Episodic North America and Pacific Plate motions

Abstract

The North America plate motion for the period 20–0 m.y. is modeled by two absolute angular velocity vectors, with a change occurring at about 9 m.y. The model incorporates new geologic constraints based on (1) the recognition of a discontinuity in North America plate motion at 9 m.y., (2) a reexamination of the Iceland and Yellowstone hotspot traces, (3) improved delineation of the present North America – Pacific relative motion, and (4) a revised history of Pacific absolute motion. The two‐stage model is consistent with several features of the late Cenozoic geologic history of North America, including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of North America. This plate model, combined with a previously established Pacific plate model, makes specific predictions concerning the 15–0 m.y. history of the North America‐Pacific plate boundary which compare favorably with the known history. These models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates. The implications of the North America plate model are the greatest for those events related to the 9 m.y. change in its motion. Among the geologic consequences of this change are rifting in the western United States, intense compressional deformation along the northern Caribbean plate boundary, a hiatus in Aleutian arc volcanic activity, and a change in the orientation of the Lesser Antilles arc. This picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances. The predictive power of the model may help in interpreting the history of the North America plate boundary in those areas where clear observations are lacking. An extra force needs to be applied to a plate to cause a change in plate motion. By treating the plate as a rigid spherical shell overlying a uniformly viscous fluid, we derive a simple class of forces sufficient to cause a given change in plate motion. Such force balance considerations indicate that an additional force applied in the northwest Pacific region, toward the northeast, would result in the observed change in North America plate motion. This force is surprisingly similar to that which apparently caused a large change in Pacific plate motion about 5 m.y. later. We propose that a common mantle process is responsible for both changes and is related to the ability of the northwest Pacific subducted lithosphere to enter into the lower mantle.