Collaborative Research: Resolving Conflicting Models for the Laramide Orogeny and the Flat-Slab Paradigm in the Southern California Batholith

了解美国西部落基山脉是如何发展的是一个至关重要的问题，因为这里蕴藏着宝贵的矿产资源，如金、铅、铜和银，这些资源对我们的国民经济至关重要。这些经济资源中的许多都是沿着大断层带到地表的，这些大断层形成于晚白垩世的造山活动期间，被称为拉腊米造山运动。一个关键的问题是，这一事件背后的动力的时间和起源以及落基山脉的形成是非常有争议的，对于这些山脉形成的构造环境没有共识。研究人员假设，了解落基山脉和拉腊米造山运动起源的答案就在南加州的圣盖博山脉。该地区具有两个重要特征，使其成为本研究的重点：(1)该地区保存了拉拉amide造山运动从最早阶段到结束的完整时间历史，这对重建事件背后的驱动力具有重要意义；(2)该地区包含一个独特的深地壳剖面，其中包括一个在拉拉amide造山运动开始时活跃的主要韧性断裂系统。对这些特征的分析将对拉腊米造山运动的开始提供重要的见解，这对于解决相互矛盾的假设至关重要。该项目的研究计划将包括在圣盖博和附近山区进行3个季节的实地工作和数据收集。从圣盖博山脉收集的岩石的实验室分析将包括地质年代学、地球化学和微观结构工作，由加州州立大学北岭激光消融和STEM实验室的拉丁分析组成。该项目将通过创新的夏季研讨会和一项名为LA-ROCS的学年指导计划来促进社会成果，这两项计划都旨在为南加州少数族裔服务机构中代表性不足的学生提供研究经验。这些为本科生提供的指导和研究机会将使他们接触到尖端的分析研究设施，并有助于培养一支多样化的、具有全球竞争力的STEM劳动力队伍。这一提议旨在通过测试关于拉腊米造山作用的起源和构造影响的相互矛盾的假设来解决有争议的拉腊米造山运动的起源，在大约8500万到7000万年前南加州岩基的演化。美国科迪勒拉发育的这一关键时期通常与南加州的浅板块俯冲有关，这被认为推动了薄皮（sevier型）向厚皮（laramide型）变形的转变。平板模型的一个关键问题是，该团队的初步地质年代学数据表明，南加州拉拉米德造山运动的开始在空间和时间上与同一地区的岩浆爆发事件有关，该地区假定发生了平板俯冲。这种矛盾使得Laramide造山运动不同于其他平板俯冲地区，包括现代阿根廷的sierra Pampeanas，其特征是岩浆缺口。为了解决这一问题，研究人员提出了一种多尺度方法，将重点放在南加州岩基的根部，并将现场工作与各种最先进的分析技术（铀-铅地质年代学、铪和锇同位素地球化学、电子背散射衍射（EBSD）和微结构分析、沿连续断面的结构分析）相结合。这些数据将使研究人员能够将南加州岩基的岩浆活动与中下地壳深度的变形和板块运动学信息联系起来，以解决与平板范式和拉腊米造山运动开始相关的冲突模型。该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。

Understanding how the Rocky Mountains developed in the western United States is a problem of vital importance because they host precious mineral resources such as gold, lead, copper, and silver, which are important for our national economy. Many of these economic resources were brought to the surface along large faults which formed in the Late Cretaceous during a mountain building event, termed the Laramide orogeny. A key problem is that the timing and origin of the driving force behind this event and the formation of the Rocky Mountains are highly controversial and there is no consensus on the tectonic setting in which these mountains formed. The investigators hypothesize that the answer to understanding the origin of the Rocky Mountains and the Laramide orogeny lies in the San Gabriel Mountains of southern California. This region contains two important features which make it the focus of this study: (1) they preserve a complete temporal history of the Laramide orogeny from the earliest phase until its termination, which is important in reconstructing the driving force behind the event, and (2) they contain a unique section of the deep crust including a major ductile fault system that was active during the beginning of the Laramide orogeny. Analysis of these features will offer critical insights on the beginning of the Laramide orogeny, which will be essential in resolving conflicting hypotheses. The research plan for this project will involve 3 seasons of field work and data collection in the San Gabriel and nearby mountains. Laboratory analyses of rocks collected from the San Gabriel Mountains will consist of geochronology, geochemistry and microstructural work at the California State University Northridge Laser Ablation and LatinXellence in STEM Laboratories. The project will advance societal outcomes through innovative summer workshops, and an academic-year mentoring program termed, LA-ROCS, both of which are aimed at providing research experiences to underrepresented students at minority-serving institutions in southern California. These mentorship and research opportunities for undergraduate students will expose them to cutting-edge analytical research facilities and will aid in developing a diverse, globally competitive STEM workforce. This proposal aims to resolve the controversial beginnings of the Laramide orogeny by testing conflicting hypotheses about the origin and tectonic effects of Laramide orogenesis on the evolution of the Southern California batholith at ca. 85-70 million years ago. This critical period in the development of the U.S. Cordillera is commonly associated with shallow-slab subduction beneath Southern California, which is thought to have driven the transition from thin-skin (Sevier-style) to thick-skin (Laramide-style) deformation. A key problem with the flat-slab model is that the team's preliminary geochronology data show that the initiation of the Laramide orogeny in Southern California is spatially and temporally linked to a magmatic flare-up event in the same region where flat-slab subduction is postulated to have occurred. This contradiction makes the Laramide orogeny distinct from other areas of flat-slab subduction, including the modern-day Sierras Pampeanas in Argentina, which is characterized by a magmatic gap. To resolve this problem, the investigators propose a multiscale approach that focuses on the roots of the Southern California batholith and combines field work with a variety of state-of-the art analytical techniques (Uranium-Lead geochronology, Hafnium- and Osmium-isotope geochemistry, electron backscatter diffraction (EBSD) and microstructural analysis, structural analyses along continuous transects). These data will allow the investigators to link magmatism in the Southern California batholith with deformation and plate-kinematic information at mid-lower crustal depths to resolve conflicting models associated with the flat-slab paradigm and the initiation of the Laramide orogeny.This 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.