Caliente formation are juxtaposed against late

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Caliente Formation are juxtaposed against Late Cretaceous granite/granodiorite of the eastern San Emigdio Mountains, including Tecuya Ridge (Davis and Duebendorfer, 1982). The San Andreas Fault trace here, though narrow, is effectively obscured by alluvium. Still, Frazier Mountain and Tecuya Ridge show great examples of rock- type influenced vegetation differences. The differences are easily seen driving on Frazier Mountain Park Road two miles west of Frazier Park (see Figure 4). (cont. pg 5) FIGURE 1 Generalized geologic map: Western Transverse Ranges. (From Kellogg, 2004 ) But, left-lateral displacement on the fault adds to the geologic complexity and stress and strain in the Frazier Park area. Dawson, et.al. (2007) document yearly (6.3 mm +/- 2mm) displacement rates and per earthquake (minimum 2.75-3.75 meter rupture offset) displacement rates for the western segment of the Garlock Fault. The average time between events on the Garlock Fault approximately 1100 years. (Dawson, et. al., 2007) Frazier Mountain was named by/for the Frazier (or Fraser) brothers, who discovered gold high on the northeast flank of the mountain in the 1860’s. Page 4
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Some recent workers, using LIDAR for detailed topographic resolution and counting the number of earthquakes capable of causing strike-slip ground rupture, suggest 1857 offset was as little as 16 feet….in areas where 31 feet has been generally accepted (Zielke, et.al., 2012). Regardless of the exact offset distance, there are several geomorphic features that represent 1857 ground rupture. Along Cuddy Valley Road, west of I-5, scarps and a few V-shaped openings in pressure ridges define the plane of 1857 offset (Figure 5), (Lynch, 2009.) At Tejon Pass, pulverized (extremely gouged) granite, juxtaposed against a sliver of sedimentary rock of uncertain age (Figure 6) defines the rupture plane. Roadcuts along Interstate 5 and Old Highway 99 (now known as Peace Valley Road) at Tejon Pass show how abrupt contacts between vastly different rock types can be within the San Andreas Fault Zone (Lynch, 2009). GORMAN POST ROAD Gorman Post Road, five miles long, traverses in and out of traces of the San Andreas Fault. It connects unin- corporated Gorman… barely more than an exit off I -5 just south of Tejon Pass… with HWY 138, an east - west road that leads to Lancaster. Gorman Post Road is aligned with several sag ponds within the fault zone. Yearround,it is common to see wet spots on the road within Gorman. (cont. on pg. 6) Geologic Features of the Tejon Pass Area by Tim Elam ...continued THE 1857 FORT TEJON EARTHQUAKE The last large earthquake in on the San Andreas Fault in southern California was the 1857 Fort Tejon Earth- quake. The earthquake was named for a small, strate- gic fort along the “grapevine” section of I -5, just south of the southern end of the San Joaquin Valley. Al- though its’ epicenter and magnitude (7.9) may be de- bated, there is general agreement that the 1857 earth- quake occurred along the Big Bend segment of the San Andreas Fault. Right lateral offset during this event has long been thought to be 31’ over much of the Big Bend (Lynch, 2009).For many years, the Big Bend segment
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