|
Traditionally, mantle convection and plate tectonics have been treated as two separate entities, with each exerting forces on the other and one (usually the plates) viewed as "driving" the other. However, in order to understand the large-scale dynamics of the plates and mantle it is important to recognize that oceanic plates are the upper thermal boundary layer of mantle convection, that subducted slabs are the major cold downwellings of mantle convection, and thus, oceanic plates and mantle convection are best treated as one combined system, not as separate entities. Furthermore, continents, although compositionally-distinct, obey the same physical laws as mantle and oceanic plates. The goal is thus to develop a plate-mantle model in which plates and regular mantle arise self-consistently from a single description of rock deformation as a function of temperature, pressure, stress, composition and deformation history.
Long-standing problems with this goal have been (i) the complexity of rock deformation- it is not clear what mechanism is most important in the formation of weak plate boundaries, and (ii) numerical difficulty in modeling realistic rock rheologies. However, advances in both areas have facilitated global mantle models in which plate-like features arise self-consistently, and recent examples of these will be presented here.
In the presented models in three-dimensional spherical geometry, plate-like behavior arises from the combination of strongly temperature-dependent viscosity giving a strong lithosphere, and a visco-plastic yield stress (representing brittle and semibrittle, semiductile deformation) allowing weak plate boundaries.
Continents may be included as compositionally-distinct material. Smoothly-evolving plate-like behavior is found over a range of yield strengths, with diffuse deformation at lower yield strength, and episodic behavior, and eventually a rigid lid, at higher yield strength (>∼150 MPa). The existence of a lower-viscosity asthenosphere improves plate quality over a range of yield stresses.
Toroidal:poloidal ratios are within the bounds observed for Earth (0.3-0.5 excluding net rotation), and are higher for spherical cases than for previous cases in cartesian geometry. Novel planforms are obtained in spherical geometry under certain conditions: a "great circle subduction" planform in which a single subduction zone spans the entire globe in approximately a great circle, and a "two hemispherical plates" planform in which a convergent boundary exists over half of a great circle and a divergent boundary covers the other half.
|
