Late Jurassic dike swarms in the southwestern Sierra Nevada Foothills terrane, California: Implications for the Nevadan orogeny and North American plate motion

Abstract

A previously unrecognized sheared dike swarm has been identified in a southern fragment of the western Foothills terrane—the Owens Mountain area of the western Sierra Nevada foothills, northeast of Fresno, California. It may be the southern extension of the Bear Mountains fault zone. The dike swarm, sheeted in places, consists predominantly of tonalitic tabular bodies and coeval, mutually cross-cutting tonalitic and mafic dikes. Textures and fabrics within the dike swarm range from partially recrystallized igneous to strongly deformed metamorphic tectonites, implying that dike emplacement occurred during ductile deformation. Mylonitization has transposed layering parallel to foliation and has greatly thinned many of the dikes. Layering and foliation dip subvertically and strike NNW–SSE. Post-tectonic annealing has destroyed most microscopic shear indicators, but macroscopic intrafolial folds are common and have steeply southeast-plunging fold axes and S-fold geometries that may indicate a sinistral sense of shear. Age data (U-Pb zircon) from the tonalites reveal that emplacement and crystallization occurred over a 7-m.y. period, from 155 Ma to 148 Ma, at an estimated depth of 10 km (from AlTotal in hornblendes). A correlation between age and degree of deformation and recrystallization of the tonalites implies syntectonic dike emplacement. Intrusion began within 5 m.y. of deposition of the strata into which the dikes were emplaced. Granitic dikes that cut the complex at 123 Ma are nondeformed. The duration of the Nevadan orogeny is shown to have lasted from Late Jurassic to Early Cretaceous (160–137 Ma and possibly to 123 Ma) and thus is more protracted that has been postulated. The regional tectonics of the Owens Mountain and other Cordilleran dike swarms can be related in a broad dynamic sense to the absolute motion of North America by using the apparent polar wander (APW) analysis of May and Butler (1986). The Nevadan orogeny may be the manifestation of drastic changes in magnitude and direction of North American motion (from ~45 km/m.y. to the NNE to ~200 km/m.y. to the NW; May and Butler, 1986). The Late Jurassic dike swarms record a complex pattern of sinistral-sense transtension-transpression that may have developed at the J2 APW cusp (ca. 150 Ma; see May and Butler, 1986) and during subsequent, rapid northwestward acceleration of North America.