CONTROLS OF PULL-APART BASIN SIZE AND SUBSIDENCE RATE ON ARC MAGMATISM: EXAMPLES FROM THE ANCESTRAL CASCADES ARC, CALIFORNIA (Invited Presentation)
The 12 to 5 Ma Sierra Crest-Little Walker arc volcanic center and pull-apart basin is as large (~4,000 km2) as the active Long Valley rift volcanic center and pull-apart basin, and similarly contains a caldera over part of the field. It formed by rifting under dextral shear at a 15 km long releasing right step. Peak strain rates coincide with the eruption of large-volume, high-K2O lavas, whose eruption rates exceed all Ancestral or modern Cascade volcanoes. These “flood andesites” were erupted from fault-controlled fissure vents scattered across the large field.
The 6.4 to 4.5 Ma Ebbetts Pass arc volcanic center and pull-apart basin is smaller, comparable in volume to the active Lassen arc volcanic center and pull-apart basin at the present day Walker Lane transtensional rift tip. It formed at a 7 km releasing right step. During initial low strain, magma output was low and restricted to basin bounding faults, and during high strain, a large central cone filled the basin. This center erupted more evolved (dacite-rhyolite) and less alkalic lavas, perhaps because the smaller basin experienced lower tensile stresses, and magmatic dikes could not extend deeply enough to tap mafic magmas, or low-degree partial melt, stored in the mid- to lower-crust.
These Miocene-Pliocene centers reveal structural controls that cannot be observed at active volcanoes, where the relevant structures are buried. We suggest that transtensional rift settings may uniquely provide rapid magma throughput.