MAGMATIC FLARE-UP AND TECTONIC EVOLUTION IN THE SIERRA NEVADA ARC DURING LATE JURASSIC OBLIQUE SUBDUCTION

The Late Paleozoic to Late Mesozoic magmatic activity in the Sierra Nevada continental arc, California, records significant bimodal calc-alkaline magmatic activity during the Late Jurassic. The mechanism triggering this sudden change in the rate and composition of magmatism remains unclear, in part due to the lack of precise and comprehensive geochronological and geochemical data. We present new CA-ID TIMS geochronology, whole-rock geochemistry, and isotopic characteristics of magmatic rocks in southern California, combined with field and structural data. High-precision zircon U–Pb ages indicate a high magmatic flux over a ~3 Myr period (152-149 Ma) following a ~7 Myr magmatic lull (159-152 Ma). The geochemical and isotopic characteristics represent a shift toward more primitive components during the period of high magmatic flux. This might have been generated by the hydration of the mantle wedge during the short lull time. Partial melting of the mantle wedge could have been induced by changing subduction angles and changes from orthogonal to oblique subduction, leading to switching from contractional to transpressional tectonics during the Late Jurassic. Rapid magma ascent and emplacement may have contributed to less crustal contamination during this period. Therefore, the generation of the high flux of magma in the Late Jurassic Sierra Nevada continental arc is genetically linked to the geodynamic evolution of western North America from high-angle convergent to more oblique subduction.