iPlex Lunch Winter-2018

Knowledge of Earth’s interior structures is important to understand its thermal, chemical, and dynamical evolution. We developed and applied seismic imaging methods based on teleseismic reflected waves to study discontinuities in the crust and mantle. Using a novel, curvelet-based, array analysis technique, we improve both the quantity and quality of SS precursors (SS waves reflecting at mantle transition zone (MTZ) discontinuities), including their travel time picks and amplitude measurements. Amplitude-distance trends in the reflectivity of SS precursors provide important constraints on the wavespeed and density contrasts across MTZ discontinuities. In conjunction with thermal dynamic modeling, our results suggest lateral variation in mantle composition near the base of the MTZ, from average pyrolitic mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. This discovery corroborates petrological predictions and numerical convection models showing compositional segregation near the 660 in high temperature, low viscosity environments produced by lower mantle upwelling. For crustal structure studies, we further developed the virtual deep seismic sounding (VDSS) method. We used data from ~1,000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States. The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected from crustal buoyancy alone) and, by implication, thermal or petrologic anomalies in the mantle. Residual topography of the western US Cordillera varies considerably. Overall, in regions to the east of the Wasatch hinge line (the eastern limit of significant extension in the North American cratonic basement) patterns of high residual topography and anomalies of low seismic wave-speeds in the upper mantle are similar, suggestive of a common, thermal origin. In contrast, such a similarity is absent in regions to the west of the hinge line, suggesting substantial effects of petrological heterogeneities in the mantle.