THE EVOLUTION AND TECTONIC SETTING OF THE EOCENE-MIOCENE IGNIMBRITE FLARE-UP OF THE GREAT BASIN, NEVADA AND UTAH

The well-established early to mid Cenozoic southward sweep of magmatism into and across the Great Basin was dominated by voluminous caldera-forming, silicic ash-flow tuff eruptions. Our work and abundant published data reveal a strong temporal-spatial pattern to the more than 100 caldera-forming eruptions between ~40 and 7 Ma in Nevada and Utah (excluding ~16-15 Ma Yellowstone hotspot calderas in NW NV). Early magmatism (~46-36 Ma) was in NE NV and western UT and dominated by arc-like intermediate volcanism that formed few (≤6) calderas. Most caldera-forming eruptions occurred between ~36 and 23 Ma in a distinctly bounded, ~200- x 600-km, NW-SE-E belt from west-central NV to southwest UT. Caldera activity and other magmatism migrated SW during this time, with caldera-forming eruptions averaging 5-7/Ma (peaking ~30 to 23 Ma). Following an apparent hiatus between ~22 and 19 Ma, caldera activity resumed with ~14 calderas forming between 19 and 14 Ma, mostly in the Caliente caldera complex, and ~8 in the SW NV volcanic field between ~15 and 7 Ma. Although rarely considered part of the earlier “ignimbrite flare-up”, calderas of the SW NV field are spatially and temporally continuous with the older activity.

The relative influence of crustal structure and mantle input on caldera distribution is unclear. Focus of caldera activity in a distinct belt and arc-like intermediate magmatism outside the belt suggest crustal influence. E.g., SW migration of caldera activity stopped at the NE edge of what is now the late-Cenozoic (≤12 Ma) Walker Lane, probably a Mesozoic strike-slip zone, while coeval and younger intermediate activity continued to the SW. However, the caldera belt in western NV extends across the craton–oceanic crust or rifted craton boundary as indicated by isotopic data (e.g., Sri 0.706). The early (46-36 Ma) intermediate activity in the craton generated few calderas. More data are needed on magmatic volumes through time.

A variety of emerging data sets indicate that calderas of the “ignimbrite flare-up” formed on a high plateau underlain by thick crust that was not substantially broken by Basin and Range extension until the middle Miocene. Caldera magmatism in the Great Basin shows similar timing and non-relation to extension to that in the Sierra Madre Occidental and other fields of western North America.