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

博士。 Nikki M. Seymour 获得了 NSF EAR 博士后奖学金，在 Martin Grove 博士和 Jeremy Hourigan 博士的指导下，分别在斯坦福大学和加州大学圣克鲁斯分校开展研究和教育计划。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挖掘历史。 Plomosas 中暴露的岩石起源于沉积在加利福尼亚以西的太平洋海底的沉积物，并在白垩纪晚期 - 古近纪拉拉米德造山运动期间俯冲并添加到北美大陆的底部，这是一个从 ~ 90 度的浅角度俯冲时期-50 Ma 与内华达山脉岩浆活动的结束、高原的建造和随后的塌陷、基底核心山脉的隆起以及广泛的火山活动有关。 Orocopia片岩的顶部在~21-15 Ma之间被Plomosa滑脱断层暴露在地表，这意味着它在白垩纪晚期穿过地壳超过40公里到达浅地壳层后暴露在地表。 -古近纪（~73-23 Ma）。浅角俯冲期间地壳底部沉积物堆积后的热历史可以揭示这些岩石如何穿过地壳。该项目将研究 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，目前在中新世断层作用（~21-15 Ma）之前将这些岩石带到浅地壳的历史并没有受到限制。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合作，扩展增强现实地形沙箱的功能，以集成地质图。成功地将地图与沙箱中的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，并将广泛应用于各个级别的地球科学教育。/pp确定 Orocopia 片岩 (OS) 的折返历史对于理解至关重要从白垩纪缩短到中新世伸展的转变，以及驱动沉积物底侵、增生和顺俯冲折返的过程。 Seymour 博士将利用细观和微观结构、地质和热年代学、岩石学和地球化学分析，系统地研究 Orocopia 片岩俯冲复合体的时间和热演化，以了解 (1) OS 吸积的深度，并了解 OS 吸积的过程。挖掘出它，（2）结构上覆盖在 OS 上的结晶片麻岩的年龄和地球化学亲和力以及它与 OS 的构造接触的重要性，以及（3）片麻岩和 OS 的折返历史。方法将包括对关键位置的 OS 和上覆片麻岩进行详细测绘和微观结构研究，以记录关键的横切关系、具有清晰运动学背景的同运动相测年，例如 U-Pb 和角闪石、白硅云母和黑云母的钛矿用于 Ar/Ar 热年代学和使用石英钛的热压计、碳质材料的拉曼光谱以及用于测温和分析的氧稳定同位素比用于气压测量的石榴石-黑云母-斜长石-石英化学物质。该方法将记录 Orocopia 片岩中保存的织物世代的数量和重要性，并确定目前暴露的糜棱岩织物是否与古近纪变质作用、中新世延伸有关，或保留了两者的元素。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。我们将特别关注片岩和片麻岩是否共享其热历史的任何部分，如果是，它们的历史在什么时候共同演化。操作系统和片麻岩之间共享的热历史将提供对控制浅板俯冲期间或之后不久的俯冲复杂折返的地球动力学过程的见解。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侵的程度以及（3）在平板俯冲期间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智力优点和更广泛的影响审查标准进行评估，被认为值得支持。/p