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Quartz Gabbro and Anorthositic Gabbro: Markers of Offset along the San Andreas Fault in the California Coast Ranges

1970; Geological Society of America; Volume: 81; Issue: 12 Linguagem: Inglês

10.1130/0016-7606(1970)81[3647

1943-2674

Donald Clarence Ross,

Geological and Geochemical Analysis

Research Article| December 01, 1970 Quartz Gabbro and Anorthositic Gabbro: Markers of Offset along the San Andreas Fault in the California Coast Ranges DONALD C ROSS DONALD C ROSS U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1970) 81 (12): 3647–3662. https://doi.org/10.1130/0016-7606(1970)81[3647:QGAAGM]2.0.CO;2 Article history received: 03 Apr 1970 rev-recd: 25 May 1970 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation DONALD C ROSS; Quartz Gabbro and Anorthositic Gabbro: Markers of Offset along the San Andreas Fault in the California Coast Ranges. GSA Bulletin 1970;; 81 (12): 3647–3662. doi: https://doi.org/10.1130/0016-7606(1970)81[3647:QGAAGM]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Large-scale lateral movement on the San Andreas fault zone is suggested by the distribution of gabbroic rocks that may be slivered remnants of oceanic crust. Distinctive and unusual hornblende quartz gabbro and anorthositic gabbro that are virtually identical both petrographically and chemically are exposed at Logan and Gold Hill in the Coast Ranges along the San Andreas fault. The hornblende quartz gabbro is made up of labradorite to bytownite plagioclase, hornblende, and quartz, with very minor biotite and pyroxene, and accessory metallic opaques and apatite. The coarse-grained anorthositic gabbro with anorthositic layers and associated gneiss is made up mainly of labradorite to bytownite plagioclase, hornblende, lesser clinopyroxene, and, locally, orthopyroxene.The present 100 mi of separation between these two gabbro bodies is probably due to displacement along the San Andreas fault of what was originally one gabbro mass. Somewhat similar quartz gabbro and anorthositic gabbro associated with ultramafic rocks near Eagle Rest Peak in the San Emigdio Mountains may represent a source for the Logan and Gold Hill slivers. This suggests a minimum right-lateral movement of about 200 mi on the San Andreas fault zone. Anorthositic gabbro clasts from Cretaceous conglomerate in the Gualala area have strong resemblance to gabbroic rocks at Logan, Gold Hill, and Eagle Rest Peak and speculatively suggest 350 mi of right-lateral movement on the fault zone.It is tentatively suggested that the gabbro clasts of Gualala may have been shed from a large area of exposed gabbroic oceanic crust, pieces of which occur as fault slivers at Gold Hill and Logan. It is further suggested that the Eagle Rest Peak locality may be a relatively in-place exposure of this gabbroic oceanic crust that lies near a continental-oceanic interface. This interface, thought to represent a fossil subduction zone between Franciscan and Sierran basements, appears to be overlain depositionally by Eocene sedimentary rocks. If these Eocene deposits do "seal over" the possible subduction zone, it suggests that movement on such a zone ceased before the Eocene deposition, and that the new and different pattern of right-lateral movement on the San Andreas fault zone probably began sometime later. 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.