LATE OLIGOCENE MAGMATISM AND EXTENSIONAL BASIN DEVELOPMENT DURING THE MID-CENOZOIC IGNIMBRITE FLARE-UP IN THE NORTHERN SIERRA MADRE OCCIDENTAL SILICIC LARGE IGNEOUS PROVINCE, CHIHUAHUA, MEXICO

The Guazapares-Cerocahui region of the northern Sierra Madre Occidental (SMO) is located adjacent to the relatively unextended core of the SMO to the east and the highly extended Gulf Extensional Province further to the west. Detailed geologic mapping and reconnaissance of a ~40 x 15 km W-E transect in this region identified numerous NNW–trending normal faults, many with vertical offsets up to ~0.5 km, with an estimated minimum of 20% total horizontal extension in the study area. The spatial distribution of faults changes along transect, with many closely spaced (~1-4 km) faults with lesser offset in the west near Guazapares, to wider spaced (~5-10 km) faults with greater offset in the east near Cerocahui.

Several normal faults bound half-graben basins with evidence of Late Oligocene synextensional deposition of volcanic and alluvial rocks. These rocks are divided into three informal formations in the study area: 1) the ca. 27.5 Ma Parajes formation (primarily welded silicic outflow ignimbrite sheets); 2) the ca. 27–24.5 Ma Témoris formation (primarily mafic-intermediate lavas and associated intrusions, alluvial deposits, and distal nonwelded silicic ignimbrites), and 3) the ca. 24.5–23 Ma Sierra Guazapares formation (silicic vent to proximal ignimbrites, lavas, and plugs).

Volcanism accompanied extension in the Guazapares-Cerocahui region and temporally overlapped with the peak of the mid-Cenozoic ignimbrite flare-up in the SMO, suggesting a general relationship between extension and silicic large igneous province magmatism. Normal faulting began by ca. 27.5 Ma, during deposition of the youngest outflow ignimbrites of the Parajes formation, but prior to magmatism in the study area. Subsequent mafic-intermediate lavas of the Témoris formation ponded in half-graben basins and on adjacent footwall blocks; continued extensional basin development resulted in uplift and erosion of these lavas and the underlying Parajes formation, which provide the primary sediment source for Témoris formation alluvial deposits. Following much of the half-graben deposition and mafic-intermediate magmatism in region by ca. 24.5 Ma, silicic fissure volcanic vents of the Sierra Guazapares formation were localized along normal faults, with rhyolitic plugs intruding along many of the faults in the region.