BIRTH OF A PLATE BOUNDARY

The transtensional eastern boundary of the Sierra Nevada microplate is a “classic” discrete plate boundary that provides an ideal natural laboratory for studying the rupture of continental lithosphere. Regional-scale plate-tectonic reconstructions show a change from more westerly motion to more northerly motion of both the Pacific plate and the Sierra Nevada microplate, relative to the Colorado Plateau, at 10-12 Ma. We use new geologic map data from the central Sierra Nevada to identify features that signal the birth of the Sierra Nevada microplate at ~10.5 Ma.

Releasing transtensional stepovers (or pull-aparts) along the plate boundary began to control the siting of large volcanic centers by ~ 10.5 Ma, and continue to do so today (e.g. Long Valley Caldera and Lassen Volcanic Center). Onset of transtension within the axis of the Ancestral Cascades arc at ~ 10.5 Ma resulted in “flood andesite” eruptions over a >40 km long segment of the new plate boundary, in the Sonora Pass to Bridgeport region (the largely trachyandesitic “Table Mountain Latite/Formation”). These “flood andesites” were erupted from 6–8 km long fissures within volcanotectonic depressions that currently lie along the Sierra Nevada range crest and range front. Individual lavas flowed distances up to 130 km, with volumes up to 20 km³, and the >200km³ lava flow field was erupted in only 28-230 kyr. The Little Walker Caldera formed at the site of maximum extension in this volcanotectonic lava depression complex, and erupted ~9.5 – 9.4 Ma large-volume trachydacite ignimbrites.

The birth of the plate boundary was also signaled by derangement of ancient drainage systems. For much of Cenozoic time, the present-day Sierra Nevada formed the western shoulder of the “Nevadaplano” and was crossed by E-W paleocanyons/paleochannels carved into the Nevadaplano, with a drainage divide >230 km to the east. Our mapping demonstrates progressive derangement of this ancient E-W drainage system by N-S plate boundary structures, including: (1) diversion of lavas and pyroclastic flows into N-S grabens at ~11-9 Ma, (2) stream capture at the western edge of the volcanotectonic graben complex, on the modern range crest, at ~9 – 6 Ma, and (3) stream capture at the eastern edge of the volcanotectonic graben, at the base of the range front, by the modern Little Walker River after 6 Ma.