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Science Tour - 3D/4D mapping of the San Andreas Fault Zone

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Cross sectional block diagrams of the East Bay region along the Hayward Fault, California

A three-dimensional (3D) mapping model illustrated: This series of block diagrams shows surficial features combined with cross sections showing the geologic interpretation at depth perpendicular (northwest to southeast) to the Hayward Fault (viewed toward the northwest). The top cross section (A) cuts through Mission Peak near Fremont (on the right) and crosses the south end of San Francisco Bay near San Jose and Mountain View (on the left). This cross section shows that at its south end the Hayward Fault dips moderately to the northeast, steepening as it merges with the Central Calaveras fault at depth (in the foothills of the Diablo Range east of San Jose in the vicinity between Alum Rock Park and Mount Hamilton). The "creeping" portion of the Hayward Fault system is shown in red. Note that the vertical scale is exaggerated about 2 times the horizontal scale. The cross section on the face of the block diagram is about 10 miles wide (15 km). The image was generated from a USGS 3D geologic map of the Hayward Fault zone.

Block B shows cross section between Hayward and Oakland in the San Leandro Area). The cross section cuts through the East Bay Hills (on the right) and the East Bay alluvial plain next to San Francisco Bay south of Oakland (on the left). By comparison to the Mission Peak area, the Hayward Fault in the San Leandro area has a nearly vertical profile. East of the Hayward Fault, a structural basin preserves a thick accumulation of Great Valley Sequence. West of the fault, the San Leandro Gabbro (Jgb) is a large intrusive igneous body that has unique physical properties compared to the Franciscan rocks (mostly volcanic and sedimentary rocks) or the Great Valley Sequence (sedimentary rocks). Gabbro has physical properties more like granitic rocks—being both harder and more brittle than sedimentary rocks.

Block C is a cross section cuts through the Berkeley area with Berkeley Hills (on the right) and the East Bay alluvial plain in the vicinity of the Bay Bridge (on the left). By comparison to the San Leandro area to the south, the body of San Leandro Gabbro pinches out to the north, and the Great Valley Sequence (east of the Hayward Fault) comes in contact with Jurassic ophiolite (mostly serpentinite) and Franciscan Formation rocks (mostly metabasalt, chert, and sandstone).

Block D is a cross section in the Richmond area that cuts through the north end of Berkeley Hills (on the right) and the East Bay alluvial plain just south of Point Pinole (to the left).


 

Legion of geologic units used in block diagram cross sections
Images generated from a San Francisco Bay regional 3D geologic model using using Earth Vision software.

Text and images modified from Stoffer, P. W., "Where's the Hayward Fault? A Green Guide to the Fault"
http://pubs.usgs.gov/of/2008/1135/

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