HR: 1340h
AN: V23B-1439    [Abstracts]
TI: 40Ar/39Ar Ages for the Sentinel-Arlington Volcanic Field, Southwestern Arizona
AU: * Cave, S R
EM: shelby.cave@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, PO Box 871404, Tempe, AZ 85287-1404, United States
AU: Greeley, R
EM: greeley@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, PO Box 871404, Tempe, AZ 85287-1404, United States
AU: Champion, D E
EM: dchamp@usgs.gov
AF: Volcano Hazards Team, U.S. Geological Survey, 345 Middlefield Rd., MS-910, Menlo Park, CA 94025, United States
AU: Turrin, B D
EM: bturrin@rci.rutgers.edu
AF: Department of Geological Sciences, Wright-Rieman Labs Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, United States
AB: The Sentinel Plains lava field and proximate small (<10 km diameter) shield volcanoes, collectively referred to as the Sentinel-Arlington Volcanic Field (SAVF) are composed of mostly basaltic lava flows with a small percentage of magmatic and phreatomagmatic tephra deposits. SAVF is located ~75 km southwest of Phoenix, Arizona, and covers ~600 km². SAVF lies on the eastern terminus of the Gila River graben within the Basin and Range physiographic province. A series of northwest-trending normal faults cut across the surrounding terrain, indicating that the loci of the SAVF eruptive centers could be controlled by structural trends. The volcanic centers of SAVF erupted near the Gila River channel, damming and diverting the river at least twice, forming small ephemeral lakes. The relative timing of the SAVF eruptions was determined in order to unravel the SAVF eruptive history as well as the timing of the ancient Gila River interactions that led to the development of the Painted Rock transverse drainage. The absolute timing was determined in order datermine causal relationships with local tectonism. The SAVF basal contact is ~30 m above the Holocene surface where exposed along the current river channel; and the lavas show similar amounts mantling by aeolian dust, development of pedogenic calcium carbonate, and subsequent incision by radial ephemeral drainages. Relative timing of eruptive events was determined by stratigraphic and embayment relationships. Continuity of distal flows exposed in cross-sections to their source vents could be established using field work, and confirmed using geomagnetic secular variation and geochemical analyses. Edifices generally corresponded to discreet geomagnetic inclination, declination, and paleointensity values. Older eruptive events exhibited normal polarity, while stratigraphically younger events exhibited reversed polarity. Most lavas were alkali olivine basalt with a range of unnormalized SiO₂ weight percentages ranging from 47.16-51.48. Geochronology using ⁴⁰Ar/³⁹Ar method revealed an age of 1.94 +/- 0.85 Ma for Painted Rock Low Shield (New Mexico Geochronology Research Laboratory), 1.64 +/- 0.14 Ma for Theba Low Shield (Rutgers University) and 1.24 +/- 0.040 Ma for Wild Horse Low Shield (Rutgers University). Some ages were precise enough to correspond to the Matuyama reversed polarity epoch, with SAVF initiation possibly within the Olduvai normal polarity event. These dates represent an overall improvement in precision and accuracy over previous dates (values corresponding to 6.20 Ma to 1.28 Ma) collected in the late 1970s and early 1980s using K-Ar technique. The ⁴⁰Ar/³⁹Ar ages correspond to expected magnetic polarities and stratigraphic sequences.
DE: 1033 Intra-plate processes (3615, 8415)
DE: 1115 Radioisotope geochronology
DE: 1194 Instruments and techniques
DE: 1522 Paleomagnetic secular variation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting