STRAIN WITHIN THE GODDARD PENDANT AND ADJACENT CRETACEOUS GRANITIC ROCKS, SOUTHERN SIERRA NEVADA

The Sierra Nevada batholith represents an archetypical arc setting that offers a unique location to investigate strain during pluton emplacement. Host rocks exposed in the eastern Sierra Nevada record multiple phases of Cretaceous strain, although less is known about how strain may have varied across the batholith. The Goddard pendant, exposed within the axial portion of the batholith, consists of Jurassic granitic rocks and Jurassic–Early Cretaceous metavolcanic rocks with foliation that is concordant with foliation in younger Cretaceous granitic rocks. The goal of this study is to characterize strain within the Goddard pendant and adjacent Cretaceous granitic rocks to define the style and timing of mid- to Late Cretaceous strain in the axial Sierra Nevada.

Our mapping from the western flank of the Goddard pendant reveals NE/SW striking foliation with subvertical lineations within Jurassic granitic rocks and cross cutting Independence dikes. Samples collected from Jurassic granitic rocks display a range of textures in thin section that vary from fine-grained aggregates of annealed feldspar interlayered with coarsely recrystallized quartz near the contact with Late Cretaceous Mt. Givens granodiorite, to mylonitic fabrics with west-side-up kinematic indicators farther east and adjacent to Cretaceous metavolcanic rocks. To the southwest, Jurassic granitic rocks are in contact with the Cretaceous granodiorite of Mt. Reinstein, which does not include Independence dikes. The contact between these two units is complex displaying both concordant and discordant relationships to foliation in the Jurassic granite, although the map-scale contact is discordant. Foliation within the Mt. Reinstein defined by hornblende and mafic enclaves is steep with NE/SW strike, although this fabric lacks obvious lineation. In thin section, Mt. Reinstein granodiorite displays weakly-oriented, coarse plagioclase with interstitial quartz, suggestive of magmatic crystallization. These observations indicate less solid-state strain within the Mt. Reinstein than Jurassic granitic rocks. Fabrics within Mt. Reinstein are cut sharply by Finger Peak granite and Mt. Givens granodiorite. Ongoing geochronologic work on key samples from the study area will further refine the timing of strain within the Goddard Pendant.