UPPER PLATE VERSUS LOWER PLATE OROGENIC COMPONENTS AND POST-SUBDUCTION ATTENUATION OF PRIMARY ARCHITECTURE (Invited Presentation)

Rocks of convergent plate margins comprise upper plate units that resided above a subduction zone during their formation, contrasted with lower plate units of subducting plate that were subsequently accreted to the upper plate when the subduction interface cut into the lower plate; lower plate units collectively comprise a subduction complex. The overwhelming volume of the lower plate subducted to the mantle; the accreted slices of lower plate oceanic material are the sole direct record of this plate. Post-subduction deformation and metamorphism mask the distinction between upper and lower plate components in many orogenic belts, and as a result obscure the record of subduction along the plate interface. Western California hosts relatively intact components that have escaped such modification. Most of the world's large ophiolite complexes formed in an upper plate (supra subduction zone or SSZ) setting and do not represent remnants of the larger oceanic plates that subducted. The latter form much smaller fault-bounded sheets within subduction complexes; such units commonly exhibit MORB or OIB geochemical affinity. The thickness of the igneous parts of such slices is commonly 500 m or less, in contrast to the SSZ ophiolite remnants that reach km in thickness. Large olistostrome blocks in lower and upper plate units further complicate evaluation of lower plate versus upper plate units, even in the absence of post-subduction structural-metamorphic overprinting. The largest such blocks may attain sizes in the km range for forearc basin (upper plate) deposits and 100s of m for subduction complex units. Olistostromal blocks are dominantly SSZ, derived from exhumed upper plate rocks, but MORB or OIB affinity blocks, derived from exhumed lower plate rocks may also be present in sedimentary melange horizons of both upper and lower plate units. In addition to structural-metamorphic overprinting, post-subduction tectonic processes also appear to attenuate upper and lower plate units; such processes may include extensional deformation, strike-slip truncation, and surface and subduction erosion. As shown by the thin (mostly <5 km) coeval upper plate and lower plate units of the northern Sierra Nevada, California and the narrow apparent arc-trench gaps preserved therein, such attenuation characterized non-collisional as well as collisional orogenic belts.