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Application of a Hillslope Development Model to Scarps Along the San Andreas Fault, Carrizo Plain, Ca lifornia

This portion of the project takes elements of the hillslope development model presented in the previous section and applies it to the calibration of the material transport rate constant (diffusivity) and the determination of the ages of fault scarps along the San Andreas Fault Zone in the Carrizo Plain, California.

Morphologic dating of scarps along the San Andreas fault, Carrizo Plain, California and progressive deformation along the Big Bend.

J R Arrowsmith (Dept. of Geology, Arizona State University, Tempe, AZ, 85287;, D. D. Pollard, G. Hilley (both at: GES Dept, Stanford, CA, 94305), and D. D. Rhodes (Dept. of Geology, Whittier College, Whittier, CA 90608)

Morphologic dating of fault scarps has been used to estimate the timing of late Cenozoic fault activity. Observed topographic profiles were compared quantitatively with those simulated by a calibrated landscape development model. Our model assumes transport-limited conditions and applies to slopes over which the material transport rate is proportional only to local slope (diffusion erosion). The material transport rate was calibrated using 9 profiles along the southwest-facing scarp southeast of Wallace Creek in the Carrizo Plain. This scarp has been exposed by lateral offset of a southeast-sloping shutter ridge along the northwest-striking San Andreas Fault (SAF), and by vertical offset related to local deformation. This reconstruction is supported by the established geologic history which provides a Holocene slip rate of 3.5 cm/yr. The initial scarp profiles were reconstructed by projection of remnants of an incised fan surface to the SAF. The lower end of each profile (at the SAF) drops at a rate determined by the difference in elevations between the inferred initial profile and the observed final profile taking into account the time since exposure of that portion of the scarp by passage of the shutter ridge. Given those assumptions, the average rate constant (diffusivity) is 8.5 +/-1.8 m^2/ka at Wallace Creek. Normal fault slip rates were determined for two graben-bounding normal faults in the Northern Elkhorn Hills in the southeastern Carrizo Plain by applying the calibrated model to determine the scarp ages. These scarps are cut into the same material, face the same direction, and presumably have the same climatic history as those at Wallace Creek (only 30 km northwest). Slip rates for these faults are a few millimeters per year, and the northwestern fault is about 12 ka old while the other is about 63 ka. We infer that the normal faults form above reverse faults that accommodate increasing contraction adjacent to the SAF as the material enters the Big Bend. Because one graben is 50 ka older than and 1 km southeast of the other, the material moves into the deformation zone associated with the Big Bend at a rate of about 2 cm/yr. The slip rate determination is incorporated into a Geographic Information System that permits us to investigate the relationships between the progessive development of the normal faults, detailed geologic and geomorphic mapping, and mechanical models of the geologic structures in the SAF zone.

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Side looking airborne radar image of the Carrizo Plain area. Look Direction is toward northeast. The rugged topography of the Temblor Range along the northeast side of the image is a contrast to the flatter Carrizo Plain. The patchwork texture of the Carrizo Plain results from variable grazing and farming practives on different properties. Soda Lake is the lowest portion of the Carrizo Plain (~1900' elevation). The San Andreas Fault (SAF) follows the southwestern foothills of the Temblor Range. Local relief along the SAF and a more westerly SAF strike increase toward the southeast. In the area of Wallace Creek in the northwest, little local relief is evident and Recent fault activity has been confined to predominantly strike-slip along a narrow zone. Toward the southeast in the Elkhorn Hills, on the other hand, local relief is evident and secondary fault activity increase, possibly the result of increased contraction across the SAF zone caused by a more westerly strike as it enters the Big Bend.

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Aerial photograph of the SAF zone in the central Carrizo Plain. The locations of the important landforms and paleoseismic sites of Wallace Creek; Phelan Creeks; and Phelan Fan are shown. The trace of the SAF is indicated by the triangles on the lower photo. Note the en echelon, right-stepping pattern of the SAF in this area. Local extension associated with a right step in the southeastern portion of the lower photograph has right-stepping pattern of the SAF in this area. Local extension associated with a right step in the southeastern portion of the lower photograph has produced a small graben. The upper two photographs are blow-ups to show the detail of Wallace Creek and Phelan Creeks. Photograph courtesy of the Fairchild Aerial Photography Collection at Whittier College. Original photograph scale 1:24,000; photograph date 26 February, 1936.

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Aerial photograph of the Northern Elkhorn Hills. This image illustrates the important structures and landforms in this area of distributed deformation adjacent to the San Andreas Fault in the southeastern Carrizo Plain. Photograph courtesy of the Fairchild Aerial Photography Collection at Whittier College. Original photograph scale 1:24,000, photograph date 26 February 1936.

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Stream offsets at Wallace Creek, Carrizo Plain, California

View northeast across the San Andreas fault showing several offset stream channels. Main channel is offset about 130 m and was incised approximately 3,700 years ago. Channel farther to left on near side of fault has been displaced approximately 350 m, is beheaded, and was incised approximately 10,000 years ago. These offsets and ages provide a long term slip rate of approximately 35 mm/yr along the San Andreas fault here (Sieh and Jahns, 1984). Small gulches at right display about 9 m offset from the 1857 earthquake. No fault creep is observed here and this section of the San Andreas fault is considered locked. Sieh and Wallace (1987) provide a detailed field description of this site. This slide is #13 from Wallace and Schulz (1983).

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Normal faulting adjacent to the San Andreas fault in the Northern Elkhorn Hills (NEH)

This photograph looks north over the northern end of the Big Graben in the NEH. The Temblor Range is in the distance. Both sides of the graben are bounded by active normal faults that began to slip at 62.7 +/- 33.3 ka. That age gives a slip rate of ~1 mm/yr based on morphologic dating of the fault scarp on the extreme right of this picture (see Arrowsmith, 1995 for a discussion of the structural geology and morphologic dating in the NEH). The truck sits on top of a younger fault scarp formed by slip along a newer fault that accomodates slip in the northwestern portion of the Big Graben, shutting down the fault to the northeast (note the more degraded scarp along that graben wall). We dug a trench across the younger fault, but no clear offset stratigraphy was found, probably because the hanging and footwalls are comprised of the same materials. Collapse pits--possibly formed along with the normal faults in response to local tension--are evident to the left of the truck in the base of the Big Graben.