HR: 14:55h
AN: T52E-06    [PDF]
TI: Constraints on Asthenospheric Flow From the Depth of Oceanic Spreading Centers
AU: * Buck, W R
EM: buck@ldeo.columbia.edu
AF: Oceanography Bldg., LDEO, Palisades, NY 10964 United States
AU: Small, C
EM: small@ldeo.columbia.edu
AF: Oceanography Bldg., LDEO, Palisades, NY 10964 United States
AU: Ryan, W B
EM: billr@ldeo.columbia.edu
AF: Oceanography Bldg., LDEO, Palisades, NY 10964 United States
AU: Carbotte, S M
EM: carbotte@ldeo.columbia.edu
AF: Oceanography Bldg., LDEO, Palisades, NY 10964 United States
AU: Muhlenkamp, B M
EM: briannam@email.arizona.edu
AF: Dept. of Geosciences, University of Arizona, Tucson, AZ 85721 United States
AU: Haxby, W F
EM: haxby@ldeo.columbia.edu
AF: Oceanography Bldg., LDEO, Palisades, NY 10964 United States
AB: The near constancy of the depth of mid-ocean ridges away from hotspots has led to the view that such ridge depths reflect a globally averaged density and pressure structure. New analysis of data on ridge depths combined with a simple model of mantle flow leads us to assert that ridge depths depend on the thickness of underlying low-density asthenosphere. The asthenosphere thickness and related isostatic ridge depth depends primarily on the divergence rate of a spreading center and also on the distance to subduction boundaries according to a thin-layer asthenospheric flow model. We assume that the asthenosphere layer is lower in density than the mesosphere by about 10 kg/m3. Following Phipps-Morgan et al., (1995) the asthenosphere is sourced from below, though we assume a uniform distribution of sources. The ridge is a sink where 100 km thick rigid lithosphere is formed from flowing asthenosphere. The mean depth of the Southern East Pacific Rise (EPR) is consistently close to 3100 m while the consistent mean depth along the Pacific-Antarctic Rise (PAR) is closer to 2700 m. For a 10,000 km by 10,000 km area of Pacific asthenosphere overlain by idealized lithospheric plates representing parts of the Pacific, Nazca and Antarctic plates the model predicts nearly constant, but different depths for the model EPR and PAR. The faster divergence rate of ~15 cm/yr for the model EPR compared to 9 cm/yr for the adjacent PAR can produce a 400 m greater mean ridge depth (i.e. average within 30 km of the axis) for the EPR for a viscosity of just over 1019 Pas and regionally averaged asthenosphere thickness near 300 km. Predicted asthenospheric strains are broadly consistent with shear wave splitting measurements. The deepest stretch of the world's mid-ocean ridges is the Austrailian Antarctic Discordance (AAD). The mean depth of the AAD is greater than 3500 m in places and there is evidence that the crustal thickness there is much less than for normal ridges. There is also evidence that the along-ridge length of the discordance is shrinking with time. We apply the asthenospheric flow model to this region under the assumption that the lithopsheric roots of the Austrailian and Antarctic continents extend to greater depths than the base of the thickest asthensophere. The divergence of the model roots combind with the asthonspheric sink of the spreading center leaves a "wake" of zero asthenospheric thickness along a section of the model spreading axis. The length of the section of little or no asthenosphere shortens in time as asthenosphere flows parallel to the axis. If the near axis upwelling of hotter or more fertile asthenosphere produces more melt than would result from melting of mesosphere then the model can explain both the depth anomalies and thin crust observed at the AAD. For model parameters similar to that assumed for the EPR-PAR model and for a spreading history based on the AAD region we predict the present day AAD length.
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8155 Plate motions--general
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2003 Fall Meeting