Guadalupe pluton–Mariposa Formation age relationships in the southern Sierran Foothills: Onset of Mesozoic subduction in northern California?

We report a new 153 ± 2 Ma SIMS U‐Pb date for zircons from the hypabyssal Guadalupe pluton which crosscuts and contact metamorphoses upper crustal Mariposa slates in the southern Sierra. A ∼950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 ± 2 Ma. Assuming a compacted depositional rate of ∼120 m/Myr, accumulation of Mariposa volcanogenic sediments, which overlie previously stranded Middle Jurassic and older ophiolite + chert‐argillite belts in the Sierran Foothills, began no later than ∼160 Ma. Correlative Oxfordian‐Kimmeridgian strata of the Galice Formation occupy a similar position in the Klamath Mountains. We speculate that the Late Jurassic was a time of transition from (1) a mid‐Paleozoic–Middle Jurassic interval of mainly but not exclusively strike‐slip and episodic docking of oceanic terranes; (2) to transpressive plate underflow, producing calcalkaline igneous arc rocks ± outboard blueschists at ∼170–150 Ma, whose erosion promoted accumulation of the Mariposa‐Galice overlap strata; (3) continued transpressive underflow attending ∼200 km left‐lateral displacement of the Klamath salient relative to the Sierran arc at ∼150–140 Ma and development of the apparent polar wander path cusps for North and South America; and (4) then nearly orthogonal mid and Late Cretaceous convergence commencing at ∼125–120 Ma, during reversal in tangential motion of the Pacific plate. After ∼120 Ma, nearly head‐on subduction involving minor dextral transpression gave rise to voluminous continent‐building juvenile and recycled magmas of the Sierran arc, providing the erosional debris to the Great Valley fore arc and Franciscan trench.