Controls of Walker Lane Belt Pull-aparts on the Plumbing System in the Ancestral Cascades Arc, Central Sierra Nevada, California

Extinct volcanic fields exposed by erosion or tectonism provide the opportunity to study structural controls on volcano-tectonic basins, vents, and plumbing systems, on a timescale of millions of years. The Miocene-Pliocene Ancestral Cascades arc in the central Sierra Nevada is spectacularly well-exposed in three dimensions over rugged topography with high structural relief. The Miocene ( 12 to 5 Ma) Sierra Crest-Little Walker volcanic center and the Miocene-Pliocene ( 6 to 4.6 Ma) Ebbetts Pass volcanic center formed in pull-apart basins in the Walker Lane belt (WLB), a NNW zone of dextral strike-slip and oblique normal faults at the western edge of the Basin and Range. The Sierra Crest-Little Walker volcanic field is as large as the active Long Valley volcanic field and the Ebbetts Pass volcanic center is as large as the active Lassen volcanic field (which also lie within the WLB). Petrographic, geochemical and geochronological data are used to understand these volcanic centers, but detailed mapping of volcanic/subvolcanic lithofacies and faults forms the core of these studies. We present a time-slice series of block diagrams that illustrate the structural controls on the Ancestral Cascades arc plumbing system in the early stages of WLB transtension. Extreme transtension in the very large Sierra Crest-Little Walker pull-apart triggered rapid ascent of low-degree partial melts, causing outpouring of high-K2O lava "flood andesites" along fissure vents that resemble flood basalt vents. These fissure vents were plumbed up NNW dextral-oblique normal faults that bound the largest graben; growth faulting relations demonstrate that the focus of subsidence shifted over the field with time synchronous with eruption from the fissure vents. Smaller, point-source vents were plumbed through faults bounding smaller NNW half grabens, as well as a series of NE-SW sinistral-oblique normal faults that define a major transfer zone. The area of maximum transtension ultimately accommodated accumulation of silicic magmas in shallow crustal levels, producing the Little Walker Caldera. The smaller Ebbetts Pass pull-apart, in contrast, plumbed early lavas through basin-margin faults that became less active with time, resulting in growth of a central volcanic edifice that eventually filled the basin margin and grew beyond it.