中的两个重要构造问题，即拉萨地块、西羌塘地块、塔里木地块和准噶尔地块晚白垩世和早第三纪古纬度的明显困境，以及东西羌塘地块之间的地体增生史，都需要可靠的古地磁资料来回答。这种方法是由这些辩论构成的。PI将在任何有前景的地区集中在火山岩中采样，以避免我们被经常影响沉积岩的虚假倾斜浅化所误导的风险。他们还将对与火山岩有关的红层进行代表性采样，并进行褶皱测试。通过这种方式，他们将能够评估红层的磁性保真度。本文提出的古地磁方案的成功完成，将首次从藏北新生代火山岩和红层获得可靠的古地磁结果，这些火山岩和红层直接参与了中亚地区新生代倾斜变浅的争论。新的古地磁结果还将提供极点位置和古纬度，以限制印度支那-亚洲会聚速率的计算。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确保了这项研究得到必要的支持，并有力地证明了成功的机会。国际和平研究所预计拟议的活动将产生以下更广泛的影响：(1)研究结果将在全国会议上提交，并向国家科学基金会报告，并发表在国际同行评议的期刊上；(2)研究的一些成果还将被引入本科生和研究生课堂；(3)研究生和本科生研究人员将直接参与实地和实验室工作；(4)关于印度支那-亚洲汇聚的程度和西藏隆起的时间的研究结果可能有助于更好地了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.