New single-grain detrital zircon U-Pb age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Sierra Nevada foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Bragdon and Baird Formations of the Eastern Klamaths and the Paleozoic miogeocline of Nevada ± sources farther to the east, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval in the Mariposa Formation; and (3) the youngest zircon ages constrain the onset of clastic deposition at 152 ± 1 Ma. Zircon age data also suggest that the local drainage divide migrated westward, resulting in a higher proportion of detritus derived from the Sierra Nevada arc over time. New geologic mapping in the central Sierra Nevada foothills shows that the Mariposa Formation thickens eastward, and that the number of coarse-grained sandstone bodies increases up section. These observations indicate that a topographically low Sierran volcanic arc gradually began to rise, providing increasing amounts of clastic debris to the Mariposa depositional basin. The Mariposa Formation was deposited in a volcanically active deep-water forearc basin and was subsequently disrupted by Nevadan orogenesis during the Late Jurassic. Inasmuch as it was located in the forearc inboard from the Middle Jurassic Coast Range ophiolite, Nevadan deformation cannot have resulted from arc-continent collision in the Sierra Nevada foothills but instead must have been related to tectonism along the plate margin.