HR: 0800h
AN: DI51A-0283 INVITED    [Abstracts]
TI: A periodic shear-heating mechanism for intermediate depth earthquakes in the mantle
AU: * Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: Columbia, LDEO, Palisades, NY 10964, United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Brown, DGS, Providence, RI 02912, United States
AU: Homburg, J
EM: jmh2166@columbia.edu
AF: Columbia, LDEO, Palisades, NY 10964, United States
AU: Warren, J
EM: jmwarren@whoi.edu
AF: WHOI, MGG, Woods Hole, MA 02543, United States
AU: Spiegelman, M
EM: mspieg@ldeo.columbia.edu
AF: Columbia, LDEO, Palisades, NY 10964, United States
AB: We first summarize results from a recent paper (Kelemen & Hirth, Nature 07). Intermediate depth earthquakes at 50-300 km in subduction zones occur below the brittle-ductile transition, where high pressures render frictional failure unlikely. Their location approximately coincides with 600 to 800 C isotherms in thermal models, suggesting a thermally activated mechanism. Some earthquakes may occur by frictional failure when high pore pressure results from metamorphic dehydration. However, because some intermediate depth earthquakes occur ~ 30 to 50 km below the paleo-seafloor, the hydrous minerals required for the dehydration mechanism may not be present. We present an alternative mechanism, the onset of highly localized viscous creep in pre-existing, fine-grained shear zones. Our numerical model uses olivine flow laws for a fine-grained, viscous shear zone in a coarse-grained, elastic half space, with initial temperatures from 600-800 C and background strain rates of 1E-12 to 1E-15 per second. When shear heating becomes important, strain rate and temperature increase rapidly to > 1/s and 1400 C. Then, stress drops dramatically, followed by low strain rates and cooling. Continued far-field deformation produces a quasi-periodic series of instabilities.

Instability at the lowest strain rates is only predicted when active shear zones are a few millimeters wide. Using data from natural samples, we are exploring the hypothesis that localization in periodically deformed ductile shear zones increases over time, because grain size reduction during high stress deformation is retained at low stress in mixed olivine-pyroxene layers due to second-phase pinning, but heals via relatively rapid grain growth in olivine-rich layers. This is consistent with the observation that, in oceanic and ophiolite peridotite mylonites, olivine grain size in millimeter- scale, mixed phase bands is much smaller than in dunite bands.

Our published models do not explicitly include the effect of enthalpy of fusion on the heating rate, nor weakening due to melting, and instead use a fixed peak temperature. New models will include heat of fusion and melt weakening to avoid the arbitrary temperature cap. New models will also investigate this mechanism using rheology for compositions other than olivine.
DE: 3902 Creep and deformation
DE: 7209 Earthquake dynamics (1242)
DE: 8033 Rheology: mantle (8162)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting