Intracontinental contraction in western North America: Continuity and episodicity

Contraction in the western North American Cordillera, exclusive of accreted terranes, occurred over a remarkably uniform duration of 110 ± 15 m.y. between approximately 165 and 55 Ma. The amount of contraction, as much as 285 km, was similar from southeastern British Columbia to northern Nevada and Utah, although irregularly partitioned between hinterland, fold and thrust belt, and Rocky Mountain foreland. Time-averaged contraction rates are 1.5 to 2.5 mm/yr. The contraction, however, appears to have occurred in pulses with cumulative duration of 75 m.y., implying rates during active periods of 2.5 to 3.8 mm/yr. Displacement rates as high as 6 mm/yr on some thrust faults suggest episodicity within pulses. Episodic contraction on 10- to 30-m.y. intervals corresponds well to episodicity in relative plate motion, suggesting rapid response of intra-continental kinematics to plate-boundary interactions. The relationship is nevertheless indirect, possibly influenced by the position and number of plates, partitioning of displacements across accreted terranes, and crustal structure.