HR: 12:05h
AN: T22E-08    [Abstracts]
TI: Do Lava Flows Drive Caldera Collapse at Spreading Centers?
AU: * Buck, W R
EM: buck@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 1000 Rt 9w, Palisades, NY 10964, United States
AB: Many fast and intermediate rate spreading centers with axial highs show sharp topographic relief on or near the spreading axis indicating offset of lithospheric blocks. These include the ~100 m deep "axial summit troughs" seen at many fast spreading segments and the ~300 m offsets of "split volcanoes" and "rifted axial highs" seen at some intermediate spreading ridges. These features were first thought to result from normal fault offset during amagmatic periods. However, seismic imaging of axial magma chambers (AMCs) under such segments indicates that magma is present. This magma should allow dikes to open and relieve stresses before significant normal fault offset. These topographic features are also hard to explain by the kind of magmatic collapse that occurs when a dike breaks out of the side of a steep-sided volcano. Topographic relief at these ridges is insufficient to drive the magma pressure drop needed to cause such collapse. The correlation of topographic relief with magma chamber depth may indicate that collapse is driven by loads related to magmatic accretion. Magma intrudes to a depth that is controlled by the magma density then solidification causes the density of the intruded dike to increase. Local isostatic compensation of this density increase would lead to a topographic drop that is proportional to the axial lithospheric thickness. Effectively local isostasy could result when collapse events occur. Magma accretion/solidification loads would bend down the lithosphere above the AMC, but collapse-forming breaks could only occur when magma allows the bent region to fail as a tension crack. Load-related bending should put the base of the axial lithosphere in tension at its center, thereby triggering dikes fed by the underlying AMC. The same bending would put the surface in tension at the sides of the axial lithosphere above the AMC. When loading has reached a sufficient level the areas of surface tension could start to crack, but those cracks would not propagate through the lithosphere unless lavas pour into them. Collapse could occur when these fluid-filled cracks open and so lose all shear strength. Simple numerical models including flexure of the axis and a parameterized magma sources are used to track the development of the observed range of collapse related relief.
DE: 8138 Lithospheric flexure
DE: 8145 Physics of magma and magma bodies
DE: 8164 Stresses: crust and lithosphere
DE: 8178 Tectonics and magmatism
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting