Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California

Abstract

Research Article| November 01, 2007 Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California Jeffrey R. Unruh; Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Search for other works by this author on: GSW Google Scholar Trevor A. Dumitru; Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Thomas L. Sawyer Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Publisher: Geological Society of America Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 The Geological Society of America, Inc. GSA Bulletin (2007) 119 (11-12): 1347–1367. https://doi.org/10.1130/B26057.1 Article history Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey R. Unruh, Trevor A. Dumitru, Thomas L. Sawyer; Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California. GSA Bulletin 2007;; 119 (11-12): 1347–1367. doi: https://doi.org/10.1130/B26057.1 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 In the California Coast Ranges east of San Francisco, post–3.5 Ma folding and erosion of the Mount Diablo anticline have created unusual three-dimensional exposures of late Mesozoic–Cenozoic stratigraphic and structural relations among rocks of the Franciscan subduction complex, Coast Range ophiolite, and Great Valley forearc basin. These relations offer new insights into the kinematics of Late Cretaceous–early Tertiary synsubduction extension, attenuation, and blueschist exhumation within the ancestral California forearc region. Map relations and subsurface data reveal that marine strata of the Great Valley forearc basin northeast of Mount Diablo accumulated in an ∼100-km-long, north-south–trending, extensional graben system. Normal faults in the graben system cut steeply downsection through Late Cretaceous and early Tertiary growth strata and terminate against the Clayton–Marsh Creek fault, which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system. The Clayton–Marsh Creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral Franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried, including the Coast Range ophiolite and basal Great Valley strata. These faults are the local expression of the Coast Range fault in this area. Franciscan rocks exposed beneath the faults were metamorphosed at ≥20 km depths after 108 Ma, based on jadeitic pyroxene and 108 Ma detrital zircons (U-Pb) in metagraywackes. Apatite fission-track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from ∼9 km to ∼3 km depths in latest Cretaceous–early Tertiary time, coeval with the subsidence and extension in the structurally overlying forearc graben system. Much of their earlier rise from ≥20 km to 9 km depth presumably also occurred coeval with graben development, although our data cannot determine this directly. These relations demonstrate that the California forearc around Mount Diablo was in an extensional regime undergoing active sedimentation during major Franciscan exhumation, and thus support models for exhumation of Franciscan blueschist facies rocks via synsubduction extensional processes, rather than via erosional processes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.