Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California

Research Article| October 01, 1974 Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California DARREL S. COWAN DARREL S. COWAN 1Shell Oil Company, P.O. Box 527, Houston, Texas 77001 Search for other works by this author on: GSW Google Scholar Author and Article Information DARREL S. COWAN 1Shell Oil Company, P.O. Box 527, Houston, Texas 77001 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (10): 1623–1634. https://doi.org/10.1130/0016-7606(1974)85<1623:DAMOTF>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 Email Permissions Search Site Citation DARREL S. COWAN; Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California. GSA Bulletin 1974;; 85 (10): 1623–1634. doi: https://doi.org/10.1130/0016-7606(1974)85<1623:DAMOTF>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 The Franciscan Complex northwest of Pacheco Pass, California, includes three fault-bounded units, each characterized by a different deformational style and suite of metamorphic mineral assemblages. Structurally highest is jadeitic pyroxene-bearing metagraywacke semischist. The areally extensive Garzas tectonic mélange separates the semischist from the structurally lowest pumpellyite-bearing Orestimba metagraywacke. The Garzas mélange is representative of Franciscan mélanges in general. These mappable bodies have an internal fabric dominated by penetrative, mesoscopic shear fractures and contain tectonic inclusions of all sizes immersed in a pervasively sheared, generally fine-grained matrix. The shear fractures record brittle deformation of consolidated rock bodies. Many mélanges contain exotic inclusions, clearly not derived from adjacent units, and metamorphic mineral assemblages and textures indicate that they were metamorphosed before being tectonically mixed with more voluminous, generally lower grade, metagraywacke inclusions.The structural units are grossly sheetlike in external form and are separated from one another by gently to steeply dipping major faults. Unlike low-angle thrusts in imbricated Cordilleran terranes, the faults do not systematically repeat or offset a normal stratigraphic sequence but rather juxtapose rock units that bear no apparent stratigraphic, deformational, or metamorphic relation to one another. The structural units were separately deformed and metamorphosed under a variety of conditions prior to their tectonic juxtaposition during late Mesozoic continental margin subduction. Field and petrographic evidence permit, but do not prove, the hypothesis that both the semischist and exotic, high-grade mélange inclusions once were more deeply buried and have been emplaced upward into their present anomalously shallow structural positions. 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.