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

EAR-PF：揭示亚利桑那州中西部 Orocopia 片岩的古近纪发掘历史

• 批准号: 1952764
• 负责人: Nikki Seymour
• 金额: $ 17.4万
• 依托单位: Seymour, Nikki M
• 依托单位国家: 美国
• 项目类别: Fellowship Award
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-08-01 至 2022-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1952764&HistoricalAwards=false
• 关键词: EAR; PF; Unraveling; Paleogene; exhumation

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

pDr。Nikki M. Seymour获得美国国家科学基金会EAR博士后奖学金，在斯坦福大学和加州大学圣克鲁兹分校分别接受Martin Grove博士和Jeremy Hourigan博士的指导进行研究和教育计划。该研究项目将确定位于亚利桑那州中西部的普洛莫萨山脉北部的发掘历史。在普洛密索斯暴露的岩石起源于加利福尼亚西部太平洋海底的沉积物，在晚白垩纪-古近纪拉拉米造山运动期间俯冲并加入到北美大陆的底部，这是一个约90-50 Ma的浅角度俯冲时期，与内华达山脉岩浆活动的结束、高高原的建设和随后的崩塌、基底核山脉的隆起和广泛的火山活动有关。在~21 ~ 15 Ma之间，Orocopia片岩顶部受Plomosa滑脱断层的影响暴露在地表，这意味着它在白垩纪-古近纪晚期（~73 ~ 23 Ma）穿过地壳到达地壳浅层后，经过了40多公里才暴露在地表。在浅角俯冲过程中，地壳底部沉积物积聚的热历史可以揭示这些岩石是如何穿过地壳的。该项目将研究来自Orocopia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，对于这些岩石在中新世（~21 ~ 15 Ma）断裂之前进入浅层地壳的历史，目前还没有明确的限制。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，扩展增强现实地形沙盒的功能，以整合地质图。在沙盒中成功地将地图与地形结合起来，将使学生能够实验沉积地层和结构的三维几何形状，并将广泛应用于各级地球科学教育。确定Orocopia片岩（OS）的发掘历史，对于理解该地区从白垩纪缩短到中新世伸展的转变，以及沉积物底沉、增生和同步俯冲发掘的过程至关重要。Seymour博士将系统地研究Orocopia片岩俯冲复合体的时间和热演化，使用中观和微观结构、地质和热年代学、岩石学和地球化学分析来了解(1)OS在什么深度被增加，并了解挖掘它的过程；(2)在结构上覆盖OS的晶体片麻岩的年龄和地球化学亲性，以及它与OS的构造接触的重要性。(3)片麻岩和OS的发掘历史。方法将包括对OS和关键位置上的片麻岩进行详细的测绘和微观结构研究，以记录关键的交叉关系，在明确的运动学背景下对同步运动相进行定年，例如对U-Pb和角闪石进行钛矿测年，对Ar/Ar热年代学进行辉云母和黑云母测年，以及使用石英中钛进行热气压测定，对碳质材料进行拉曼光谱分析。氧稳定同位素比值用于测温，石榴石-黑云母-斜长石-石英化学成分用于测气压。该方法将记录Orocopia片岩中保存的几代织物的数量和意义，并确定目前暴露的糜棱岩织物是否与古近纪变质作用、中新世伸展作用有关，或者两者都保留了元素。将这些结果与结晶片麻岩的热历史相比较，将为两种岩石类型之间的构造关系提供关键的细节。特别要注意的是片岩和片麻岩的热历史是否有共同的部分，如果有，它们的历史是在什么时候共同演化的。OS和片麻岩之间共同的热历史将为我们了解在浅板俯冲期间或之后不久控制俯冲复合体发掘的地球动力学过程提供见解。这些观测结果对(1)美国西南部的区域构造，(2)浅角俯冲期间俯冲底板的范围，以及(3)在平板俯冲期间或之后不久控制俯冲复合体发掘的地球动力学过程具有启示意义。/p这个奖项反映了美国国家科学基金会的法定使命，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