2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 2
Presentation Time: 8:00 AM-12:00 PM


THOMPSON, Ren1, GRAUCH, V.J.S.1, SAWYER, David2 and HUDSON, Mark1, (1)USGS, Box 25046, Denver Federal Center, Denver, CO 80225, (2)USGS, MS 980, Box 25046, Denver, CO 80225, rathomps@usgs.gov

Aeromagnetic expression of volcanic rocks is commonly dominated by remanent magnetization. Volcanic rocks with strong, reverse-polarity remanent magnetization produce distinctive, high-amplitude negative anomalies. Recognition of these distinctive anomalies in relation to mapped volcanic rock units, combined with paleomagnetic measurements, can be useful in locating flow contacts, determining aerial flow extent, and assigning ages to undated rock units. Lavas and related pyroclastic deposits of the Cerros del Rio field have a surface expression of 700 km2 and reflect eruption of minimally 120 km3 of rift-related mafic magma pre- and postdating large-volume silicic eruptions of the Jemez volcanic field. Eruptive centers of the Cerros del Rio volcanic field are typically central vent volcanoes ranging from low-relief shield centers to steep-sided, breached cinder cone remnants. Lavas range from 49-64 wt% SiO2 and exhibit strong geomorphic correlation with whole-rock chemistry. Low-silica, subalkaline basaltic lavas are thin (< 3-4 m), far-traveled, and erupted from broad shield volcanoes; whereas transitional to mildly alkaline basalts and basaltic andesites occur as thick (as much as 30 m), discontinuous lavas erupted from high-relief, steep-sided, dissected vents. Dacitic lavas are related to late-stage dome growth and eruption of thick (as much as 50 m) blocky lava flows from poorly defined vent areas. Aeromagnetic data for the Cerros del Rio volcanic field were compiled from two surveys flown with 400 m and 150 m line spacing and observation heights of 73 m and 150 m above ground, respectively. The data were analytically continued to a common observation surface 100 m above ground, merged, and reduced-to-pole. Aeromagnetic signatures of the Cerros del Rio volcanic field express many small, mostly negative, magnetic anomalies of large amplitude, relating to magnetizations inherited during cooling of individual lava flows and associated pyroclastic deposits. Magnetic anomalies correlate positively with preliminary paleomagnetic site means for the volcanic field showing both reversed and normal magnetic polarities corresponding to signatures of both Gauss and Matuyama magnetic chrons. These results are consistent with 40Ar/39Ar ages and detailed geologic mapping of the volcanic field.