HR: 11:15h
AN: V21F-04    [PDF]
TI: The `Plate-Like' Subsidence of the East Pacific Rise - South Pacific Superswell System
AU: * Hillier, J K
EM: johnh@earth.ox.ac.uk
AF: Oxford University, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Watts, T B
EM: tony.watts@earth.ox.ac.uk
AF: Oxford University, Department of Earth Sciences, Parks Road, Oxford, OX1 3PR United Kingdom
AB: The separation of small-scale features from the regional seafloor depth is an important problem in the geosciences, especially as it impacts our understanding of mid-plate topographic swells and subsidence away from mid-ocean ridges. In the south Pacific ocean, for example, the removal of these features from the bathymetry using modal techniques has revealed a large and unusually shallow region of the seafloor, which at $\sim$3000 km wide and up to 1 km high has been dubbed a ``Superswell''. Modal analysis, however, does not completely isolate and remove small-scale features such as oceanic islands, seamounts, oceanic plateaus and localised hot-spot swells from the regional bathymetry. This is because these features are superimposed upon the unperturbed ridge-generated regional bathymetry, accordingly a technique is required that underlines topographic constructs rather than passing through them as is the tendency of any average (mean, median or mode). We have therefore developed an algorithm that reproducibly simulates manual interpretation ({\bf MiMIC}), thereby removing the superimposed features and revealing larger scale trends. Application of {\bf MiMIC} to grids of bathymetric data in the region 12-26$^\circ$S, 200-243$^\circ$E shows that seafloor of all ages (0.5-112Ma) deepens slowly (initially $\sim$218 mMa$^{-1/2}$) and in essence monotonically from the East Pacific Rise (EPR). Although initially deep (-2712m) with respect to a standard plate model (-2500m, 125km, 1350$^\circ$C), the low subsidence rate reduces the negative depth anomaly with time until it becomes a positive anomaly west of $\sim$234$^\circ$E ($\sim$20-25Ma) that increases to a maximum of 712$\pm$66m at 98Ma, not 1300m at $\sim$65Ma as previously observed. Most significantly though, the Superswell appears to be part of a larger scale, monotonic and `plate-like' subsidence trend that extends to the EPR, not an isolated shallowing that reverses subsidence and causes uplift between 40-80Ma. The continuous nature of the EPR-Superswell subsidence trend suggests to us a common causal mechanism that links processes acting at the Superswell with those at the EPR. Previous studies at the EPR show asymmetry in seafloor subsidence and other observables such as seismic velocity and electrical conductivity which have been interpreted in terms of an across-axis temperature gradient, possibly sustained by a flow of hotter material from the west. Thus, a lateral temperature gradient may exist across the entire EPR-Superswell system. We have tested this hypothesis using a model in which the isostatic and thermal effects of a lateral sub-lithospheric temperature gradient are allowed to perturb the subsidence of a standard plate. Our preferred model has a depth of isostatic compensation of 320km and a linear temperature gradient of only 0.014$^\circ$C/km. Such a model better explains the regional seafloor depths of the EPR-Superswell system than published cooling plate models and is in accord with constraints from elastic thickness, heat flow, seismic tomographic and long-wavelength gravity anomalies.
UR: http://www.earth.ox.ac.uk/~johnh/AGU2003.html
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3045 Seafloor morphology and bottom photography
DE: 8100 TECTONOPHYSICS
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting