DISTRIBUTION, TIMING, AND CONTROLS OF THE MID-CENOZOIC IGNIMBRITE FLAREUP IN WESTERN NORTH AMERICA

Geologic and Ar-Ar data reveal regional patterns of timing and migration of caldera activity during the North American Cenozoic ignimbrite flareup. Caldera-forming ash-flow eruptions began as early as 49 Ma (Challis), 45 Ma (Great Basin, Sierra Madre Occidental), and continued to at least 7 Ma in the GB. Peak activity was 37 to 22 Ma in all provinces (SMO, GB, Southern Rocky Mountain Volcanic Field, Mogollon-Datil, Trans-Pecos Texas), diminishing as regional extension became dominant. Based on geologic mapping, tuff stratigraphy, geochronology, and estimates from outcrop area (SMO), 500-600 calderas formed from the southern SMO to the northern GB. Approximate numbers of calderas are SMO (300-400), GB (120), SRMVF (31), MD (28), and TPT (17). In all areas caldera magmatism generally migrated to the SW through time. Gaps in caldera magmatism in SRMVF, MD, and TPT (31.4-29.3; 26.8-25.1) were times of major activity in the GB. The most voluminous parts (SMO, GB) form NW-striking belts parallel to the subduction zone, align when later extension is restored, but are separated by the amagmatic Colorado Plateau. MD and TPT form a NW belt farther NE, and SRMVF is farthest NE. The variation in distance between caldera belts and paleotrench is ascribed to segmentation of the subducted Farallon slab. The SW migration is consistent with generation of caldera-related magmas by rollback of the shallow slab leading to upwelling of athenosphere and heating of overlying hydrated lithosphere. The regular pattern of migration is consistent with rollback of a slab fragmented into a few major panels, not independent collapse of many small pieces. NE-striking boundaries between GB and SRMVF and between SRMVF and MD approximately coincide with Farallon-plate transforms and also closely follow major lithospheric discontinuities. Contemporaneous SW migrations across TPT and SMO suggest that the slab broke along a NW trend into two parts that rolled back independently. Broad similarities of the disparate loci on a continental scale, yet complex space-time compositional relations when examined closely, are inferred to reflect overlapping influences of late Cretaceous compressional geometry, Precambrian structural features in North American lithosphere, and evolving structures in the subducted slab as compression gave way to extension.