HR: 1340h
AN: G13A-0800    [Abstracts]
TI: What is the Strength of the Plastic Lower Crust? Comparing the Evidence From Postseismic Deformation and Laboratory Experiments
AU: * Montesi, L G
EM: montesi@whoi.edu
AF: Woods Hole Oceanographic Institution, Dpt. Geology and Geophysics, Woods Hole, MA 02453 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, Dpt. Geology and Geophysics, Woods Hole, MA 02453 United States
AB: The plastic strength of many rock types has been well studied in the laboratory. These data suggest that stresses in excess of a few 100 MPa are required to allow deformation at tectonic strain rates near the brittle-ductile transition. The microstructures observed in naturally-deformed rocks agree with such stress levels. By contrast, earthquake-induced stress perturbations, which scale with stress drop, are not more than 10 MPa. Moreover, they decrease rapidly with distance from the edges of an earthquake rupture. Hence, the earthquake cycle produces stress variations of only a few percent in the aseismic plastic crust and mantle. Unless the rock rheology is very different from what is observed in the laboratory, such small stress variations should not produce large creep transients. Yet, transient postseismic deformation in the lower crust and upper mantle is now commonly observed in the aftermath of earthquakes of magnitude 6 and higher. This implies that the strength of the aseismic regions of the lithosphere is much less than inferred from laboratory. We tested the validity of this reasoning with numerical models of postseismic deformation using one of the weakest laboratory-determined flow laws, that of quartzite. The resolution of this paradox may reside in the presence of localized shear zones that differ in their mineral assemblage, fabric, or grain size from the host rock, or possibly a temperature anomaly associated with the downward continuation of brittle faults.
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting