The Jemez Mountians volcanic field (JVF) is centered astride the western flank of the Rio Grande rift in north central New Mexico. Near the center of the volcanic field are the Valles and Toledo calderas, the sites of two cattaclysmic eruptions of silicic tephra and subsequent caldera collapses. These calderas are among the largest and most recent (1.1 and 1.4 Ma) of the world's giant volcanic calderas. The Valles caldera, especially, has associated highly developed hydrothermal systems and may still contain residual magma within a cooling, compositionally zoned pluton at shallow crustal depths. The main goal of our study was to estimate the complex shallow crustal structure beneath the Jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field. A time term technique was used to process first arrival time data from the network of temporary seismic stations of the Caldera and Rift Deep Seismic Experiment (CARDEX) refraction experiment. The data set, consistimg of 290 time-distance pairs, was processed to yield time terms at 84 locations (78 recording stations and 6 explosion sites). The P wave velocity of the Precambrian basement refractor was estimated to be 5.86 ¿ 0.01 km/s. Station time terms are highly correlated to Bouguer gravity anomalies and thus to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the JVF and dips generally eastward to depths of at least 600 m below sea level beneath sediment-filled axial basins of the rift.