Introduction

During the 1960s there were a wide variety of studies on continental drift and its relationship to mantle convection. One of the major contributors was J. Tuzo Wilson. Wilson (1963a, b, 1965a, b) used a number of geophysical arguments to delineate the general movement of the ocean floor associated with seafloor spreading. He argued that the age progression of the Hawaiian Islands indicated movement of the Pacific plate. He showed that earthquakes on transform faults required seafloor spreading at ridge crests. During this same period other geophysicists outlined the general relations between continental drift and mantle convection (Orowan, 1964, 1965; Tozer, 1965a; Verhoogen, 1965). Turcotte and Oxburgh (1967) developed a boundary layer model for thermal convection and applied it to the mantle. According to this model, the oceanic lithosphere is associated with the cold upper thermal boundary layer of convection in the mantle; ocean ridges are associated with ascending convection in the mantle and ocean trenches are associated with the descending convection of the cold upper thermal boundary layer into the mantle. Despite these apparently convincing arguments, it was only with the advent of plate tectonics in the late 1960s that the concepts of continental drift and mantle convection became generally accepted.

Plate tectonics is a model in which the outer shell of the Earth is broken into a number of thin rigid plates that move with respect to one another. The relative velocities of the plates are of the order of a few tens of millimeters per year. Volcanism and tectonism are concentrated at plate boundaries. The basic hypothesis of plate tectonics was given by Morgan (1968); the kinematics of rigid plate motions were formulated by McKenzie and Parker (1967) and Le Pichon (1968). Plate boundaries intersect at triple junctions and the detailed evolution of these triple junctions was given by McKenzie and Morgan (1969). The concept of rigid plates with deformations primarily concentrated near plate boundaries provided a comprehensive understanding of the global distribution of earthquakes (Isacks et al., 1968).

The distribution of the major surface plates is given in Figure 2.1; ridge axes, subduction zones, and transform faults that make up plate boundaries are also shown. Global data used to define the plate tectonic model are shown in Figures 2.2-2.9. The distribution of global shallow and deep seismicity is shown in Figure 2.2, illustrating the concept of shallow seismicity defining plate boundaries. Figure 2.3 shows the distribution of ages of the ocean crust obtained from the pattern of magnetic anomalies on the seafloor. The distribution of crustal ages confirms that ridges are the source of ocean crust and also establishes the rates of seafloor spreading in plate tectonics. Figures 2.4-2.6 show geoid height variations - the topography of the equilibrium sea surface, which correlates closely with seafloor topography

A.i Subduction 20ne ------Uncertain plate boundary

-Strike-slip {transform | faults Direction of plate motion

Ridge axis

Figure 2.1. Distribution of the major surface plates. The ridge axes, subduction zones, and transform faults that make up the plate boundaries are shown. After Bolt (1993).

JUAN >1 Dt FUCA J PLATE

San Ancieas fault

Kuril trench

Japan caribbean ~ , plate -

Anatolian fault

Macanas ireficr

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