Paper No. 8
Presentation Time: 10:45 AM


HILDEBRAND, Robert S., Department of Geology, University of California, Davis, CA 95616-8605,

The 125-82 Ma Sierra Nevada batholith is one of several composite magmatic belts of similar age located in the North American Cordillera. Plutons are mostly biotite and hbd-bearing tonalite, granodiorite, and granite with sharp, dominantly concordant contacts. By 1959 J.G. Moore recognized that the more mafic plutons outcrop west of more intermediate composition bodies. Others have confirmed that the Sierra Nevada can be divided into older western and younger eastern parts based on geochemistry, magnetic susceptibility, age, radiogenic and stable isotopes, wall rock provenance, and basement types.

As the batholith has been studied in considerable detail and is accessible, it has come to serve as a template for the emplacement of multicomponent batholiths and plutons, at least for North American geologists. Despite widespread younger deformation throughout the adjacent regions, all of the existing models for the three-dimensional shapes of plutons within the Sierra Nevada rely on one simple precept: their present shape is the original shape; that is, they are basically upright and geological maps represent plan-view sections through the original 3-dimensional bodies. Within this paradigm any folding of the wallrocks must predate emplacement of the plutons. However, as volcanic and sedimentary rocks, dated as 100-98 Ma, are isoclinally folded about northerly axes and locally overturned, all older plutons must also be folded.

Concordance of intrusive contacts with single stratigraphic units for many kilometers; local occurrence of two transecting cleavages that cut intrusive contacts in even the youngest plutons; folded cleavage and internal contacts; as well as folding and thrusting of latest-Cretaceous sedimentary rocks which sit unconformably upon Sierran granites, all suggest that plutons within the Sierran batholith were originally sheet-like bodies that are folded by two sets of folds, northerly and easterly. The older northerly trending folds formed during collision of the eastern and western halves of the batholith, at ~98 Ma, similar to ~100 Ma collisions between Wrangellia and Stikinia in the Coast plutonic complex and the Alisitos arc with an eastern block in the Peninsular Ranges batholith. 98-85 Ma plutonic suites occur in all three batholiths and predate Laramide (?) cross-folding at 80±5 Ma.