Theoretical studies of the geodynamics of accretion boundaries in the plate tectonics

Abstract

Various aspects of the physical processes occurring at the accretion plate boundary in plate tectonics have been investigated. Regional stresses have been investigated, arising from lateral density contrasts in the ocean lithosphere. Elastic, visco-elastic and elastic/visco-elastic models predict regional stresses in the ocean basin of the order of 0.25 kb. Investigation of the thermal stresses created in the oceanic lithosphere as a consequence of the cooling of the ocean lithosphere as it moves away from the ridge axis, shows that tensional stresses occur in the upper lithosphere and compressional stresses in the lower lithosphere. An elastic/viscous model of the lithosphere predicts deviatoric stresses of the order of 3 kb. in the upper crust. The temperature distribution beneath the ocean ridge with magma solidifying to form crustal layer 3 has been investigated. Numerical models show that the width of the magma chamber and the thickness of the dyke complex depends on half spreading rate. If there is significant crystal settling, the width of the chamber is predicted to be considerably reduced. A critical half spreading rate of 0.45 cm/yr is predicted, below which the intruded material solidifies instantaneously. Computations support Cann's petrological model. Investigation of the magnitude of the stresses caused by the buoyancy of a magma chamber in the lower crust at the ridge axis suggest that the magma chamber is unable to cause crustal fracture and is, alone, a dynamically stable structure. The additional stresses due to the upthrust of molten upper mantle material is required to cause crustal fracture and a zone of fracture of less than 5 km wide is predicted. The stress field created in the oceanic lithosphere by a mantle plume has been calculated analytically. Estimates of the plume dimensions and velocity suggested by Morgan are predicted to be just sufficient to cause fracture of the lithosphere above the plume axis.