EVIDENCE FOR THREE DISTINCT METASEDIMENT EMPLACEMENT MECHANISMS IN AN ARC CRUST SECTION, SOUTHERN SIERRA NEVADA, CALIFORNIA

Metasediments are found in all continental arc sections. However, their origins remain unclear: sediments may be derived from the upper plate, either as pre-existing basement, or emplaced later through return flow or retro-arc underthrusting. Alternatively, sediments may be sourced from the down-going plate and delivered to the lower crust via relamination or subduction underplating.

We have conducted a detailed detrital zircon (DZ) study of metasediments in the southern Sierra Nevada, CA to evaluate these hypothesized emplacement mechanisms. The southern Sierra Nevada is an ideal location for this study: it preserves a continuous cross section of arc crust from ~3 kbars to >10 kbars and contains metasediments at all depths. Moreover, much of the magmatic section was built rapidly at ~100 Ma, and zircon in metasediments throughout the section have 100-105 Ma rims, indicating all metasediments were emplaced prior to arc shutoff. Finally, each emplacement mechanism predicts distinct sedimentary protoliths with unique and well-characterized DZ spectra.

Our DZ study reveals three distinct metasediment emplacement mechanisms with increasing depth in the southern Sierra Nevada. At shallow pressures, Mesozoic DZ dominate, similar to shallow roof pendants in the central Sierra Nevada, suggesting a similar emplacement via return-flow. Mid-crustal metasediments are dominated by Proterozoic zircon with no Mesozoic grains, similar to sedimentary rocks found east of the Sierra Nevada; these mid-crustal metasediments are likely samples of pre-existing basement. Finally, dominant Mesozoic peaks with subordinate Proterozoic populations characterize the DZ in lower crustal metasediments, analogous to the Pelona-Orocopia-Rand schists, indicating that lower crustal metasediments are derived from the down-going plate. Additionally, the lower crustal samples preserve an inverse stratigraphy, with the age of the Mesozoic zircon peak younging with increasing depth. This trend, along with emplacement prior to the termination of arc magmatism, strongly supports relamination over underplating as the dominant lower crustal emplacement mechanism. We explore the implications of these distinct processes for magmatic and tectonic processes in the Sierra Nevada, and for the formation of continental crust.