P-Wave Residual Differences and Inferences on an Upper Mantle Source for the Silent Canyon Volcanic Centre, Southern Great Basin, Nevada

Abstract

Summary Teleseismic P-wave travel-time residuals from six large nuclear detonations in the caldera of the Silent Canyon volcanic centre of the Nevada Test Site have been reduced by corresponding P-wave residuals of a nearby reference event (Dumont) in an attempt to map the deep structure of this volcanic centre. Each set of residual difference data is plotted on a projection of the lower focal hemisphere of Dumont. After corrections were applied for known low velocity rocks in the Silent Canyon caldera, data clusters of at least -1.0 s were found to dominate observations in the distance range 60–90 degrees. These large negative delays indicate the existence of a major high velocity anomaly that is directly beneath the Silent Canyon volcanic centre. Teleseismic P-wave residuals of two additional nuclear explosions, outside both the Silent Canyon caldera and the region of the reference event, were also reduced by Dumont residuals. The resulting data are distinct from the Silent Canyon residual difference fields and serve as a control on the primary experiment. The observed differential residuals, independent data on trace and major element chemistry of Silent Canyon lavas, and direct observations of effective Q argue that the high velocity anomaly is in the upper mantle. A model that satisfies the observed residual differences and which is consistent with additional seismic data possesses an anomalous zone which begins beneath the Mohorovicic discontinuity, at a depth of 40–50 km, extends to a depth of 170–190 km, and has a volume of 350000–600000 km3. This zone is interpreted as representing upper mantle material that has been largely depleted of its partial melt component by various differentiation processes and by associated eruption of the partial melt material through the Silent Canyon volcanic centre.