Sandstone matrix olistostrome deposited on intra-subduction complex serpentinite, Franciscan Complex, western Marin County, California

Detailed field mapping in the Franciscan Complex in western Marin County, California, offers insight into the origins and development of mélanges, as well as accretionary complex architecture. From structurally high to low, the study area units comprise a structurally high sandstone matrix olistostrome, serpentinite, a structurally intermediate sandstone matrix olistostrome, Early Cretaceous Nicasio Reservoir Terrane greenstone, and coherent Cretaceous greywacke, all exhibiting prehnite–pumpellyite burial metamorphism. The structurally high sandstone matrix olistostrome includes volcanic blocks up to 700 m in diameter and chert blocks up to 100 m in diameter, whereas the structurally intermediate olistostrome unit comprises a lithic-rich sandstone matrix enclosing volcanic blocks 1–20 m in diameter. The youngest detrital zircons in the high olistostrome and the coherent greywacke have U–Pb ages of ≈ 20 m.y. older than the inferred depositional ages of the units themselves, suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older, previously accreted sediments. The matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage. These findings demonstrate introduction of Franciscan exotic blocks and development of Franciscan block-in-matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix mélanges. Additionally, these data suggest that sandstone matrix olistostromes/mélange may constitute a larger part of the Franciscan and other accretionary complexes than previously believed.The structurally high olistostrome preserves a depositional contact on serpentinite, verifying an intra-subduction complex origin for the serpentinite. The serpentinite lacks block-in-matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe, rather than a sedimentary serpentinite deposit. Multi-kilometer scale west-vergent overturned folds with subhorizontal fold axes deform the nappes of the field area, and a later generation of open, northeast-trending folds overprints the earlier folds.

► We report sandstone matrix olistostrome units in the Franciscan Complex. ► A depositional contact atop serpentinite verifies intra-Franciscan serpentinite. ► Kilometer-scale overturned folds give insight into subduction complex architecture. ► Sedimentary processes may introduce exotic blocks to subduction complex units. ► We reinforce sedimentary interpretations for more deformed mélanges.