GUADALUPE PLUTON-MARIPOSA FORMATION AGE RELATIONSHIPS IN THE SOUTHERN SIERRAN FOOTHILLS: ONSET OF LATE MESOZOIC PLATE CONVERGENCE IN NORTHERN CALIFORNIA

We report a new 153 ± 1 Ma SIMS U-Pb date for zircons from the Guadalupe hypabyssal pluton that confirms its prior TIMS U-Pb emplacement age (Saleeby et al., 1989). This granitic body crosscuts upper crustal Mariposa slates in the southern Sierran Foothills. The igneous age calls into question the onset of Mariposa deposition at a maximum age of 152 ± 2 Ma inferred by Snow and Ernst (2008) based on detrital zircon SIMS U-Pb geochronology. A section of dark metashales ~950 m thick lies below sandstone horizons from which the clastic zircons were analyzed. Assuming a relatively high (compacted) depositional rate of ~120 m/Myr, sedimentation of Mariposa dark shales probably commenced at ~160 Ma. Regionally, the Mariposa volcanogenic sediments—with Logtown Ridge-Gopher Ridge extrusives beneath and interlayered Copper Hill volcanics above—overlap previously stranded Middle Jurassic and older ophiolite + chert-argillite belts in the Sierran Foothills; correlative Oxfordian-Kimmeridgian volcanogenic 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 chiefly but not exclusively dextral strike-slip and docking of oceanic terranes, with a brief period of underflow producing calcalkaline igneous rocks + blueschists at ~170 Ma; to (2) a plate junction step-out at ~160-155 Ma and formation of an incipient arc + Mariposa-Galice erosional overlap strata attending onset of transpression; then (3) continued underflow during ~200 km left-lateral displacement of the Klamath Mountains salient at ~155-145 Ma (time of the J2 cusp in the apparent polar wander path of North America); followed by (4) nearly orthogonal convergence during the mid and Late Cretaceous. The latter gave rise to voluminous continent-building juvenile magmas of the Sierran arc and its erosional products, the Great Valley forearc and coeval Franciscan trench deposits.