USArray Imaging of Continental Crust in the Conterminous United States

Abstract

The thickness and bulk composition of continental crust provide important constraints on the evolution and dynamics of continents. Crustal mineralogy and thickness both may influence gravity anomalies, topographic elevation, and lithospheric strength, but prior to the inception of EarthScope's USArray, seismic measurements of crustal thickness and properties useful for inferring lithology are sparse. Here we improve upon a previously published methodology for joint inversion of Bouguer gravity anomalies and seismic receiver functions by using parameter space stacking of cross correlations of modeled synthetic and observed receiver functions instead of standard H‐κ amplitude stacking. The new method is applied to estimation of thickness and bulk seismic velocity ratio, vP/vS, of continental crust in the conterminous United States using USArray and other broadband network data. Crustal thickness variations are reasonably consistent with those found in other studies and show interesting relationships to the history of North American continental formation. Seismic velocity ratios derived in this study are more robust than in other analyses and hint at large‐scale variations in composition of continental crust. To interpret the results, we model the pressure‐/temperature‐dependent thermodynamics of mineral formation for various crustal chemistries, with and without volatile constituents. Our results suggest that hydration lowers bulk crustal vP/vS and density and releases heat in the shallow crust but absorbs heat in the lowermost crust (where plagioclase breaks down to pyroxene and garnet resulting in higher seismic velocity). Hence, vP/vS variations may provide a useful proxy for hydration state in the crust.

Physical State and Structure of the Crust Beneath the Western‐Central United States From Multiobservable Probabilistic Inversion

“…Interestingly, our results at depths ≥15 km (i.e., middle and lower crustal depths) beneath the SRM and southern boundaries of the CP do not confirm the values reported in Lowry and Pérez‐Gussinyé () and Ma and Lowry (). At these depths, V p / V s varies between 1.75–1.9, suggesting the presence of either intermediate to mafic lithologies, high temperatures and/or partial melts.…”

Crustal Composition and Moho Variations of the Central and Eastern United States: Improving Resolution and Geologic Interpretation of EarthScope USArray Seismic Images Using Gravity

“…Other features of the density field estimates can be useful to interpret phase dynamics of the region. Along a profile through the thickest crust in the Illinois basin and the Granite‐Rhyolite province (the Line PP′ in Figure ), we estimate temperatures to be 510 ± 86 °C at 35 km depth and 622 ± 140 °C at 55 km, based on a conductive thermal model combining surface heat flow and mineral physics of Pn seismic velocity (Ma & Lowry, ; Schutt et al, ). Corresponding pressures would be 990 and 1,580 MPa, respectively, assuming 15 km of granitic upper crust (2,750 kg/m 3 ) underlain by gabbro (3,000 kg/m 3 ).…”

“…These are supplemented with CRUST1.0 (Laske et al, ) crustal thickness estimates outside the USArray footprint. The seismic imaging does not take into account other crustal and upper mantle interfaces because that requires additional information not included in single‐layer H‐κ receiver function stacks afforded by EARS (e.g., Ma & Lowry, ). Since sedimentary basins could contribute significantly to gravity anomalies, we calculate basin anomalies (Figure ) combining basin thickness estimates from several sources (Ellett & Naylor, ; Hinze et al, ; Laske et al, ; Mooney & Kaban, ) with basin density‐depth variations suggested by Mooney and Kaban ().…”

“…These parameters are significantly different for the western United States as shown in Figure , where , Δρ Moho , and are estimated at 70, 1,180, and −100 kg/m 3 , respectively. The differences likely reflect warmer uppermost mantle (Schutt et al, ) and removal of western U.S. lower crustal garnet phases by hydrous melting (Ma & Lowry, ; Schmandt et al, ). The western U.S. relationships were discussed in the study of Lowry and Pérez‐Gussinyé (), whereas our present study is focused on the central and eastern United States.…”

The Role of Crustal Strength in Controlling Magmatism and Melt Chemistry During Rifting and Breakup

“…There is relatively little change in the observed ratios of La/Yb and Sm/Yb from the observed compositions in the southern Main Ethiopian Rift (Figures 10b and 10c): Gerta Kele (15 Ma;George & Rogers, 2002), Chencha (12 Ma;Rooney, 2010) and Arba Minch (Quaternary; Rooney, 2010). The strong crustal model is capable of matching the observed compositions at Gerta Kele and Chencha (Figures 10b and 10c, stars and triangles), but becomes increasing depleted in the light relative to heavy REEs as the rift evolves.…”