HR: 10:35h
AN: S41F-02 INVITED     [PDF]
TI: Brittle to Viscous Transition, Modeling the Evolution of the Andean Plateau
AU: * McQuarrie, N
EM: nmcq@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, MC 100-23, Pasadena, Ca 91125 United States
AU: Lavier, L L
EM: luc@utig.ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd., Austin, Tx 78759 United States
AB: Although numerical and analytical models of plateau development stress the importance of viscous deformation as a requirement for plateau growth, we present a series of numerical 2-D experiments that illustrate the formation of brittle plateaus as emphasized by geologic studies. The role of brittle deformation in plateau formation is particularly apparent for the Andean Plateau in Bolivia where balanced cross-sections through the Andean fold-thrust belt indicate upper crustal shortening of 300-330 km through predominantly eastward propagating brittle fault systems. Although associated mantle deformation is not fully understood, only half of the original 880 km length of mantle lithosphere is preserved in the orogen. Much of the preserved mantle lithosphere is interpreted to be Brazilian shield. The models of plateau growth that we present show both the development of a plateau through brittle deformation processes and the importance of intracontinental subduction of mantle lithosphere for the eastward propagation of deformation. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions. The brittle parts of the lithosphere are modeled as a frictional and cohesional material. The ductile lithosphere is modeled as a non-Newtonian Maxwell visco-elastic material. The initial conditions for the model include a strong cold lithosphere adjacent to a younger, warmer lithosphere that corresponds to the 400-200 Ma passive margin/ extensional basin on the western edge of the South American continent. Subduction of the Nazca oceanic plate is included as the driving mechanism applied as compressional velocity boundary conditions on the model. Initial deformation in the model is focused in the center of the passive margin basin, on a pre-existing weakness in both the crust and lithosphere. Westward dipping intracontinental subduction of mantle lithosphere pulls down the overlying crust and facilitates the eastward propagation of brittle deformation. With time heat associated with a radiogenic, thickened crust decreases the viscosity of the lower crust and facilitates plateau growth through viscous processes. The force history curve of the experiments shows a continuous decrease in the force required to drive the deformation that is due to the negative buoyancy generated by the subducting lithosphere. This process is mainly responsible for the generation of space that allow the eastward propagation of plateau. The curve flattens when the plateau reaches the edge of the cold lithosphere, the subducting slab stalls, and plateau growth is continued through viscous-dominated deformation.
DE: 8000 STRUCTURAL GEOLOGY (New field, replaces single entry 8165)
DE: 8102 Continental contractional orogenic belts
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 9360 South America
SC: Seismology [S]
MN: 2003 Fall Meeting