BIRTH OF A PLATE BOUNDARY: TRANSTENSIONAL TECTONICS AND MAGMATISM, SIERRA NEVADA MICROPLATE AND GULF OF CALIFORNIA RIFT

We identify the key conditions that localized active transtensional continental rifts in the Gulf of California (Mexico) and along the eastern boundary of the Sierra Nevada microplate (California), and elucidate the geologic signals of transtensional continental breakup.

Geodetic studies show that the transtensional eastern boundary of the Sierra Nevada microplate (Walker Lane Belt) accommodates about 25% of the plate motion between North America and the Pacific plate (Unruh et al., 2003). This boundary is a reasonable approximation to a “classic” plate boundary, because it is discrete relative to its north and west boundaries, which are diffuse and complex due to structural interleaving by compressional and transpressional tectonics, respectively. The eastern microplate boundary is thus ideal for determining when the microplate formed, and for identifying the timing of features that signal the birth of a microplate. This has important implications for understanding the processes involved in the rupturing of continental lithosphere, similar to the well-funded MARGINS study in the Gulf of California (Lizarralde et al., 2007). Much of the geologic record of the early rift stage of the latter now lies below sea level in the Gulf of California, making it difficult and expensive to study in detail. In contrast, rocks and structures of the early rift stage are superbly exposed on land along the eastern boundary of the Sierra Nevada microplate. Although rifting has not yet succeeded in forming new sea floor there, we suggest that it 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. Rifting led to seafloor spreading very quickly in the Gulf of California (by 6 Ma), because the spreading center between the plate subducting under Mexico and the huge Pacific plate froze offshore of Mexico; the Pacific plate then pulled the dead slab northwestward, dragging the upper plate of the slab (Baja California) with it. In contrast, continental breakup is still in progress in California.

We use new geologic map data from the central Sierra Nevada to identify features that signal the birth of the Sierra Nevada microplate at ~12 - 11 Ma. This timing is consistent with the plate-tectonic reconstructions of McQuarrie and Wernicke (2005), which 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. Releasing transtensional stepovers (or pull-aparts) along the plate boundary began to control the siting of large volcanic centers by ~ 11 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 ~ 11 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 Sierra Nevada 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”, a high broad uplift formed by crustal shortening during Cretaceous low-angle subduction; this shoulder 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.

The geologic signals of transtensional continental breakup described here must be taken into account when constructing theoretical, experimental, or geophysically-constrained models for the rupture of continental lithosphere.