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San Andreas Fault System in Southern California (SAFSOC)

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WHAT…is the purpose of the project? What is it about?

Produce high-quality, multi-purpose digital geologic maps and accompanying databases and reports to solve diverse land-use problems in high-priority areas. Make geologic map information more accessible to the public by providing geologic maps, reports, and databases in a variety of digital forms. Earthquake Hazard Assessments, Hydrogeologic Framework, desert science applications, and information resources related to natural resources and hazards, and associated impacts on the economy. Project tasks include developing and maintaining geologic map databases, GIS, and decision-support systems, and dissemination of Earth sciences information and outreach. The project combines traditional surficial geologic mapping with geophysical, hydrological, and paleontological research. Mapping products will have widespread application for studies of earthquake and fault motion behavior, as well as applications relevant to impacts on infrastructure and civil planning and development, and education.
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Coachella Valley in southern California as seen from Keyes View in Joshua Tree National Park, CA
Coachella Valley in southern California as seen from Keyes View in Joshua Tree National Park, CA.

Statement of Problem:
The San Andreas Fault (SAF) zone is the dominant structural feature along the transform boundary between the North America and Pacific plates. Plate-margin strain is, however, broadly distributed across a complex array of other structures that interact in various ways with the San Andreas. Patterns of faulting and associated magmatism in this array reflect encroachment of the oceanic-continental plate boundary into the continent. Inception of the transform platemargin occurred at about 30 Ma and spatial patterns of interacting fault sets have evolved in a way that reflects both episodic changes in seafloor spreading dynamics and the differential strain response of various crustal blocks along the continental margin. Development of the San Andreas Fault system in southern California is particularly well suited to discriminating the effects of seafloor spreading episodes and spatial and temporal variations in crustal response mechanisms.

Plate-margin deformation leads either directly (through seismic events) or indirectly (by creating steep range-front relief) to high-risk of natural hazards in southern California, home to more than 16 million people. USGS and USGSsponsored studies have made and continue to make essential contributions to our understanding of the overall geologic and tectonic framework for these hazards. Considering the influence of pre-San Andreas system crustal structure on the evolution of interacting fault patterns, there is a clear need to continue these studies across the span of plate-margin evolution to gain the greatest insight into the distribution of seismic hazards. Although much is being learned by analyzing seismic and paleoseismic events in the region, the portion of San Andreas history elucidated by these studies represents only a small fraction of the overall development of the plate margin transform system. Everything that we know about the evolution of the plate-margin transform system prior the earliest evidence for individual seismic events derives from geologic mapping and related topical studies, including geomorphic, geochronologic, paleontologic, stratigraphic, geophysical, and geohydrologic studies. Moreover, mapping-constrained palinspastic reconstruction of the San Andreas fault system provides paleogeologic and paleogeographic maps that help to better understand aspects of geologic history during the evolution of the transform margin and prior to its inception.

(1) Conduct geologic mapping and stratigraphic studies that contribute to our understanding both of fault interactions in the greater San Andreas system in southern California and of the overall evolution of the fault system. Publish geologic maps.
(2) Conduct geomorphic mapping and carry out tectonic landform analyses that help us understand landscape evolution, plate-margin strain-partitioning, and hazards in southern California.
(3) Conduct paleontologic and paleogeographic studies of benthic foraminifers in Eocene, Miocene, and Pliocene strata along the San Andreas fault: (a) Use findings from the Miocene and Pliocene strata (Imperial, Bouse, and related formations) to refine ages of deposition and timing of the opening of the Gulf of California and to constrain magnitude and timing of dextral offsets in the Trough; (b) Use findings from Eocene marine formations which can be used to correlate tectonic blocks in southern and central California and to define piercing points that constrain reconstruction of the San Andreas Fault system.

Relevance and Impact:
The framework mapping and topical studies proposed in this document support • USGS Science Strategy directions involving natural hazards and water resources • Needs of other agencies for geologic information for resource management and interpretive presentations, including National Park Service, U.S. Forest Service, and California Geological Survey, National Resources Conservation Service, and Department of Defense • Needs of the public and local government agencies for geologic mapping data

Stategy and approach:
Our strategy and approach will be to generate and publish geologic mapping in areas that encompass key fault interactions in the plate margin of southern California and to use that mapping to develop a better understanding of shifting and evolving crustal blocks in the region. Four key areas are defined in separate but interrelated tasks: (1) San Gorgonio Pass/Santa Rosa Mts – Interactions among the dextral San Andreas and San Jacinto fault zones, the sinistral Pinto Mountain fault, and the extensional West Salton Detachment Fault set. This area encompasses the conjunction of the uplifted Peninsular Ranges and Transeverse Ranges blocks and the foundering Salton Trough.
(2) Joshua Tree National Park – Role of the left-lateral faults of the Eastern Transverse Ranges province as a sinistral domain within the dextral Eastern Transverse Ranges province and the interaction of that domain with the San Andreas Fault Zone. This faulting domain links the Eastern California Shear Zone to the San Andreas Fault Zone.
(3) Eastern California Shear Zone in Twentynine Palms area – Interaction between dextral faults in the southern Mojave Desert province and sinistral faults in the Eastern Transverse Ranges province. This area straddles an apparent transition from greater seismicity in the Mojave block to lesser seismically in the Eastern Transverse Ranges block.
(4) Southwestern San Gabriel Mts – Interaction of the dextral San Gabriel and Vasquez Creek fault zones, the sinistral Raymond and related faults, the Sierra Madre-Cucamonga thrust fault zone, and steep normal faults along the range front. This area encompasses the transition from the uplifted crystalline San Gabriel Mts block of the Transverse Ranges into the deep-basin domain of the San Gabriel and San Fernando valleys and the Los Angeles Basin. A better grasp of how these various faults have intereacted is important for understanding segmentation along this high-relief range-front. In addition, we have created a task that encompasses paleontological studies along the San Andreas Fault from the Gulf of California into central California (offset from southern California) and that also affords an opportunity for project staff interactions in the field and in comparing various fault reconstruction models based on paleogeologic patterns in the crystalline rocks of southern California and their overlying cover strata:
5. Paleontologic studies – Detailed micropaleontologic study of sedimentary strata yields both time and position data that reflect tectonic events coeval with growth of the basins in which the strata were deposited. For southern and central California, tectonic events by which the plate boundary changed from a convergent margin to a transform margin, and subsequent events that resulted in the evolution of the San Andreas Fault system, are recorded in strata in basins along that system. For marine strata, paleontologic studies of foraminifers can indicate time of deposition and yield paleoenvironmental information about latitude at time of deposition, as well as proximity to the coastline and depth below sea level. We will match faunal assemblages in displaced blocks of marine Eocene, Miocene, and Pliocene strata to define piercing "points" that constrain timing and magnitude of strike-slip offset on various faults of the San Andreas system.

WHAT…scientific questions hope to be addressed?

WHAT…tasks are part of this project?

The project will be organized into five tasks:

TASK 1) - San Andreas Fault System in Northwestern Salton Trough Region
TASK 2) - Joshua Tree National Park Studies
TASK 3) - Eastern California Shear Zone in Twentynine Palms Area
TASK 4) - Paleontologic Studies Along the San Andreas Fault System
TASK 5) - Fault Interactions in the Southwestern San Gabriel Mountains

WHEN did the project start? October 1, 2010

WHEN is the project anticipated to be completed? September 30, 2015

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