GEOLOGIC SIGNALS OF THE INITIATION OF CONTINENTAL RIFTING WITHIN THE ANCESTRAL CASCADES ARC

The transtensional eastern boundary of the Sierra Nevada microplate is a “classic” discrete plate boundary and so provides an ideal natural laboratory for studying processes that result in the rupture of continental lithosphere. We suggest that this boundary has many features in common with the Gulf of California rift, including: (1) timing of initiation, at about 12 Ma; (2) localization of rifting due to thermal weakening in the axis of a subduction-related arc undergoing extension due to slab rollback; and (3) enhanced thermal weakening in the arc, due to stalling of the trenchward-migrating precursor arc against a thick Cretaceous batholithic crustal profile on its western boundary. Much of the geologic record of the early arc-rift stage of the Gulf of California now lies below sea level, but structures and volcanoes of the arc-rift stage are superbly exposed on land along the eastern boundary of the Sierra Nevada microplate.

The geologic signals of continental breakup within the axis of the ancestral and modern Cascades arc include:

(1) Siting of large arc volcanic centers on releasing transtensional stepovers. These include the ~ 9 Ma Little Walker Caldera, the newly identified ~ 5 Ma Ebbetts Pass Volcanic Center, and the ~3 Ma to Recent Lassen Volcanic Center (Muffler et al., AGU abstr, 2008), as well as Long Valley Caldera (Bursik, Geofísica Internacional, 2009), which is not an arc volcanic center.

(2) Accumulation of unusually large-volume, widespread volcano collapse deposits in grabens and rectilinear volcano-tectonic subsidence structures. This is because vents were sited along very active NNW-trending and lesser NE-trending faults, and volcanoes collapsed repeatedly down fault scarps and through lateral ramps into grabens.

(3) Extreme effusive eruptions along fault-controlled fissures, including intermediate-composition fissure eruptions of “flood lava”. Trachyandesites and trachybasaltic andesites were erupted from 6–8 km long fissures within volcanotectonic depressions that currently lie along the Sierra Nevada range crest and range front, forming a >200km³ lava flow field in only 28-230 kyr. Lesser volumes of trachydacite and basalt also erupted from faults.

(4) Abrupt derangement of ancient E-W drainage systems on the western flank of the “Nevadaplano”, by development of north-south grabens.