HR: 1340h
AN: U43B-0836 INVITED     [Abstracts]
TI: Diffuse Oceanic Plate Boundaries, Thin Viscous Sheets of Oceanic Lithosphere, and Late Miocene Changes in Plate Motion and Tectonic Regime
AU: * Gordon, R G
EM: rgg@rice.edu
AF: Rice University, Earth Science--MS 126, 6100 Main St., Houston, TX 77005 United States
AU: Royer, J
EM: jyroyer@univ-brest.fr
AF: CNRS UBO-IUEM Domaines Oc‚aniques, Place Copernic, Plouzane, 29280 France
AB: Diffuse plate boundaries are often viewed as a characteristic only of continental lithosphere and as a consequence of its rheology, while narrow boundaries and plate rigidity are viewed as characteristic of oceanic lithosphere. Here we review some of the evidence that shows that deformation in the ocean basins is in many places just as diffuse as deformation in the continents. Moreover, we argue that the best description of these oceanic deforming zones is the un-plate tectonic-like representation as a fluid. Diffuse oceanic plate boundaries are deforming zones that are typically thousands of kilometers long (along strike) and hundreds to thousands of kilometers wide (across strike). These plate boundaries also appear to have deformation that is broadly distributed with no single fault or system of faults taking up most of the relative plate motion. Consequently the spatially averaged strain rates across diffuse oceanic plate boundaries are orders of magnitude lower than in narrow plate boundaries. One of Earth's best examples of a diffuse oceanic plate boundary is located in the equatorial Indian Ocean. A fluid-like representation of deformation in this diffuse boundary explains many observations, including the steadiness of the deformation process, the characteristic across-strike width of deformation relative to the along-strike length of the deforming zone, and the change of style in deformation across the 86°E fracture zone. In addition, poles of relative rotation between adjacent component plates tend to lie within the diffuse plate boundary that separates them; this is also predicted by models of diffuse plate boundaries that assume a power-law fluid approximation, irrespective of rheology (for power-law rheologies between Newtonian and plastic end-members). A change in behavior of the lithosphere from elastic or visco-elastic to that of a fluid may be interpreted as a phase change, not in microscopic but in megascopic properties, above a certain threshold of force per unit length applied to the lithosphere. There remain many outstanding fundamental kinematic, dynamical, and rheological questions, the answers to which would enhance our understanding of diffuse oceanic plate boundaries. These questions include the timing of initiation and acceleration of motion across various diffuse oceanic plate boundaries, the relative and absolute strengths of the upper and lower oceanic lithosphere, changes in torques across diffuse oceanic plate boundaries and their role in causing rapid changes in plate motion. Of particular interest is the role of diffuse oceanic plate boundaries in the widespread re-organization of plate motion and tectonic regime that occurred at ~8 Ma in the Tibetan Plateau, Indian Ocean, Pacific Ocean, and western North America. Diffuse plate boundaries, especially in the oceans, are excellent natural laboratories, not only as windows on the mechanical and rheological properties of the lithosphere, but also--at least in the oceans--for investigating a variety of styles of widely distributed deformation that is ignored by traditional plate tectonics.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8149 Planetary tectonics (5475)
DE: 8150 Plate boundary: general (3040)
DE: 8155 Plate motions: general (3040)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Union [U]
MN: Fall Meeting 2005