The Potrillo Volcanic Field Seismic Experiment

Introduction | The Seismic Experiment | Photo Essay | Acknowledgements

Introduction

On May 16-18, 2003, the geophysics group at the University of Texas at El Paso conducted a wide-angle seismic refraction/reflection experiment across a portion of the southern Rio Grande Rift centered on the Potrillo Volcanic Field (PVF).  The PVF is perhaps best known for its xenolith localities at two maar volcanoes within the field, Kilbourne Hole and Potrillo Maar.  The experiment is part of a joint project to study magmatic contributions to crustal evolution in an intracontinental setting using the southern Rio Grande Rift as a natural laboratory.  The project is funded primarily by the Texas Advanced Research Program in the form of grants to principal investigators Kate Miller at UTEP and Calvin Barnes at Texas Tech.

The crust that underlies the southern Rio Grande Rift has undergone a protracted history of magmatic modification including  the modern extensional environment plus four earlier episodes of magmatism at 1400 Ma, 1100 Ma, 500 Ma, and 47-28 Ma.  Each event produced distinct magmas, and each was broadly extensional, with some subduction-related activity during Tertiary time.  The region is an excellent place to study magmatic modification because it is a location where comprehensive geophysical data sets and a rich suite of basement samples can be obtained. In fact, all crustal levels are accessible, with (1) shallow basement exposed in nearby mountain ranges, (2) a remarkably diverse deep- and middle-crustal xenolith suite in two Quaternary maar volcanoes, and (3) a middle crustal xenolith suite in Eocene intrusions near El Paso. Thus, these data sets provides a fantastic opportunity to establish the relative importance of magmatic input to formation of the crust by integrating the geophysical structure of the region with detailed geobarometry, geothermometry, geochronology, and chemistry of surface and xenolith samples.

The Seismic Experiment




The seismic experiment was conducted along a ca. 225 km long corridor from El Paso, Texas to the bootheel of New Mexico.  Nearly 800 Texan seismographs owned by UTEP and IRIS-PASSCAL were deployed along the transect to record the seismic signal from shots detonated at seven locations (black stars).  Seismograph spacing varied between 100 and 600 m.  A spacing of 100 m was used for a 40 km stretch centered on the Potrillo Volcanic Field in order to obtain detailed information on the crustal structure of that region.  A spacing of 200 m was used over ca. 20 km east of the Franklin Mountains on Ft. Bliss in order to obtain detailed information on basin geometry there.  In all other areas, seismograph spacing was approximately 600 m.   Shot sizes range from as little as 1000 lbs to 4000 lbs on each end of the profile.



A preliminary record section (above) from the easternmost shotpoint, shows that the seismic signal quality is excellent and that the energy travelled across the entire sensor array.



A portion of a shot gather centered on the Potrillo Volcanic Field shows excellent near-vertical incidence reflections from the Moho at 11 to 12 s.  This gather has been filtered to remove low frequency surface waves.  Once the metadata from the experiment are finalized, the data set will be archived at the IRIS Data Management Center and will be made openly available by June 2005.  






Full analysis of data from the experiment is currently underway.  Our goal is to significantly improve our understanding of the structure of the crust in comparison to the existing information (above) which is based on much lower resolution seismic data collected in the early 1980s (Sinno et al., Journal of Geophysical Research, 1986).  Our analysis will also add significantly to other ongoing studies of the lithospheric structure of New Mexico including that of the RISTRA and CD-ROM projects.  

Photo Essay

Acknowledgements

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