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Published August 1992 | public
Journal Article

Jurassic Cordilleran dike swarm-shear zones: Implications for the Nevadan orogeny and North American plate motion

Abstract

A cogenetic and coeval tonalitic and mafic dike swarm has been identified within a southern fragment (the Owens Mountain area) of the western Foothills terrane (California). The dikes were mylonitized and transposed (rotated into subparallel orientation) during emplacement, from 155 to 148 Ma (U-Pb zircon data), which coincides in time with the Nevadan orogeny. Steeply southeast-plunging fold axes and S-fold geometries indicate a sinistral-sense of shear, possibly with some dip-slip motion as well. This shear zone may be the southern and possibly deeper extension of the Bear Mountains fault zone. This and other Late Jurassic Cordilleran dike swarms record a complex pattern of sinistral-sense transtension-transpression that developed at the apparent-polar-wander J2 cusp (∼150 Ma) and during subsequent, rapid, northwestward acceleration of North America. The Late Jurassic Nevadan orogeny is a manifestation of these dramatic changes in magnitude and direction of North American motion.

Additional Information

© 1992 Geological Society of America Manuscript received December 20, 1991; Revised manuscript received April 13, 1992; Manuscript accepted April 22, 1992. Supported by National Science Foundation grants EAR 8904063 and EAR 9105692. The field work could not have been done without the permission and hospitality of ranch owner Morgan Blasingame. The mapping in the present study was based in part on unpublished reconnaissance work done by Bill Power. North American plate motion vectors were calculated using programs and suggestions from Joe Kirschvink. We thank Richard Gordon and John Oldow for helpful reviews. Caltech contribution no. 5149.

Additional details

Created:
August 22, 2023
Modified:
October 17, 2023