Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico

Abstract

The Cretaceous Zarza Intrusive Complex, located in the Peninsular Ranges of Baja California Norte, Mexico, is perhaps the bestpreserved multiple-center, cone-sheet‐bearing ring complex documented in North America. The 7 km 2 elliptical complex hosts three nested, non-concentric intrusive centers that are successively younger to the south. The northern and central centers show the same evolutionary sequence of (1) intrusion of concentric gabbroic cone sheets, (2) intrusion of massive core gabbros, and (3) development of subvertical, ductile ring faults. Ring-fault kinematics indicate that both centers moved down relative to the surrounding country rocks, suggesting collapse into an underlying magma chamber. The southern center is composed of approximately equal proportions of gabbro and tonalite and lacks cone sheets. Aluminumin-hornblende barometry on the tonalite indicates a maximum emplacement depth of 2.3 ± 0.6 kbar. The Zarza Intrusive Complex is surrounded by a ductile deformation aureole, and bedding is inward dipping and inward younging around the entire complex. Excellent preservation of the intrusive history allowed us to evaluate the origin of the aureole, and the three most applicable models are (1) collapse of the complex into its underlying magma chamber, (2) sinking of the complex and its chamber after solidification, and (3) formation of the aureole prior to emplacement of the complex. The preserved structural and intrusive relationships provide information on the dynamic evolution of subvolcanic magma chambers and suggest that the complex may have been overlain by a caldera.