New Mexico Geological Society Annual Spring Meeting — Abstracts

We invert P and S teleseismic travel-time residuals from the USArray and more than 1800 additional stations for 3-D mantle velocity perturbations to a depth of 1200 km. The inversion uses recent advances in western U.S. crust models to better isolate the mantle component of travel-time residuals, and frequency-dependent 3-D sensitivity kernels to map residuals, measured in multiple frequency bands, into velocity structure. In addition to separate Vp and Vs models, we jointly invert the two datasets for Vp/Vs perturbations by imposing a smoothness constraint on the dlnVs/dlnVp field. The joint inversion helps identify regions where partial melt is probable. The amplitude of Vp, Vs, and Vp/Vs variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and widespread occurrence of small-scale convection. Unreasonably large mantle temperature variations, up to ~900 C at 100 km depth, are required if the entire magnitude of velocity structure is attributed to temperature. The results of the joint inversion delineate mantle volumes where partial melt contributes strongly to the imaged velocity structure, and these regions are highly correlated with young volcanic fields. The inferred depth extent of partial melt is consistent with volatile enrichment in the upper mantle and locally elevated temperatures beneath the eastern SRP and Yellowstone. A northeast trending swath of high-velocity upper mantle extends from the central Colorado Plateau to northeastern Wyoming suggesting that compositional heterogeneity in the lithosphere is necessary to reconcile the high mean elevation and negligible geoid anomaly of the region. This swath of high-velocity mantle is juxtaposed against generally low-velocity upper mantle beneath the Basin and Range, Jemez Lineament, Rio Grande Rift, and Colorado Rockies. High-velocity anomalies at sub-lithospheric depths, which are expected to be subducted slabs, are highly dissected and the volume of imaged slab fragments is inadequate to account for plate convergence since the beginning of the Laramide. Several short-wavelength (100-200 km) high-velocity anomalies in the uppermost mantle beneath a diverse range of geologic provinces must represent foundering of lithosphere. We suggest that North America lithosphere and remaining fragments of the subducted Farallon plate are possible origins of these structures. Viscosity reduction resulting from addition of volatiles and advection of heat during emplacement and subsequent removal of the flat Laramide slab provides a potential stimulus for foundering of NA lithosphere. Broader geodynamic implications of our tomography are that the spatial spectrum of convection in the upper mantle appears to have high power at short wavelengths (~100-200 km), and there is strong correlation between small-scale mantle structures and locations of pronounced magmatic activity, surface uplift, and crustal deformation. Thus, small-scale upper mantle convection in addition to classical horizontal tectonic forcing appears to be of fundamental importance to the distribution of surface geologic activity in the western U.S.