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Shallow structure and geomorphology along the offshore northern San Andreas Fault, Tomales Point to Fort Ross, California

Samuel Y. Johnson and Jeffrey W. Beeson
Shallow structure and geomorphology along the offshore northern San Andreas Fault, Tomales Point to Fort Ross, California
Bulletin of the Seismological Society of America (March 2019) 109 (3): 833-854

Abstract

We mapped a poorly documented 35-km-long section of the northern San Andreas fault (NSAF) zone between Tomales Point and Fort Ross, California. Mapping is largely based on high-resolution seismic-reflection profiles (38 fault crossings), multibeam bathymetry, and onshore geology. NSAF strike in this section is nearly parallel to plate motion, characterized by a slight ( approximately 2 degrees ) northerly (transtensional) bend in the south between Tomales Bay and the Bodega isthmus, and a northwesterly (transpressional) approximately 5 degrees bend in the north between the Bodega isthmus and Fort Ross. The southern transtensional bend is the northern part of the now-submerged, linear, approximately 50-km-long and 1-2-km-wide, Tomales-Bodega valley. The valley floor is cut by a complex zone of subparallel, variably continuous fault strands, and the deformed valley fill is an inferred mix of late Quaternary marine and nonmarine strata. In the northern part of this elongate valley, Holocene fault offset occurred on two fault strands about 740 m apart. The northern transpressional bend is characterized by narrow, elongate, asymmetric basins containing as much as 56 m of inferred latest Pleistocene to Holocene sediment. Between Bodega Head and Fort Ross, the gently dipping ( approximately 0.8 degrees ) shelf includes two large (4.8 and 5.9 km (super 2) ) zones of sediment failure that we speculatively correlate with the 1906 San Francisco NSAF earthquake. Similar sediment-failure zones should be common along offshore reaches of the NSAF and other nearshore fault zones but have apparent limited preservation potential. Onland geomorphic impacts of the mainly offshore NSAF include: (1) northward upwarping of uplifted marine terraces in the transpressional zone north of Bodega Bay; and (2) blocking of littoral sediment transport by uplifts on the west flank of the NSAF at Bodega Head and Tomales Point, resulting in rapidly accreting beaches and large coastal sand dune complexes.


ISSN: 0037-1106
EISSN: 1943-3573
Coden: BSSAAP
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 109
Serial Issue: 3
Title: Shallow structure and geomorphology along the offshore northern San Andreas Fault, Tomales Point to Fort Ross, California
Affiliation: U. S. Geological Survey, Santa Cruz, CA, United States
Pages: 833-854
Published: 20190326
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 114
Accession Number: 2019-029451
Categories: Quaternary geologyApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch maps, sects.
N37°47'60" - N38°49'60", W123°34'60" - W122°22'00"
Secondary Affiliation: Fugro USA Marine, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, Copyright, Seismological Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201916
Program Name: USGSOPNon-USGS publications with USGS authors
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