Collaborative research: Lithospheric foundering beneath the Sierra Nevada constrained by analysis of an anomalous Pn shadow zone

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

Mountain ranges are supported by lower than usual densities in the outermost parts of the Earth: that is, a lower density crust, thicker crust, or low densities in the shallowest mantle will support elevated terrain. This project explores how the Sierra Nevada mountains in California came to be supported by the low density materials. Understanding of how higher density materials were removed is still unknown to scientists. Historical observations of peculiarities of such seismic waves originally led seismologists to the erroneous conclusion that the Sierra Nevada had a thick crust; although we know that to not be true, those observations still have not been explained. The researchers suggest that this material likely remains or has been focused in the western part of the mountain range. They will use seismic information to detect whether dense material exists where they predict. This comparison is accomplished by expanding software that can compute seismic waveforms in the presence of complex geologic structures. Thus this project improves our understanding of how mountains can be made as well as improving our seismological toolkit for exploring what happens at the boundary between the crust and mantle in continents. Lithospheric foundering has been invoked to explain orogenesis in a number of regions around the globe, but in most cases the means by which lithosphere founders remains largely unknown. The Sierra Nevada in eastern California is often cited as one of these regions, and hypotheses for how lithosphere foundered there range from lithospheric delamination to localized convective instability. A key piece of evidence for discriminating between these competing hypotheses lies in the presence or absence of certain arc-related lithologies (a.k.a. "arclogite") and the geometry of such dense lithologies beneath the western Sierran foothills. The presence of arclogite should result in large and potentially abrupt changes in the thickness and wavespeed of the lower crust and upper mantle, and these are likely to influence the passage of Pn energy across the Sierra. Indeed, such effects can provide an explanation for a long-lived conundrum in seismology, namely: what is the cause of a Pn shadow zone produced by the Sierra? The specific objective of the project is to apply recently developed full waveform inversion (FWI) techniques to evaluate trial models of the crust and upper mantle beneath the Sierra that may be responsible for the Pn shadow zone. One hypothesis is that arclogitic lithologies are responsible for this zone, and hence that modeling the nature of the zone will allow determination of their geometry and extent beneath the Sierra. This in turn will place constraints on the roles of delamination and convective instability in this region. The primary source of data to be analyzed was collected during the Sierra Nevada Earthscope Project (SNEP) from May 2005 to mid-2007, supplemented by recordings from a dense array of broadband seismometers across Yosemite National Park (~37.8°N) during the summer of 2007. The analysis will combine full waveform modeling of teleseismic receiver functions and surface waves with local and regional (for Pn) arrivals to ensure that any model produced is consistent with observations that are mostly likely to be sensitive to the crust-mantle transition. This project benefits from an abundance of trial models from prior studies in the region, and this approach will be to use these as starting models in a formal joint inversion to examine how well they can explain the Pn shadow zone while still being consistent with teleseismic observations. Ultimately, the study's results will help resolve the role of "arclogite" in lithospheric foundering and hence determine whether delamination of the lithosphere or a convective instability can better explain lithospheric foundering beneath the Sierra.