CRUST AND UPPER-MANTLE P-WAVE VELOCITY STRUCTURE BENEATH VALLES CALDERA, NEW-MEXICO - RESULTS FROM THE JEMEZ TELESEISMIC TOMOGRAPHY EXPERIMENT

New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative P wave arrival times recorded on 50 po rtable stations to determine velocity structure to depths of 40 km. Th e three principle features of our model for Valles caldera are: (1) ne ar-surface low velocities of -17% beneath the Toledo embayment and the Valle Grande, (2) midcrustal low velocities of -23% in an ellipsoidal volume underneath the northwest quadrant of the caldera, and (3) a br oad zone of low velocities (-15%) in the lower crust or upper mantle. Crust shallower than 20 km is generally fast to the northwest of the c aldera and slow to the southeast. Near-surface low velocities are inte rpreted as thick deposits of Bandelier tuff and postcaldera volcanicla stic rocks. Lateral variation in the thickness of these deposits suppo rts increased caldera collapse to the southeast, beneath the Valle Gra nde. We interpret the midcrustal low-velocity zone to contain a minimu m melt fraction of 10%. While we cannot rule out the possibility that this zone is the remnant 1.2 Ma Bandelier magma chamber, the eruption history and geochemistry of the volcanic rocks erupted in Valles calde ra following the Bandelier tuff make it more likely that magma results from a new pulse of intrusion, indicating that melt flux into the upp er crust beneath Valles caldera continues. The low-velocity zone near the crust-mantle boundary is consistent with either partial melt in th e lower crust or mafic rocks without partial melt in the upper mantle. In either case, this low-velocity anomaly indicates that underplating by mantle-derived melts has occurred.