Joint 70th Rocky Mountain Annual Section / 114th Cordilleran Annual Section Meeting - 2018

Paper No. 46-5
Presentation Time: 2:50 PM


HOWARD, Keith A., U.S. Geological Survey, 345 Middlefield Rd, MS/973, Menlo Park, CA 94025 and FERGUSON, Charles A., Arizona Geological Survey, PO Box 210184, Tucson, AZ 85721

The pioneering works of Spencer and of McClelland in the Sacramento Mts provided the framework for newer studies resulting in 3 principal new observations: 1) 14.3-Ma rhyolite lava overlies the main detachment fault (Simpson et al., 1991) and early Miocene rocks. 2) Early Miocene (~20-17.5 Ma) volcanic rocks and strata are structurally detached from the Stepladder Mts on the W and the Sacramento Mts metamorphic core on the east, and 3) they occur in two main fault-bounded detached SW-tilted panels that include fragments of the source caldera of the 18.8-Ma Peach Spring Tuff (PST). The PST outflow facies ignimbrite grades southward in these panels into intracaldera (mega- and meso-breccia bearing) facies. The transition in both panels, interpreted as the caldera’s margin, lies on an extensional vector of 035°, consistent with slickenlines on the Sacramento detachment and an extension vector that would restore the segments to continuity with the main caldera in the Black Mts 50 km to the NE (Ferguson et al., 2013). The more northeasterly panel, nearly horizontal, appears to be a remnant that first was tilted northeasterly and subsequently was rotated back. Early Miocene strata in the structurally lower more westerly panel dip steeply SW and exhibit a large cutoff angle with their underlying detachment, implying the panel and fault are strongly tilted. Miocene plutonic equivalents of the tilted PST are lacking from basement exposed in these panels. But the Sacramento Mts core, which restores structurally to the NE, has dated PST-age plutons (Pease et al., 1999) that could relate to the PST. Thick conglomerate and sedimentary megabreccia that accumulated during major post-PST extensional faulting are seen now to overlie the volcanic rocks stratigraphically. The new perspectives support (1) the PST caldera correlation, (2) the surprising NNE extensional dismemberment of the caldera, and (3) strong tilting of upper-plate panels and their underlying detachment.