Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 8:00 AM-12:00 PM


IRWIN, William P., U.S. Geological Survey, 345 Middlefield Road, MS977, Menlo Park, CA 94025 and WOODEN, Joseph L., U. S. Geol Survey, 345 Middlefield Road, MS977, Menlo Park, CA 94025,

Compilaton of published and new U/Pb isotopic dates of plutonic and other rocks provides a useful database for studying the plutonic and accretionary history of the Sierra Nevada. The north-trending Sierra Nevada batholith consists of a multitude of intermingled plutonic intrusions ranging in age from ~125 to ~82 Ma .and is generally thought to represent the root of a Cretaceous volcanic arc. The batholith cuts, at a low angle, a preexisting series of northwest–trending accreted terranes consisting of lower Paleozoic to Upper Jurassic rocks. Plutons exposed in the terranes west of the batholith are either preaccretionary plutons that were original parts of a terrane or are accretionary plutons that intruded as a result of the generation of magma during an accretionary episode. The most voluminous plutons are ~140 Ma , and although clearly accretionary, the accretionary event to which they are related is obscure. They intrude the greatest number of terranes, including the Smartville, Central belt, Mother Lode , and Calaveras, postdating all the known accretionary units west of the Feather River terrane and Calaveras-Shoo Fly fault. Other preliminary observations west of the batholith are: (1) Plutons ~150 Ma are nearly restricted to small areas near the southern ends of the Smartville and Central belt terranes. (2) Plutons ~160 Ma cut the Central belt and Calaveras terranes, and in the Northern Sierra terrane an ~160 Ma pluton intrudes the Shoo Fly, Taylorsville, and Jurassic volcanic rock subterranes. Similar ages of ~160 Ma have been measured on elements of the Smartville Complex. (3) Plutons ~170 Ma are in the Calaveras terrane except for one in the ShooFly terrane. They are east of the Melones fault, and two are terminated by the fault. (4) Small plutons ~200 Ma are present at various places along the length of the Central belt. (5) Plutons of 385 Ma and 378 Ma in the Northern Sierra terrane suggest a Devonian subduction event.