Collaborative Research: Relationships Between Seismic and Petrological Moho and Implications for Lithospheric Processes

Terrestrial land surfaces rise above the ocean surface because they are underlain by crustal rocks, a layer of material with relatively low density sitting on top of the higher density mantle rocks. The thickness of the crust is important for investigations of earthquake hazards, and the growth and evolution of continents through time. Energetic waves caused by earthquakes travel faster through mantle rocks than crustal rocks. Seismologists typically observe a sharp change in wave velocities at a boundary named Mohorovicic discontinuity, abbreviated to Moho, and has been interpreted as an abrupt boundary between the crust and the mantle. Since the crust-mantle boundary is too deep to be observed directly via methods like drilling, such seismic imaging is the primary method to observe it. However, rare surface exposures of crust-mantle boundaries and fragments of rock brought up in volcanic eruptions suggest that the transition from mantle to crustal rocks may be more gradual than suggested by the sharpness of the Moho. This could mean that the seismically observed Moho may sometimes instead be a boundary within the crust and does not represent the crust-mantle transition. This project combines several types of seismic waves to characterize the seismic wavespeeds above and below the seismic Moho and compares them to calculated seismic wavespeeds for different rock compositions to assess the sharpness of the Moho and its relationship to the crust-mantle boundary. The investigation is carried out using already collected seismic recordings across the contiguous U.S. and Tibet, which provide contrasting geological settings. A more accurate characterization of the boundary provides clues to the evolution of continents and distribution of critical elements as well as improved constraints for calculations requiring crustal thickness. Anticipated results are of interdisciplinary interest for tectonic processes of continental crust formation and evolution and will be disseminated in peer-reviewed journals, at conferences, and via an interactive map and calculation tool product. Funding supports a graduate student and all-early career and/or soft money PIs; funds and mentoring for an undergraduate student at CU Boulder who will present their research project at AGU, a high school intern working on the project through the Simons Summer Research Program and a summer undergraduate intern, both at Stony Brook University. The crust-mantle boundary is seismically defined by the Mohorovic?ic´ discontinuity (Moho), interpreted as separating shallower seismic velocities representative of continental crust lithologies and higher velocities typical of ultramafic mantle lithologies. Only in the idealized case does the seismic Moho correspond to a petrological Moho juxtaposing crustal against mantle lithologies. Recent research produces paradoxical features such as a brighter Moho indicative of a large velocity contrast in hotter areas where one expects a smaller velocity contrast. Furthermore, sub-Moho velocities beneath large continental portions are significantly slower than expected. These observations could be explained by anomalously fast lower crust, crustal and mantle compositional effects such as hydration, or a diffuse petrological Moho. The proposed research aims to test these hypotheses by interpreting these paradoxical observations through a systematic combination of surface wave tomography, receiver function analysis, and petrophysical modeling. Accurately constraining the depth, width, and physicochemical state of the crust-mantle boundary is important to the transfer of stress from the mantle to the surface, topographic support, inversion constraints in seismology, and the composition and structure of the crust. The proposed work is to combine surface waves and receiver functions with complementary sensitivities to determine 1. Shear velocity contrast at the Moho; 2. Absolute shear velocities in the lower crust and uppermost mantle; 3. Moho depth and character, all at existing seismic stations with available data under the contiguous U.S. and Tibet. Method development includes modeling of absolute receiver function Moho amplitudes with near-surface corrections and reduction of the nonuniqueness of gradients near the Moho in surface wave inversions by adding receiver function constraints. The scientific product from this proposal will be an interactive map and tool to calculate and display tradeoffs between shear wave velocity, temperature, Moho depth, and compositional structure, in agreement with geophysical data beneath each analyzed seismic station in the contiguous US and Tibet. This product will elucidate the tectonic evolution of continental lithosphere from the Archean to the present as well as provide improved constraints and parameterization necessary for future geoscience studies on the continental lithosphere. 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.