Kinematics of plate convergence deduced from Mesozoic structures in the Western Cordillera
Authors: Oldow John S. , Lallemant Hans G. Avé , Schmidt William J. 
Published: February 2008

Mesozoic structures in the western Cordillera of the United States yield important constraints for kinematic reconstruction of plate motions relative to North America. Structures in the Mesozoic arc(s) of the northern Sierra Nevada and of northeastern Oregon are remarkably uniform in timing, style, and with Late Cretaceous and/or Cenozoic rotations removed, orientation. Rocks of both regions experienced three major episodes of contraction occurring in the Triassic‐Early Jurassic, the Late Jurassic, and the Early Cretaceous. Cross folds were developed after the first two deformations but were not associated with significant shortening. The three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular (NE‐SW) to the regional trend of the arc system. In the back arc region of the northwestern Great Basin, deposition was continuous during the Triassic and Early Jurassic, and the region was shielded from tectonism in the arc to the west. Development of a regional fold‐thrust belt associated with major NW‐SE shortening was initiated in the back arc region during the Middle or Late Jurassic and continued through the Early Cretaceous. Corresponding deformation did not occur in the arc, and the arc and back arc were decoupled by a major left‐lateral fault system that subparallelled the arc axis. In the mid‐Cretaceous (approximately 100 m.y. B.P.), regional compression in the back arc abruptly changed to NE‐SW, and both the arc and back arc experienced major shortening. Development of homoaxial structures in the Sierran arc is probably analogous to shortening in present‐day arc systems undergoing oblique subduction, in which the shortening axis is parallel to the normal component of convergence. Thus the dominant structures in the Sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence. Strike‐slip faults and related compressional structures, such as the back arc left‐lateral fault and associated fold‐thrust belt, are probably the response to the strike‐slip component of convergence and indicate a period of left‐oblique subduction. Relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right‐lateral components of motion for the last 100 m.y. Before 100 m.y. B.P., relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left‐oblique convergence from the Middle or Late Jurassic to approximately 100 m.y. B.P. During the Triassic and Early Jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left‐oblique convergence occurred throughout the late Paleozoic and early Mesozoic (until 100 m.y. B.P.). Based on the available data, we cannot discount the possibility that a complex convergence history existed, however, with changes from right to left convergence in the Middle or Late Jurassic and from left to right convergence in the mid‐Cretaceous.