HR: 13:55h
AN: T43D-02 INVITED     [Abstracts]
TI: The Strength of Fracture Zones from their Bathymetric and Gravitational Evolution
AU: * Hall, C
EM: chall@gps.caltech.edu
AF: Seismo Lab, Cal Tech, MS 252-21, Pasadena, CA 91125
AU: Gurnis, M
EM: gurnis@gps.caltech.edu
AF: Seismo Lab, Cal Tech, MS 252-21, Pasadena, CA 91125
AB: Depending on their strength, fracture zones (FZs) may play an important role in accommodating changes in plate motions and creating new plate boundaries. All previous models of FZ evolution have imposed the bathymetric offset at FZs kinematically with thin plates; here we model FZs dynamically so that they are free to slip. The models use a visco-elastoplastic rheology incorporating the influence of fault friction on FZ slip history and thus model the transition from elastic to viscous behavior more realistically than assuming a transition isotherm. By comparing synthetic gravity profiles to free-air gravity across FZs, we find that the amplitude of the gravity jump across FZs is best fit by models with weak faults that have depth-averaged yield strengths less than 10 MPa. Small-scale convective removal of the lowermost (i.e., viscous) thermal lithosphere cannot explain the older-side highs observed at many Pacific FZs. Many FZs do not fit plate subsidence models with locked or slipping faults, but are better fit by systems that are tectonically deformed by modest amounts of extension, presumably due to minor stage pole re-orientations. Longer-lived tectonic adjustments that place a FZ in compression may result in the conversion of the FZ into an incipient subduction zone. We show that $\sim$10 MPa depth-averaged strength is a critical point for plate boundary evolution: FZs with strengths less than $\sim$10 MPa can evolve into self-sustaining subduction zones with an intense phase of back-arc basin extension, as found for the Mariana's when it transitioned from a FZ to a subduction zone.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 8160 Rheology--general
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting