CONTROLS OF PULL-APART BASIN SIZE AND SUBSIDENCE RATE ON ARC MAGMATISM: EXAMPLES FROM THE ANCESTRAL CASCADES ARC, CALIFORNIA (Invited Presentation)

Pull-apart basins are common in continental arcs, and many volcanic centers are sited in such. We show for the first time that erupted composition, style and volume are linked to size and subsidence rates of the structural basins in which volcanic centers are contained. We establish this link using detailed mapping and ⁴⁰Ar/³⁹Ar geochronology from two Ancestral Cascade volcanic centers that are sited within adjacent Walker Lane transtensional rift pull-apart basins. All offsets and kinematic indicators show that these faults are oblique, and strain was not partitioned. Thus, we use subsidence rates as a proxy for strain rates. In both pull-apart basins, peak erupted volumes correspond to peak fault-controlled subsidence rates of 2000-3000 m/myr (an order of magnitude faster than orthogonal rift basins).

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.