BASIN AND RANGE FISSURE-SOURCE ORIGIN FOR VOLUMINOUS IGNIMBRITES OF THE SIERRA MADRE OCIDENTAL, MEXICO

The Sierra Madre Occidental (SMO) is the largest continuous ignimbrite province in the world. It is at least 1,200 km long and 200-500 km wide, from the U.S.-Mexico border (31°N) to its intersection with the Mexican Volcanic Belt (21°N), from where SMO-age ignimbrites continue into southern Mexico as discontinuous outcrops. Considering the average thickness of 1,000 m for the ignimbrite plateau, a conservative estimate of the physical volume of the SMO ignimbrites is about 360,000 km3 (1,200x300x1 km). Swanson and McDowell (1984) estimated that at least 350 calderas the size of those at San Juan Volcanic Field in Colorado, would be needed to have produced these ignimbrites. However, less than ten calderas have been identified in the SMO. Field evidence indicates that much of the ignimbrite´s volume was related to fissure-type eruptions, most of them coinciding with Basin and Range faults. These fissures were several kilometers long, and similarly as the caldera-related conduits, are marked by rhyolite or pyroclastic-flow dikes, aligned lava domes, and by elongated co-ignimbrite lithic-lag breccias occuring next to Basin and Range faults. Considering that the Basin and Range extensional orogeny coincided in time and space with the ignimbrite flare-up, we propose a model in which batholith-sized magma chambers reached shallow crustal levels, and were evacuated when Basin and Range normal faults cut the roof of these chambers; this caused fast decompression when the system was opened, and erupted explosively voluminous silicic magmas through these faults. Finally, devolatized rhyolitic magmas were emplaced as domes or dikes. Both types of sources, calderas and fissures occurred in the SMO, with fissures apparently predominating in areas more affected by extension (the margins of SMO and the whole southern SMO at Tepíc-Zacatecas-San Luis Potosí), and calderas occurring in relatively less extended areas (the core of SMO at Chihuahua-Sonora).