Abstract
The Caopas, Rodeo, and Nazas formations exposed in the San Julián uplift of northern Zacatecas are distinguished principally on the basis of style and intensity of deformation; they are parts of the same early Mesozoic(?) volcanogenic suite. This suite was the source for overlying volcaniclastic conglomerate and sandstone (La Joya Formation) that appears transitional into succeeding Late Jurassic (Oxfordian?) Zuloaga Limestone. Deformation that was contemporaneous with the deposition of the lower part of the Zuloaga produced an asymmetrical northwest trending fold nappe that was driven southwestward. The massive quartz‐porphyry core of the structure (Caopas Formation) moved somewhat independently of the encasing, more ductile rocks (Rodeo and Nazas formations). Phyllonite developed in the nose of the nappe; the upright limb was attenuated and ruptured. In zones where deformation due to the nappe‐forming process was weaker, a gently dipping foliation, possibly related to an older episode of recumbent folding, is preserved. Orientation, age, and location of the nappe suggest that it is a manifestation of transpressional stress developed along the Mojave‐Sonora megashear as left‐lateral movement carried the Jurassic arc toward the southeast.
Published Version
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