Paleomagnetic evidence for Jurassic deformation of the McCoy Mountains Formation, southeastern California and southwestern Arizona

Abstract

The Mesozoic McCoy Mountains Formation is a 7.3-km-thick deformed clastic sequence exposed in six mountain ranges in southeastern California and southwestern Arizona. Interbedded with Jurassic volcanic rocks at its base, the McCoy Mountains Formation had been assigned a Cretaceous age based upon fossil angiosperm wood found in the upper third of the section. Characteristic natural remanent magnetism (NRM) from 145 oriented samples from 18 sites within the sedimentary terrane yield an in situ mean direction: I = 20.6°, D = 335.1°, α 95 = 7.7° (uncorrected for structural tilting). Opaque mineralogy and a failed fold test indicate that the NRM is a chemical remanence acquired post-folding. The paleomagnetic pole position calculated from the in situ mean direction falls adjacent to poles from the Summerville Formation and Canelo Hills Volcanics. We interpret these data to indicate that deformation, mild metamorphism, and resultant magnetization of the McCoy Mountains Formation occurred during Jurassic time. It is suggested that the McCoy Mountains Formation and underlying Jurassic volcanics were deposited adjacent to, and then deformed between, the North American craton and an outlying allochthonous terrane during Jurassic time.