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Basin Research (1999) 11, 59–73 Steady, balanced rates of uplift and erosion of the Santa Monica Mountains, California A. Meigs,* N. Brozovic† and M. L. Johnson‡ *Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA †Department of Geology and Geophysics, University of California, Berkeley, Berkeley, CA 94720, USA ‡University of Nevada, Reno Seismological Laboratory, Mackay School of Mines, University of Nevada, Reno, Reno, NV 89557, USA ABSTRAC T Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 104 to 106 years were constrained with geological cross-sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First-order morphology of the Santa Monica Mountains is set by large-scale alongstrike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment-scale relief are #300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr−1. Uplift rate across the south flank of the range is #0.5±0.4 mm yr−1 and across the north flank is 0.24±0.12 mm yr−1. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment-scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady-state form, comparison of the time-scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady-state form. INTROD UCTION That topography resulting from active deformation is a function of the interplay between structural and geomorphic processes is self-evident. What is less evident is how to invert the erosional and deformational components from landscape form because they can be dependent and in-phase and independent and out-of-phase, both in space and in time (Hack, 1960; Schumm, 1963; Ahnert, 1970; Bull, 1991; Kooi & Beaumont, 1996). Coseismic– interseismic deformation, for example, is a continuous tectonic process that varies in magnitude with time (King et al., 1988; Stein et al., 1988; Wells & Coppersmith, 1994). Although seismically induced denudation is, by default, in phase with deformation (Keefer, 1984; Pearce © 1999 Blackwell Science Ltd & Watson, 1986; Keefer, 1994), climate forces erosion independently of tectonism and varies on time scales ranging from 101 to 105 years (Wolman & Miller, 1960; Schumm, 1963; Bull, 1991). Consequently, the temporal and spatial scales of erosion and deformation themselves hinder direct measurement of topographic change at meaningful scales (i.e. at the range scale). Time-averaged implications of the interaction between uplift and denudation are suggested by numerical models of landscape evolution (see, for example, Anderson, 1994; Koons, 1994; Beaumont et al., 1996; Kooi & Beaumont, 1996; Howard, 1997; Densmore et al., 1998). As inferred from observational studies (Gilbert, 1877; Hack, 1960; Ahnert, 1970), topography tends towards a steady-state form with time in many models. How much time is 59 A. Meigs et al. required to attain a steady-state form depends on uplift and erosion rates (Ahnert, 1970; Kooi & Beaumont, 1996; Densmore et al., 1998). The steady-state form is characterized by local relief (at the scale of individual catchments), or topographic relief (at the scale of a range with respect to adjacent base level), reaching a maximum value (Hack, 1960; Ahnert, 1970). Because a topographic feature develops only if rates of denudation are less than rock uplift rates initially, denudation rates must approach uplift rates as deformation proceeds (Fig. 1; Ahnert, 1970). Steady-state topography can be sustained only if uplift and erosion rates are balanced (Koons, 1989; Kooi & Beaumont, 1996). The objective of this study is to understand the topographic development of an actively growing mountain range, the Santa Monica Mountains, southern California, in the context of interactions between uplift and erosion. Questions addressed in the analysis include: (1) What is the present form of landscape? How has that form changed with time?; (2) What are erosion rates? What are rock uplift rates? How do they compare temporally?; (3) How are bedrock lithological variations reflected by topography?; (4) Can uplift rates, erosion rates and the present topography be used to make inferences about topographic change? We integrate landscape characterization from a digital elevation model (DEM), bedrock geology and structure, surface geomorphology, and estimates of erosion and uplift rates. Topographic change in the Santa Monica Mountains is Fig. 1. Conceptual model illustrating the relationship between local relief (A), uplift rate and erosion rate (B) and topographic development (C) for an actively growing structure. The uplift rate is set arbitrarily as constant to emphasize the point that as erosion rate approaches uplift rate, the rate of topographic change slows and the landscape approaches a steady-state configuration (similar in concept to Ahnert, 1970). Uplift is characterized as a simple asymmetric function that grows selfsimilarly. Note that the erosion function scaled by a percentage of uplift rate and wavelength. Local relief (catchment scale) and erosion rate are coupled and display positive feedback. A lag between the initial stage when erosion rate is lower than uplift rate (t ) and when erosion rate approximates uplift rate (t ) 1–2 3–4 modulates the rate of topographic change with time. 60 argued to be relatively slow (approaching steady-state?) given the present form, duration of deformation, and erosion and rock uplift rates. R EGIONAL SE TTING Lying to the north and west of downtown Los Angeles, the Santa Monica Mountains extend 90 km from the Los Angeles River on the east to the Oxnard Plain on the west (Fig. 2). Investigations of the connection between tectonics and geomorphology in the Santa Monica Mountains extend back more than 70 years (Eaton, 1926; Tieje, 1926; Vickery, 1927; Hoots, 1931; Davis, 1933; Grant & Sheppard, 1939). Of those investigators who have considered the large-scale form of the range, most accept the interpretation of Davis (1933) that the range is a dissected remnant of a recently uplifted peneplain, which implies that erosion rates are significantly lower than uplift rates (Hoots, 1931; Dibblee, 1982). R A NG E -SCAL E S TRUC TURE AND B EDR OCK G EOLOGY A complex mosaic of active strike-slip and thrust faults is deforming the Los Angeles basin as the consequence of transpressive motion between the Pacific and North American plates (Wright, 1991). Many of the active structures are marked at the surface by structural anticlinoria (Hauksson & Jones, 1989; Lin & Stein, 1989; Dolan et al., 1995; Yeats & Huftile, 1995). The Santa Monica Mountains anticlinorium is one of these anticlinoria and is interpreted to have formed due to displacement on a blind thrust system at depth (Davis et al., 1989, 1996; Davis & Namson, 1994). Slip on the thrust system is inferred to have been accommodated by a combination of hangingwall folding and displacement on fault splays off the principal thrust (Dibblee, 1982; McGill, 1989; Wright, 1991; Dolan & Sieh, 1992; Weber, 1992; Hummon et al., 1994; Dolan et al., 1995; Davis et al., 1996; Schneider et al., 1996). The anticlinorium is a broad structural culmination extending from the Los Angeles River on the east to the Oxnard Plain on the west and between the synclines beneath the San Fernando Valley on the north and the northern edge of the Los Angeles basin on the south (Hoots, 1931; Wright, 1991; Hummon et al., 1994; Yeats & Huftile, 1995; Davis et al., 1996; Schneider et al., 1996) (Fig. 3). Syntectonic sediments indicate fold growth initiated at #5 Ma (Fig. 3b) (Schneider et al., 1996). It is important to emphasize that the anticlinorium comprises three distinct physiographical domains: the Santa Monica Mountains, the northern margin of the Los Angeles basin and Santa Monica Bay (Fig. 3). A fault system exposed at the surface, the Malibu, Santa Monica, Hollywood fault system (MSH), separates the Santa Monica Mountains on the north from the other topographic domains on the south (Fig. 2) (Vickery, 1927; Hoots, 1931; Dibblee, 1982; McGill, 1989; Weber, © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains Fig. 2. Geological map of the Santa Monica Mountains and adjacent areas. Geomorphic surfaces are differentiated from bedrock lithological units. The location of Potrero Canyon in Pacific Palisades is indicated by a black dot (Fig. 10). Specific details on ages and types of geomorphic surfaces was compiled form the literature (Eaton, 1926; Tieje, 1926; Vickery, 1927; Hoots, 1931; Davis, 1933; Jennings & Strand, 1969; Birkeland, 1972; Campbell, 1975; Lajoie et al., 1979; McGill, 1989; Dolan & Sieh, 1992; Weber, 1992; Levi & Yeats, 1993; Johnson et al., 1996). Individual bedrock stratigraphic units are lumped into large groups: Mesozoic through Lower Tertiary sequence includes the Santa Monica Slate, Tuna Canyon Formation, Coal Canyon Formation, Llajas Formation, and the Sespe Formation; the Miocene sedimentary rocks includes the Vaqueros, Topanga Canyon, Monterey, Calabasas, Puente and Modelo Formations; the Miocene Volcanic rocks are the Conejo Volcanics; and the Pliocene through Pleistocene includes the Fernando, Repetto and Pico Saugus Formations (Tieje, 1926; Hoots, 1931; Jennings & Strand, 1969; Dibblee, 1982; Wright, 1991; Levi & Yeats, 1993; Weigand et al., 1993; Davis & Namson, 1994; Davis et al., 1996; Schneider et al., 1996). Note that the Miocene sedimentary rocks are exposed around the periphery of the range whereas the interior is dominated by the Mesozoic through Lower Tertiary sequence in the east and the Conejo Volcanics in the west. Modified from Hoots (1931), Jennings & Strand (1969), Lamar (1970), Dibblee Foundation maps, Dolan & Sieh (1992), Levi & Yeats (1993), Hummon et al. (1994) and Schneider et al. (1996). Figures 3, 10, 11 and 12 are indicated. 1992; Dolan et al., 1997). Structurally, the anticlinorium is characterized by a relatively simple asymmetric anticline cut by the MSH between the Oxnard Plain and Santa Monica (Figs 2 and 3A) (Davis et al., 1996). The anticline is doubly plunging, has forelimb dips that range from 20° to 65° south, and a backlimb dip of #20° north (Figs 2 and 3) (Hoots, 1931; Dibblee, 1982). Structural interference between the anticlinorium and structures lying to the west of the San Fernando Valley obscure the north flank of the range (Fig. 2). Structural complexity increases to the east of Santa Monica (Fig. 2). Although the anticlinorium persists as a broad structural high, a change in the position of the MSH from the forelimb to © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 the crest of the anticlinorium and the superposition of folds alters the cross-sectional geometry (Figs 2 and 3). These folds and faults are inferred to have formed after 1 Myr and be evidence that displacement on the eastern MSH, hangingwall folding, and footwall deformation occurred coevally (Fig. 2) (Hummon et al., 1994; Schneider et al., 1996; Meigs & Oskin, 1997). Surface exposures of bedrock are dominated by four rock types: (a) Mesozoic intrusive rocks, (b) Mesozoic to Lower Tertiary metasedimentary and sedimentary rocks ( Jurassic Santa Monica Slate through the Oligocene Sespe Formation), and (c) Miocene–Pliocene sedimentary rocks including (d) an interbedded sequence of volcanic 61 A. Meigs et al. Fig. 3. Geological cross-section across the Santa Monica Mountains and San Fernando Valley. Cross-section (A) traverses the central part of the range whereas section (B) traverses the eastern portion of the range (modified from Davis et al., 1996; Schneider et al., 1996, respectively). Note the change in position of the range-front fault system (MSH: the Malibu Coast, Santa Monica and Hollywood fault system), from the forelimb to the crest of the anticlinorium from west to east, respectively. Unit K – M. Mio. is Cretaceous through Miocene rocks that include the portions of the Mesozoic through Lower Tertiary and Miocene sedimentary rocks of Fig. 3. Unit Plio. – Q includes both the Pliocene through Pleistocene sedimentary sequence and modern alluvial surfaces of Fig. 2. rocks (Vaqueros, Topanga, Puente and Modelo Formations and the Conejo Volcanics) (Fig. 2) (Dibblee, 1982; Weigand et al., 1993). Surrounding the mountains on all sides are a series of surfaces that are thought to be Pleistocene in age and are overlain by younger alluvial deposits locally (Tieje, 1926; Vickery, 1927; Hoots, 1931; Davis, 1933; Dibblee, 1982; Dolan & Sieh, 1992). The distribution of these bedrock units reflects the anticlinal structure of the bedrock. Miocene sedimentary rocks are exposed around the periphery of the range, and bedrock exposure in the core changes systematically, from Mesozoic intrusive rocks on the east, to Mesozoic through Oligocene sedimentary rocks, to Miocene volcanic rocks on the west (Fig. 2). LANDSC APE CHAR A CTE RISTICS OF THE S ANTA MONICA MOUNTAINS First-order topography in the Santa Monica Mountains is set by the large-scale bedrock structure and varies as a function of the along-strike structural changes (compare Figs 2 and 4A). West of Santa Monica, the range is 62 characterized by relatively constant width and relief (Fig. 4B). East of Santa Monica, a distinct eastward taper in width and decrease in relief mark the form of the mountains. The surface of the Los Angeles basin to the south is characterized by a dissected low-relief surface (Tieje, 1926; Vickery, 1927; Grant & Sheppard, 1939; Dolan & Sieh, 1992) (Figs 2 and 4A). A natural subdivision within the range is created by an east–west trending drainage divide (Fig. 4A). The drainage divide does not correspond with the structural axis; the divide lies from 4 to 10 km to the north (Hoots, 1931). Further subdivision of the range into four distinct topographic domains, one north and three south of the drainage divide, is suggested by topography. The domain defined north of the divide is characterized by northdraining basins (Fig. 4A). Three domains lying to the south of the drainage divide, an eastern domain, the Malibu Creek basin and a western domain, have basins that drain southward. Local drainage divides separate the eastern and western domains from the Malibu Creek drainage. Base level is set by the Los Angeles basin for most of the eastern domain and by Santa Monica bay for the three westernmost catchments in the eastern domain, the Malibu Creek basin, and the western domain. Base level for the northern domain, the San Fernando Valley, lies at a higher elevation than that for the south-draining basins (Fig. 4A). Overall, the fact that the south-draining basins cover most of the range, are more elaborated and have greater areas than the north-draining basins (Fig. 4D) reflects greater dissection of the south flank than the north flank. Most of the denudation, and consequently form, of the range is dictated by the southflank basins. The following discussion focuses on the region lying to the south of the drainage divide. Mean elevation, catchment-scale relief, slope and hypsometry were calculated from the 30-m grid spacing digital elevation model (DEM) available from the United States Geological Survey (Figs 4–6). Calculations of mean elevation and slope involved iterative calculation of a subset of the data followed by a one-pixel (data point) shift of the calculation space across and down the data array. Mean elevation (sum of the elevations divided by the number of data points) was calculated for (900 m)2 scrolling windows (30×30 subset of data array; Fig. 4B). Mean slope angles were calculated by fitting a leastsquare deviation best-fit plane to groups of five-by-five pixels, corresponding to approximately (120 m)2 surface area (Fig. 4C). Slope angles are sensitive to both DEM grid spacing and to the length scale over which slope is measured (Zhang & Montgomery, 1994). In general, slope angles decrease with increasing measurement length. For the 30-m DEM used in this study, slope angles are underestimated by this technique, although multiple hillslopes will not generally be averaged by an individual calculation. Recognizing this limitation of slope determined from the DEM, we place more significance on trends in slope angles than on their absolute values. Individual catchment properties including length, width, © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains Fig. 4. (A) USGS 30-m digital elevation model (DEM) of the Santa Monica Mountains and adjacent areas with major drainages, the drainage divide and structural axis. Note that the drainage divide lies consistently to the north of the structural axis. Abbreviations denote Santa Monica (SM), the eastern and western domains of the south flank basins (ED and WD, respectively; the southern boundaries are marked by the blue–red colour change), the domain of north-draining basins (ND; the northern boundary is marked by the purple–blue colour change) and the Malibu Creek basin (MCb). White line indicates position of topographic profile in Fig. 7. Sharp north–south- and east–west-trending breaks in the image are artefacts of the data sets and do not represent real topographic features. (B) Mean elevation map. (C) Slope map. See text for details of the calculation procedure for mean elevation and slope maps. (D) Map showing the boundaries of the principal catchments imaged by the 30-m DEM. © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 63 A. Meigs et al. Fig. 5. Hypsometry, or frequency distribution of elevations per 100-m bin, for the eastern, western and combined domains. The bimodal distribution of the combined domains reflects the fact that the eastern domain is dominated by lower elevations than the western domain. Note that the mean elevation of the eastern domain is higher than the mode but less than the combined mean elevation. The mean elevation for the western domain is higher than both the mode and the mean elevation of the combined data. area and relief (maximum minus minimum elevation) were calculated based on the principal catchments identifiable from the DEM (Fig. 4D). Hypsometry is nor- malized to percentage area in order that variably sized subsets of the data may be compared directly (Fig. 6). Hypsometry, maximum elevation, mean elevation, relief, basin length–width aspect ratio, drainage pattern, drainage spacing and underlying bedrock lithology distinguish the eastern from the western domains (Figs 4–6). Topography in the eastern domain is characterized by a maximum elevation of about 600 m and a mean elevation of 164 m (Figs 4 and 5). Parallel drainages, typified by length–width aspect ratios of #651, are spaced #1.6 km apart in the east (Fig. 4D). Easterndomain catchments tend to have areas less than 10 km2 and relief less than 500 m (Fig. 6A–C). Bedrock is dominated by Mesozoic to Lower Tertiary metasedimentary and sedimentary rocks (Fig. 2). In contrast, the western domain has elevations up to 1000 m and a mean elevation of 387 m (Figs 4 and 5). Dendritic drainages with #351 aspect ratios, areas up to 30 km2 and relief as high as 900 m have developed primarily on Middle Miocene volcanic rocks (Figs 4D and 6A–C). Drainages in the west have an average spacing of 2.4 km. Slope, in contrast, does not vary appreciably between the east and west. Close inspection of Fig. 4(C) shows that the highest slopes occur on interfluves, regardless of underlying bedrock lithology. Calculated slope angles are in general agreement with, although consistently lower than, field measurements (Campbell, 1975). Fig. 6. Plot of drainage basin attributes along the strike length of the range from the Los Angeles River to the Oxnard plain on the east and west, respectively. Basin properties were calculated for identifiable basins in the 30-m DEM (Fig. 4A). (A) Approximately 75% of the south-draining basins have areas of less than 10 km2. Excluding the Malibu Creek basin, #60–70% of the range is denuded by the south-draining basins (Fig. 4A,D). (B) Note that the north domain basins (ND) are differentiated from north-draining basins in the western region of structural interference (Figs 2 and 4A). (C) Drainage basin relief, defined as the difference in elevation between the drainage divide and mouth of each catchment (Hovius, 1996; Talling et al., 1997). An arrow marks the along-strike point at which base level for the south-draining basins changes from the Los Angeles basin in the east to Santa Monica Bay in the west. (D) Relief vs. area highlights the distinction between the eastern and western domains. 64 © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains Two key differences between the eastern and western domains are highlighted by a topographic profile along the length of the range (Fig. 7). First, the average topographic relief, as measured by the difference between the mean elevation and the base level elevation for each domain, is significantly larger in the west than in the east (Fig. 7). Second, an upper limit to drainage basin relief can be inferred from the integration of slope angle with drainage spacing (Fig. 8). Relief is given by the product of one half the drainage spacing and the tangent of the average slope angle (Fig. 8). Relief is particularly sensitive to slope and has a maximum equal to half the drainage spacing as slope approaches 45°. Drainage spacing is the variable that differentiates maximum possible relief calculated for the eastern and western domains (360–550 m and 540–810 m, respectively; Fig. 8), given slope angles are similar in each domain (25°–35°; Fig. 4C). Measured catchment-scale relief is within the range of that predicted for each domain (compare Figs 4C and 8). Malibu Creek is noteworthy when compared with the other south-draining basins because its area (>340 km2) is an order of magnitude larger than that of any other basin (Fig. 4D). Overall, the majority of the Malibu Creek basin is characterized by low relief and low slopes (Fig. 4C). Valley bottoms contain Upper Pleistocene (?) to Recent alluvial and fluvial fill (Fig. 2). The only bedrock-channel has developed in a narrow gorge where Malibu Creek drains across the south flank of the range (Fig. 4A). Relief in the gorge reaches 450 m and the channel gradient is up to 5% locally. Topography immediately to the east and west of the gorge is similar. Reaches of Malibu Creek and its tributaries immediately north of the gorge have incised #3–4 m into alluvial fill (Fig. 2) ( Jennings & Strand, 1969). Defeat of a local drainage divide and capture of an upland basin is inferred Fig. 7. Topographic profile along the strike length of the south flank of the range from the Oxnard Plain (OP) to the Los Angeles River (LAr) (Fig. 4A). The profile intersects the canyons at the points of high local relief in order to illustrate canyon depth and extent of dissection. Note that the canyons in the west are more deeply incised and have greater relief than those in the east, that the valley bottoms in the east are higher than in the west and that mean elevation is higher in the west than in the east (see Figs 4B and 6C). The mean elevation and elevation of base level for the eastern (164 and 115 m, respectively) and western (387 and 0 m, respectively) contrast between the two domains. Malibu Creek (MC) and Topanga Creek (TC) are indicated for reference. Note that although Topanga Creek lies within the eastern domain, its base level is set by Santa Monica Bay (the change to base-level control by Santa Monica Bay is indicated by an arrow in Fig. 6C). Fig. 8. Idealized along-strike topographic profile using observed drainage spacing (Fig. 7) and 25–35° values for typical interfluve slopes (Fig. 4C). A characteristic relief and accordant summits like those observed in the Santa Monica Mountains are predicted by this model (Hoots, 1931). See text for discussion. This model creates similar peak, interfluve and ridge top elevations that could be interpreted as remnants of an original planation surface (Davis, 1899; Hoots, 1931; Davis, 1933; Dibblee, 1982). A number of lines of evidence suggest that the combined effects of dissection and uplift, given the age of the structure, would have obliterated any evidence of an older planation surface. © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 65 A. Meigs et al. to explain the anomalous area of the Malibu Creek drainage basin. This interpretation contrasts with that of Dibblee (1982), who believed that Malibu Creek was antecedent to uplift of the Santa Monica Mountains. Specific features of the topography of the Santa Monica Mountains can now be related to structural variation, differential relief with respect to base level and bedrock lithology. The first-order width and length of the range are controlled by along-strike variations in bedrock structure. Greater differential relief across the south flank of the range than the north flank has resulted in the development of larger basins and greater dissection in the south (Figs 4 and 5). The south limb of the anticlinorium is steeper structurally than the north limb, which probably amplifies the north–south asymmetry in dissection. A combination of lower base level and higher average slope, imposed by the fold asymmetry, suggests that the south-draining streams may have higher stream power (in a qualitative sense), and thus greater erosive capability, than north-draining streams with the same areas. In this way, differences in base level combine with fundamental structural asymmetry to force the drainage divide to the north of the structural axis (Fig. 4A). Base-level control is also implied by comparison of hypsometry for each domain (Fig. 5). Basin length–width aspect ratio, drainage pattern and drainage spacing vary as a function of underlying bedrock (compare Figs 2 and 4). The sedimentary and metasedimentary rocks exposed in the east are inferred to be relatively ‘weaker’, more susceptible to erosion, than the volcanic rocks exposed in the west. Although the regular spacing of drainage outlets documented by Hovius (1996) implies that bedrock may not be important in the definition of drainage networks, the correlation between drainage geometry and underlying bedrock in the Santa Monica Mountains is most simply interpreted as a bedrock-erodibility phenomenon. Slope, on the other hand, shows no obvious systematic variation related to bedrock, and is most likely controlled by processes or the sum of processes operating at scales below the resolution of the DEM (Dietrich et al., 1993; Anderson, 1994; Zhang & Montgomery, 1994). The relationship between specific hillslope processes and bedrock lithologies is poorly constrained (Campbell, 1975). TIME- S CALE S A ND R ATES O F DE NUD ATION VS. U PLIFT Having differentiated the relative contributions that structure, base level and bedrock lithology have had in shaping the present topography, we now address the more difficult problem of calibration of denudation and uplift rates and their variation over the time-scale of growth of the anticlinorium. Denudation Sediment yield estimates for catchments throughout southern California were compiled from the literature and 66 unpublished Los Angeles County Department of Water and Power records, normalized by basin area, and divided by length of record to approximate regional denudation rates (Fig. 9). Two hundred and seven independent estimates over time-scales from 1 to 70 years in length and from basins with areas extending from <0.01 km2 to #1000 km2 indicate that denudation rates vary from 0.02 to 40 mm yr−1 regionally. The highest rates come Fig. 9. Denudation rate data for southern California (A) and the Santa Monica Mountains (B). Data for (A) include 1-year (closed circles; Scott & Williams, 1978) and multiyear records (open circles; Ferrell, 1959; Lustig, 1965; Scott & Williams, 1978; Taylor, 1981; and unpublished Los Angeles County Department of Public Works). Denudation rate is calculated by dividing sediment yield data (km3) by basin area (km2) and then by the number of years of the record. These data are useful for defining the expected range of denudation rate. Note that no clear correlation between drainage basin area and denudation rate is seen for basins less than #10 km2, for either single- or mullet-year records (Brozovic et al., 1997). (B) Measurements and estimates of denudation rate as a function of drainage basin area for the Santa Monica Mountains. Denudation estimates based on relief and basin area for Santa Monica basins (Fig. 5) after the linear regression model of Taylor (1981) (denudation rate=0.0936*L3.11*Area−0.141, where L is a landscape factor related to qualitative estimates of relief; L=1, relief<#100 m, L=2, #100 <relief <1000 m, L=2.7, relief >#1000 m). Rather than strictly interpreted as absolute values, this analysis provides an order-of-magnitude estimate on denudation rate in a landscape with relief similar to that of the Santa Monica Mountains (0.5±0.3 mm yr−1). Black squares are measured values and shaded symbols are model calculations based on the basin areas depicted in Figs 4D and 6A. Note that (A) is a log– log plot, whereas (B) is a log–linear plot. © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains from data collected after a significant rainfall year (1968–69; Scott & Williams, 1978). No strong correlation between rate and basin area is revealed by the data, although basins with areas less than or equal to 10 km2 show a greater range of rates than those with areas greater than 10 km2. Whether these rates represent primary bedrock erosion rates depends on the time-scale of sediment storage relative to the record length. That time has been inferred to be short for basins smaller than 10 km2 (Brozovic et al., 1997), thus implying that rates from these catchments represent primary bedrock erosion rates. Data from catchments outside southern California, however, suggest that sediment production and storage on 105-year time-scales can significantly influence sediment yields from basins with areas ranging from <1 km2 to >104 km2 (Langbein & Schumm, 1958; Church & Slaymaker, 1989; Reneau et al., 1990). Some of the 1year data comes from catchments with areas <10 km2 and that contain incised valley fills. Clearly, sediment yields from these basins have a mixed signal and do not reflect primary bedrock erosion (Fig. 9A). These data therefore provide order-of-magnitude constraints on the range of probable denudation rates. Nine of the estimates are from basins within the Santa Monica Mountains and indicate that erosion rates likely vary between #0.25 and 1.1 mm yr−1 (Fig. 9B). The potential range of denudation rate can be narrowed using the inferred relationship between relief and denudation rate (Langbein & Schumm, 1958; Schumm, 1963; Ahnert, 1970). Regression of denudation rate against basin area for some of the southern California data (Fig. 9A) suggested a correlation between sediment yield relief (relief was loosely defined on the basis of topographic relief of Taylor, 1981). From this analysis, Taylor (1981) developed a simple model relating denudation rate to basin area and relief. A mean denudation rate of 0.2–0.8 mm yr−1 is revealed when basin area extracted from the DEM (Fig. 5) and values of relief between #100 and <1000 m are used in the model (Fig. 9B). This range of rates captures #66% of the measured rates (6 of 9 basins). Denudation on the time-scale of these data, ≤70 years, appears to be dominated by the 50-year-return storm (Ferrell, 1959; Lustig, 1965; Scott et al., 1968; Campbell, 1975; Scott & Williams, 1978; Taylor, 1981). Shallow landsliding from hillslopes and hollows accounts for a significant fraction of sediment delivered from catchments after one such storm cycle in the winter of 1968–69 (Campbell, 1975). Other significant factors influencing sediment yields include fire cyclicity and landsliding forced by earthquake shaking, orientation of stratigraphic layering and base-level lowering (Lustig, 1965; Scott et al., 1968; Campbell, 1975; Scott & Williams, 1978; Weber, 1992; Brozovic et al., 1997; Schwarz, 1997). River incision River incision plays a critical role in long-term, rangescale denudation (Schumm, 1963; Koons, 1989; © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Densmore et al., 1998). To know or approximate river incision rate allows rates of relief development to be estimated (Schumm, 1963; Ahnert, 1970; Chappell, 1974b; Burbank et al., 1996; Densmore et al., 1998). Incision rates have been measured using the differential relief between fill and/or strath river terraces of known ages (Merritts et al., 1994; Anderson et al., 1996; Burbank et al., 1996; Granger et al., 1997) and the differential relief of rivers set into other geomorphic surfaces with known ages, such as marine terraces or lava flows, provided the catchment lies entirely within that surface (Ruxton & McDougall, 1967; Chappell, 1974b; Rosenbloom & Anderson, 1994; Seidl et al., 1994). A flight of at least three marine terraces has been etched into the south flank of the Santa Monica Mountains, permitting the latter technique to be applied (Hoots, 1931; Davis, 1933; Birkeland, 1972; Lajoie et al., 1979; McGill, 1989). The marine terraces have been recognized since the 1920s and are well described, correlated and reasonably well dated (Hoots, 1931; Davis, 1933; Birkeland, 1972; Lajoie et al., 1979; McGill, 1989; Weber, 1992; Johnson et al., 1996). The abandoned platforms are correlated with oxygen isotope stages 5e, 7 and 9 (#125, 225–240, 325–340 kyr, respectively; Imbrie et al., 1984), based on uranium-series dating of material deposited on individual platforms (Lajoie et al., 1979; McGill, 1989; Weber, 1992). Potrero Canyon is a small drainage basin lying entirely within the lowest terrace at Pacific Palisades (stage 5e age, area=0.7 km2, length=1.7 km, and relief (drainage head to the outlet)=91 m, Fig. 2). Because the stream is younger than the terrace, a maximum incision rate of 0.5 mm yr−1 for the stream over the past 125 kyr can be calculated at the point of maximum height (#62 m) between the modern stream and the terrace surface (Fig. 10). It is likely that this rate has varied on shorter time-scales owing to variations in the rate of migration of the drainage head (mean rate of #10.5 mm yr− 1; Fig. 10) and sea-level oscillations (Merritts et al., 1994; Seidl et al., 1994). Because erosion rate is related to drainage basin size and geometry and climate (Schumm, 1963), this may be a maximum rate given that nearly all Fig. 10. Stream and interfluve (to the east and west of Potrero Canyon) of a drainage incised onto the 125-ka marine terrace at Pacific Palisades (Fig. 2). Note that incision is measured perpendicular to the stream profile near the canyon mouth at the point of maximum relief. 67 A. Meigs et al. Table 1. Time-scale, uplift rate, data, source. Time-scale Rate (mm yr−1) Data Source 10 kyr 100–400 kyr 0.1 0.2–0.9 Offset soils Uplifted marine terraces 800 kyr–1 Myr 0.3–0.4 Minimum fault displacement 1–5 Myr 0.5–1.0 Uplifted syntectonic strata and pre-tectonic strata Dolan et al. (1997) Lajoie et al. (1979) McGill (1989) Weber (1992) Hummon et al. (1994) Dolan et al. (1997) Meigs & Oskin (1997) Schneider et al. (1996) Davis et al. (1996) this study Average 0.5±0.4 other significant drainage basins are at least an order of magnitude larger (Fig. 6A). Uplift rates Estimates of short-, intermediate- and long-term uplift rates for the Santa Monica Mountains anticlinorium are in general agreement (Table 1) (McGill, 1989; Weber, 1992; Johnson et al., 1996; Dolan et al., 1997; Meigs & Oskin, 1997). Intermediate-term rates are constrained by the ages and present elevation of the three marine terrace levels (Fig. 11). Terrace age and elevation with respect to sea level provide a measure of uplift rate after eustatic sea-level change is subtracted from terrace elevation (Chappell, 1974a). After correction, the Santa Monica terraces yield an average uplift rate of 0.22 mm yr−1 since 340 ka: since 125 ka rates have varied from 0.2 mm yr−1 at Pt. Dume on the west to 0.9 mm yr−1 at Pacific Palisades on the east (Birkeland, 1972; Lajoie et al., 1979; McGill, 1989; Weber, 1992; Johnson et al., 1996). These rates are consistent with short- and intermediate-term rates of 0.1–0.4 mm yr−1 inferred from palaeoseismological and structural arguments, respectively (Hummon et al., 1994; Dolan et al., 1997; Meigs & Oskin, 1997). Geological cross-sections can be used to determine Fig. 11. Profiles at the same position through Point Dume from the DEM and mean elevation data sets and the height of the 340-ka terrace (Davis, 1933; Birkeland, 1972; Lajoie et al., 1979; McGill, 1989; Weber, 1992; Johnson et al., 1996). At least 680 m of topographic relief may have been present by 340 ka according to this profile. 68 long-term rock uplift rates if age of fold initiation can be established. Age of formation has been established at 5 Ma (Schneider et al., 1996). Finding a suitable marker in the pre-uplift strata for measuring uplift is notoriously difficult in southern California because pre-uplift strata typically vary in thickness and are time transgressive owing to deposition over pre-existing topography (Sullwold, 1960; Blake, 1991; Wright, 1991; Schneider et al., 1996). Mohnian-aged strata (the Modelo Formation, Sullwold, 1960), used to calculate uplift on Fig. 12, share many of these traits. A number of unique characteristics of the specific position of our section, however, allow reasonable confidence to be placed on the uplift estimate using these strata. First, the unit is nearly continuously exposed from the north flank across the crest to the south flank of the range (Fig. 2) (Dibblee, 1991). Its projection into the subsurface of the San Fernando Valley north of the range is constrained by well-data (Dibblee, 1982; Division of Oil & Gas, 1991). On the south, projection into the subsurface is based on structural geometry and information from an adjacent section (Davis & Namson, 1994; Davis et al., 1996). Second, detailed stratigraphic analyses in the area of the section include mapping of a sand bed exposed continuously above the basal unconformity, mapping of the lateral extent of individual sand beds within the unit, closely spaced measured sections and biostratigraphic constraints on palaeo-water depths (#900 m) (Sullwold, 1960). Two key assumptions are necessary in order to use Mohnian strata on the line of section to measure structural relief: (1) the same bed or suite of beds can be projected across the fold; and (2) that those beds were deposited at approximately the same depth. Whereas the latter is not well constrained as palaeontological data are only available for strata on the north limb (Sullwold, 1960), the former is justifiable. Bedding on the north flank of the anticlinorium is concordant and dip is relatively constant (20–25° north) across #900 m of section. The base of these beds is continuous with flatlying strata on the crest to the south (Dibblee, 1991). Measured sections indicate that this unit thins by © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains Fig. 12. Detailed geological crosssection across the Santa Monica Mountains drawn just to the west and parallel to the cross-section of Fig. 3. Uplift is indicated by the base of Mohnian-aged strata (heavy line) exposed on the flanks and crest of the range and the nearby subsurface (structure below the Mohnian strata from Davis & Namson, 1994). Data sources include (Hoots, 1931; Sullwold, 1960; Dibblee, 1982, 1991; Blake, 1991; Division of Oil & Gas, 1991; Wright, 1991; Davis et al., 1996; Schneider et al., 1996). #600 m eastward (Sullwold, 1960); this thickness variation provides a generous error estimate for across-strike changes in thickness. Because the strata are interpreted to have been deposited in middle to upper bathyal depths, 600 m also encompasses nearly the entire range of possible water depths during deposition. Uplift rates are therefore assigned an error of ±0.12 mm yr−1 (0.6 km/5 Myr= 0.12 mm yr−1). It is the unique circumstances of this specific section that justifies the use of the Mohnian as a strain marker. Mohnian aged strata are not well-suited as a strain marker regionally, in general, because of uncertainties in palaeo-water depths and thickness variations (Sullwold, 1960; Blake, 1991; Wright, 1991). The cross-section reveals a complex pattern of uplift of the Santa Monica Mountains anticlinorium with respect to adjacent basins (Fig. 12). Three uplift rates can be calculated: (1) the crest of the anticlinorium relative to the Los Angeles basin on the south, (2) the crest of the anticlinorium relative to the San Fernando Valley on the north and (3) the San Fernando Valley relative to the Los Angeles basin. A mean uplift rate of 0.52±0.12 mm Myr−1 is given by the #2600 m differential relief of the Mohnian between the crest and the Los Angeles basin (Fig. 12). This rate is consistent with, although lower than, those indicated on crosssections by Wright (1991) (0.6 mm yr−1), Schneider et al. (1996) (0.8–1.0 mm yr−1) and Davis et al. (1996) (1 mm yr−1). These rates imply that uplift across the south flank has not varied more than ±0.4 mm Myr−1 about the long-term mean (Table 1). Differential relief of the Mohnian across the north flank of the range (1210 m) is smaller, resulting in a lower uplift rate (0.24±0.12 mm yr−1). Different rates of uplift across the north and south flanks of the anticlinorium are predicted by the inference that the blind thrust fault-dip angle decreases from steep to gentle beneath the mountains (Fig. 3A) (Davis & Namson, 1994; Davis et al., 1996). Both regions will be uplifted owing to displacement on the fault at depth, but basic geometry requires that the amount of vertical uplift will be greater on a highangle segment of the fault than a low-angle segment given the same net horizontal shortening (Huftile & © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Yeats, 1995). Finally, the 1380 m structural relief between the Mohnian in the subsurface of the San Fernando Valley and Los Angeles basin gives an uplift rate of 0.28±0.12 mm yr−1 (Fig. 12). T OP OGRAPHIC C HANGE : IMP LICAT IO NS OF PR ESENT T OP OGRAPHY, U PLIF T A ND D EN U DAT I ON R ATES One of the most difficult unresolved questions is the issue of topographic change. An estimate of the time required to create the present landscape is offered by the uplift and denudation rates. Measured catchment-scale relief varies from #300–900 m (Fig. 6C), potential catchment-scale relief given by drainage spacing and slope angle varies from #400 to 800 m (Fig. 8), and mean topographic relief averages between 300 and 600 m (Fig. 4B). Roughly 0.6–1.6 Myr are required to create between 300 and 900 m of relief if #0.5 mm yr−1 characterizes both the average catchment-scale denudation rate and fluvial incision rate. Thus the present catchment- and range-scale relief could have developed by #3.4 Ma given that the anticlinorium began forming at 5 Ma. Interestingly, the topographic relief above the stage 9 terrace at Pt. Dume is #680 m (Fig. 11). One interpretation of this observation is that the range at the longitude of Pt. Dume had attained this relief before #400 ka. Dividing the topographic relief by the lower 0.22 mm yr− 1 end-member uplift rate suggests that as much as 2.7 Myr were required to create that relief. This argument is predicated on the structural model for the range in which uplift of points on the forelimb (the structural position of the terraces) equals crestal uplift (Davis et al., 1996). These arguments do not apply to those portions of the structure that remained below sea level and/or in the subsurface for the length of structural development (south end, Fig. 3B), however (Wright, 1991; Schneider et al., 1996). If uplift and denudation rates have been sustained at approximately the same rates, isostatic uplift is expected to be minor and crustal deflection related to this loading is sustained after #1.6 Myr of uplift 69 A. Meigs et al. (provided rates do not vary on the time-scale of isostatic response). Are similar uplift and denudation rates a sound basis for suggesting this landscape has achieved steady-state (Hack, 1960; Ahnert, 1970)? Rates of denudation and uplift are relatively slow and vary about #0.5 mm yr−1 (Figs 9 and 10 and Table 1). It is intriguing that the amount of time required to create either the present topographic or catchment relief, using either the rate of uplift or denudation, is between 30 and 55% (1.6–2.7 Myr) of the total duration of folding (5 Myr). Comparative studies (Ahnert, 1970) and some models of landscape evolution (Anderson, 1994; Kooi & Beaumont, 1996) indicate that more than #1.5 Myr is required to develop steady-state topography. Steady state is reached in 1–1.5 Myr in models in which erosion and uplift rates are in the range 0.5–1.0 mm yr−1 (Densmore et al., 1998). If so, it is plausible that the topography approximates a quasi-equilibrium form. Rates of uplift and denudation must be steady on time-scales as short as the landscape response time if this interpretation is valid (Hack, 1960; Schumm, 1963; Ahnert, 1970; Kooi & Beaumont, 1996; Densmore et al., 1998). Deformation on time-scales up to #tens of kyr is strongly discontinuous (Dolan et al., 1995, 1997; Dolan & Pratt, 1997). Available data suggest that earthquakes on the frontal fault system have long recurrence intervals (#1 kyr). The time interval between #100 ka and 10 ka may have been characterized by uplift rates nearly twice the mean (Table 1) (McGill, 1989), succeeded by a drop to #20% of the mean rate since 10 ka (Dolan et al., 1997). Southern Californian sediment fluxes and river profiles covary with climatic fluctuations on time-scales ranging from 101 to 105 years (see Bull, 1991, and references therein). Thus it is difficult to say how catchment-scale and fluvial denudation rates have responded to short-term climate change and the deceleration of uplift rate. If denudation rates have persisted at #0.5 mm yr−1, the present landscape must be interpreted as a transient form changing in response to a slowing of uplift rate. Climatic or tectonic-rate changes induce a landscape response and the response time, the time required to established equilibrium with the new conditions, is dictated by the rates of erosional processes at and below the catchment scale (Hack, 1960; Anderson, 1994; Densmore et al., 1998). Relatively short response times are inferred for regions characterized by widespread bedrock landsliding (Anderson, 1994; Schmidt & Montgomery, 1995; Burbank et al., 1996; Schmidt & Montgomery, 1996; Densmore et al., 1997, 1998). Bedrock landsliding is related to rock strength and local relief (Strahler, 1950; Schmidt & Montgomery, 1996). Correlation of drainage spacing, drainage geometry and relief, to a lesser extent, with bedrock serves as a proxy for bedrock strength variations in the Santa Monica Mountains. Inferred differences in rock strength implies that the eastern domain may be characterized by a lower threshold than 70 is the western domain because bedrock landsliding is a threshold response dictated by bedrock strength and relief production (Strahler, 1950; Anderson, 1994; Schmidt & Montgomery, 1996; Densmore et al., 1997). Whether bedrock landsliding plays a central role in longterm hillslope erosion is uncertain, however. The magnitude of topographic departures from a mean form depends on response time (Kooi & Beaumont, 1996). Regions undergoing persistent, rapid uplift are argued to maintain a steady-state form (or show small fluctuation about a mean form) because bedrock landsliding dominates hillslope erosion (Anderson, 1994; Schmidt & Montgomery, 1995, 1996; Burbank et al., 1996; Densmore et al., 1997, 1998). For model landscapes dominated by bedrock landsliding, but with low uplift and denudation rates comparable with those of the Santa Monica Mountains, landscape response time is #100 kyr (Densmore et al., 1998). Existing data indicate that shallow landsliding plays a central role in hillslope erosion in the Santa Monica Mountains (Campbell, 1975) and thus represents a dominant signal in the debris-basin sediment yields (Fig. 9) (Ferrell, 1959; Lustig, 1965; Scott et al., 1968; Scott & Williams, 1978). Shallow landslides often involve regolith and regolith-production rates tend to be lower than bedrock-landslide erosion rates (Anderson & Humphrey, 1989; Rosenbloom & Anderson, 1994). If hillslope erosion rate is set primarily by shallow landsliding and regolith production rates are less than 0.3 mm yr−1 as suggested by some models (Rosenbloom & Anderson, 1994), landscape response time is probably greater than 100 kyr in the Santa Monica Mountains. Despite the inference that the duration of uplift has been sufficient for the landscape to approach a steady-state form, the likelihood that both uplift and denudation rates have changed significantly over the past 100 kyr and that landscape response time is longer than 100 kyr suggests that the present topography is a departure from a mean form. C O N CLU S I O N S 1 Bedrock structure varies along strike from a simple anticline in the west to a compound structural culmination in the east. The change in geometry is strongly influenced by the change in position of the emergent frontal fault system from the forelimb of the anticlinorium to its crest from west to east, respectively. 2 Four discrete physiographic domains can be differentiated within the Santa Monica Mountains. A range-long drainage divide separates a northern north-draining domain from three south-draining domains. The northdraining basins are characterized by low differential relief with respect to base level in the San Fernando Valley. Of the south-draining basins, the Malibu Creek basin is anomalously large (340 km2), is marked by low internal relief and has low slopes in comparison with either the western or the eastern domain. Base level set by Santa Monica Bay, a mean elevation of 387 m, topographic © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains relief of #600 m with respect to base level, drainage spacing of 2.4 km, dendritic drainage geometries with aspect ratios of 351, catchment-scale relief of 300–900 m and bedrock dominated by volcanic rocks characterize the western domain. In contrast, the eastern domain is marked by base level set by the Los Angeles basin on the south of the range, a mean elevation of 164 m, topographic relief of #300 m with respect to base level, parallel drainage geometries with 651 aspect ratios, 1.6 km drainage spacing, catchment-scale relief of 200– 500 m and bedrock dominated by metasedimentary and sedimentary rocks. 3 Range-scale topographic characteristics of the Santa Monica Mountains are controlled by along-strike changes in structural geometry. The position of the drainage divide is dictated by differential relief across the flanks of the range and the fundamental asymmetry of the underlying structure. Differences in drainage geometry and spacing between the eastern and western domains are controlled by the underlying bedrock lithology. 4 Mean catchment-scale denudation rate based on debris basin sediment yield data from the last 70 years is 0.5±0.3 mm yr−1. River incision rates over the last 100 kyr may have been of the order of 0.5 mm yr−1. This is a maximum rate given that determined from a catchment with an area smaller than most of the other catchments in the range. 5 Published uplift rates of the crest of the anticlinorium relative to the Los Angeles basin on the south are #0.5±0.4 mm yr−1 on time-scales from 104 to 105 years. Mean uplift rate at 106-year time-scales based on structural relief is 0.52±0.12 mm yr−1. This rate is consistent with but lower than published long-term uplift rates of 0.6, 0.8 and 1 mm yr−1. Uplift rate of the crest of the anticlinorium relative to the San Fernando Valley on the north is 0.24±0.12 mm yr−1. Structural relief between the San Fernando Valley and Los Angeles basin gives an uplift rate of 0.28±0.12 mm yr−1. 6 On the basis of the present topographic and catchmentscale relief and the uplift and denudation rates, between 1.6 and 2.7 myr are required to create the present landscape. This represents 30–55% of the total length of folding (5 Myr). Because uplift rates may have varied since 100 ka and landscape response time is probably >100 kyr, the present landscape is probably close to a quasi-equilibrium form, although changing in response to the new uplift/denudation-rate ratio. Outstanding questions include the role of bedrock landsliding in this landscape and landscape response times. ACKNOWLE DGEME NTS A.J.M. was supported by a Postdoctoral Fellowship from the Division of Geological and Planetary Sciences, California Institute of Technology and from the Southern California Earthquake Center (SCEC). M.L.J. was partially supported by an SCEC Summer Internship. Michael Bohlander of the Los Angeles County Division © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 of Public Works provided the unpublished sediment yield data that form part of the data set presented in Fig. 9. Doug Burbank is thanked for discussions, suggestions and a review of this study. Kerry Sieh, Jim Dolan, Doug Yule and Eric Fielding are thanked for assistance and discussions along the way. Peter Talling and two other anonymous reviewers are thanked for keeping the analysis focused and honest. This research was supported by the Southern California Earthquake Center. SCEC is funded by NSF Cooperative Agreement Ear-8920136 and USGS Cooperative Agreements 14-08-0001-A0899 and 1434-HQ-97ag01718. This is SCEC contribution #415. R EFER EN CE S A, F. (1970) Functional relationships between denudation, relief, and uplift in large mid-latitude basins. Am. J. Sci., 268, 243–263. A, R.S. (1994) Evolution of the Santa Cruz Mountains, California through tectonic growth and geomorphic decay. J. Geophys. Res., 99, 20 161–20 179. A, R.S. & H, N.F. (1989) Interaction of weathering and transport processes in the evolution of arid landscapes. In: Quantitative Dynamic Stratigraphy (ed. T.A. Cross), pp. 349–361, Prentice Hall, Englewood Cliffs, N. J. A, R.S., R, J.L. & D, G.S. (1996) Explicit treatment of inheritance in dating depositional surfaces using in situ 10Be and 26Al. Geol, 24, 47–51. B, C., K, P.J.J., H, J. & F, P. (1996) The continental collision zone, South Island, New Zealand: Comparison of geodynamical models and observations. J. Geophys. Res., 101, 3333–3359. B, P.W. (1972) Late Quaternary eustatic sea-level changes along the Malibu coast, Los Angeles County, California. J. Geol., 80, 432–448. B, G.H. (1991) Review of the Neogene biostratigraphy and stratigraphy of the Los Angeles basin and implications for basin evolution. In: Active Margin Basins (Ed. by K.T. Biddle), pp. 135–184. American Association of Petroleum Geologists, Tulsa. B, N., B, F., B, M.J. & D, W.E. (1997) A seventy year record of erosion and sedimentation from the San Gabriel Mountains, southern California. EOS, Trans., Am. Geophy. Union, 78, F306. B., W.B. (1991) Geomorphic Response to Climate Change. Oxford University Press, New York. B, D.W., L, J., F, E., A, R.S., B, N., R, M. & D, C. (1996) Bedrock incision, rock uplift and threshold hillslopes in the northwestern Himalayas. Nature, 379, 505–510. C, R.H. (1975) Soil Slips, Debris Flows, and Rainstorms in the Santa Monica Mountains and Vicinity, Southern California. United States Geological Survey Professional Paper 851. United States Geological Survey, Washington. C, J. (1974a) Geology of corral terraces, Huon Peninsula, New Guinea: a study of Quaternary tectonic movements and sea level changes. Geol. Soc. Am. Bull., 84, 553–570. C, J. (1974b) The geomorphology and evolution of small valleys in dated coral reef terraces, New Guinea. J. Geol., 82, 795–812. C, M. & S, O. (1989) Disequilibrium of 71 A. Meigs et al. Holocene sediment yield in glaciated British Columbia. Nature, 337, 452–454. D, W.M. (1899) The geographic cycle. Geogr. J., 14, 481–504. D, W.M. (1933) Glacial epochs of the Santa Monica Mountains, California. Bull. Geol. Soc. Am., 44, 1041–1133. D, T.L. & N, J.S. (1994) A balanced cross-section of the 1994 Northridge earthquake, southern California. Nature, 372, 167–169. D, T.L., N, J.S. & G, S. (1996) Structure and hydrocarbon exploration in the transpressive basins of southern California. In: Pacific Section of the American Association of Petroleum Geologists Field Conference Guide 1996 (Ed. by P.L. Abbott and J.D. Cooper), pp. 189–238. Pacific Section of the Society of Economic Paleontologists and Mineralogists, Los Angeles. D, T.L., N, J. & Y, R.F. (1989) A cross-section of the Los Angeles area: seismically active fold and thrust belt, the 1987 Whittier Narrows earthquake, and earthquake hazard. J. Geophys. Res., 94 (B7), 9644–9664. D, A.L., A, R.L., MA, B.G. & E, M.A. (1997) Hillslope evolution by bedrock landslides. Science, 275, 369–372. D, A.L., E, M.A. & A, R.S. (1998) Landsliding and the evolution of normal-fault-bounded mountain fronts. J. Geophys. Res., 103 (B7), 15 203–15 219. D, T.W.J. (1982) Geology of the Santa Monica Mountains and Simi Hills, southern California. In: Geology and Mineral Wealth of the California Transverse Ranges (Ed. by D.L. Fife and J.A. Minch), pp. 94–130. South Coast Geological Society, Santa Ana. D, T.W. (1991) Geologic Map of the Beverly Hills Quadrangle, Los Angeles County, California. Dibblee Geological Foundation, 1524 000 (1524 000). D, W.E., W, C.J., M, D.R. & MK, J. (1993) Analysis of erosion thresholds, channel networks, and landscape morphology using a digital terrane model. J. Geol., 101, 259–278. D O G. (1991) California Oil and Gas Fields: Southern, Central Coastal, and Offshore California. California Department of Conservation, Sacramento. D, J.F. & P, T.L. (1997) High-resolution seismicreflection profiling of the Santa Monica fault zone, west Los Angeles, California. Geophys. Res. Lett., 24 (16), 2051–2054. D, J.F. & S, K. (1992) Tectonic geomorphology of the northern Los Angeles basin: Seismic hazards and kinematics of young fault movement. In: Engineering Geology Field Trips: Orange County, Santa Monica Mountains, and Malibu (Ed. by P.L. Ehlig and E.A. Steiner), pp. B-20–26. Southern California Section of the Association of Engineering Geologists, Los Angeles. D, J.F., S, K., G, P., M, G. & R, T.K. (1997) Active tectonics, paleoseismology, and seismic hazards of the Hollywood fault, southern California. Geol. Soc. Am. Bull., 12, 1595–1616. D, J.F., S, K., R, T.K., Y, R.S., S, J., S, J., H, G.J. & G, E.M. (1995) Prospects for larger or more frequent earthquakes in the Los Angeles metropolitan region. Science, 267, 199–205. E, J.E. (1926) A contribution to the geology of the Los Angeles basin, California. Am. Assoc. Pet. Geol. Bull., 10, 753–767. 72 F, W.R. (1959) Debris reduction studies for mountain watersheds. Los Angeles County Flood Control. G, G.K. (1877) Report on the geology of the Henry Mountains. In: United States Geographical and Geological Survey of the Rocky Mountains Region (Ed. by J.W. Powell), pp. 160. U.S. Government Printing Office, Washington, DC. G, D.E., K, J.W. & F, R.C. (1997) Quaternary downcutting rate of the New River, Virginia measured from differential decay of cosmogenic 26Al and 10Be in cave-deposited alluvium. Geology, 25, 107–110. G, U.S. & S, W.E. (1939) Some recent changes of elevation in the Los Angeles basin of southern California, and their possible significance. Bull. Seis. Soc. Am., 29 (2), 299–326. H, J.T. (1960) Interpretation of erosional topography in humid regions. Amer. J. Sci., 258-A, 80–97. H, E. & J, L.M. (1989) The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: seismological and tectonic analysis. J. Geophys. Res., 94 (B7), 9569–9589. H, H.W. (1931) Geology of the Eastern Part of the Santa Monica Mountains Los Angeles County, California. United States Geological Survey, Washington. H, N. (1996) Regular spacing of drainage outlets from linear mountain belts. Basin Res., 8, 29–44. H, A.D. (1997) Badland morphology and evolution: Interpretation using a simulation model. Earth Surf. Proc. Landf., 22, 211–227. H, G.J. & Y, R.S. (1995) Convergence rates across a displacement transfer zone in the western Transverse Ranges, Ventura basin, California. J. Geophys. Res., 100 (B2), 2043–2067. H, C., S, C.L., Y, R.S., D, J.F., S, K.E. & H, G.J. (1994) Wilshire fault: earthquakes in Hollywood? Geology, 22, 291–294. I, J., H, J.D., M, D.G., MI, A., M, A.C., M, J.J., P, N.G., P, W.L. & S, N.J. (1984) The orbital theory of Pleistocene climate: Support from a revised chronology of the marine ∞O18 record. In: Milankovitch and Climate, Part 1 (Ed. by A.L. Berger et al.), pp. 269–305. D. Reidel, Hingham, MA. J, C.W. & S, R.G. (1969) Geologic map of California Los Angeles sheet. California Division of Mines and Geology. J, M., D, J.F. & M, A. (1996) Geomorphologic and structural analysis of the stage 5e marine terrace, Malibu coast, California suggests that the Santa Monica Mountains blind thrust fault is no longer a major seismic hazard. EOS, Trans., AGU, 77, F461. K, D.K. (1984) Landslides caused by earthquakes. Geol. Soc. Am. Bull., 95, 406–421. K, D.K. (1994) The importance of earthquake-induced landslides to long-term slope erosion and slope-failure hazards in seismically active regions. Geomorphology, 10, 265– 284. K, G.C.P., S, R.S. & R, J.B. (1988) The growth of geological structures by repeated earthquakes 1. Conceptual framework. J. Geophys. Res., 93 (B11), 13 07–13 313 318. K, H. & B, C. (1996) Large-scale geomorphology: Classical concepts reconciled and integrated with contemporary ideas via a surface processes model. J. Geophys. Res., 101 (B2), 3361–3386. © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 Uplift and erosion of the Santa Monica Mountains K, P.O. (1989) The topographic evolution of collisional mountain belts: a numerical look at the southern Alps, New Zealand. Am. J. Sci., 289, 1041–1069. K, P.O. (1994) Three-dimensional critical wedges: Tectonics and topography in oblique collisional orogens. J. Geophys. Res., 99 (B6), 12301–12315. L, K.R., K, J.P., W, J.F., K, G.L., M, S.A., S-W, A.M., Y, R.F. & MC, P.F. (1979) Quaternary marine shorelines and crustal deformation, San Diego to Santa Barbara, California. In: Geological Society of America Annual Meeting Field Trip Guidebook: Quaternary Terraces and Crustal Deformation in Southern California (Ed. by K.R. Lajoie and J.P. Kern), pp. 140–152. Geological Society of America, Denver. L, D.L. (1970) Geology of the Elysian Park–Repetto Hills Area, Los Angeles County, California. California Division of Mines Special Report 101, San Francisco. L, W.B. & S, S.A. (1958) Yield of sediment in relation to mean annual precipitation. Trans. Am. Geophys. Un., 39 (6), 1076–1084. L, S. & Y, R.S. (1993) Paleomagnetic constraints on the initiation of uplift on the Santa Susana fault, western Transverse Ranges, California. Tectonics, 12 (3), 688–702. L, J. & S, R.S. (1989) Coseismic folding, earthquake recurrence, and the 1987 source mechanism at Whittier Narrows, Los Angeles basin, California. J. Geophys. Res., 94, 9614–9632. L, L.K. (1965) Sediment yield of the Castaic Watershed, Western Los Angeles County, California – a quantitative geomorphic approach. USGS Prof. Pap., 422-F, F1–23. MG, J.T. (1989) Geologic maps of the Pacific Palisades area, Los Angeles, California. United States Geological Survey. M, A.J. & O, M.E. (1997) Northwest by west: making sense of the structure of Hollywood. EOS, Trans., Am. Geophy. Union, 78, F453. M, D.J., V, K.R. & W, E.J. (1994) Long river profiles, tectonism, and eustacy: a guide to interpreting fluvial terraces. J. Geophys. Res., 99 (B7), 14 031–14 050. P, A.J. & W, A.J. (1986) Effects of earthquakeinduced landslides on sediment budgets and transport over a 50-yr period. Geology, 14, 52–55. R, S.L., D, W.E., D, D.J., J, A.J.T. & R, M. (1990) Late Quaternary history of colluvial deposition and erosion in hollows, central California Coast Ranges. Geol. Soc. Am. Bull., 102, 969–982. R, N.A. & A, R.S. (1994) Hillslope and channel evolution in a marine terraced landscape, Santa Cruz, California. J. Geophys. Res., 99 (B7), 14 013–14 014 029. R, B.P. & MD, I. (1967) Denudation rates in northeast Papua from Potassium-Argon dating of lavas. Am. J. Sci., 265, 545–561. S, K.M. & M, D.R. (1995) Limits to relief. Science, 270, 617–620. S, K.M. & M, D.R. (1996) Rock mass strength assessment for bedrock landsliding. Environ. Engin. Geosci., 2 (3), 325–338. S, C.L., H, C., Y, R.S. & H, G.J. (1996) Structural evolution of the northern Los Angeles basin, California, based on growth strata. Tectonics, 15 (2), 341–355. S, S.A. (1963) The disparity between present rates of denudation and orogeny. USGS Prof. Pap., 454-H, H1–13. © 1999 Blackwell Science Ltd, Basin Research, 11, 59–73 S, K.M. (1997) Variable hillslope erosion in a post-fire chaparral environment. EOS, Trans., Am. Geophy. Union, 78, F306. S, K.M., R, J.R. & K, J.M. (1968) Sedimentation in the Piru Creek Watershed Southern California. United States Geological Survey, Washington. S, K.M. & W, R.P. (1978) Erosion and sediment yields in the Transverse Ranges, southern California. United States Geological Survey, Washington, DC. S, M.A., D, W.E. & K, J.W. (1994) Longitudinal profile development into bedrock: an analysis of Hawaiian channels. J. Geol., 102, 457–474. S, R.S., K, G.C.P. & R, J.B. (1988) The growth of geological structures by repeated earthquakes 2. Field examples of continental dip-slip faults. J. Geophys. Res., 93 (B11), 13 319–13 331. S, A.N. (1950) Equilibrium theory of erosional slopes approached by frequency distribution analysis. Am. J. Sci., 248, 673–696. S, H.H. (1960) Tarzana Fan, deep submarine fan of Late Miocene age Los Angeles county, California. Bull. Am. Assoc. Pet. Geol., 1960, 433–457. T, P.J., S, M.D., S, C.P., G, S. & V, S.J. (1997) Regular spacing of drainage outlets from linear fault blocks. Basin Res., 9, 275–302. T, B.W. (1981) Inland sediment movements by natural processes. In: Sediment Management for Southern California Mountains, Coastal Plains, and Shoreline, pp. B1–81. California Institute of Technology Environmental Quality Laboratory, Pasadena. T, A.J. (1926) The Pliocene and Pleistocene history of the Baldwin Hills, Los Angeles County, California. Bull. Am. Assoc. Pet. Geol., 10, 502–512. V, F.P. (1927) The interpretation of the physiography of the Los Angeles coastal belt. Bull. Am. Assoc. Pet. Geol., 10, 417–424. W, G.E. (1992) Determination of the initiation of slide movement Big Rock Mesa landslide, Malibu, California. In: Engineering Geology Field Trips: Orange County, Santa Monica Mountains, and Malibu (Ed. by P.L. Ehlig and E.A. Steiner), pp. C-45–53. Southern California Section of the Association of Engineering Geologists, Los Angeles. W, P.W., F, A.E. & D, G.E. (1993) Depositional and Volcanic Environments of Middle Tertiary Rocks in the Santa Monica Mountains, Southern California. Pacific Section Society of Economic Paleontologists and Mineralogists, Bakersfield. W, D.L. & C, K.J. (1994) New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement. Bull. Seis. Soc. Am., 84, 974–1002. W, M.G. & M, J.P. (1960) Magnitude and frequency of forces in geomorphic processes. J. Geol., 68, 54–74. W, T.L. (1991) Structural geology and tectonic evolution of the Los Angeles basin, California. In: Active Margin Basins (Ed. by K.T. Biddle), pp. 35–134. American Association of Petroleum Geologists, Tulsa. Y, R.S. & H, G.J. (1995) The Oak Ridge fault system and the 1994 Northridge earthquake. Nature, 373, 418–420. Z, W.H. & M, D.R. (1994) Digital elevation model grid size, landscape representation, and hydrologic simulations. Water Res. Res., 30, 1019–1028. 73