HR: 1330h
AN: B12A-0744    [PDF]
TI: Modeling Crustal Thickness Variations Beneath the East Pacific Rise: Mantle Diapirs or Plate Kinematics?
AU: * George, S A
EM: sgeorge@newberry.uoregon.edu
AF: University of Oregon, 1272 Geological Sciences, Eugene, OR 97403
AU: Toomey, D R
EM: drt@newberry.uoregon.edu
AF: University of Oregon, 1272 Geological Sciences, Eugene, OR 97403
AB: Geophysical studies along the East Pacific Rise between the Siqueiros and Clipperton fracture zones reveal along- and cross-axis variations in crustal thickness whose origins are poorly understood. By one view, variations in crustal thickness are the result of three-dimensional upwelling of the mantle associated with a melt-rich diapir centered at $9\deg$50'N. Alternatively, it has been proposed that the migration of the $9\deg$03'N overlapping spreading center (OSC) alters the thickness of crust by increasing the amount of time that a crustal unit resides near the spreading axis. In this case, crustal thickness variations arise from plate kinematics, and not from three-dimensional variations in mantle upwelling. We report on a modeling study designed to explore how the evolution of OSCs may alter the thickness of newly-formed crust. OSC propagation is modeled using the kinematic algorithm developed by Wilson [1990], modified to track parcels of crust through time. Given an OSC's kinematic history and two-dimensional descriptions of the melt flux out of the mantle (i.e. invariant along the rise), we predict relative variations in crustal thickness. Our modeling assumes that underplating increases the thickness of the crust and/or Moho transition zone as long as a crustal unit resides over the source of mantle-derived melt. Results suggest two general kinematic mechanisms whereby variations in crustal thickness can occur: those due to an offset between the mantle-level magmatic system and the spreading axis, and those due to any relative reduction in the velocity of a crustal unit as it moves off axis. Offset-induced crustal thickness variations are manifest as long-wavelength ($\sim$50 km), low-amplitude cross-axis asymmetries. Local slowing of crustal units as they move off axis -- in direct association with the OSC and its overlap basins -- results in relatively short-wavelength ($\sim$10 km), high-amplitude variations in crustal thickness. Using a kinematic history appropriate for the $9\deg$03'N OSC, we predict both long- and short-wavelength variations in crustal thickness that are comparable to those observed in seismic refraction data [Canales et al., 2003]. We conclude that a combination of plate kinematics and underplating of crustal material is a viable mechanism for generating observed variations in crustal thickness.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
SC: Biogeosciences [B]
MN: 2003 Fall Meeting