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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.
Study Area
| Publication type | Article |
|---|---|
| Publication Subtype | Journal Article |
| Title | Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crustal source near Eagle Rest Peak, California |
| Series title | Journal of Research of the U.S. Geological Survey |
| Volume | 1 |
| Issue | 1 |
| Year Published | 1973 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Publisher location | Reston, VA |
| Contributing office(s) | Geology, Minerals, Energy, and Geophysics Science Center |
| Description | 8 p. |
| First page | 45 |
| Last page | 52 |
| Country | United States of America |
| State | California |
| City | Gualala |
| Other Geospatial | Eagle Rest Peak |
| Online Only (Y/N) | N |
| Additional Online Files (Y/N) | N |
| Google Analytic Metrics | Metrics page |