Search

Abstract: We present observations of both null and non-null SKS splitting from temporary deployments across the southeastern United States in order to evaluate the relative contributions of lithospheric deformation and asthenospheric flow to regional anisotropy. Data for this study come from four temporary broadband seismic deployments: the Appalachian Seismic Transect (AST), the Test Experiment for Eastern North America (TEENA), the South Carolina Earth Physics Project (SCEPP), and the Florida to Edmonton Array (FLED). In general, we find fast directions aligned roughly parallel to absolute plate motion of the North American plate (APM) within and west of the Southern Appalachians, whereas to the southeast, we find a broad area dominated by complex splitting patterns consisting of well-constrained null splitting measurements over a range of backazimuths along with a very small number of resolved non-null measurements. This change in splitting patterns is consistent with a transition from drag induced asthenospheric flow beneath the older sections of the North American continent to vertical or incoherent mantle flow, likely in combination with complex lithospheric anisotropy, beneath the younger accreted terranes to the southeast. In addition to these general patterns, we find a number of non-null splitting measurements that are not aligned with APM, but are instead aligned with prominent magnetic anomalies that may correspond to ancient continental suture zones or faults. This would suggest that in these areas, a strongly anisotropic (but localized) lithospheric fabric dominates over any ambient asthenospheric anisotropic signature. In areas with generally strong APM parallel splitting, this would imply a thick sheared mantle lithosphere whose deformation-induced anisotropy is strong enough to overprint the anisotropy induced by APM, and is aligned with the shallower crustal structures responsible for generating the observed magnetic anomalies. In the southeastern areas dominated by null splitting measurements, there may be no strong signature from asthenospheric anisotropy to override, but a substantial lithospheric thickness is still required to generate the magnitude of the observed SKS splitting (∼1s). More data are required to verify these results, but future datasets including data from USArray may be able to exploit the correlations between null and non-null SKS splitting measurements and magnetic lineaments to better constrain the provenance of the regional anisotropic signature. [Copyright &y& Elsevier]

We assess the ability of predicted seismic velocities (Vp, compressional wave; Vs, shear wave; and Vp/Vs) to identify regions in the upper mantle that are enriched in orthopyroxene relative to normal melt-depleted peridotite compositions. Orthopyroxene enrichment has been found in mantle xenoliths from a number of locations, including the Colorado Plateau and the Kaapvaal craton. We find that the Vp/Vs ratio is very sensitive to orthopyroxene concentration, but it is not sensitive to depletion level. We compare these predicted velocities and Vp/Vs ratios to the high Vs, low Vp/Vs anomaly found above the central Chile-Argentina fl at slab. Within error, the predicted velocities of some of the orthopyroxene-enriched xenoliths match the observed velocities from the Chile-Argentina upper mantle. Because the anomaly above the central Chile-Argentina fl at slab does not conform to known terrane boundaries but does align with the downgoing Juan Fernandez ridge track, we suggest that the orthopyroxene enrichment in this area may be related to fl at slab processes, which would have implications for our understanding of lithospheric silica enrichment in the western U.S. and elsewhere. [ABSTRACT FROM AUTHOR]