A prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival, repeatedly observed on seismograms recorded in the central Rio Grande Rift from intermediate- and deep-focus earthquakes at distances near 90¿, is identified as a probable compressional to shear wave conversion from a lower crustal root of the Socorro mid-crustal magma body (SMB). This phase is inconsistent with previously determined crustal models containing a simple sill-like midcrustal magma body with a negligible conduit system. Finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19-km-deep upper surface of the magma body down into the lower crust of the southwestern portion of the Albuquerque--Belen basin. The location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the SMB and associated seismicity is occurring via an off-axis conduit system located beneath the active margin of the Albuquerque--Belen basin. ¿ American Geophysical Union 1996