CORRELATION OF THE KLAMATH MOUNTAINS AND SIERRA NEVADA

Compilation of both published and new data supports the early belief of J. S. Diller and various later geologists that the Klamath Mountains (KM) and Sierra Nevada (SN) have similar histories and are probably parts of the same orogenic belt. A new map has been prepared to aid the study of the relations between these two composite terranes. A working hypothesis is that early in their histories, each of them consisted mainly of early to middle Paleozoic volcanic and sedimentary rocks resting on slabs of ultramafic rock. These old rocks (the Yreka subterrane in the KM; the Shoo Fly Complex in the SN) and their ultramafic base (the Trinity Complex in the KM; the Feather River terrane in the SN) acted as a backstop for a succession of accretionary episodes. Most or perhaps all of these episodes involved the subduction of oceanic volcanic and sedimentary rocks which, when remobilized at depth, were recycled as plutonic intrusions and volcanic eruptions in the overlying accreted and backstop terranes . The longest coherent records of the igneous pulses caused by the subduction episodes are found in the multiple volcanic sequences (the Redding section in the KM; the Taylorsville, Permian, and Jurassic volcanic arc sequences in the Northern Sierra terrane of the SN) that formed on the backstop terranes rather than on the successively younger westerly terranes. These volcanic sequences range in age from early and middle Paleozoic to Jurassic. The plutonic rocks generated by the various accretionary episodes generally appear more widely distributed than are their volcanic counterparts, and their intrusive relations are important to an understanding of the accretionary history. The Sierra Nevada Batholith represents a major tectonic change from the Jurassic and older accretionary history of the SN. It is composed of plutons ranging mainly from 125 Ma to 82 Ma, cuts across the southern half and eastern side of the SN, and is not represented in the KM. An important new element relating to the tectonic history of the KM is the recent discovery of Neoproterozoic fossils in Antelope Mountain Quartzite (Yreka subterrane ) of the KM by N. Lindsley-Griffin and associates (oral commun., 2001), which should motivate a search for similar fossils in possibly correlative rocks (Lang sequence(?), Shoo Fly Complex) of the Northern Sierra terrane.