HR: 16:30h
AN: V52G-03    [PDF]
TI: Megabreccias, Early Lakes, and Duration of Resurgence Recorded in Valles Caldera, New Mexico
AU: * Goff, F
EM: fraser@lanl.gov
AF: EES-Division, Los Alamos National Labortory, Los Alamos, NM 87545 United States
AU: Goff, C J
EM: candf@swcp.com
AF: EES-Division, Los Alamos National Labortory, Los Alamos, NM 87545 United States
AU: Phillips, E H
EM: ehp@nmt.edu
AF: Department of Earth and Environmental Science & New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: Kyle, P R
EM: kyle@nmt.edu
AF: Department of Earth and Environmental Science & New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: McIntosh, W C
EM: mcintosh@nmt.edu
AF: Department of Earth and Environmental Science & New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM 87801 United States
AU: Chipera, S
EM: chipera@lanl.gov
AF: EES-Division, Los Alamos National Labortory, Los Alamos, NM 87545 United States
AU: Gardner, J N
EM: jgardner@lanl.gov
AF: EES-Division, Los Alamos National Labortory, Los Alamos, NM 87545 United States
AB: New 1:24,000 scale geologic mapping combined with previous and ongoing geoscientific studies are revealing significant new findings on intracaldera stratigraphy and structure, initial development of intracaldera lakes, and the duration of resurgence within the ca. 1.25 Myr Valles caldera. The caldera is about 22 km in diameter and contains a resurgent dome that is a northeast-trending oval roughly11 x 9 km in dimension. Maximum resurgence (uplift) was more than 1000 m, during which the dome split into three principal segments herein named the Redondo Peak, Redondo Border, and Valle San Luis segments. These segments are separated from each other by long, narrow grabens herein called the Redondo Creek, Jaramillo Creek, and San Luis Creek grabens. Differential uplift accompanied by intense faulting has exposed large, rootless megabreccia (Mbx) blocks composed of precaldera rocks submerged in densely welded, intracaldera upper Bandelier Tuff. The largest Mbx blocks are roughly 0.2 to 2.0 km long and consist primarily of Abo Fm (Permian), Gallisteo Fm (?) (Eocene), Santa Fe Group (Miocene), Paliza Canyon Fm (late Miocene) and lower Bandelier Tuff (ca. 1.62 Ma). Deep geothermal wells drilled within the Redondo Creek graben from 1970 to 1983 penetrate as much as 2032 m of intracaldera Bandelier Tuff and post-Bandelier rocks before intersecting caldera floor rocks (average = 1646 m, n = 23 wells). Evidence that a lake developed within the caldera depression is preserved in finely laminated lacustrine beds and rhyolitic, hydromagmatic tuffs that overlie intracaldera Bandelier Tuff on the resurgent dome. The lacustrine rocks contain organic remains and the hydromagmatic tuffs contain accretionary lapilli. In some locations, lacustrine and hydromagmatic rocks are interbedded. Earliest post-caldera rhyolite lavas (Deer Canyon Member) display occasional pepperite and pillow textures. Many lavas contain significant amounts of fine, opalized flow breccia indicating interaction with water. Associated Deer Canyon tuffs are altered to variable mixtures of silica, smectite, clinoptilolite, mordenite and other phases. Slightly younger rhyolite lava flows (Redondo Creek Member) occasionally display upper flow surfaces in which cracks are filled with zeolitized mud. The combined geologic evidence indicates that the initial Valles lake was widespread and relatively shallow, containing waters with neutral to alkaline pH and relatively high K/Na ratios. 40Ar/39Ar dating of sanidine separates from Deer Canyon and Redondo Creek rhyolites yields ages that are statistically indistinguishable from the age of underlying upper Bandelier Tuff. These results indicate that the intracaldera lake developed immediately after the caldera formed and that the resurgent dome rose out of a lake. Most resurgence occurred after Redondo Creek rhyolite was erupted because the unit is intensely faulted and associated lacustrine beds are now as much as 500 m above the undeformed caldera moat. In contrast, rhyolite lavas of the first post-caldera moat complex, Cerro del Medio (about 1.22 Ma) show no apparent deformation or uplift due to resurgence. Within the errors of the various 40Ar/39Ar dates, the apparent duration of resurgence was no longer than about 50,000 years yielding a minimum resurgence rate of about 2 cm/y.
DE: 8429 Lava rheology and morphology
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting