THE CENOZOIC IGNIMBRITE FLAREUP IN WESTERN NORTH AMERICA: DISTRIBUTION, TIMING, VOLUME, AND TECTONIC RELATIONS

Calderas of the ignimbrite flareup, the most voluminous Cenozoic magmatism in North America, form a semi-continuous belt from at least as far north as Oregon and Idaho southward at least to the southern end of the Sierra Madre Occidental. Peak activity was 37 to 22 Ma in all major parts of the belt (Sierra Madre Occidental, Great Basin, Southern Rocky Mountain Volcanic Field, Mogollon-Datil, Trans-Pecos Texas), and this period is generally accepted to constitute the flareup. However, caldera-forming ash-flow eruptions began as early as 49 Ma (Challis), 45 Ma (GB, SMO), and continued to at least 7 Ma in the GB. We estimate ~500-600 total calderas in the belt, mostly in the SMO, with a possible total erupted volume of silicic magma of ~600,000 km³. In all subareas, voluminous caldera magmatism migrated to the SW through time, which is best documented in the most thoroughly studied parts, SRMVF, MD, GB, and TPT. SW migration is consistent with generation of caldera-related magmas by rollback of the formerly shallow Farallon slab, leading to upwelling of athenosphere and heating of overlying hydrated lithosphere. The caldera belt goes around the Colorado Plateau, a unique crustal-mantle province, but small-volume felsic laccoliths and ultramafic diatremes in the eastern part of the plateau follow the time-space progression of caldera activity. The regular pattern of migration is consistent with rollback of the slab fragmented into a few major panels, not independent collapse of many small pieces. NE-striking boundaries between the SRMVF and GB and between SRMVF and MD-TPT approximately coincide with Farallon-plate transforms and also closely follow major lithospheric zones. Caldera magmatism mostly preceded major extension in the GB part of the Basin and Range Province, which began ~16-17 Ma, and in and south of the Rio Grande rift zone, where major extension began no earlier than ~30 Ma. The core SMO, having probably the greatest density of calderas, has undergone no to locally minor extension, and the caldera belt constitutes some of the least extended parts of the GB. Caldera magmatism, localized in thick crust, was a precursor to major extension in weaker crustal zones.