Late Triassic through Early Cretaceous detrital zircon separated from Lower Cretaceous sedimentary strata provides a record of arc magmatism that is not obscured by products of the mid- to Late Cretaceous surge, which dominate the exposed Sierra Nevada batholith. Matching U-Pb age-probability maxima to U-Pb dates of exposed arc plutonic rocks provides confirmation that the detrital zircon was sourced in the erupting and eroding Sierra Nevada arc. These data suggest that magmatic productivity in the southwestern arc increased steadily through the Middle Jurassic, from an Early Jurassic lull through the Late Jurassic. The detrital-zircon record documents an original footprint of the Early Cretaceous arc extending from its current exposure in the western Sierra Nevada foothills northwestward into the eastern Sacramento Valley, where its relatively mafic roots are presumably buried beneath younger sedimentary strata infilling the Great Valley. The sparse record of Late Triassic magmatism preserved in the analyzed intra-arc and forearc deposits likely reflects greater separations in both time and space between the Early Cretaceous basins and the Triassic arc. Analysis of an atypically dense sample set from the Goldstein Peak Formation intra-arc basin deposits, in conjunction with new data from the Lower Cretaceous Gravelly Flat Formation and published data from other Lower Cretaceous forearc strata of the Great Valley Group, suggests that an even greater density and broader geographic distribution of detrital-zircon samples are needed to more completely reconstruct the record of arc magmatism.

The Lovejoy basalt represents the largest eruptive unit identified in California, and its age, volume, and chemistry indicate a genetic affinity with the Columbia River Basalt Group and its associated mantle-plume activity. Recent field mapping, geochemical analyses, and radiometric dating suggest that the Lovejoy basalt erupted during the mid-Miocene from a fissure at Thompson Peak, south of Susanville, California. The Lovejoy flowed through a paleovalley across the northern end of the Sierra Nevada to the Sacramento Valley, a distance of 240 km. Approximately 150 km 3 of basalt were erupted over a span of only a few centuries. Our age dates for the Lovejoy basalt cluster are near 15.4 Ma and suggest that it is coeval with the 16.1–15.0 Ma Imnaha and Grande Ronde flows of the Columbia River Basalt Group. Our new mapping and age dating support the interpretation that the Lovejoy basalt erupted in a forearc position relative to the ancestral Cascades arc, in contrast with the Columbia River Basalt Group, which erupted in a backarc position. The arc front shifted trenchward into the Sierran block after 15.4 Ma. However, the Lovejoy basalt appears to be unrelated to volcanism of the predominantly calc-alkaline Cascade arc; instead, the Lovejoy is broadly tholeiitic, with trace-element characteristics similar to the Columbia River Basalt Group. Association of the Lovejoy basalt with mid-Miocene flood basalt volcanism has considerable implications for North American plume dynamics and strengthens the thermal “point source” explanation, as provided by the mantle-plume hypothesis. Alternatives to the plume hypothesis usually call upon lithosphere-scale cracks to control magmatic migrations in the Yellowstone–Columbia River basalt region. However, it is difficult to imagine a lithosphere-scale flaw that crosses Precambrian basement and accreted terranes to reach the Sierra microplate, where the Lovejoy is located. Therefore, we propose that the Lovejoy represents a rapid migration of plume-head material, at ~20 cm/yr to the southwest, a direction not previously recognized.