HR: 17:15h
AN: T34B-06 INVITED     [Abstracts]
TI: Numerical Simulations of Faulting and Magmatism at Ridges Illustrates Conditions for Oceanic Detachment Faults
AU: * Buck, W R
EM: buck@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9w, Palisades, NY 10964 United States
AU: Qin, R
EM: rqin@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt 9w, Palisades, NY 10964 United States
AU: Lavier, L L
EM: luc@utig.ig.utexas.edu
AF: Jackson School of Geosciences, Institute of Geophysics, University of Texas, 4412 Spicewood Springs Rd., Austin, TX 78759 United States
AU: Behn, M D
EM: mbehn@whoi.edu
AF: Woods Hole Oceanographic Instition, 360 Woods Hole Road - Mail Stop #22, Woods Hole, MA 02543 United States
AB: The offset and spacing of oceanic normal faults vary greatly with mid-ocean ridge spreading rate and even vary strongly with position along a spreading segment. The oceanic normal faults with the largest offsets, typically tens of kilometers, are detachment faults. The detachment faults are more than the end-member of a continuum of fault offsets and styles of tectonic deformation; they appear to be qualitatively and quantitatively distinct from high-angle, abyssal-hill bounding faults. Our working hypothesis is that oceanic detachments form initially like more typical normal faults, but are offset sufficiently that the footwall is rotated nearly flat. The offset of ridge normal faults is likely to depend on the thermally defined strength structure of the ridge axis and on the fraction (M) of the total plate separation rate that occurs by magmatic widening of dikes. We use numerical models of plate separation for two dimensional, cross-sections of idealized ridge axes. The brittle plate separation is assumed to occur either by normal fault offset or intrusion of magma into a dike. Normal faults are allowed to develop spontaneously where stresses reach a defined yield stress and the strength of the faults decrease with strain. In published work we treated dikes as vertical zones of depth-independent, constant rate widening located at the center of a fixed axial thermal; structure. Here we report results of models with a more sophisticated dike simulation allowing for periodic dike intrusion. The dikes are instantly filled with inviscid magma at a set pressure and the elastic widening of the dike is computed. For both model procedures the offset and spacing of faults depends strongly on M, and detachment-style, large-offset faults only form for m close to 0.5. Models are being developed to simulate ridge faulting with a self-consistent, time-varying axial thermal structure and we expect the model fault offset to continue to depend on magma input. Finally, we speculate on why the magma budget may be close to that needed to produce detachments in many places.
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 8020 Mechanics, theory, and modeling
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005