TELESEISMIC P-WAVE IMAGE OF CRUST AND UPPER-MANTLE STRUCTURE BENEATH THE VALLES-CALDERA, NEW-MEXICO - INITIAL RESULTS FROM THE 1993 JTEX PASSIVE ARRAY

Teleseismic P-wave relative arrival-time data, collected from a tempor ary array during the 1993 Jemez Tomography Experiment (JTEX), have bee n inverted to image velocity anomalies beneath the Valles caldera in n orthern New Mexico. Instruments were deployed in two 30-km-long profil es, one of 8 and one of 9 stations. These profiles crossed the caldera trending at azimuths of N46-degrees-W and N60-degrees-E, respectively . Two-dimensional teleseismic relative arrival time inversion of the 1 993 data set, supplemented with data from an overlapping 1987 profile, confirms the existence of a mid-crustal low velocity region (-30%) be neath the Valles caldera in the depth range of 8 to 13 km (below sea l evel), with about a 6 km horizontal extent. This feature is interprete d to be the seismic expression of the remnant magma chamber. A shallow low velocity anomaly beneath San Antonio Mountain coincides with the region of highest thermal gradient values in the caldera. A lower crus t/upper mantle low velocity anomaly is imaged but is not as well const rained due to the limited length of the profile. We tentatively correl ate this anomaly with the thermal effects of basaltic magmas ponded at the crust-mantle boundary.