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.

Abstract Petrologic, structural, geochronologic, and geochemical data from rocks exposed in the western foothills of the Sierra Nevada batholith just south of the western Foothills metamorphic belt (the Stokes Mountain region) provide new insight regarding several poorly understood aspects of the development of the compound Sierra Nevada arc. Exposures of three different metasedimentary packages together document transitions in the depositional environments present along the outboard edge of this arc segment throughout the Mesozoic Era, culminating in the emergence of the Cretaceous continental margin arc. Exposures of mafic cumulates and associated differentiates of the Early Cretaceous batholith permit investigation into the earliest stages of differentiation of depleted-mantle–derived arc magmas. Together with the surrounding Kings-Kaweah ophiolite belt, these Mesozoic plutonic and metasedimentary rocks are proposed to form an analog for the crystalline basement underlying much of the eastern half of the Great Valley forearc basin.

Abstract Fluid inclusion geothermometry is useful for establishing detailed thermal histories of sedimentary rocks that cannot be gleaned from other techniques. The fluid inclusion technique requires careful attention to petrography and evaluation of thermal reequilibration. The fluid inclusion assemblage approach (FIA) is most important in evaluating the extent to which fluid inclusions have been altered. Fluid inclusion geothermometry can be accomplished in both low- and high-temperature sedimentary systems. At low temperature, it can be used in paleoclimate work by generating bubbles from metastable inclusions, using cooling and femtosecond laser techniques. At elevated paleotemperature, homogenization temperatures (Th) can be measured from inclusions composed of high-temperature aqueous liquid without dissolved gas, high-temperature aqueous liquid with dissolved gas, gas, and petroleum. Given consistent FIAs and an understanding of the pressure-volume-temperature (PVT) relations for the inclusions, fluid inclusion Th data can be pressure-corrected given certain constraints. Inclusions can be used to evaluate the detailed spatial, temporal, tectonic, and fluid composition history of a system that cannot be determined in other ways. Fluid inclusions are used as a tool for determining the maximum temperature achieved in a sample, but this commonly would be an underutilization of the data generated. Unlike other techniques, fluid inclusions can link thermal history to fluid flow, as a means of evaluating whether heating was associated with normal burial conditions, hydrothermal systems, or cool fluids flowing into warmer rocks. Hydrothermal systems can be identified when geothermometry indicates: (1) repeated increases and decreases in temperature inconsistent with the burial and unroofing history; (2) paleotemperatures higher than what the most liberal burial history analysis will allow; (3) paleogeothermal gradients or pressure-temperature data inconsistent with normal burial; (4) evidence for locally increased temperature at the same depth within a region; and (5) geothermometric evidence that higher temperatures are focused in fracture, fault, or stratigraphic conduits for paleofluid flow.