Superimposition and timing of deformations in the Mount Morrison roof pendant and in the central Sierra Nevada, California

Abstract

The Mount Morrison roof pendant, the only roof pendant in the central Sierra Nevada containing Paleozoic fossils, is complexly deformed and contains three generations of structures, including folds, reverse faults, schistosities, and lineations. All three generations of structures occur in the Ordovician-Silurian(?) metasedimentary rocks, whereas only the younger two are recorded in the Pennsylvanian-Permian(?) metasedimentary rocks and the Permian(?)-Jurassic(?) metavolcanic rocks. The average strike directions of axial planes of folds are north-south, N25°W, and N60°W in the first, second, and third generations, respectively. Generations of structures having similar styles, orientations, and relative age relations occur in other pendants of the central Sierra Nevada. The pendant is interpreted as a thin sequence with tight isoclinal folds instead of a thick homoclinal sequence. The first deformation occurred during Devonian or Mississippian time, perhaps during the Antler orogeny. Uplift, erosion, and volcanism occurred in Late Permian time between the first and second deformations, perhaps as an expression of the Sonoma orogeny. The second generation structures formed in several pulses between Early Triassic and Early Cretaceous time, as indicated by temporal relations between deformed wall rocks and younger, crosscutting granitic plutons. The third generation structures formed between Early and Late Cretaceous time, during which these structures were crosscut by granitic rocks. The wall rocks of the batholith may form an anticlinorium instead of a synclinorium. Other roof pendants in the axial portion of the batholith may be relatively old, because they contain the same three sets of structures as found in the Ordovician-Silurian(?) rocks of the Mount Morrison roof pendant. Locally, various age belts of granitic rocks have shielded roof pendants from subsequent deformation.