Geologic Map of Lassen Volcanic National Park and Vicinity, Northern California

We display a new geologic map of the Lassen area in the southern Cascade Range. The map is in press and will soon be available as U.S. Geological Survey SIM 2899. The 1:50,000 scale mapping is compiled from field mapping at 1:24,000. The 1:24,000 GIS dasebase will be included in electronic format on disk accompanying the printed map and has already proven of great use in a wide range of derivative products (Muffler and Clynne, this session). The map area, which includes 1,900 km2 centered on Lassen Volcanic National Park, is dominated by volcanic rocks ranging in age from about 3.5 Ma to Holocene. The map displays nearly 300 volcanic units, which were defined as individual or related groups of eruptive deposits. Stratigraphy is constrained by traditional mapping methods, but nearly 100 new K-Ar and 40Ar/39Ar radiometric ages provide absolute age control for the relative stratigraphy. Additionally, some Holocene and latest Pleistocene deposits were dated by radiocarbon. Major-element chemical analyses used to characterize rock units were published separately. Glacial deposits are extensive, and the deposits of 5 recent glacial advances are mapped. Normal faults are common in the Lassen area, where the Cascade arc is intersected by the western margin of the extending Basin and Range province and the northward propagating Walker Lane belt. Products of two modes of volcanism are present in the Lassen area: rocks of mafic to intermediate composition related to distributed regional volcanism, and rocks of intermediate to felsic composition where focused volcanism has generated large volcanic centers. Two types of primitive basaltic magmas are present in the regional suite: a diverse array of low- to high-K calc-alkaline basalts generated by subduction of the Juan de Fuca plate system, and low-potassium olivine tholeiite related to the Basin and Range extensional province. Regional volcanoes range from small volume (<1 to a few km3) monogenetic cinder cones and lava flows to slightly larger (up to a few 10s of km3), short lived, lava cones and shields. The magmatic systems of the individual regional volcanoes are petrographically simple and compositionally limited; evidence for multiple magma batches and magma mixing is scarce. In contrast, volcanic centers are large (100-200 km3), long-lived (up to 1 Ma) loci of volcanism that result from focusing of the regional volcanism. Their magmatic systems are petrographically and compositionally complex and involve fractionation of regional mafic magmas combined with crustal melting; evidence for magma mixing is ubiquitous. Five volcanic centers are recognized in the Lassen area, including four late Pliocene to Pleistocene centers and the active Lassen Volcanic Center, which last erupted in 1915.