A LOWER CRUSTAL EXTENSION TO A MIDCRUSTAL MAGMA BODY IN THE RIO-GRANDE RIFT, NEW-MEXICO

A prominent and spatially localized phase arriving approximately 4 s a fter the initial compressional arrival, repeatedly observed on seismog rams recorded in the central Rio Grande Rift from intermediate- and de ep-focus earthquakes at distances near 90 degrees, is identified as a probable compressional to shear wave conversion from a lower crustal r oot of the Socorro midcrustal magma body (SMB). This phase is inconsis tent with previously determined crustal models containing a simple sil l-like midcrustal mama body with a negligible conduit system. Finite e lement synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends sever al kilometers from the 19-km-deep upper surface of the magma body down into the lower crust of the southwestern portion of the Albuquerque-B elen basin. The location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing i nflation of the SMB and associated seismicity is occurring via an off- axis conduit system located beneath the active margin of the Albuquerq ue-Belen basin.