Ortigalita fault zone, Los Banos Valley section (Class A) No. 52b
Last Review Date: 2000-05-09
Compiled in cooperation with the California Geological Survey
citation for this record: Bryant, W.A., and Cluett, S.E., compilers, 2000, Fault number 52b, Ortigalita fault zone, Los Banos Valley section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 08/08/2022 08:13 PM.
The Ortigalita fault zone is a major Holocene dextral strike-slip fault in the central Coast Ranges that is an eastern part of the larger San Andreas fault system. The Ortigalita fault zone extends from about 20 km northwest of San Luis Reservoir southeast to the vicinity of Panoche Valley. The Ortigalita fault zone is characterized by en echelon fault traces separated by pull-apart basins. Anderson and others (1982 #5344; 1982 #5345) excavated trenches along each of the four sections, demonstrating that the Ortigalita fault zone is latest Pleistocene to Holocene (active) and that the Little Panoche Valley section is late Holocene active. Most of the Ortigalita fault zone is delineated by geomorphic evidence of latest Pleistocene to Holocene dextral strike-slip displacement and is locally marked by complex dextral normal offset, primarily at the section boundaries marked by pull-apart basins (Anderson and others, 1982 #5344; 1982 #5345; Hart, 1985 #5350; Manson, 1985 #5355). Late Quaternary slip rates and recurrence intervals are unknown, although Anderson and others (1982 #5344) concluded that the recurrence interval for the entire Ortigalita fault zone is about 2–5 k.y. Clark and others (1984 #2876) reported a minimum vertical slip rate of 0.01–0.04 mm/yr. The dextral slip component is probably greater than the vertical component, but this has not been documented.
Sections: This fault has 4 sections. Anderson and others (1982 #5344) defined four segments of the Ortigalita fault, from north to south: Cottonwood Arm, Los Banos Valley, Piedra Azul, and Little Panoche. There is insufficient data to define these as seismogenic fault segments. The names and segment boundaries from Anderson and others (1982 #5344) are adopted as sections for this compilation and include the Cottonwood Arm [52a], Los Banos Valley [52b], Piedra Azul [52c], and Little Panoche [52d] sections.
General: The Ortigalita fault zone was first recognized and mapped by Anderson and Pack (1915 #5347) for a fault that juxtaposes Cretaceous Franciscan Complex rocks against Upper Cretaceous Chico Group along the eastern side of Ortigalita Peak. The fault was first named by Taliaferro (1943 #5356), who identified a steeply west-dipping reverse fault (cross-section VII) that he named the Ortigalita Thrust. The fault has also been referred to as the Tesla-Ortigalita fault (e.g., Cotton, 1972 #5348). This compilation refers to the Ortigalita fault zone (most common name) as the zone of faults from about 20 km northwest of San Luis Reservoir southeast to Panoche Valley that are characterized by predominantly dextral late Pleistocene and Holocene strike-slip.
Section: Los Banos Valley section corresponds to Los Banos Valley segment of Anderson and others (1982 #5344). This section extends from the large right-releasing step in the San Luis Reservoir area (San Luis Creek) southeast to Carrisalito Flat.
Fault ID: Refers to number 214 (Ortigalita fault) of Jennings (1994 #2878) and number L03 (Ortigalita fault) of Working Group on Northern California Earthquake Potential (1996 #1216).
|County(s) and State(s)||
MERCED COUNTY, CALIFORNIA
|Reliability of location||
Compiled at 1:62,500 scale.
Comments: Locations based on digital revisions to Jennings (1994 #2878) using original mapping by Dibblee (1975 #4832) at 1:62,500 scale and mapping by Anderson and others (1982 #5345), Lettis (1982 #5353), and Manson (1985 #5355) at 1:24,000 scale.
The Ortigalita fault zone consists of near vertical dipping dextral strike-slip faults located along the eastern border of the central Diablo Range (Anderson and others, 1982 #5344; Lettis, 1985 #5354). This fault zone separates the Franciscan core of the range from the Great Valley Sequence of the eastern foothills of the range (Anderson and others, 1982 #5344). The fault's style of displacement locally is complex with minor reverse and normal components to the predominant dextral strike-slip displacement (Anderson and others, 1982 #5344; 1982 #5345; Manson, 1985 #5355). Cumulative dextral displacement is unknown, but the large-scale dextral displacement of Los Banos Creek suggests at least 5 km of offset (Manson, 1985 #5355). Anderson and La Forge (1990 #5346) estimated that at least 1–2 km of dextral slip is indicated by the size of pull-apart basins associated with the Ortigalita fault zone. Lettis (1985 #5354) argued that the location of Basalt Hill, an outlier of the late Miocene Quien Sabe volcanic field, limits the amount of significant late Cenozoic dextral displacement along the Ortigalita fault zone. Elevations of the Quien Sabe volcanic rocks on either side of the Ortigalita fault zone also limit the amount of late Cenozoic vertical displacement (Lettis, 1985 #5354).
|Length (km)||This section is 35 km of a total fault length of 71 km.|
|Average strike||N32°W (for section) versus N28°W (for whole fault)|
|Sense of movement||
Comments: Los Banos Valley section is characterized by dextral normal offset near its northern end. Trench exposures near the northern end of this section reported in Anderson and others (1982 #5344; 1982 #5345) indicate normal drag of sedimentary deposits of the Tulare (?) Formation into a steely dipping fault. Further southeast, dextral displacement is indicated by geomorphic features that show a consistent sense of dextral displacement. La Forge and Lee (1982 #5352) reported that dextral strike-slip focal mechanisms characterize seismicity associated with the Ortigalita fault zone.
Comments: La Forge and Lee (1982 #5352) reported that focal mechanism of small earthquakes associated with the Los Banos Valley section are characterized by near vertical dipping dextral strike-slip faults. Trench exposures indicate vertical to steely northeast dipping fault planes along strands of the Los Banos Valley section (Anderson and others, 1982 #5344), (1982 #5345).
Anderson and others (1982 #5344; 1982 #5345) conducted a detailed study of the fault zone, and excavated about nine fault-normal trenches along traces of the Cottonwood Arm, Los Banos Valley, Piedra Azul, and Little Panoche Valley sections of the Ortigalita fault zone. Two study sites were examined along Los Banos Valley section.
Site 52-2 involved excavation of two fault-normal trenches. Trench T-7 exposed a 20-m-wide shear zone marked by steeply NE-dipping faults (with horizontal slickensides) that offset latest Pleistocene colluvium. Trench T-3 exposed steeply dipping faulting and drag folding of Tulare Formation sediment, indicating a normal component of displacement.
Site 52-3 involved the excavation of one fault-normal trench across the base of a SW-facing scarp. The fault offsets colluvium derived from sediment of the Tulare Formation. The steeply NE-dipping fault flattens upward and is overlain by unfaulted colluvium of undetermined age.
Los Banos Valley section is delineated by moderately well-defined geomorphic indicators of dextral strike-slip displacement, such as dextrally deflected drainages, linear scarps, linear tonal contrasts on Holocene alluvium, aligned saddles, side-hill benches, and a closed depression (Anderson and others, 1982 #5345; Manson, 1985 #5355).
|Age of faulted surficial deposits||Faults of Los Banos Valley section offset latest Pleistocene and Holocene alluvium as mapped by Lettis (1982 #5353).|
|Most recent prehistoric deformation||
latest Quaternary (<15 ka)
Comments: Anderson and others (1982 #5344; 1982 #5345) reported that the most recent paleoevent along Los Banos Valley section occurred since 10-15 ka. This is based on a trench exposure of faulted colluvium interpreted to be 10–15 ka on the basis of soil profile development. The overlying (undated) soil is not offset.
Comments: Anderson and others (1982 #5344) concluded that the average recurrence interval for movement somewhere along the entire fault is about 2–5 k.y. based on their observations that there have been at least 4 major surface rupturing earthquakes on the Ortigalita fault zone in the past 15 k.y. There is no documentation for recurrence intervals on specific fault sections, although Anderson and others (1982 #5344) state that recurrence of large earthquakes on individual sections may be on the order of 10–15 k.y.
Between 1.0 and 5.0 mm/yr
Comments: Clark and others (1984 #2876) reported a late Pleistocene vertical displacement rate of about 0.01 mm/yr for Los Banos Valley section; the dextral component of slip is unknown. Clark and others (1984 #2876) qualify this vertical rate by stating that the rate is an extreme minimum, that trench exposures and geomorphic expression of the fault zone suggest a much greater dextral rate on the order of 0.5–1.0 mm/yr or greater. Petersen and others (1996 #4860) used a probable slip rate of 1±0.5 mm/yr, so the 1–5 mm/yr category seems to fit best.
|Date and Compiler(s)||
William A. Bryant, California Geological Survey
Sereyna E. Cluett, California Geological Survey
#5344 Anderson, L.W., Anders, M.H., and Ostenaa, D.A., 1982, Late Quaternary faulting and seismic hazard potential, eastern Diablo Range, California, in Hart, E.W., Hirschfeld, S.E., and Schulz, S.A., eds., Proceedings, Conference on earthquake hazards in the eastern San Francisco Bay area: California Division of Mines and Geology Special Publication 62, p. 197-206.
#5346 Anderson, L.W., and LaForge, R., 1990, Comment on "The style of late Cenozoic deformation at the eastern front of the California Coast Ranges" by C.M. Wentworth and M.D. Zoback: Tectonics, v. 9, no. 5, p. 1263-1265.
#5345 Anderson, L.W., LaForge, R., and Anders, M.H., 1982, Seismotectonic study of the San Luis area, eastern Diablo Range, California, for San Luis Dam, O'Neill Dam, Los Banos Detention Dam, and Little Panoche Detention Dam, San Luis Unit, Central Valley Project: U.S. Bureau of Reclamation Seismotectonic Report 82-2, 82 p., 4 pls., scale 1:24,000.
#5347 Anderson, R., and Pack, R.W., 1915, Geology and oil resources of the west border of the San Joaquin Valley north of Coalinga, California: U.S. Geological Survey Bulletin 603, p. 220.
#2876 Clark, M.M., Harms, K.H., Lienkaemper, J.J., Harwood, D.S., Lajoie, K.R., Matti, J.C., Perkins, J.A., Rymer, M.J., Sarna-Wojcicki, A.M., Sharp, R.V., Sims, J.D., Tinsley, J.C., III, and Ziony, J.I., 1984, Preliminary slip rate table and map of late Quaternary faults of California: U.S. Geological Survey Open-File Report 84-106, 12 p., 5 plates, scale 1:1,000,000.
#5348 Cotton, W.R., 1972, Preliminary geologic map of the Franciscan rocks in the central part of the Diablo Range, Santa Clara and Alameda Counties, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-343 (Basic Data Contribution 39), 2 sheets, scale 1:62,500.
#4832 Dibblee, T.W., Jr., 1975, Geologic maps of the Pacheco Pass, Hollister, Quien Sabe, Ortigalita Peak, San Benito, Panoche Valley, and "Tumey Hills" quadrangles, San Benito, Santa Clara, Merced, and Fresno Counties, California: U.S. Geological Survey Open-File Report 75-394, 7 sheets, scale 1:62,500.
#5350 Hart, E.W., 1985, Ortigalita fault (northwest segment): Stanislaus County, California: California Division of Mines and Geology Fault Evaluation Report FER-166, Supplement No. 1, microfiche copy in Division of Mines and Geology Open-File Report 90-11, 3 p., scale 1:24,000.
#2878 Jennings, C.W., 1994, Fault activity map of California and adjacent areas, with locations of recent volcanic eruptions: California Division of Mines and Geology Geologic Data Map 6, 92 p., 2 pls., scale 1:750,000.
#5352 LaForge, R., and Lee, W.H.K., 1982, Seismicity and tectonics of the Ortigalita fault and southeast Diablo Range, California, in Hart, E.W., Hirschfeld, S.E., and Schulz, S.A., eds., Proceedings, Conference on earthquake hazards in the eastern San Francisco Bay area: California Division of Mines and Geology Special Publication 62, p. 93-101.
#5353 Lettis, W.R., 1982, Late Cenozoic stratigraphy of the western margin of the central San Joaquin Valley, California: U.S. Geological Survey Open-File Repot 82-526, 203 p., scale 1:24,000.
#5354 Lettis, W.R., 1985, Late Cenozoic stratigraphy and structure of the west margin of the central San Joaquin Valley, California, in Weide, D.L., and Faber, M.L., eds., Soils and Quaternary geology of the southwestern United States: Geological Society of America Special Paper 203, p. 97-114.
#5355 Manson, M.W., 1985, Ortigalita fault, Fresno, Merced, San Benito and Stanislaus Counties, California: Department of Conservation, Division of Mines and Geology Fault Evaluation Report 166, microfiche copy in California Division of Mines and Geology Open-File Report 90-11, 12 p.
#4860 Petersen, M.D., Bryant, W.A., Cramer, C.H., Cao, T., Reichle, M.S., Frankel, A.D., Lienkaemper, J.J., McCrory, P.A., and Schwartz, D.P., 1996, Probabilistic seismic hazard assessment for the State of California: California Department of Conservation, Division of Mines and Geology Open-File Report 96-08 (also U.S. Geological Open-File Report 96-706), 33 p.
#5356 Taliaferro, N.L., 1943, Geologic history and structure of the central Coast Ranges of California, in Geologic formations and economic development of the oil and gas fields of California: California Division of Mines Bulletin 118, part 2, p. 119–163.
#1216 Working Group on Northern California Earthquake Potential (WGNCEP), 1996, Database of potential sources for earthquakes larger than magnitude 6 in northern California: U.S. Geological Survey Open-File Report 96-705, 40 p.