UC Santa Barbara

Silicic large igneous provinces are significant in the geologic record, due to their unusually extensive areal coverage (>100,000 km^2) and large volumes (>250,000 km^3), and may be characteristic of continental regions undergoing broad lithospheric extension. The Sierra Madre Occidental of northwestern Mexico is the biggest and best-preserved silicic large igneous province of the Cenozoic and is considered part of the extensive mid-Cenozoic ignimbrite flare-up that affected much of the southwestern North American Cordillera. Despite its size and preservation, very little is known about the geology of the Sierra Madre Occidental, and the timing and spatial extent of ignimbrite flare-up volcanism in relation to crustal extension is relatively unknown. This study presents new geologic mapping, stratigraphy, zircon U-Pb laser ablation ICP-MS dating, modal analysis, and geochemical data from the Guazapares Mining District and Cerocahui basin regions, two adjacent areas of the northern Sierra Madre Occidental in western Chihuahua. The rock exposure and topographic relief in this previously unmapped ~450 km^2 area make it ideal for studying the relationships between silicic large igneous province volcanism and crustal extension.

Three informal formations are identified in the study area: (1) the ca. 27.5 Ma Parajes formation, a ~1-km-thick succession of primarily welded silicic outflow ignimbrite sheets erupted from sources within ~50-100 km of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) the ca. 27-24.5 Ma Témoris formation, composed primarily of locally erupted mafic-intermediate lavas and associated intrusions with interbedded alluvial deposits, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental following the Early Oligocene ignimbrite pulse; and (3) the ca. 24.5-23 Ma Sierra Guazapares formation, composed of silicic vent to proximal facies ignimbrites, lavas, plugs, and reworked equivalents that record the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The Guazapares Mining District and Cerocahui basin regions share this stratigraphy, but the rocks in the Cerocahui basin consist of a much higher proportion of alluvial deposits.

The main geologic structures in the Guazapares Mining District and Cerocahui basin regions are NNW-trending normal faults, with an estimated minimum of 20% total horizontal extension. Many normal faults bound half-graben basins that show evidence of syndepositional extension. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse of the ignimbrite flare-up to the east and before magmatism began in the study area. Preexisting normal faults localized mafic-intermediate volcanic vents of the Témoris formation and silicic vents of the Sierra Guazapares formation, and were active during deposition of these formations. In addition, the localization and timing of epithermal mineralization in the Guazapares Mining District appears to be favored where pre-to-synvolcanic extensional structures are in close association with Sierra Guazapares formation rhyolite plugs.

The timing of extensional faulting and magmatism in the Guazapares Mining District and Cerocahui regions is consistent with regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and extension in the northern Sierra Madre Occidental. Extension accompanied mafic-intermediate and silicic volcanism in the study area, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is likely a relationship between lithospheric extension and silicic large igneous province magmatism.