Collaborative Research: RUI: Leveraging isotopic compositions of igneous systems to resolve the geometry and distribution of subducted sediment in the southern U.S. Cordillera

Occidental College, Los Angeles CA

Investigators

Abstract

Quartz-rich sediment is eroded from continents and transported by rivers to the oceans. Some of this sediment is deposited in deep oceanic trenches and subsequently subducted beneath the continents. Because these sediments are not dense enought to sink into the mantle, much of the subducted sediment is thought to become re-incorporated into the continents. This re-addition of subducted sediment to the base of continents can help understanding of the patterns of deformation and the distribution of elements within the continent. It can potentially reduce the strength of tectonic plates, so that some zones may be more prone to faulting and seismic activity. Introducing sediment into the lithosphere can also recharge the elements that have previously been depleted by tectonic processes, including critical minerals. One of the fundamental challenges this research addresses is determining the distribution and geometry of subducted sediment in the continental crust. This project establishes a new collaboration and provides support for two early-career researchers and five undergraduate students from historically marginalized and underrepresented groups at two different universities. The students receive training in a variety of geologic field methods, conduct chemical and isotopic analyses at multiple university laboratories outside of their home institutions, and develop effective science communication skills. End-member models for the geometry of subducted sediment at depth include replacement of the crustal materials with uniform, sheet-like layers of sediment and non-uniform, diapir-like bodies distributed throughout the crust. This research will test these models in southern California and Arizona, which experienced widespread sediment subduction and accretion during the Late Cretaceous to early Paleogene. The subducted sediment, locally called the Pelona-Orocopia-Rand Schist (‘Orocopia Schist’), has been tectonically exhumed to the surface allowing for direct investigation into its structural and chemical properties. The Orocopia Schist has a unique combination of juvenile radiogenic isotopic compositions and heavy stable oxygen isotope ratios, which distinguishes it from other major isotopic reservoirs in the area. Recent studies and preliminary data indicate that igneous rocks that transversed the crust during the mid-Cenozoic ignimbrite flare-up in areas where the Orocopia Schist is present record the unique isotopic signature, whereas magmatism outside of this area does not. Additionally, igneous rocks that intruded into the crust prior to accretion of the Orocopia Schist do not feature the unique isotopic fingerprint. As a result, this research uses igneous rocks in the region as a “deep-crustal probe” to identify refine the geometry of subducted sediment in the subsurface. 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.