No abstract

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The Jemez Mountains 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 cataclysmic 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, consisting 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 tend 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. INTRODUCTION The Jemez Mountains volcanic field (JVF) covers about 4500 km 2 and is centered astride the western flank of the Rio Grande rift, about 60 km northwest of Santa Fe, New Mexico (Figure 1). It is the largest and most complex of seven major volcanic fields that extend northeast from east central Arizona to northeastern New Mexico and define what is commonly known as the Jemez lineament [Aldrich and Laughlin, 1984; Smith and Luedke, 1984]. In contrast to the dominantly basaltic fields elsewhere along the lineament, the JVF is mainly silicic in composition, probably a result of its unique location at the intersection of the deep-seated zones of crustal weakness of both lineament and rift structures [Smith and Luedke, 1984]. When considered also in relation to the volume of volcanic rocks along the mainly north-south trending Rio Grande rift, the dominance of the JVF again is overwhelming. Rift associated volcanism outside the JVF is mainly basaltic, is younger than 5 Ma, shows a range of compositions suggestive of only localized melting in small regions in the asthenosphere, and involves surprisingly small total volumes of magma [Baldridge et al., 1984; Olsen et al., 1986]. Because of their sizes and ages, most of the volcanic centers along the Jemez lineament (and especially the JVF) cannot be presumed to be extinct and thus constitute the Paper number 5B5490. 0148-0227/86/005B 5490505.00 zone of greatest volcanic potential in the southwestern U...

DE5?R!f3JTlQN OF THIS BQCUMENT I S UNLIMITED DISCLAIMER This report was .prepared as a n account of work sponsored by an agency of t h e United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes a n y legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by t h e United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of t h e United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Preface This report presents the final results and conclusions drawn from geophysical studies begun in November 1993, information vital to administrative and technical staff responsible for environmental planning and remediation at the Pilot Plant. Two technologies, not listed in the work plan, were added to the study to improve diagnostic interpretations, and two were removed because they were deemed unnecessarily redundant. The two technologies added are (1) bistatic ground-penetrating radar, which was used for interpreting site stratigraphy and detecting of paleochannels, and (2) EM61, which was used for detecting metallic objects buried close to metal buildings. The technologies deleted are seismic and resistivity sounding. Staff at Aberdeen Proving Ground, Directorate of Safety, Health, and Environment, and Argonne National Laboratory guided the work scope and its objectives.

Presented in this technical memo are preliminary results of ongoing work or work that is more limited in scope and depth than that described in formal reports issued by the ES Division.