EVIDENCE FOR FOREARC BASIN INITIATION FROM THE GREAT VALLEY FOREARC, CALIFORNIA

Forearc basins often preserve the long-term record of the tectonic history of a convergent margin, yet the fundamental tectonic mechanisms driving forearc basin initiation remain enigmatic. We present new field observations, petrographic, geochronologic, and thermochronologic data that bare on the age and processes that drove the development of the Great Valley forearc (GVF) basin, California. In the northernmost GVF, a 200-1000 m-thick matrix-supported pebble-cobble polymictic conglomerate and ophiolitic breccia separate forearc strata from its underlying basement, the Coast Range Ophiolite (CRO). Detrital zircon U-Pb age spectra of the upper breccia are dominated by Jurassic age grains, with subordinate pre-Mesozoic age populations. Its maximum depositional age (MDA), derived from the youngest U-Pb zircon age population, is 151.4 ± 0.53 Ma, ~ 5 Myr older than overlying GVF strata. We interpret the formation of the ophiolitic breccia to be deposited by cohesive or plastic debris flows that resulted from localized normal faulting proximal to North America, reflecting localized dismemberment and uplift of the middle Jurassic CRO. This hypothesis is consistent with synsedimentary normal-fault systems <10 km to the north that were active by ~151 Ma, and may provide a mechanism for early basin development. Additional evidence for extension during the early stages of forearc development is found where sedimentary serpentinite bodies intermingle with forearc stratigraphy. A volcanic clast from a forearc detrital serpentinite deposit yields a weighted mean age of 145.00 ± 1.02 Ma. Zircon (U-Th)/He dates from this deposit range from 126.66 ± 1.63 to 123.55 ± 1.61 Ma, which is younger than the MDAs of surrounding and overlying forearc strata. Specifically, a sandstone directly overlying the CRO exposed 7 km to the North yields a MDA of 145.4 ± 0.63 Ma. Together, this indicates the volcanic clast was likely part of the serpentinite body (versus derived from the proximal Sierra Nevada) and emplacement of the serpentinite was coeval with early forearc development. We propose these serpentinite bodies are analogous to the modern Marianas forearc where serpentinite mud volcanoes are found along normal faults and reflect extension during convergence.