Implications of the northwestwardly younger age of the volcanic rocks of west-central California

Abstract

Research Article| May 01, 1985 Implications of the northwestwardly younger age of the volcanic rocks of west-central California KENNETH F. FOX, JR.; KENNETH F. FOX, JR. 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ROBERT J. FLECK; ROBERT J. FLECK 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GARNISS H. CURTIS; GARNISS H. CURTIS 2Department of Geology and Geophysics, University of California at Berkeley, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar CHARLES E. MEYER CHARLES E. MEYER 3U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (5): 647–654. https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation KENNETH F. FOX, ROBERT J. FLECK, GARNISS H. CURTIS, CHARLES E. MEYER; Implications of the northwestwardly younger age of the volcanic rocks of west-central California. GSA Bulletin 1985;; 96 (5): 647–654. doi: https://doi.org/10.1130/0016-7606(1985)96<647:IOTNYA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Erosional remnants of volcanic fields in west-central California form a linear northwest-trending belt growing younger in age to the northwest. Major fields within the belt are represented by the Neenach Volcanics, Pinnacles Volcanic Formation, Quien Sabe Volcanics, volcanic rocks in the Berkeley Hills, Tolay Volcanics, Sonoma Volcanics, and Clear Lake Volcanics. Dispersion in the age-distance relation is reduced by restoration of inferred offsets on transecting right-lateral fault systems. The offsets include 115 km on the San Gregorio–Hosgri fault, 314 km on the San Andreas fault, 43 km on the Hayward-Rodgers Creek fault, and 28 km on the Carneros-Franklin-Sunol-Calaveras fault. On the basis of the age and restored position of the volcanic rocks, we judge that the locus of initial active volcanism migrated northwestward ∼3.75 cm/yr from 25 to 12 Ma, and ∼1.35 cm/yr from 12 Ma to the present.The volcanic rocks apparently formed south of the northwardly retreating edge of the subducted part of the Juan de Fuca plate, corroborating one corollary of a published model of an expanding hole in the subducted Farallon-Juan de Fuca-Cocos plate. The present position of the locus of melting at Clear Lake, California, requires substantial overthrusting of the Juan de Fuca plate by the Pacific plate, as was postulated on the basis of foreshortening of magnetic anomalies in the Gorda basin. The change in rate of northwestward migration ∼12 Ma reflects a change in spreading direction of the Juan de Fuca plate vis-à-vis the Pacific plate, previously recognized from changes in orientation of oceanic magnetic anomalies. From the migration rates, it can be inferred that the relative movement between the Pacific plate and the westernmost fringe of the North American plate averaged ∼3.5 cm/yr from 27 m.y. ago to the present. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.