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Modeling the Effects of the San Andreas, Garlock Faults and Sierra Nevada Frontal Fault Zone on the Uplift of the S

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2011 Friends of the Pleistocene Field Trip Stop 1-1c: Modeling the effects of the San Andreas, Garlock faults and Sierra Nevada frontal fault zone on the uplift of the Sierra Nevada mountains, California Andrea Figueroa, California State University, Fullerton David Bowman, California State University, Fullerton Jeffrey Knott, California State University, Fullerton Abstract 2002; Unruh, 1991; Wakabayashi and Sawyer, 2001). However, controversy continues Hypotheses that explain the mechanism of uplift regarding the mechanism driving the late for the Sierra and subsidence of the adjacent Cenozoic uplift (Small and Anderson, 1995; San Joaquin Valley have largely failed to Unruh, 1991; Wakabayashi and Sawyer, 2001; consider the influence of the San Andreas Fault Wernicke et al., 1996). (SAF). To examine the influence of the SAF, Garlock and Sierra Nevada Frontal Fault Zone The question proposed herein is: Can the fault (SNFFZ) on late Cenozoic Sierra uplift geometry and slip rates produce the southern dislocation models of regional crustal Sierra and San Joaquin valley topography? In deformation were constructed and run using this paper, a 3D crustal dislocation model is geologically determined slip rates. Slip on only presented that describes strain generated by the the SNFFZ generates the observed mountain interaction of the SAF – Garlock – SNFFZ fault front-piedmont intersection, but not the observed systems. The goal of the modeling is to use subsidence of the San Joaquin Valley. Models various time-dependent fault slip rates and fault with combined slip on the Garlock, SAF and interaction to replicate the modern topography. vertical slip on the SNFFZ generate the observed greater subsidence in the southern Methods San Joaquin. Based on modeling, pre-Cenozoic Modeling Physical Parameters uplift along the SNFFZ could define the large- Faults are modeled as dislocations in an elastic scale crustal blocks and relict topography. The half-space (Okada, 1992). In this large-scale post-5 Ma Sierra and San Joaquin Valley model the lithosphere is assumed to behave in deformation may be produced by a complex an elastic fashion (Hubert-Ferrari at al., 2003). interaction between SAF and Garlock strike-slip Faults are areas where dislocations are imposed motion and SNFFZ normal faulting, with SNFFZ on the model. The resulting elastic deformation strike-slip faulting having little influence. is calculated. Fault locations are from Jennings Introduction (1994). The Sierra Nevada Mountains of eastern San Andreas Fault California (Sierra) were a prominent topographic In the model, the SAF is a vertical (Eberhart- high in the early Cenozoic that has undergone a Phillips et al., Snay et al., 196; Zhu 2000), right lesser, yet significant, late Cenozoic uplift lateral fault with variable slip rates. For modeling (Bateman and Wahrhaftig, 1966; House et al., purposes, the SAF slip rate is estimated to be 10 2001; Huber, 1981; Poage and Chamberlain, mm/yr pre-5 Ma and 30 mm/yr after 5 Ma 28 2011 Friends of the Pleistocene Field Trip (Atwater, 1970; Atwater and Stock, 1998; beginnings of SNFFZ normal faulting and Sierra Eberhart-Phillips et al., 1990; Jones and uplift should be simultaneous. Estimates of the Wesnousky, 1992; Niemi and Hall, 1992; initiation of late Cenozoic uplift vary from 10 Ma Petersen and Wesnousky, 1994; Sedlock and to 5 Ma, while onset of normal faulting ranges Hamilton, 1991; Snay et al., 1996 Weldon and from 2.3 to 7 Ma (Bachman, 1978; Monastero et Sieh, 1985; Wesnousky, 1986; Zhu, 2000). al., 2002; Reheis and Sawyer, 1997). Garlock Fault The vertical component of slip, as measured on normal and oblique faults, varies from 0.1 to 2.5 In the model, the Garlock fault is a vertical (Astiz mm/yr (Berry, 1997; Clark and Gillespie, 1993; and Allen, 1983), left-lateral strike slip fault Clark et al., 1984; Martel et al., 1987; Zehfuss et extending at least 265 km eastward from the al., 2001). River incision and tilted strata in the SAF (Figures 2 and 3; (Hutton et al., 1991). A San Joaquin Valley reveal a longer-term rate of slip rate of 10 mm/yr was assigned to the 0.28º per million years for the last 5 Ma (Unruh, Garlock fault since estimates of slip rate vary 1991). To produce this tilt requires a 0.5-mm/yr from 7 to 13 mm/yr (Clark et al., 1984; Eberhart- vertical slip rate on the SNFFZ. Thus, a model Phillips et al., 1990; McGill and Sieh 1993; slip rate of 0.5 mm/yr was chosen. Petersen and Wesnousky, 1994; Smith, 1962; Smith et al., 2002; Snay et al., 1996). It is Strike Slip and Oblique Slip on the SNFFZ. assumed in the model that slip on the Garlock Oblique (dextral, normal) motion is common fault began at 5 Ma when the slip rate on the along the SNFFZ (Table 2). The transition from SAF increased (Hill and Dibblee, 1953). vertical to oblique motion within the SNFFZ occurred after the onset of normal faulting Sierra Nevada Frontal Fault Zone (SNFFZ) between 3.5 and 2 Ma and propagated The SNFFZ was modeled as one continuous northward (Monastero et al., 2002). Estimates of fault with an easterly dip of 70º. This dip angle the dextral slip rate on the Owens Valley fault and direction is consistent with the 80 ± 15º dip (southern section of SNFFZ) range from 1.5 to estimated from 1872 rupture (Beanland and 8.5 mm/yr (Beanland and Clark, 1982; Gan et Clark, 1982). al., 2000; Lee et al., 2001a; Lee et al., 2000; Lee Normal Slip Component of SNFFZ. If late et al., 2001b; Reheis and Dixon, 1996) with most Cenozoic uplift of the Sierra is the result of values close to 2 mm/yr. For the model, a right- normal faulting along the SNFFZ, then the lateral strike-slip rate of 2 mm/yr was chosen. Table 1. Model Slip Rates. Slip rates (mm/year) for fault zones used for each model version. Model Conditions San Andreas Garlock SNFFZ Lateral SNFFZ Normal Figure Only uplift 0 0 0 0.5 1a No SNFFZ activity 30 10 0 0 1b 5 - 3 Ma 30 10 0 0.5 1c 3 Ma to present 30 10 2 0.5 1d 29 2011 Friends of the Pleistocene Field Trip Model Construction Modeling of elastic deformation was done using Nutcracker v6.3 (http://geology.fullerton.edu/ bowman/downloads.html). The model was constructed to cover the area 35.2º N 121.2º W to 38.8º N to 117.8º W. All calculations of elastic deformation in response to slip on the faults were done at a grid spacing of 5 km. Model Results Scenario A Many studies have proposed that uplift of the Sierra is the result of normal slip on the SNFFZ along the east side of the range (e.g., Huber, 1981; 1987; Wakabayashi and Sawyer, 2001). Figure 3a. Model results with normal slip on SNFFZ To test this hypothesis, Scenario A included 0.5 only. Contours show elevation change in meters. mm/yr of west-side-up slip on the SNFFZ and no Yellow tints are uplift; green and blue tints are lateral slip on the SAF, SNFFZ and Garlock subsidence. faults (Figure 1a). Scenario A shows both uniform uplift perpendicular to SNFFZ and uniform subsidence in the San Joaquin Valley parallel to the axis of the modern valley. However, drawbacks to Scenario A are that it does no uplift along the SAF nor does it produce greater subsidence in the southern San Joaquin Valley. Scenario B Scenario B models deformation generated by slip on the Garlock and SAF alone (Table 1) with no slip on the SNFFZ. As such, Scenario B tests whether the SAF and Garlock faults, without the SNFFZ, could generate Sierra uplift or San Joaquin valley subsidence. Scenario B produces Figure 1b. Model results with strike slip on SAF and a region of subsidence in the southern San Garlock. Joaquin Valley that extends across the southern Sierras (Figure 1b). Uplift produced by this Scenario C model is found along the SAF extending Scenario C models the 5-3 Ma tectonic eastward into the northern San Joaquin Valley; conditions by using slip rates of 30 mm/yr for the however, Scenario B does not produce uplift of SAF, 10 mm/yr for the Garlock, and 0.5 mm/yr of the Sierras. west side up slip for the SNFFZ (Figure 1c). In Scenario C, the southern Sierra has a narrower 30 2011 Friends of the Pleistocene Field Trip Scenario D Scenario D depicts crustal deformation similar to Scenario C with the addition of 2 mm/yr of dextral slip along the SNFFZ. The 2 mm/yr of strike slip motion on the SNFFZ (Figure 1d) results in a more restricted subsidence in the southern San Joaquin Valley along with the accompanying greater area of uplift of the southern Sierra and along the SAF. Discussion Many studies have invoked uplift along the SNFFZ as the key mechanism producing the late Cenozoic Sierra and San Joaquin topography (e.g., Huber, 1981; 1987; Unruh, 1991; Figure 1c. Model results with slip rates from 5-3 Ma Wakabayashi and Sawyer, 2001). Scenario A – see Table 1 for rates. uses uplift by normal slip on the SNFFZ only; area of uplift compared to the northern Sierras. however, critically absent is the generation of the Subsidence is generated in the southern Sierra topographic low in the southern San Joaquin and San Joaquin Valley, and there is uplift of the Valley. Scenario A accurately predicts the region along the SAF. variation in width of the Sierra with latitude, which is likely a relict topographic feature generated by pre-5 Ma slip on the SNFFZ.
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