EAR-PF: Unraveling the Paleogene exhumation history of the Orocopia Schist in west-central Arizona

pDr.尼基·M Seymour已获得NSF博士后奖学金，分别在斯坦福大学和加州大学圣克鲁斯的Martin格罗夫博士和Jeremy Hourigan博士的指导下开展研究和教育计划。这项研究项目将确定位于亚利桑那州中西部的北方普洛莫萨山脉的挖掘历史。普洛莫萨斯中暴露的岩石起源于沉积在加州以西太平洋海底的沉积物，并在晚白垩世-古近纪Laramide造山运动期间俯冲并添加到北美大陆的底部，这是一个约90-50 Ma的浅角度俯冲时期，与内华达州山脉岩浆活动的结束、高原的建造和随后的崩塌有关，隆起的基底核山脉，和广泛的火山活动。奥罗莫皮亚片岩的顶部在~21-15 Ma之间通过普洛莫萨剥离断层暴露在地表，这意味着它在地壳中行进超过40 km后暴露在地表，以在最新的古近纪（~73-23 Ma）达到浅地壳水平。在浅角俯冲过程中，地壳底部沉积物的积累所伴随的热历史可以揭示这些岩石是如何穿过地壳的。该项目将研究奥罗莫皮亚片岩的变质锆石边缘，以记录沉积物俯冲和添加到北美底部的时间。然而，目前还没有限制的历史，使这些岩石到浅地壳之前中新世断层（~21-15马）。作为该项目更广泛影响的一部分，Seymour博士将与旧金山的软件开发人员协调，扩大增强现实地形沙箱的功能，以整合地质图。在沙箱中成功地将地图与地形相结合，将使学生能够实验沉积地层和结构的3D几何形状，并将广泛适用于各级地球科学教育。pp确定奥罗科普亚片岩（OS）的折返历史对于理解从白垩纪缩短到中新世伸展的过渡以及驱动沉积物底侵、增生和同俯冲折返的过程至关重要。Seymour博士将系统地研究奥罗科普亚片岩俯冲复合体的时间和热演化，使用中-和微结构，地质-和热年代学，岩石学和地球化学分析，以了解（1）在什么深度的OS是增生，并了解挖掘它的过程，（2）构造上覆结晶片麻岩的时代、地球化学亲合性及其与造山带构造接触的意义;（3）片麻岩和OS的折返历史。方法将包括OS和关键位置上覆片麻岩的详细绘图和显微结构研究，以记录关键的交叉关系，具有明确运动学背景的同运动学阶段的定年，如用于U-Pb和角闪石的钛铁矿，用于Ar/Ar热年代学的多硅白云母和黑云母，以及使用石英中的钛的热压测量法、含碳材料的拉曼光谱法和用于温度测量的氧稳定同位素比率以及用于压力测量的石榴石-黑云母-斜长石-石英化学。这种方法将记录的数量和意义的构造世代保存在奥罗莫皮亚片岩，并确定目前暴露的糜棱岩组构是否与古近纪变质作用，中新世的扩展，或保留元素。将这些结果与结晶片麻岩的热历史进行比较，将为两种岩石类型之间的构造关系提供关键细节。特别注意的是，片岩和片麻岩是否共享其热历史的任何部分，如果是这样的话，在什么时候他们的历史一起演变。OS和片麻岩之间共享的热历史将提供对控制俯冲复合体折返的地球动力学过程的见解，在浅板俯冲期间或之后不久。这些观测结果对（1）美国西南部的区域构造，（2）浅角俯冲过程中俯冲底侵的程度，以及（3）在平板俯冲过程中或之后不久控制俯冲复合体折返的地球动力学过程具有影响。pp该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。/ p

pDr. Nikki M. Seymour has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education plans at Stanford University and the University of California Santa Cruz under the mentorships of Dr. Martin Grove and Dr. Jeremy Hourigan respectively. This research project will determine the exhumation history of the northern Plomosa Mountains, located in west-central Arizona. The rocks exposed in the Plomosas originated as sediments deposited on the Pacific Ocean floor west of California and were subducted and added to the base of the North American continent during the late Cretaceous-Paleogene Laramide Orogeny, a period of shallow-angle subduction from ~90-50 Ma that has been tied to the end of Sierra Nevada magmatism, construction and subsequent collapse of high plateaus, uplift of basement-cored mountain ranges, and extensive volcanism. The top of the Orocopia Schist was exposed at the surface by the Plomosa detachment fault between ~21-15 Ma, meaning it would have been exposed at the surface after traveling more than 40 km through the crust to reach shallow crustal levels in the latest Cretaceous-Paleogene (~73-23 Ma). The thermal history that follows the buildup of sediment at the base of the crust during shallow-angle subduction can reveal how these rocks moved through the crust. This project will study metamorphic zircon rims from the Orocopia Schist to document the timing of sediment subduction and addition to the bottom of North America. However at present there are no constraints on the history that brought these rocks to the shallow crust prior to Miocene faulting (~21-15 Ma). As part of the broader impacts of this project Dr. Seymour will coordinate with San Francisco-based software developers to expand the capabilities of the Augmented Reality Topographic Sandbox to integrate geologic maps. Successfully integrating maps with topography in the sandbox will allow students to experiment with the 3D geometries of sedimentary formations and structures and will be widely applicable throughout all levels of geoscience education./ppDetermining the exhumation history of the Orocopia Schist (OS) is critical to understanding the transition from Cretaceous shortening to Miocene extension, as well as the processes driving sediment underplating, accretion, and syn-subduction exhumation. Dr. Seymour will systematically investigate the temporal and thermal evolution of the Orocopia Schist subduction complex using meso- and microstructural, geo- and thermochronological, petrological, and geochemical analyses to understand (1) at what depth the OS was accreted and understand the processes that exhumed it, (2) the age and geochemical affinity of the crystalline gneiss that structurally overlies the OS and the significance of its tectonic contact with the OS, and (3) the exhumation histories of the gneiss and OS. Methods will include detailed mapping and microstructural study of the OS and the overlying gneiss at key locations to document crucial cross-cutting relationships, dating of syn-kinematic phases with clear kinematic context such as titanite for U-Pb and hornblende, phengite, and biotite for Ar/Ar thermochronology, and thermobarometry using Titanium-in-quartz, Raman spectroscopy of carbonaceous material, and oxygen stable isotope ratios for thermometry and garnet-biotite-plagioclase-quartz chemistries for barometry. This approach will document the number and significance of fabric generations preserved in the Orocopia Schist, and determine whether the presently exposed mylonitic fabric is related to Paleogene metamorphism, Miocene extension, or preserves elements of both. Comparing these results to the thermal history of the crystalline gneiss will provide critical details to tectonic relationship between the two rock types. Particular attention will be paid to whether the schist and gneiss share any portion of their thermal history, and if so, at what point their histories evolved together. A shared thermal history between the OS and gneiss would provide insights into the geodynamic processes that control subduction complex exhumation during or shortly following shallow-slab subduction. These observations have implications for (1) the regional tectonics of the southwestern USA, (2) the extent of subduction underplating during shallow-angle subduction, and (3) the geodynamic processes that control subduction complex exhumation during or shortly following flat-slab subduction./ppThis award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria./p