THE GREAT BASIN ALTIPLANO AND THE MYTH OF LARGE-MAGNITUDE REGIONAL EXTENSION DURING THE IGNIMBRITE FLAREUP

Many workers have concluded that by the early Cenozoic the area that is now Nevada and western Utah was a topographically high orogenic plateau underlain by unusually thick (60-70 km) crust created by episodes of contractile deformation since the mid-Paleozoic. Some workers believe this Great Basin altiplano was collapsing by large magnitude extensional faulting during the middle Cenozoic ignimbrite flareup from 36 to 18 Ma. To test whether regional tectonic extension did indeed accompany the flareup in the east central Great Basin we examined 36 stratigraphic sections that consist principally of ignimbrite, are mostly far removed from volcanic centers, and consist of ≥4 ignimbrite sheets in this age range. Conglomerate of rounded Paleozoic sedimentary clasts derived from nearby topographic highs is found in some sections below the ignimbrites and locally is interbedded within the lower part of the section. But only one section has an intercalated layer of eroded (or epiclastic?) ignimbrite clasts that might have been shed off a nearby uplifted fault block and one section has an inter-tuff angular unconformity of tens of degrees. All other sections comprise an essentially conformable stack of ignimbrite sheets that lack intercalated erosional debris and are therefore consistent with tectonic quiescence. If regional tectonic extensional faulting had occurred during the flareup to produce topographic highs and lows the great areal distribution of the ignimbrite sheets around their sources would not have been possible, especially ones only a few tens of meters thick. Chemical and Sr-isotopic compositions of 376 samples of intermediate composition lavas throughout the Great Basin extruded during the flareup have been compared to >6000 analyses of late-Cenozoic lavas in continental arcs around the world where the thickness of the crust is known. The Great Basin lavas clearly have compositions most like arcs founded on crust that is unusually thick (e.g., 60-70 km), as in the central Andes, and distinctly different from arcs on crust like the present ~30 km thickness. The geologic evidence indicates persistence of an altiplano whose crust was not thinned by large-magnitude, regional tectonic extensional faulting until mostly after the flareup.