SP RECEIVER FUNCTION IMAGING OF LITHOSPHERE-ASTHENOSPHERE BOUNDARY AND MID-LITHOSPHERIC DISCONTINUITY UNDER WESTERN US WITH TRANSPORTABLE ARRAY DATA

Seven years of 70 km spaced Earthscope Transportable Array data in conjunction with more densely spaced PASSCAL arrays provides an opportunity to map the velocity contrasts in the upper mantle with forward scattered S_dp phase receiver function (RF) analysis and common conversion point (CCP) imaging. Preliminary results reveal a persistent negative shear velocity gradient beneath the Moho at 90 km depth, with variations on ~300 km lateral regional scales in both the thickness of the layer and sharpness of impedance contrast. One hypothesis for the origin of this anomaly is that the negative shear velocity gradient demarks the base of a thermal boundary layer (TBL), hence signifying the Lithosphere-Asthenosphere Boundary (LAB). A very different interpretation the low velocity layer is that ascending carbonate-rich partial melts have accumulated over the last 2 Ga in the deep continental lithosphere (Sleep, 2009), forming a layer referred to as the Mid-Lithospheric Discontinuity (MLD). These two explanations for the observed anomalies have differing implications for our understanding of upper-mantle dynamics, convective flow, temperature variations, and tectonic processes. Sp RF images under western US support the second hypothesis, with evidence that a MLD resides at approximately the same depth across the study region, invariant with respect to crustal thickness.

To better constrain the nature of shear velocity contrasts in this region, S_dp receiver functions have the advantage over P_ds RFs in this depth range because the S to p converted arrivals are precursory with respect to direct S and SKS arrivals and hence not contaminated by free-surface multiples from crustal reflections prevalent in P_ds RF imaging. Furthermore, rather than using the P-component seismogram as an approximation for the source, a deconvolution method (Mercier et al, 2006; Hansen and Dueker, 2010) is used to isolate individual source-spectrum estimates, allowing for a more accurate approximation of the Earth’s Sp Green’s function.

Combining overlapping data from PASSCAL arrays (e.g. SNEP, CREST, CROMCAR, etc.) and the Transportable Array (TA) also provides an opportunity to assess how image quality modulates with respect to sampling density and data fold.