The Quaternary and Pliocene Yellowstone plateau volcanic field of Wyoming, Idaho, and Montana

Early Permian volcanic sections preserved in north-central Queensland to lie within volcanic subsidence structures; many of these features clearly are volcanic cauldrons or subvolcanic ring structures, but some are quite large graben-like structures. Oversby and others (1980) later pointed out that these large linear blocks are not all necessarily related to specific eruptive events but are more complex synvolcanic subsided blocks. They regarded the blocks as first-order volcano-tectonic subsidence features that include the individual cauldrons, which they considered to be second-order volcanic subsidence structures. Mackenzie (1987; 1993; Mackenzie and others, 1993) further emphasized the extensional tectonic setting of the volcanism and the range of styles of associated subsidence—graben-like, sag-like, and ring-faulted. The volcanic history and structural pattern of the Yellowstone-Snake River Plain region suggests the further possibility that some of the large linear blocks of northern Queensland, largely filled with volcanic deposits, may be extensional tectonic features analogous to the downdropped fault blocks of the region around Yellowstone and the eastern Snake River Plain, in which are preserved sections of welded tuffs erupted from the associated ring and cauldron complexes and emplaced as ash flows into adjacent tectonic fault basins. Some of the larger subsided blocks might have been synchronous with volcanism but related to regional tectonic stresses rather than to subsidence resulting directly from the transfer of magma from the crust to the surface. The associated cauldron complexes would represent the subvolcanic parts of calderas related to individual volcanic cycles climaxed by major ashflow eruptions. Furthermore, some of the associated granites might be cogenetic late-stage discordant plutons such as I have suggested to be associated with the polybatholithic complex beneath Yellowstone.