|Western Earth Surface Processes Team|
Extensional Fault Zones
Normal Faults of the Coachella and Morongo Valleys
Faults that have normal dip-slip separations occur in the southern Indio Hills and in the northern Coachella Valley. Clark (1984, p. 4-5, map sheet 1) attributed the distinctive zone of normal faults in the southern Indio Hills to uplift northeast of the San Andreas fault, which he in turn related to displacements across the San Andreas. Northeast-oriented faults in the northern Coachella Valley have not been studied; the faults have both north- and south-facing scarps, and break late (but not latest) Quaternary alluvium. Northeast-oriented faults and lineaments in the Morongo Valley area, including the Morongo Valley fault, also appear to have normal dip-slip separations. The kinematic role of normal faults in this region has not been documented.
Beaumont Plain Fault Zone
We apply the name Beaumont Plain fault zone to a series of northwest-trending en-echelon fault scarps that traverse late Quaternary alluvial deposits in the vicinity of Beaumont. Most of the scarps face northeast, but one short scarp segment faces southwest. We have not documented the style or history of faulting that created these scarps; however, they appear to have formed by normal dip-slip displacements and probably represent an extensional strain field. This interpretation is strengthened by closely spaced northeast- and southwest-facing scarps northeast of Beaumont which bound a down-dropped block that forms a graben. Similar faults having northwest trends and northeast-facing scarps occur elsewhere in the San Gorgonio Pass region for example, scarps that have been referred to the modern trace of the San Andreas fault at Burro Flat, and scarps near Oak Glen and Wildwood Canyon. We do not understand the kinematic role of these faults, but they may represent a family of related features formed by regional extension.
Crafton Hills Horst-and-Graben Complex
We apply the name Crafton Hills fault zone to a system of normal dip-slip faults that forms a horst-and-graben complex in the vicinity of Redlands and Yucaipa. These faults bound the west and east flanks of the Crafton Hills, and break late Quaternary alluvium in the valleys of Oak Glen and Wilson Creeks. The faults trend northeast in the vicinity of the Crafton Hills (Matti and others, 1992), but adopt more easterly trends near the San Bernardino strand of the San Andreas fault and south of Redlands (Morton, 1978b). Normal faulting within this zone coincides geographically with a series of conspicuous left steps in the San Bernardino strand, and with the western termination of the San Gorgonio Pass compressional fault system.
The Crafton Hills horst-and-graben complex is a neotectonic structural element that has been active during both late Pleistocene and Holocene time, although not all faults in the zone break Holocene alluvial deposits. The complex represents a zone of extensional faulting in a region where right-lateral and reverse faulting are the most obvious expressions of crustal deformation.
Dip-slip Faults of the San Bernardino Valley and San Bernardino Mountains
The Peters and Tokay Hill faults occur south of the San Bernardino strand of the San Andreas fault in the Devore area southeast of Cajon Pass (Morton and Miller, 1975; Morton and Matti, 1990b). The faults break Holocene deposits and probably have normal dip-slip displacements. This geometry is complicated locally where the Tokay Hill fault intersects the San Bernardino strand of the San Andreas fault: there, the Tokay Hill fault dips southwest and appears to have a reverse dip-slip geometry (relations observed by us in a trench excavated by G. Rasmussen and Associates). Similar faults traverse the southwest-facing escarpment of the western San Bernardino Mountains; they are east-trending, and almost all have north-facing scarps with down-on-the-north displacements (Weldon, 1985b, pl. 12). The Peters and Tokay Hill faults and those in the western part of the San Bernardino Mountains form a family of related dip-slip faults that reflect an extensional strain field.Continue to Neotectonic Framework of the South-central Transverse Ranges and Vicinity.
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