Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crustal source near Eagle Rest Peak, California

Donald C. Ross; Carl M. Wentworth; Edwin D. McKee

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USGS · 70159646

Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crustal source near Eagle Rest Peak, California

Donald C. Ross·Carl M. Wentworth·Edwin D. McKee

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Abstract

Upper Cretaceous mafic conglomerate and quartz-plagioclase arkose that crop out on the southwest side of the San Andreas fault near Gualala, Calif., may have been eroded from a gabbroic terrane that now lies about 350 miles to the southeast, on the opposite side of the San Andreas fault. The plagioclase arkose near Gualala contains little or no K-feldspar, and the conglomerate is characterized by quartz-bearing mafic rocks that lack K-feldspar volcanic rocks, diabase, and diorite to gabbro. Hornblendes from these clasts yield K/Ar ages of 141±4,175±7, and 186±7 m.y. The arkose and conglomerate appear to have been eroded from a chert-poor ophiolite (oceanic crust) sequence that, according to paleocurrent evidence, lay east of the present San Andreas fault. Near Eagle Rest Peak, 350 miles southeast of Gualala, similar mafic quartz-bearing volcanic rocks, diabase, and gabbro are exposed in a small structurally isolated area that abuts the San Andreas fault on the southwest. These rocks yield hornblende K/Ar ages of 134±4, 165±4, and 207±10 m.y. They may also be the source of two small fault slivers of similar mafic rocks, which yield hornblende K/Ar ages between 144 and 172 m.y. These slivers now lie 100 and 200 miles to the northwest along the San Andreas fault at Gold Hill and Logan.

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Journal of Research of the U.S. Geological Survey Article

Source-reported bounding extent: 38.734134099386395° to 38.7819893767876° latitude; -123.55662345886229° to -123.48074913024902° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Donald C. Ross, Carl M. Wentworth, Edwin D. McKee. 1973. Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crustal source near Eagle Rest Peak, California. https://pubs.usgs.gov/publication/70159646

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