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

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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.