PLATE MOTION CHANGES, THE HAWAIIAN-EMPEROR BEND, AND THE APPARENT SUCCESS AND FAILURE OF GEODYNAMIC MODELS

Geodynamic models explain present-day plate motions in terms of mantle buoyancy forces arising from subducted lithosphere and lithospheric t hickening, or from velocity anomalies mapped by seismic tomography. Ho wever, such models do not account for sudden plate tectonic reorganiza tions, such as the dramatic change in Pacific plate motion implied by the sharp bend in the Hawaiian-Emperor seamount chain about 43 million years ago. Candidate mechanisms for the Hawaiian-Emperor bend, such a s subduction initiation, ridge subduction, or the possibly time-coinci dent collision between India and Asia, remain weakly formulated and la rgely untested. We test the India-Asia collision hypothesis using a mo del for Cenozoic plate motions driven mainly by buoyancy forces introd uced at paleo-subduction zones, and we show that Pacific plate motion is virtually unaffected by the development of compressional stresses a long the India-Asia margin. Geologic evidence suggests subduction init iation at about 43-48 Ma along a transform boundary on the western Pac ific plate margin, but this mechanism is difficult to test. We specula te that transform boundaries may guide plate motions for long periods of time, and that rapid plate motion changes may result from the creat ion or destruction of major transform faults. Speculations aside, our results show that the character of global plate motion changes is not adequately explained by current geodynamic models.