Fissure ignimbrites:: Fissure-source origin for voluminous ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting

The Sierra Madre Occidental is mostly composed of middle Tertiary large-volume ignimbrites. From the United States-Mexico border (similar to31degreesN), the Sierra Madre Occidental extends southward to its intersection with the Mexican volcanic belt (similar to21degreesN). Ignimbrites of equivalent age extend into southern Mexico as discontinuous outcrops. Considering the average thickness of 1000 m for these ignimbrites based on representative measured sections, a conservative estimate of their total volume is similar to393,000 km(3). Fewer than 15 calderas have been identified in this province, and the source of most of these ignimbrites has been an unsolved problem. We present geologic evidence indicating that fissures, most of them with the regional trend of Basin and Range faults, served as conduits for the ignimbrites. These fissures can be several kilometers long and are represented by pyroclastic (ignimbrite) dikes, rhyolitic lava dikes, linearly aligned lava domes, and elongated coignimbrite lithic-lag breccias adjacent to Basin and Range faults. Considering that the Basin and Range extension overlapped in time and space with the ignimbrite flare-up, we propose a model in which batholith-sized magma chambers reached shallow crustal levels and erupted their contents when they reached Basin and Range normal faults. The faults acted as vents and caused fast decompression when the system was opened, and large volumes of silicic magmas were explosively erupted. Finally, devolatilized rhyolitic magmas were emplaced as domes or dikes. We propose the term "fissure ignimbrites" for ignimbrites formed in this way.