The delamination of mantle lithosphere from continental crust is a fundamental tectonic process that is difficult to study. The goal of this project is to examine the removal of mantle lithosphere from under the Sierra Nevada. The PIs assert that the Sierras are an excellent place to look because delamination appears to have happened rather recently, and may still be happening. Preliminary studies indicate possible mantle "drips" at both ends of the Sierra. The proposal(s) seeks to identify the removal process and the fate of the delaminated portion ..., i.e. the drips. Through combined magnetotelluric (MT), geochemistry, geomorphic studies and geodynamic modeling, integrated with existing and proposed passive seismic data, the PIs seek to gain understanding of how mantle lithosphere is removed, and the conditions that facilitate such removal. The hope is that this understanding can be applied elsewhere. Removal of mantle lithosphere may have played a key role in regions with histories and structures as diverse as the southern Sierra Nevada, the Alboran Sea, the Tibetan Plateau, the Apennines, the Appalachians, central Andes, the Carpathians, Precambrian South Africa, the North China Craton, and the Basin and Range. However, in many of these other regions where mantle lithosphere may have been removed, the removal is either too old or in an environment too complex, or logistically too difficult to expect a record as clear as in the Sierra. The proposed project will build on the results of two previously funded CD projects, the Southern Sierra Continental Dynamics Project (that found that the southern part of the range was supported not by thick crust, but by a hot, low-density upper mantle) and the Sierran Paradox Experiment (that discovered that eclogite had been removed from the base of the crust in the latest Miocene or Pliocene time).
A strength of the project is the PI's intent to conduct a joint analysis of MT and seismic data. As stated, this joint analysis will use a more quantitative coupling of MT and seismic results based on the underlying physical properties which produce the observables (e.g. petrophysical behavior of mantle olivine and pyroxene based rocks with and without water). The PIs seek to tie seismic velocities and electrical resistivity via poisson's ratio, attenuation, and anisotropy. Such a joint inversion method, particularly if it can provide some measure of mantle hydration, would advance the ability to geophysically image the mantle lithosphere.
This project is paired with a project that has been funded by EarthScope (Sierra Nevada EarthScope Project - SNEP) to conduct a seismological investigation with the FlexAray component of EarthScope in the central Sierra Nevada. The PIs of SNEP are G. Zandt, T. Owens, and C. Jones. Currently focused on the northern edge of the region where lithosphere is known to have been removed, the seismological work will place limits on the extent of lithosphere removal and delineate the seismological structures resulting from that removal. This CD project explores the variations in geological and other geophysical observations across the same region with the goal of identifying those features clearly associated with removal of the lithosphere. Although each of these studies will yield useful results without the other, the combination of them should exceed the sum of the separate parts.
