Garlock-Hosgri transfer zone

Garlock-Hosgri transfer zone
Name	Garlock-Hosgri transfer zone
Type	transfer zone / strike-slip-transpressional structure
Location	Southern California coast, Transverse Ranges, Mojave Desert
Coordinates	approx. 35°N, 119°W
Length	~150 km (approx.)
Movement	left-lateral transtensional/transpressional transfer
Notable	linkage between fault systems

Garlock-Hosgri transfer zone The Garlock-Hosgri transfer zone is a major crustal linkage that transfers displacement between the Garlock Fault and the Hosgri Fault along the southern California margin, connecting structural domains influenced by the San Andreas Fault, Eastern California shear zone, and the Transverse Ranges. It occupies a critical position between the Mojave Desert and the Santa Maria Basin, interfacing with the Sierra Nevada forearc and offshore structures adjoining the Pacific Plate boundary. Research into the zone integrates data from institutions such as the United States Geological Survey, California Institute of Technology, and regional universities.

Overview and Geological Setting

The transfer zone lies where onshore structures of the Mojave Block step west to meet offshore systems in the Santa Barbara Channel, the San Joaquin Basin, and the Santa Maria Basin, juxtaposing crustal blocks mapped by the California Geological Survey, United States Navy seismic surveys, and academic studies from Stanford University and University of California, Santa Barbara. Regional geology shows interaction among the Sierra Nevada microplate, the Pacific Plate, and the North American Plate along strands related to the San Andreas Fault System, the Garlock Fault, and the Hosgri Fault complex. The setting includes topographic elements such as the Transverse Ranges, the Tehachapi Mountains, and the offshore Channel Islands structural province, with sedimentary basins like the Kern Basin and Santa Maria Basin recording syn-tectonic deposition.

Structural Anatomy and Kinematics

Structurally the transfer zone comprises anastomosing strike-slip faults, oblique reverse faults, and accommodation zones documented in studies by the National Earthquake Information Center and seismic reflection programs of the Scripps Institution of Oceanography. Kinematic models invoke left-lateral motion linking the right-lateral Garlock Fault to the right-lateral Hosgri Fault, mediated by bends, relay ramps, and stepovers observed in field mapping by teams from Caltech and University of Southern California. Component structures include fault splays that interact with folds like the San Andreas fold belt and thrusts related to the Santa Barbara fold-and-thrust belt, producing transpressional uplift and localized transtensional basins comparable to features described in the Eastern California shear zone.

Tectonic Evolution and Seismicity

Tectonic evolution reflects Mio-Pliocene reorganization following initiation of the San Andreas Fault system and the clockwise rotation of crustal blocks documented in paleomagnetic studies from University of California, Berkeley and thermochronology from Stanford University. The transfer zone has been implicated in redistributing slip during major ruptures on the San Andreas Fault and offshore events recorded in catalogs maintained by the USGS and the International Seismological Centre. Notable regional earthquakes influencing stress transfer include historic events associated with the 1857 Fort Tejon earthquake, the 1971 San Fernando earthquake, and offshore events near the Arroyo Grande Fault system, with aftershock sequences and Coulomb stress modeling by researchers at Scripps Institution of Oceanography and UC Santa Cruz.

Stratigraphy and Stratigraphic Correlation

Stratigraphic records across the transfer zone tie onshore sedimentary sequences such as the Monterey Formation, Fernando Formation, and Vaqueros Formation to offshore reflector packages imaged in industry seismic lines acquired by companies like Chevron and government-funded seismic programs. Correlation integrates biostratigraphy using microfossil zonations from specialists affiliated with Smithsonian Institution collections and lithostratigraphic mapping by the USGS and California Department of Conservation. Syn-kinematic growth strata preserved in the Santa Maria Basin and Kern Basin provide temporal constraints for phases of activity during the Pliocene and Pleistocene recorded by magnetostratigraphy and radiometric dating undertaken at Caltech and UC Santa Barbara laboratories.

Geophysical and Geodetic Investigations

Geophysical imaging via reflection seismic surveys executed by the United States Navy and commercial contractors, crustal-scale tomography from the Southern California Earthquake Center, and potential-field mapping by the USGS reveal fault geometry and depth extent linking the onshore and offshore systems. Geodetic constraints from Global Positioning System networks operated by UNAVCO, continuous GPS stations at Scripps Institution of Oceanography, and InSAR studies by NASA and Jet Propulsion Laboratory quantify present-day strain partitioning across the transfer zone and adjacent blocks like the Mojave Block and Peninsular Ranges. Marine geodesy and multibeam bathymetry from the Monterey Bay Aquarium Research Institute and NOAA map submarine fault scarps and submarine landslide deposits that document active deformation.

Paleoseismology and Slip History

Paleoseismic trenching onshore by teams from USGS and Caltech and submarine paleoseismic studies using piston cores analyzed at Woods Hole Oceanographic Institution and Scripps Institution of Oceanography constrain recurrence intervals and slip per event. Events inferred from turbidites in the Santa Barbara Channel and stratigraphic offsets in the Monterey Formation have been correlated with regional rupture scenarios proposed by investigators at Southern California Earthquake Center and UC Santa Cruz, with slip rates estimated by cosmogenic nuclide studies at University of Washington and paleoseismic chronologies calibrated against radiocarbon ages processed at Lawrence Livermore National Laboratory.

Implications for Regional Fault Systems and Hazards

The transfer zone influences seismic hazard models developed by the USGS National Seismic Hazard Model, because it mediates rupture propagation between the San Andreas Fault and offshore faults such as the Hosgri Fault and San Simeon Fault. Urban and infrastructure exposure involves regions served by agencies like the California Office of Emergency Services and the Federal Emergency Management Agency, and energy infrastructure operated by companies including Pacific Gas and Electric Company and Shell Oil Company. Understanding fault linkage, cumulative slip, and potential multi-fault rupture scenarios informs hazard mitigation for population centers near Los Angeles, Santa Barbara, and San Luis Obispo and supports planning by regional authorities including the Association of Bay Area Governments and county emergency planners.

Category:Geology of California Category:Seismotectonics Category:Fault zones