HR: 14:40h
AN: T52E-05    [PDF]
TI: Deep Continental Lithosphere Keels as Impediments to Asthenosphere Flow and Cause of Ocean Crust Depth Anomalies
AU: * Ryan, W B
EM: billr@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Muhlenkamp, B M
EM: briannam@email.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building, Tucson, AZ 85721-0077 United States
AU: Haxby, W F
EM: bill@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Carbotte, S M
EM: carbotte@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Buck, W R
EM: buck@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AB: The Australian-Antarctic Discordance (AAD) lies directly in the wake of a deep keel of 2.5 billion year old Australian lithosphere that severed from and moved northward in the hot spot reference frame from a near-stationary Antarctica. Adopting the model of a plume-fed sub-ocean asthenosphere (Morgan, 1971,1972; Phipps Morgan,et al, 1995), this keel, like others of similar ancient age beneath South Africa and Laurentia could have served as a dam to isolate different pools of asthenosphere and thus allow each pool to evolve independently from their supplying plumes. A NSF-funded Antarctic database effort has allowed us to digitize a large set of seismic reflection profiles, map sediment thickness and remove the subsidence caused by sediment load. This effort confirms not only that the ocean crust between the keels of Australia and Antarctica was always manufactured at an anomalously deep spreading center, but the depth anomaly was greatest right after continental separation, and the crust beneath the faster moving Australian plate is deeper for all ages than counterparts on the Antarctic Plate. The same depth asymmetry and a similar geoid anomaly are observed in the South Atlantic where South America is traveling faster than Africa relative to the mantle. Upon separation of the continents, asthenosphere from both sides of the keel flows into the wake of the moving continent to replenish the void created by the initial dam. The Pacific asthenosphere, with its distinct geochemical composition, is currently observed to be propagating into the AAD. Since asthenosphere is consumed as it is frozen onto the diverging oceanic lithosphere, a dammed region with a limited asthenosphere supply might act as a brake, or governor, for seafloor spreading. We point out that spreading started very slowly upon the breakup of Australia and Antarctica, the aborted spreading center in the Labrador Sea occurs in a narrow ocean between very ancient cratons, and the boundary between the geochemical distinct Indian and Atlantic mid-ocean ridges occur where deep continental keels were severed.
DE: 1025 Composition of the mantle
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8155 Plate motions--general
DE: 9310 Antarctica
DE: 9330 Australia
SC: Tectonophysics [T]
MN: 2003 Fall Meeting