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Southern California Geology

Major Faults of Southern California
Inland Empire Region

Text from USGS Open-File Report 92-354

San Andreas Fault Zone

 

index map of faults and basement terranes Click on this thumbnail to view an index map of faults and basement terranes in the Inland Empire region of San Bernardino and Riverside Counties. The file is a 135Kb pdf graphic that requires a portable-document file reader to examine and read. Link to Adobe Acrobat Reader to download a cost-free version of a pdf reader. Depending on your browser's setup, you may read the file now or save it to your disk.

Table of Contents for the San Andreas Fault Zone (click on any item to jump to that section

General Statement

Fault Nomenclature

Mojave Desert Segment, San Andreas Fault

Geologic Setting in the San Gabriel Mountains

Faulting Chronology

Displacement History

Coachella Valley Segment, San Andreas Fault

Displacement History

San Bernardino Mountains Segment, San Andreas Fault

 

Geologic Setting

Wilson Creek and Mission Creek faults

Wilson Creek and Mission Creek faults in the southeastern San Bernardino Mountains

Wilson Creek fault along the southwest margin of the San Bernardino Mountains

Mission Creek fault along the southwest margin of the San Bernardino Mountains

Mill Creek Strand

San Bernardino Strand

Cajon Pass to Mill Creek segment

Mill Creek to Banning Canyon segment

Banning Canyon to Burro Flat segment

Geologic History

Sequencing Relations

Faulting Chronology

Wilson Creek and Mission Creek Strands

Mill Creek Strand

San Bernardino Strand

Amount of Displacement: Previous Interpretations

Amount of Displacement: New Possibilities

San Bernardino Strand

Mill Creek strand

Mission Creek and Wilson Creek strand

Synthesis

Discussion

 

General Statement

The main strand of the San Andreas fault in southern California consists of two segments (fig. 1): (1) the Mojave Desert segment that mainly separates rocks of San Gabriel Mountains-type from rocks of San Bernardino Mountains-type, and (2) the Coachella Valley segment that separates rocks of Peninsular Ranges-type from rocks of San Bernardino Mountains- and San Gabriel Mountains-type. In plan view the main strand has a left-stepping geometry: the Mojave Desert segment has been stepped left (west) about 15 km from the Coachella Valley segment, with the step occurring in the San Gorgonio Pass region of the southeastern San Bernardino Mountains (Matti and others, 1985). This left step forms a structural knot in the San Andreas that has influenced the evolution of the entire transform-fault system and has led to multiple fault strands that evolved sequentially. Multiple strands also have developed along the Mojave Desert segment of the San Andreas (Barrows and others, 1985, 1987). This strand complexity has made it difficult to identify the distribution and displacement history of faults in the San Andreas family, and has led to variable fault nomenclature and to conflicting fault-movement scenarios.


Fault Nomenclature

The complex pattern of late Cenozoic right-lateral faults in southern California has led to nomenclature for the San Andreas fault that is more complex than for central California. There, most if not all Miocene and younger displacement on the San Andreas has occurred within a narrow, singular (although complex) zone that extends the length of central California to Hill and Dibblee's (1953) "Big Bend" at the latitude of the Garlock fault. The name "San Andreas fault" has been used by most workers in central California (Hill, 1981), and little confusion has arisen with regard to which geologic structure bears the name "San Andreas fault" or whether multiple fault strands have generated sequential displacements. By contrast, the San Andreas fault in southern California consists of multiple strands, each representing some portion of the geologic history allocated to the more singular zone in central California (Noble, 1932; Dibblee, 1954, 1968a; Allen, 1957; Crowell, 1962; Woodburne, 1975; Matti and others, 1985). This structural complexity has presented two challenges: (1) to document the distribution of the various fault strands and to determine their sequencing and amount of right-lateral displacement, and (2) to establish nomenclature that provides a logical framework for understanding faulting history and for relating fault segments having similar and/or dissimilar movement histories.

A nomenclatural framework for the San Andreas fault in southern California has evolved through the efforts of many workers (see the historical review by Hill, 1981). Early workers recognized that the characteristic geomorphic and geologic features of the San Andreas rift zone in central California extend southeast beyond Hill and Dibblee's Big Bend region and intervene between the Mojave Desert and the massifs of Liebre Mountain and the San Gabriel Mountains. On this basis, the name "San Andreas fault" originally was extended into southern California. However, Noble (1926, 1932) was among the first to observe that the San Andreas in southern California splays into several major branches--some occurring close to each other within the narrow Mojave Desert zone, others forming discrete strands that follow independent traces many kilometers apart. As Allen (1957) pointed out, this strand complexity creates a dilemma: which strand should bear the name San Andreas fault and which strand generated the large displacements proposed by Hill and Dibblee (1953) for the fault in central California?

Whenever possible, we refer to the San Andreas fault proper in southern California by one of the specific strand names identified in Table 1. However, because each of these strands merged with or fed into the San Andreas fault in central California, each southern California strand during its lifetime represented the San Andreas before it was abandoned and succeeded by the next "San Andreas." This iterative pattern has culminated in the modern San Andreas fault in southern California--the genetically related set of strands that most workers believe originated onshore in response to Pliocene opening of the Gulf of California by sea-floor spreading and transform faulting (Atwater, 1970). The name San Andreas fault usually is applied to this modern strand. Within this complex geologic and nomenclatural framework, we use the term "San Andreas (sensu lato)" for all strands of the San Andreas in southern California that have fed into the central California segment of the fault and contributed to its total history; we use the term "San Andreas (sensu stricto)" for those strands in southern California that have contributed to displacements on the fault only in Pliocene and Quaternary time. Thus, we can refer to the San Andreas fault generically without reference to a particular strand, but at the same time distinguish between broader vs. narrower interpretations of the name.


Continue to Mojave Desert Segment, San Andreas Fault


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