HR: 16:45h
AN: G14A-04    [Abstracts]
TI: Into the Future: Continuing Evolution of the Pacific-Juan de Fuca-North America Plate System
AU: * McCrory, P A
EM: pmccrory@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Wilson, D S
EM: dwilson@geol.ucsb.edu
AF: University of California, Department of Earth Science, Santa Barbara, CA 93106, United States
AU: Stanley, R G
EM: rstanley@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: The death of a series of spreading ridge segments adjacent to California and Mexico starting about 28.5 Ma led to the piecewise destruction of a subduction regime and the opening of slab windows beneath the continent. The formation, growth, and healing of these slab windows has been a transient process marked at the surface by overprinted pulses of volcanism in the former forearc triggered by asthenospheric upwelling behind subducting slab edges. In the subsurface, where windows opened beneath thin forearc lithosphere, the shallow asthenosphere promoted a plastic mode of mechanical behavior in the lithosphere characterized by fault-block deformation. This plastic mode of deformation resulted mainly from (1) asthenosphere in direct contact with the lower crust instead of stronger lithospheric mantle; and (2) lower crust shifting from brittle to ductile behavior as a result of conductive heating. Our finite-rotation model built from quantitative reconstruction of Farallon-North America slab windows and their magmatic signatures allows us to investigate past and future fault-block kinematics of the North America continental margin. While the shallow asthenosphere thermally weakened coastal California and promoted plastic deformation in the lower crust, the actual fragmentation of the upper crust into fault blocks occurred in concert with retrograde motion of a partially subducted Monterey plate fragment following its capture by the Pacific plate ca. 19 Ma. This abrupt change from convergent to transtensional deformation initiated the pulling apart and clockwise pivoting of the adjacent western Transverse Ranges and California Borderlands region.  A similar reversal in plate motion occurred adjacent to coastal Mexico following capture of a Magdalena plate fragment by the Pacific plate ca. 12.5 Ma. In this case, the transtensional strain that initiated Baja California pulling away from the Mexican continental margin was localized along already thermally weakened Comondu volcanic arc crust. In California, running our kinematic model into the future suggests that the current locus of strike-slip motion may shift to the east side of the Sierra Nevada Mountains, starting about 2 My from now as a result of convergence between the Sierra Nevada and Peninsular Range batholiths. Alternatively, the locus of fault slip may remain on the San Andreas fault, requiring new fragments to be broken from the southern Sierra Nevada and Mojave blocks, to make room for the Peninsular Range to slip past on their west side. Or perhaps, eastern parts of the Peninsular Range may transfer back to the North America plate, with the Whittier-Elsinore fault accommodating most of the relative plate motion.
DE: 1209 Tectonic deformation (6924)
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8106 Continental margins: transform
DE: 8158 Plate motions: present and recent (3040)
DE: 8178 Tectonics and magmatism
SC: Geodesy [G]
MN: 2007 Fall Meeting