Mesozoic transpression,transtension, subduction and metallogenesis in northern and central California

Middle Paleozoic to Middle Jurassic terrane assemblies in the Klamaths and Sierran Foothills consist of mafic–ultramafic complexes + fine‐grained terrigenous strata derived from previously accreted continental‐margin belts. Sutured oceanic terranes reflect c. 230 Myr of margin‐parallel slip involving chiefly transtension and transpression. Quartzofeldspathic clastic rocks and blueschists ± eclogites are very rare. Little devolatilization occurred at magmagenic depths; hence, coeval hydrothermal ore deposits and granitoids are uncommon. In contrast, nearly head‐on Cretaceous subduction of the Farallon plate generated the massive Klamath–Sierra Nevada volcanic–plutonic arc, reflecting dewatering of the eastward descending oceanic lithosphere in the magmagenic zone. Immature Great Valley forearc and Franciscan trench deposits shed from the arc record c. 70 Myr. of rapid crustal growth. Au‐bearing solutions rising from magmagenic depths, exsolved from plutons, and expelled from heated wall rocks were mobilized attending arc construction. Precipitation of gold‐bearing quartz veins occurred where H₂O + CO₂‐bearing fluids encountered major geochemical discontinuities in the wall rocks.