HR: 09:45h
AN: G51D-08    [Abstracts]
TI: Constraints on Lithospheric Rheology From Fault Displacement Rate Histories and Numerical Experiments
AU: * Lavier, L L
EM: luc@ig.utexas.edu
AF: Jackson School of Geosciences, University of Texas Institute for Geophysics , 4412 Spicewood Springs Road, #600, Austin, TX 78759 United States
AU: Bennett, R
EM: rab@geo.arizona.edu
AF: University of Arizona, Department of Geosciences , Tucson, AZ 85721-0077 United States
AB: Several models of strain accumulation for parallel strike-slip faults have been defined. The most commonly used model is the viscoelastic coupling model which has a fault cutting the seismogenic layer (schizosphere) coupled to a non-seismogenic weak viscous layer (plastosphere). Other models include the deep slip model in which a ductile shear zone cuts across the crust in middle to lower crust. Based on fits to both the short-term geodetic and long-term geologic rates on faults it has been shown that geodetic rates and displacement rate histories alone are not able to differentiate between different strain accumulation models for details. Moreover the assumption used in the previous models that geodetic and geological displacement rates are similar is now being questioned by new data. Recent observations of long and short term displacement rate histories in the Basin and Range Province and in Southern California suggest that a set of faults "share" a large fraction of the total displacement at a plate boundary on time-scales of 100 to 10 kyr. This brings additional constraints on models of strain accumulation for parallel faults. There are several lines of evidence including geological, geodetic and seismic reflection observations that strike slip faults such as the San Andreas extend in a deep ductile shear zone in the crust. We propose a parameterization of the rheology of the lithosphere, and we use this parameterization in self-consistent dynamic models of faulting to simulate the formation and the variations in displacement rate histories on a set of strike-slip faults in 2.5D. We first model a crust only system. Initially the model behaves like the visco-elastic coupling strain. Modeled displacement histories show that the displacement rates accelerate or decelerate over faults in the upper crust and fast flow areas in the lower crust over 100 kyr time scale. As the faults form and accumulate enough strain to develop a deep ductile shear zone the system develops in blocks accumulating a constant amount of strain. The 100 kyr time scale is imposed by the behavior at a critical point at brittle ductile transition. We then analyze the effect on displacement rates of the presence of a weak or strong mantle lithosphere. For a weak mantle the faults define crustal blocks such that geodetic rates and geologic rates are similar. There is no exchange of displacement between faults in such a system. The only time scale variation occurs at the time scale of 100 to 1000 yr on each individual faults each sharing an equal amount of displacement. For a strong mantle rate changes over the 100 kyr time scale occurs between blocks in the crust and the mantle lithosphere with periods of acceleration/deceleration of 10-20 kyr. These periods of constant rate displacement followed by an increase/decrease of activity on faults may be critical to our understanding of earthquake hazard.
DE: 1209 Tectonic deformation (6924)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 4255 Numerical modeling (0545, 0560)
DE: 8160 Rheology: general (1236, 8032)
SC: Geodesy [G]
MN: Fall Meeting 2005