HR: 16:30h
AN: V14B-03    [Abstracts]
TI: Caldera Ellipticity Through Regional Tectonic Deformation
AU: Holohan, E P
EM: holohane@tcd.ie
AF: The University of Dublin, Trinity College, The Department of Geology, School of Natural Sciences, Dublin, 2 Ireland
AU: * Troll, V R
EM: trollv@tcd.ie
AF: The University of Dublin, Trinity College, The Department of Geology, School of Natural Sciences, Dublin, 2 Ireland
AU: Walter, T R
V14B-03 AF: MGG/RSMAS, University of Miami, Miami, FL 33149 United States
AU: van Wyk de Vries, B
V14B-03 AF: Universite Blaise Pascal, 34 Avenue Carnot, Clermont-Ferrand, BP18563006 France
AU: Byrne, P K
V14B-03 AF: The University of Dublin, Trinity College, The Department of Geology, School of Natural Sciences, Dublin, 2 Ireland
AB: Collapse calderas are delimited by reverse ring faults and surrounded by peripheral concentric normal faults. In the simplest scenario, circular magma chambers produce circular calderas. Many calderas are elliptical in shape, however, particularly those in highly active tectonic settings. Several factors may explain caldera ellipticity in such regimes: 1) Initial geometry of magma chamber(s) 2) Distribution and orientation of pre-existing regional faults, and 3) Influence of the regional stress field on caldera fault geometries. To better understand relationships between caldera morphology, reservoir geometry and regional tectonics, we conducted two analogue experimental series: One series investigated the influence of orthogonal tectonic stresses on caldera and chamber shapes. In all cases where tectonic stress was applied across circular chambers (balloons), elliptical calderas were produced. Pre-existing basement structures also influenced the shape of calderas, either increasing or reducing elongation. Intrusion of silicon gel into tectonically active sand piles showed that silicon gel chambers responded systematically to applied tectonic stress, and that associated calderas would be elliptical in shape. A second series examined the effect of strike slip faulting on magma chambers and associated calderas. We used sand to simulate brittle crust and cream honey to simulate granitic magma. With a sufficiently high transtensive component, pull-apart-like half grabens formed above the passive honey chamber. Chamber evacuation following strike-slip deformation produced arcuate reverse faults that were again occasionally affected by regional structures. From our results, we identify a number of controls for elliptical caldera formation in tectonically active settings, including initial chamber geometry, caldera fault distortion, and interaction with pre-existing structures. Our results indicate that the final caldera surface expression will be the result of interplay between these processes, and may deviate significantly from the underlying plan-view chamber geometry.
DE: 8000 STRUCTURAL GEOLOGY
DE: 8400 VOLCANOLOGY
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005