WESTWARD YOUNGING OF DEFORMATION RELATED TO THE NEVADAN OROGENY

The Nevadan Orogeny was originally defined (e.g. Hinds, 1934) as a singular event that resulted in the isoclinal folding and slaty cleavage in Upper Jurassic strata of the westernmost central and northern Sierra Nevada foothills. However, more detailed studies suggest a period of deformation instead of a singular event. The western Sierra Nevada foothills is composed largely of metamorphic rocks of Paleozoic and Mesozoic protolith intruded by Jurassic and Triassic plutons and bisected north to south by the Bear Mountains Fault Zone (BMFZ). The ~150 Ma Guadalupe igneous complex that intrudes the Upper Jurassic Mariposa Slate east of the BMFZ is largely undeformed, at scales ranging from outcrop to thin section, except along the western margin at the BMFZ, suggesting that isoclinal folding and development of penetrative cleavage there largely preceded pluton emplacement. In contrast, the ~147 Ma Santa Cruz Mountain pluton west of the BMFZ is reported to be pervasively deformed (e.g. Saleeby and others, 1989), suggesting that pluton emplacement there was coeval with or preceded regional deformation. Thus, the Nevadan orogeny was a period of westward younging regional deformation that extended from prior (just prior given the age of the Mariposa Slate protolith) to 150 Ma through 147 Ma. This trend might reflect deformation in successive packages of fringing arc volcanic and sedimentary rocks as they were accreted to the western margin of North America. If so, the BMFZ is the boundary between two such packages (that including the Mariposa Slate and underlying rocks to the east and that including Salt Spring Slate and underlying rocks to the west), and may have initially been a shallowly dipping fault in the accretionary prism that was steepened to its present orientation along with or following deformation of the package to the west (see also Ratschbacher et al., 2024 for evidence that the BMFZ was initially under the Guadalupe igneous complex). Existing data is not sufficient to distinguish the Late Jurassic strata east and west of the BMFZ (for example to differentiate between the Salt Spring Slate to the west and the Mariposa Slate to the east), but the next step is to collect detrital zircon localities in the strata to the west to compare with those previously collected from the Mariposa Slate (e.g. Snow and Ernst, 2008).