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Stop 1-1b: Tectonic of the western , CA Andrea Figueroa, State University, Fullerton Jeffrey Knott, California State University, Fullerton

Abstract Hypotheses regarding uplift of the Sierra Nevada Mountains (Sierra), California, USA, vary from a single, westward tilting block to rapid Pliocene and/or Quaternary uplift by various mechanisms. To test these hypotheses, we examined geomorphic indices of the western Sierra. We interpret longitudinal profiles of the larger westerly flowing rivers, front sinuosity, valley floor-to-width to height ratio, and relief ratio to show that there are two tectonically active segments of the western Sierra mountain front. Relative tectonic activity is greatest in the southern Sierra near the Gorge . In the central Sierra, the geomorphology indicates lower, but still active, tectonic activity that we hypothesize is consistent with Pliocene delamination. We hypothesize that the locus of tectonism in the south is most consistent with Figure 1. Sierra Nevada and vicinity. Dotted line is post-Pliocene interaction of the San Andreas, edge of San Joaquin and Sacramento River valleys. Garlock, and Sierra Nevada Frontal fault zones. Long dash line is approximate location of Sierra crest with Mt. Whitney the highest point. The circle is the Introduction approx. location of the mantle drip (Saleeby and Several different hypotheses for the style and Saleeby, 2004). Major faults are San Andreas (SA), Garlock (G), Owens Valley frontal fault (OWFF), mechanism of late Cenozoic Sierra Nevada White Wolf (WW) and Kern River Gorge (K). Ball and Mountains, California, USA (Sierra) surface uplift bar and barb are on hanging wall for normal and are proposed (e.g., Huber, 1981; Unruh, 1991; thrust faults, respectively. Strike-slip motion shown by Wakabayashi and Sawyer, 2001; Stock et al., arrows. Precipitation contours for 150 cm/yr and 75 2004). In this study, we apply geomorphic indices cm/yr are generalized from National Atlas to the western mountain front and west-flowing (www.nationalatlas.gov). Limits of glaciation is Tahoe rivers of the Sierra (Fig. 1). We chose stage of Blackwelder (1931) taken from Atwater et al. (1986). geomorphic indices based on their likelihood to record perturbations or reach equilibrium over side of the Sierras (e.g., Huber, 1981; Unruh, late Cenozoic time scales. 1991). Huber (1981) proposed that the Sierras The most frequently cited mechanism is westerly experienced uniform westward tilting of the entire tilting attributed to normal faulting along the east Sierra from north to south (Fig. 2e).


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Wakabayashi and Sawyer (2001) supported this al., 2000; Niemi, 2003). As the lower crust hypothesis by noting that relatively uniform uplift delaminated and sank into the mantle, rising rates inferred from river incision are seen from asthenosphere induced uplift (Ducea and the Yuba River south to the San Joaquin River Saleeby, 1998; Liu and Shen, 1998). The (Fig. 1). delamination is modeled as a singular event initiated in the late Miocene or early Pliocene (Manley et al., 2000) with the subsequent tectonic impact decreasing through the Quaternary (Fig. 2a). The locus of delamination is found in the central Sierras near the Kings, Kaweah and Tule Rivers (Farmer et al., 2002; Saleeby and Foster, 2004). The delamination and subsequent mantle instability would have generated a mantle drip structure in the mantle below the Tulare Basin Figure 2. Hypothetical relations between tectonic geomorphic indices and differing (Saleeby and Foster, 2004). This mantle drip scenarios: a) delamination caused by Basin and may be pulling down the crust near the Kings, Range extension (Ducea and Saleeby, 1998; Liu and Kaweah, and Tule Rivers (Fig. 2d), generating a Shen, 1998; Manley et al., 2000; Niemi, 2003); b) basin in this region of the San Joaquin Valley. If northward migration of the Mendicino triple junction Pliocene delamination is driving uplift, then the (Crough, 1977); c) delamination in the central and greatest tectonic activity would be found in the northern Sierras (Farmer, 2002); d) mantle drip central Sierras (Fig. 2c); however, the effects of (Saleeby, 2004); e. uniform uplift related to slip on the Owens Valley frontal fault (Wakabayashi and Sawyer, any Pliocene delamination in would have 2001); f) climate-driven isostatic rebound (Small and decreased through the Quaternary. Anderson, 1995). Another hypothesis infers that tilting of the Alternatively, Sierran uplift may be in response to Sierras may be in response to isostatic rebound the northward migration of the Mendocino triple from high rates of glacial erosion (Montgomery, junction (Crough and Thompson, 1977; Jones et 1994; Small and Anderson, 1995). According to al., 2004). According to this hypothesis, because Small and Anderson (1995), this hypothesis the triple junction moved from south to north, the predicts that the greatest amounts of denudation, southern Sierra have been rising the longest and and therefore isostatic response, occurred in have therefore experienced the most total uplift areas with the greatest glaciation. As a result, (Fig. 2b). the southernmost Sierra near the Kern River would have had the least glaciation as it has Niemi (2003) proposed that Sierra uplift initiated lower elevations and precipitation and, because of westward migration of Basin and consequently, relatively lower uplift rates (Fig. Range extension. Basin and Range extension is 2f). thought to have propagated NW since the Miocene and continued through the Quaternary. Methods Uplift may also be related to delamination of the Digital elevation models (DEMs) were used to upper mantle from the lower crust (Ducea and calculate geomorphic indices. All river length Saleeby, 1998; Liu and Shen, 1998; Manley et measurements were made using Rivertools, as


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were all river longitudinal profiles. Mapinfo v7.0 Isabella has a convex upward profile showing at was used in the measurement and calculation of least 600 m of incision below equilibrium at the all other values mountain front where the Kern River Gorge fault is found. Tectonic activity induces topographic and base- level changes in fluvial systems that may be The Tule River has an overall concave upward used to decipher relative tectonic activity among shape (Fig. 5A). A knickpoint indicates at least mountain ranges (e.g., Bull and McFadden, 200 m of incision is found near the river mouth. 1977) and segmentation along a single mountain The Kaweah River also has a concave upward range (e.g., Wells et al., 1988). Quantification of longitudinal profile (Fig. 5B), but a knickpoint is geomorphic response to tectonic activity may be found at the confluence with a tributary, showing done by using geomorphic indices (Bull, 1984). that the river is not completely in equilibrium. In this study the following indices were used: The Kaweah River knickpoint (Fig. 5C) is also at mountain front sinuosity (Bull and McFadden, an elevation of 2000 m, within the glaciated 1977), valley floor width to height ratio and relief portion of the Sierra. ratio (Bull and McFadden, 1977; Rockwell et al., The Kings River has an overall concave upward 1985; Wells et al., 1988). In addition, river profile, but several knickpoints are found at the profiles were created because they record long confluence between tributaries and the trunk term system equilibrium (Ritter et al. 2001). streams in both the middle and south forks, and Results at the confluence between the two forks (Fig. 5C). The one knickpoint with a low enough Both the mountain-front sinuosity and valley elevation to rule out as glacial in origin shows at floor width to height ratio are the lowest near the least 200 m of incision. The San Joaquin Kern River and increase with increasing latitude longitudinal is largely a result of three reservoirs (Fig. 3A and B). The highest relief ratios are along the river‘s course. found at the Tule and Kaweah Rivers, respectively (Fig. 3C). The Merced River longitudinal profiles are a good example of the effects of glaciation. The main (north) fork of the Merced has several prominent knickpoints that are the result of glacial downcutting near Yosemite Valley (Fig. 6B). Discussion Variations in the geomorphic indices along the Figure 3. Plots of (A) mountain front sinuosity, (B) western Sierra suggest different uplift histories valley floor width to height ratio and (C) relief ratio and base level changes with latitude. We divide versus distance northeast of the Kern River. Gray the western Sierra into three areas based on bars indicate rivers shown on Fig. 1 and as labeled tectonic geomorphology: (i) north of the Kings on A. River, (ii) the Tule-Kaweah-Kings area and (iii) The longitudinal profile of the main fork of the the Kern River area – south of the Tule River. Kern River (Fig. 4A) is fault controlled above The mountain front north of the Kings River is Lake Isabella (Webb, 1955; Nadin and Saleeby, inactive. The mountain front sinuosity and basin 2005). The Kern River reach below Lake relief ratios are >2, which is consistent with an


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Figure 1. Longitudinal profiles of the Kern River (A) and its tributary Sweetwater Creek (B). Distances are measured downstream from the basin divide. Gray bars below profile show the lateral extent of Pleistocene glaciation and reaches influenced by the Kern Canyon fault. Dam location (D) and relative movement (U=upthrown block; D=downthrown block) of fault are also shown.

Figure 6. Profile of Merced River with relict glacial topography.

Figure 2. Profiles of Tule, Kaweah and Kings Rivers. 25

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inactive mountain front using the criteria of Bull incision below an equilibrium profile. All of and McFadden (1977). Longitudinal profiles of these geomorphic ratios are consistent with an rivers in this region show a concave up profile active, class 1 mountain front according to Bull that approximate an equilibrium profile. and McFadden (1977). The 600 m of uplift is consistent with the basement uplift estimated In the Tule-Kaweah-Kings River area the by Hart et al. (1984) for the Kern River Gorge mountain front sinuosity is 1.8-1.9. The Tule fault at the mountain front. The relief ratio of the and Kings Rivers also have knickpoints in their Kern River is relatively low; however, this may longitudinal profiles are unrelated to glaciation. reflect structural control of the basin. Farmer et This region also has higher basin relief ratios al. (2002) interpreted geochemical data from consistent with greater tectonic activity. the Kern volcanic field as inconsistent with The Tule-Kaweah-Kings mountain front delamination in the Kern River area. As a qualifies as a class 2, or an active mountain result, we interpret the geomorphology as front using Bull and McFadden‘s criteria; consistent with active uplift along the Kern however, the mountain front sinuosity and relief River Gorge fault. ratio are not consistent. The Tule-Kaweah- To explain the Kings River area coincides with anomalous two-fold topography (more linear mountain front) and of the southern the mantle drip (Fig. 1) related to Pliocene Sierra, we crustal delamination (Saleeby and Foster, hypothesize that 2004). in the Tule-Kaweah-Kings the Pliocene area is geochemically consistent with delamination and lithospheric delamination (Farmer et al., 2002). mantle drip Along the Kings River, Stock et al. (2004) generated rapid described the Kings River as experiencing uplift in the Tule- more rapid Pliocene incision followed by slower Kaweah-Kings uplift during the Quaternary, possibly related to Figure 7. Hypotheticall River region summation of geomorphic the Pliocene crustal delamination. We interpret (Saleeby and indices. these knickpoints to be evidence of Quaternary Foster, 2004). tectonic activity along the mountain front. Isostatic response to delamination was Although we have no analogs, we suggest that relatively rapid and has since decreased the geomorphology, which indicates an active (Saleeby and Foster, 2004; Stock et al., 2004). mountain front, but at a comparatively low uplift Subsequently, we suggest that uplift of the rate, might be the result of a singular isostatic southern Sierra over the last 5 Ma is related to pulse of uplift related to Pliocene delamination San Andreas plate margin tectonics has whose expression has been subsequently caused rapid subsidence in the Tulare Basin subdued by Quaternary erosion. and incision of the lower reaches of the Kern The lowest mountain front sinuosity values River along the Kern Gorge Fault (Fig. 7). This along the western Sierra mountain front (1.3) is combination of mechanisms is consistent with found in the southernmost area near the Kern the geomorphic indices. River. The valley floor width to height ratio is close to 0. The longitudinal profile of the lower Kern River is convex up and shows ~600 m of


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Conclusions Jones, C.H., Farmer, G.L., Unruh, J., 2004. Tectonics of Pliocene removal of lithosphere of the Sierra Nevada, A combination of Pliocene lower crust/upper California. Geological Society of America Bulletin 116 (11/12), 1408-1422. mantle delamination followed by late Pliocene Liu, M., and Shen, Y., 1998. Sierra Nevada uplift: a ductile to present San Andreas plate boundary link to mantle upwelling under the Basin and Range province. Geology 26 (4), 299-302 deformation is most consistent with Sierra Manley, C.R., Glazner, A.F., Farmer, G.L., 2000. Timing of geomorphology. The delamination and mantle volcanism in the Sierra Nevada of California: drip hypotheses explain the lack of crustal root evidence for Pliocene delamination of the batholithic root? Geology 28(9), 811-814. in the central Sierras, as well as the higher Monastero, F.C., Walker, J.D., Katzenstein, A.M., Sabin, elevations in the Kings River region. Because A.E., 2002. Neogene evolution of the Indian Wells Valley, east-central California. In: Glazner, A.F., delamination occurred in the Pliocene, the Walker, J.D.,Bartley, J.M. (Eds.), Geologic Evolution effects have waned during the Quaternary. This of the Mojave Desert and Southwestern Basin and Range. Geological Society of America Memoir 195, is consistent with uplift rates in the central and Boulder, CO, pp. 199-228. northern Sierras (Stock et al., 2004). The higher Montgomery, D. R. 1994 Valley incision and the uplift of tectonic activity in the southwestern Sierras and mountain peaks. Journal of Geophysical Research 99(B7), 13,913-13,921. southern San Joaquin Valley is a consequence Nadin, E., Saleeby, J., 2005. Recent motion on the Kern of San Andreas, Garlock, and Sierra Nevada Canyon fault, southern Sierra Nevada. EOS, Transactions of the American Geophysical Union Frontal fault interactions (Figueroa et al., this 86(52).National Atlas of the United States, 2010. volume; Figueroa, 2005) beginning in the late http://www.nationalatlas.gov/printable/images/pdf/prec Pliocene and continuing to the present. ip/pageprecip_ca3.pdf. Niemi, N.A., 2003. Active faulting in the southern Sierra References Nevada: initiation of a new Basin and Range normal fault? Geological Society of America Abstracts with Bull, W.B., 1984. Tectonic geomorphology. Journal of Program 35(6), 581. Geological Education 32, 310-324. Ritter, D.F., Kochel, R.C., Miller, J.R., 2002. Process Bull, W.B., McFadden, L.D., 1977. Tectonic Geomorphology. Waveland Press, Long Grove, IL. geomorphology north and south of the , Saleeby, J., Foster, Z., 2004. Topographic response to California. In: Doehring, D.C. (Ed.), Geomorphology in mantle lithosphere removal in the southern Sierra Arid Regions, Proceedings 8th Annual Nevada region, California. Geology 32(3), 245-248. Geomorphology Symposium, State University of New Small, E.E., Anderson, R.S., 1995. Geomorphically driven York, Binghamton, NY, pp. 115-137. Late Cenozoic rock uplift in the Sierra Nevada, Crough, S.T., Thompson, G.A., 1977. Upper mantle origin California. Science 270, 277-280. of Sierra Nevada uplift. Geology 5, 396-399. Stock, G.M., Anderson, R.S., Finkel, R.C., 2004. Pace of Ducea, M., Saleeby, J.B., 1998. A case for delamination of landscape evolution in the Sierra Nevada, California the deep batholithic crust beneath the Sierra Nevada, revealed by cosmogenic dating of cave sediments. California. International Geology Review 40(1), 78-93. Geology 32(3), 193-196. Farmer, G.L., Glazner, A.F., Manley, C.R., 2002. Did Unruh, J.R., 1991. The uplift of the Sierra Nevada and lithospheric delamination trigger late Cenozoic implications for the late Cenozoic epeirogeny in the potassic volcanism in the southern Sierra Nevada, western Cordillera. Geological Society of America California. Geological Society of America Bulletin Bulletin 103, 1395-1404. 114(6), 754-768. Wakabayashi, J., Sawyer, T.L., 2001. Stream incision, Figueroa, A.M., 2005. Elastic modeling and tectonic tectonics, uplift and evolution of topography of the geomorphic investigation of the Late Cenozoic Sierra Sierra Nevada, California. Journal of Geology 109, Nevada (California) uplift. M.S. Thesis, California 539-562. State University-Fullerton, CA. Webb, R.W., 1955. Kern Canyon lineament. In: G.B. Hart, E.W., Bryant, W.A., Smith, T.C., 1984. Summary Oakeshott, G.B., Jenkins, O.P. (Eds.), in Report: Fault Evaluation Program, 1983 Area (Sierra Kern County, California, during 1952. California Nevada Region). California Division of Mines and Division of Mines and Geology Bulletin 171, pp. 35- Geology, Sacramento, CA. 36. Huber, N.K., 1981. Amount and Timing of Late Cenozoic Wells, S.G. Bullar, T.F., Menges, C.M., Drake, P.G., Kara, Uplift and Tilt of the Central Sierra Nevada, California- P.A., Kelson, K.I., Ritter, J.B., Wesling, J.R., 1988. Evidence from the upper San Joaquin River Basin. Regional variations in tectonic geomorphology along U.S. Geological Survey Professional Paper 1197, a segmented convergent plate boundary, Pacific Washington, D.C., pp. 1-28. coast of Costa Rica. Geomorphology 1, 239-365.