(1999) 11, 59–73 steady, balanced rates of uplift and ...€¦ · santa monica mountains,...

Basin Research (1999) 11, 59–73

Steady, balanced rates of uplift and erosion of theSanta Monica Mountains, CaliforniaA. Meigs,* N. Brozovic† and M. L. Johnson‡*Division of Geological and Planetary Sciences, CaliforniaInstitute of Technology, Pasadena, CA 91125, USA†Department of Geology and Geophysics, University ofCalifornia, Berkeley, Berkeley, CA 94720, USA‡University of Nevada, Reno Seismological Laboratory,Mackay School of Mines, University of Nevada, Reno, Reno,NV 89557, USA

ABSTRACT

Topographic change in regions of active deformation is a function of rates of uplift anddenudation. The rate of topographic development and change of an actively upliftingmountain range, the Santa Monica Mountains, southern California, was assessed usinglandscape attributes of the present topography, uplift rates and denudation rates. Landscapefeatures were characterized through analysis of a digital elevation model (DEM). Uplift rates attime scales ranging from 104 to 106 years were constrained with geological cross-sections andpublished estimates. Denudation rate was determined from sediment yield data from debrisbasins in southern California and from the relief of rivers set into geomorphic surfaces ofknown age. First-order morphology of the Santa Monica Mountains is set by large-scale along-strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesserextent relief are controlled by bedrock strength. Dissection of the range flanks and position ofthe principal drainage divide are modulated by structural asymmetry and differences instructural 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 is0.5±0.3 mm yr−1. Uplift rate across the south flank of the range is #0.5±0.4 mm yr−1 andacross the north flank is 0.24±0.12 mm yr−1. At least 1.6–2.7 Myr is required to create eitherthe present topographic or the catchment-scale relief based on either the mean rates ofdenudation or uplift. Although the landscape has had sufficient time to achieve a steady-stateform, comparison of the time-scale of uplift and denudation rate variation with probablelandscape response times implies the present topography does not represent the steady-stateform.

& Watson, 1986; Keefer, 1994), climate forces erosionINTRODUCTION

independently of tectonism and varies on time scalesranging from 101 to 105 years (Wolman & Miller, 1960;That topography resulting from active deformation is a

function of the interplay between structural and geo- Schumm, 1963; Bull, 1991). Consequently, the temporaland spatial scales of erosion and deformation themselvesmorphic processes is self-evident. What is less evident is

how to invert the erosional and deformational components hinder direct measurement of topographic change atmeaningful scales (i.e. at the range scale).from landscape form because they can be dependent and

in-phase and independent and out-of-phase, both in Time-averaged implications of the interaction betweenuplift and denudation are suggested by numerical modelsspace and in time (Hack, 1960; Schumm, 1963; Ahnert,

1970; Bull, 1991; Kooi & Beaumont, 1996). Coseismic– of landscape evolution (see, for example, Anderson, 1994;Koons, 1994; Beaumont et al., 1996; Kooi & Beaumont,interseismic deformation, for example, is a continuous

tectonic process that varies in magnitude with time (King 1996; Howard, 1997; Densmore et al., 1998). As inferredfrom observational studies (Gilbert, 1877; Hack, 1960;et al., 1988; Stein et al., 1988; Wells & Coppersmith,

1994). Although seismically induced denudation is, by Ahnert, 1970), topography tends towards a steady-stateform with time in many models. How much time isdefault, in phase with deformation (Keefer, 1984; Pearce

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required to attain a steady-state form depends on uplift argued to be relatively slow (approaching steady-state?)given the present form, duration of deformation, andand erosion rates (Ahnert, 1970; Kooi & Beaumont, 1996;

Densmore et al., 1998). The steady-state form is charac- erosion and rock uplift rates.terized by local relief (at the scale of individual catch-ments), or topographic relief (at the scale of a range with REGIONAL SETTINGrespect to adjacent base level), reaching a maximum value(Hack, 1960; Ahnert, 1970). Because a topographic fea- Lying to the north and west of downtown Los Angeles,

the Santa Monica Mountains extend 90 km from the Losture develops only if rates of denudation are less thanrock uplift rates initially, denudation rates must approach Angeles River on the east to the Oxnard Plain on the

west (Fig. 2). Investigations of the connection betweenuplift rates as deformation proceeds (Fig. 1; Ahnert,1970). Steady-state topography can be sustained only if tectonics and geomorphology in the Santa Monica

Mountains extend back more than 70 years (Eaton, 1926;uplift and erosion rates are balanced (Koons, 1989; Kooi& Beaumont, 1996). Tieje, 1926; Vickery, 1927; Hoots, 1931; Davis, 1933;

Grant & Sheppard, 1939). Of those investigators whoThe objective of this study is to understand thetopographic development of an actively growing moun- have considered the large-scale form of the range, most

accept the interpretation of Davis (1933) that the rangetain range, the Santa Monica Mountains, southernCalifornia, in the context of interactions between uplift is a dissected remnant of a recently uplifted peneplain,

which implies that erosion rates are significantly lowerand erosion. Questions addressed in the analysis include:(1) What is the present form of landscape? How has that than uplift rates (Hoots, 1931; Dibblee, 1982).form changed with time?; (2) What are erosion rates?What are rock uplift rates? How do they compare RANGE-SCALE STRUCTURE ANDtemporally?; (3) How are bedrock lithological variations BEDROCK GEOLOGYreflected by topography?; (4) Can uplift rates, erosionrates and the present topography be used to make A complex mosaic of active strike-slip and thrust faults

is deforming the Los Angeles basin as the consequenceinferences about topographic change? We integrate land-scape characterization from a digital elevation model of transpressive motion between the Pacific and North

American plates (Wright, 1991). Many of the active(DEM), bedrock geology and structure, surface geomor-phology, and estimates of erosion and uplift rates. structures are marked at the surface by structural anticli-

noria (Hauksson & Jones, 1989; Lin & Stein, 1989; DolanTopographic change in the Santa Monica Mountains iset al., 1995; Yeats & Huftile, 1995). The Santa MonicaMountains anticlinorium is one of these anticlinoria andis interpreted to have formed due to displacement on ablind thrust system at depth (Davis et al., 1989, 1996;Davis & Namson, 1994). Slip on the thrust system isinferred to have been accommodated by a combinationof hangingwall folding and displacement on fault splaysoff 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 abroad structural culmination extending from the LosAngeles River on the east to the Oxnard Plain on thewest and between the synclines beneath the San FernandoValley on the north and the northern edge of the LosAngeles basin on the south (Hoots, 1931; Wright, 1991;Hummon et al., 1994; Yeats & Huftile, 1995; Davis et al.,Fig. 1. Conceptual model illustrating the relationship between

local relief (A), uplift rate and erosion rate (B) and topographic 1996; Schneider et al., 1996) (Fig. 3). Syntectonic sedi-development (C) for an actively growing structure. The uplift ments indicate fold growth initiated at #5 Ma (Fig. 3b)rate is set arbitrarily as constant to emphasize the point that as (Schneider et al., 1996).erosion rate approaches uplift rate, the rate of topographic It is important to emphasize that the anticlinoriumchange slows and the landscape approaches a steady-state comprises three distinct physiographical domains: theconfiguration (similar in concept to Ahnert, 1970). Uplift is Santa Monica Mountains, the northern margin of thecharacterized as a simple asymmetric function that grows self-

Los Angeles basin and Santa Monica Bay (Fig. 3). Asimilarly. Note that the erosion function scaled by a percentagefault system exposed at the surface, the Malibu, Santaof uplift rate and wavelength. Local relief (catchment scale) andMonica, Hollywood fault system (MSH), separates theerosion rate are coupled and display positive feedback. A lagSanta Monica Mountains on the north from the otherbetween the initial stage when erosion rate is lower than uplifttopographic domains on the south (Fig. 2) (Vickery,rate (t1–2) and when erosion rate approximates uplift rate (t3–4)

modulates the rate of topographic change with time. 1927; Hoots, 1931; Dibblee, 1982; McGill, 1989; Weber,

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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 bedrocklithological units. The location of Potrero Canyon in Pacific Palisades is indicated by a black dot (Fig. 10). Specific details onages 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 throughPleistocene 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; Schneideret al., 1996). Note that the Miocene sedimentary rocks are exposed around the periphery of the range whereas the interior isdominated by the Mesozoic through Lower Tertiary sequence in the east and the Conejo Volcanics in the west. Modified fromHoots (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 the crest of the anticlinorium and the superposition offolds alters the cross-sectional geometry (Figs 2 and 3).is characterized by a relatively simple asymmetric anti-

cline cut by the MSH between the Oxnard Plain and These folds and faults are inferred to have formed after1 Myr and be evidence that displacement on the easternSanta Monica (Figs 2 and 3A) (Davis et al., 1996). The

anticline is doubly plunging, has forelimb dips that range MSH, hangingwall folding, and footwall deformationoccurred coevally (Fig. 2) (Hummon et al., 1994;from 20° to 65° south, and a backlimb dip of #20° north

(Figs 2 and 3) (Hoots, 1931; Dibblee, 1982). Structural Schneider et al., 1996; Meigs & Oskin, 1997).Surface exposures of bedrock are dominated by fourinterference between the anticlinorium and structures

lying to the west of the San Fernando Valley obscure the rock types: (a) Mesozoic intrusive rocks, (b) Mesozoic toLower Tertiary metasedimentary and sedimentary rocksnorth flank of the range (Fig. 2). Structural complexity

increases to the east of Santa Monica (Fig. 2). Although ( Jurassic Santa Monica Slate through the OligoceneSespe Formation), and (c) Miocene–Pliocene sedimentarythe anticlinorium persists as a broad structural high, a

change in the position of the MSH from the forelimb to rocks including (d) an interbedded sequence of volcanic

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characterized by relatively constant width and relief (Fig.4B). East of Santa Monica, a distinct eastward taper inwidth and decrease in relief mark the form of themountains. The surface of the Los Angeles basin to thesouth 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 byan east–west trending drainage divide (Fig. 4A). Thedrainage divide does not correspond with the structuralaxis; the divide lies from 4 to 10 km to the north (Hoots,1931). Further subdivision of the range into four distincttopographic domains, one north and three south of thedrainage divide, is suggested by topography. The domaindefined north of the divide is characterized by north-draining basins (Fig. 4A). Three domains lying to thesouth of the drainage divide, an eastern domain, theMalibu Creek basin and a western domain, have basinsthat drain southward. Local drainage divides separate theeastern and western domains from the Malibu Creekdrainage. Base level is set by the Los Angeles basin formost of the eastern domain and by Santa Monica bay forFig. 3. Geological cross-section across the Santa Monicathe three westernmost catchments in the eastern domain,Mountains and San Fernando Valley. Cross-section (A)the Malibu Creek basin, and the western domain. Basetraverses the central part of the range whereas section (B)level for the northern domain, the San Fernando Valley,traverses the eastern portion of the range (modified from Davis

et al., 1996; Schneider et al., 1996, respectively). Note the lies at a higher elevation than that for the south-drainingchange in position of the range-front fault system (MSH: the basins (Fig. 4A). Overall, the fact that the south-drainingMalibu Coast, Santa Monica and Hollywood fault system), basins cover most of the range, are more elaboratedfrom the forelimb to the crest of the anticlinorium from west to and have greater areas than the north-draining basinseast, respectively. Unit K – M. Mio. is Cretaceous through (Fig. 4D) reflects greater dissection of the south flankMiocene rocks that include the portions of the Mesozoic than the north flank. Most of the denudation, andthrough Lower Tertiary and Miocene sedimentary rocks of Fig.

consequently form, of the range is dictated by the south-3. Unit Plio. – Q includes both the Pliocene throughflank basins. The following discussion focuses on thePleistocene sedimentary sequence and modern alluvial surfacesregion lying to the south of the drainage divide.of Fig. 2.

Mean elevation, catchment-scale relief, slope and hyp-sometry were calculated from the 30-m grid spacingdigital elevation model (DEM) available from the Unitedrocks (Vaqueros, Topanga, Puente and Modelo

Formations and the Conejo Volcanics) (Fig. 2) (Dibblee, States Geological Survey (Figs 4–6). Calculations ofmean elevation and slope involved iterative calculation of1982; Weigand et al., 1993). Surrounding the mountains

on all sides are a series of surfaces that are thought to be a subset of the data followed by a one-pixel (data point)shift of the calculation space across and down the dataPleistocene in age and are overlain by younger alluvial

deposits locally (Tieje, 1926; Vickery, 1927; Hoots, 1931; array. Mean elevation (sum of the elevations divided bythe number of data points) was calculated for (900 m)2Davis, 1933; Dibblee, 1982; Dolan & Sieh, 1992). The

distribution of these bedrock units reflects the anticlinal scrolling windows (30×30 subset of data array; Fig. 4B).Mean slope angles were calculated by fitting a least-structure of the bedrock. Miocene sedimentary rocks are

exposed around the periphery of the range, and bedrock square deviation best-fit plane to groups of five-by-fivepixels, corresponding to approximately (120 m)2 surfaceexposure in the core changes systematically, from

Mesozoic intrusive rocks on the east, to Mesozoic through area (Fig. 4C). Slope angles are sensitive to both DEMgrid spacing and to the length scale over which slopeOligocene sedimentary rocks, to Miocene volcanic rocks

on the west (Fig. 2). is measured (Zhang & Montgomery, 1994). In general,slope angles decrease with increasing measurementlength. For the 30-m DEM used in this study, slopeLANDSCAPE CHARACTERISTICS OFangles are underestimated by this technique, althoughTHE SANTA MONICA MOUNTAINSmultiple hillslopes will not generally be averaged by anindividual calculation. Recognizing this limitation of slopeFirst-order topography in the Santa Monica Mountains

is set by the large-scale bedrock structure and varies as determined from the DEM, we place more significanceon trends in slope angles than on their absolute values.a function of the along-strike structural changes (compare

Figs 2 and 4A). West of Santa Monica, the range is Individual catchment properties including length, width,



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 northernboundary is marked by the purple–blue colour change) and the Malibu Creek basin (MCb). White line indicates position oftopographic profile in Fig. 7. Sharp north–south- and east–west-trending breaks in the image are artefacts of the data sets and donot represent real topographic features. (B) Mean elevation map. (C) Slope map. See text for details of the calculation procedurefor mean elevation and slope maps. (D) Map showing the boundaries of the principal catchments imaged by the 30-m DEM.


A. Meigs et al.

malized to percentage area in order that variably sizedsubsets 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 dis-tinguish the eastern from the western domains (Figs4–6). Topography in the eastern domain is characterizedby a maximum elevation of about 600 m and a meanelevation of 164 m (Figs 4 and 5). Parallel drainages,typified by length–width aspect ratios of #651, arespaced #1.6 km apart in the east (Fig. 4D). Eastern-domain catchments tend to have areas less than 10 km2and relief less than 500 m (Fig. 6A–C). Bedrock isdominated by Mesozoic to Lower Tertiary metasedimen-

Fig. 5. Hypsometry, or frequency distribution of elevations per tary and sedimentary rocks (Fig. 2). In contrast, the100-m bin, for the eastern, western and combined domains. western domain has elevations up to 1000 m and a meanThe bimodal distribution of the combined domains reflects the elevation of 387 m (Figs 4 and 5). Dendritic drainagesfact that the eastern domain is dominated by lower elevations with #351 aspect ratios, areas up to 30 km2 and reliefthan the western domain. Note that the mean elevation of the as high as 900 m have developed primarily on Middleeastern domain is higher than the mode but less than the

Miocene volcanic rocks (Figs 4D and 6A–C). Drainagescombined mean elevation. The mean elevation for the westernin the west have an average spacing of 2.4 km. Slope, indomain is higher than both the mode and the mean elevation ofcontrast, does not vary appreciably between the east andthe combined data.west. Close inspection of Fig. 4(C) shows that the highestslopes occur on interfluves, regardless of underlying

area and relief (maximum minus minimum elevation) bedrock lithology. Calculated slope angles are in generalwere calculated based on the principal catchments ident- agreement with, although consistently lower than, field

measurements (Campbell, 1975).ifiable from the DEM (Fig. 4D). Hypsometry is nor-

Fig. 6. Plot of drainage basin attributes along the strike length of the range from the Los Angeles River to the Oxnard plain onthe 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) aredifferentiated from north-draining basins in the western region of structural interference (Figs 2 and 4A). (C) Drainage basinrelief, defined as the difference in elevation between the drainage divide and mouth of each catchment (Hovius, 1996; Tallinget al., 1997). An arrow marks the along-strike point at which base level for the south-draining basins changes from the LosAngeles basin in the east to Santa Monica Bay in the west. (D) Relief vs. area highlights the distinction between the eastern andwestern domains.



Two key differences between the eastern and western of that predicted for each domain (compare Figs 4Cand 8).domains are highlighted by a topographic profile along

the length of the range (Fig. 7). First, the average Malibu Creek is noteworthy when compared with theother south-draining basins because its area (>340 km2)topographic relief, as measured by the difference between

the mean elevation and the base level elevation for each is an order of magnitude larger than that of any otherbasin (Fig. 4D). Overall, the majority of the Malibudomain, is significantly larger in the west than in the

east (Fig. 7). Second, an upper limit to drainage basin Creek basin is characterized by low relief and low slopes(Fig. 4C). Valley bottoms contain Upper Pleistocene (?)relief can be inferred from the integration of slope angle

with drainage spacing (Fig. 8). Relief is given by the to Recent alluvial and fluvial fill (Fig. 2). The onlybedrock-channel has developed in a narrow gorge whereproduct of one half the drainage spacing and the tangent

of the average slope angle (Fig. 8). Relief is particularly Malibu Creek drains across the south flank of the range(Fig. 4A). Relief in the gorge reaches 450 m and thesensitive to slope and has a maximum equal to half the

drainage spacing as slope approaches 45°. Drainage spac- channel gradient is up to 5% locally. Topographyimmediately to the east and west of the gorge is similar.ing is the variable that differentiates maximum possible

relief calculated for the eastern and western domains Reaches of Malibu Creek and its tributaries immediatelynorth of the gorge have incised #3–4 m into alluvial fill(360–550 m and 540–810 m, respectively; Fig. 8), given

slope angles are similar in each domain (25°–35°; Fig. (Fig. 2) ( Jennings & Strand, 1969). Defeat of a localdrainage divide and capture of an upland basin is inferred4C). Measured catchment-scale relief is within the range

Fig. 7. Topographic profile along the strike length of the south flank of the range from the Oxnard Plain (OP) to the LosAngeles River (LAr) (Fig. 4A). The profile intersects the canyons at the points of high local relief in order to illustrate canyondepth and extent of dissection. Note that the canyons in the west are more deeply incised and have greater relief than those inthe 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 theeast (see Figs 4B and 6C). The mean elevation and elevation of base level for the eastern (164 and 115 m, respectively) andwestern (387 and 0 m, respectively) contrast between the two domains. Malibu Creek (MC) and Topanga Creek (TC) areindicated for reference. Note that although Topanga Creek lies within the eastern domain, its base level is set by Santa MonicaBay (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 interfluveslopes (Fig. 4C). A characteristic relief and accordant summits like those observed in the Santa Monica Mountains are predictedby this model (Hoots, 1931). See text for discussion. This model creates similar peak, interfluve and ridge top elevations thatcould be interpreted as remnants of an original planation surface (Davis, 1899; Hoots, 1931; Davis, 1933; Dibblee, 1982). Anumber of lines of evidence suggest that the combined effects of dissection and uplift, given the age of the structure, would haveobliterated any evidence of an older planation surface.


A. Meigs et al.

to explain the anomalous area of the Malibu Creek unpublished Los Angeles County Department of Waterdrainage basin. This interpretation contrasts with that of and Power records, normalized by basin area, and dividedDibblee (1982), who believed that Malibu Creek was by length of record to approximate regional denudationantecedent to uplift of the Santa Monica Mountains. rates (Fig. 9). Two hundred and seven independent

Specific features of the topography of the Santa Monica estimates over time-scales from 1 to 70 years in lengthMountains can now be related to structural variation, and from basins with areas extending from <0.01 km2differential relief with respect to base level and bedrock to #1000 km2 indicate that denudation rates vary fromlithology. The first-order width and length of the range 0.02 to 40 mm yr−1 regionally. The highest rates comeare controlled by along-strike variations in bedrock struc-ture. Greater differential relief across the south flank ofthe range than the north flank has resulted in thedevelopment of larger basins and greater dissection inthe south (Figs 4 and 5). The south limb of the anticlin-orium is steeper structurally than the north limb, whichprobably amplifies the north–south asymmetry in dissec-tion. A combination of lower base level and higheraverage slope, imposed by the fold asymmetry, suggeststhat the south-draining streams may have higher streampower (in a qualitative sense), and thus greater erosivecapability, than north-draining streams with the sameareas. In this way, differences in base level combine withfundamental structural asymmetry to force the drainagedivide to the north of the structural axis (Fig. 4A).

Base-level control is also implied by comparison ofhypsometry for each domain (Fig. 5). Basin length–widthaspect ratio, drainage pattern and drainage spacing varyas a function of underlying bedrock (compare Figs 2 and4). The sedimentary and metasedimentary rocks exposedin the east are inferred to be relatively ‘weaker’, moresusceptible to erosion, than the volcanic rocks exposedin the west. Although the regular spacing of drainageoutlets documented by Hovius (1996) implies that bed-rock may not be important in the definition of drainagenetworks, the correlation between drainage geometry andunderlying bedrock in the Santa Monica Mountains is

Fig. 9. Denudation rate data for southern California (A) andmost simply interpreted as a bedrock-erodibility phenom-the Santa Monica Mountains (B). Data for (A) include 1-yearenon. Slope, on the other hand, shows no obvious(closed circles; Scott & Williams, 1978) and multiyear recordssystematic variation related to bedrock, and is most likely(open circles; Ferrell, 1959; Lustig, 1965; Scott & Williams,

controlled by processes or the sum of processes operating 1978; Taylor, 1981; and unpublished Los Angeles Countyat scales below the resolution of the DEM (Dietrich Department of Public Works). Denudation rate is calculated byet al., 1993; Anderson, 1994; Zhang & Montgomery, dividing sediment yield data (km3) by basin area (km2) and then1994). The relationship between specific hillslope pro- by the number of years of the record. These data are useful forcesses and bedrock lithologies is poorly constrained defining the expected range of denudation rate. Note that no(Campbell, 1975). clear correlation between drainage basin area and denudation

rate is seen for basins less than #10 km2, for either single- ormullet-year records (Brozovic et al., 1997). (B) Measurements

TIME-SCALES AND RATES OF and estimates of denudation rate as a function of drainage basinDENUDATION VS. UPLIFT area for the Santa Monica Mountains. Denudation estimates

based on relief and basin area for Santa Monica basins (Fig. 5)Having differentiated the relative contributions thatafter the linear regression model of Taylor (1981) (denudationstructure, base level and bedrock lithology have had inrate=0.0936*L3.11*Area−0.141, where L is a landscape factorshaping the present topography, we now address therelated to qualitative estimates of relief; L=1, relief<#100 m,more difficult problem of calibration of denudation andL=2, #100 <relief <1000 m, L=2.7, relief >#1000 m).

uplift rates and their variation over the time-scale of Rather than strictly interpreted as absolute values, this analysisgrowth of the anticlinorium. provides an order-of-magnitude estimate on denudation rate in

a landscape with relief similar to that of the Santa MonicaMountains (0.5±0.3 mm yr−1). Black squares are measuredDenudationvalues and shaded symbols are model calculations based on the

Sediment yield estimates for catchments throughout sou- basin areas depicted in Figs 4D and 6A. Note that (A) is a log–log plot, whereas (B) is a log–linear plot.thern California were compiled from the literature and



from data collected after a significant rainfall year Densmore et al., 1998). To know or approximate river(1968–69; Scott & Williams, 1978). No strong correlation incision rate allows rates of relief development to bebetween rate and basin area is revealed by the data, estimated (Schumm, 1963; Ahnert, 1970; Chappell,although basins with areas less than or equal to 10 km2 1974b; Burbank et al., 1996; Densmore et al., 1998).show a greater range of rates than those with areas greater Incision rates have been measured using the differentialthan 10 km2. Whether these rates represent primary relief between fill and/or strath river terraces of knownbedrock erosion rates depends on the time-scale of ages (Merritts et al., 1994; Anderson et al., 1996; Burbanksediment storage relative to the record length. That time et al., 1996; Granger et al., 1997) and the differentialhas been inferred to be short for basins smaller than relief of rivers set into other geomorphic surfaces with10 km2 (Brozovic et al., 1997), thus implying that rates known ages, such as marine terraces or lava flows,from these catchments represent primary bedrock erosion provided the catchment lies entirely within that surfacerates. Data from catchments outside southern California, (Ruxton & McDougall, 1967; Chappell, 1974b;however, suggest that sediment production and storage Rosenbloom & Anderson, 1994; Seidl et al., 1994). Aon 105-year time-scales can significantly influence sedi- flight of at least three marine terraces has been etchedment yields from basins with areas ranging from <1 km2 into the south flank of the Santa Monica Mountains,to >104 km2 (Langbein & Schumm, 1958; Church & permitting the latter technique to be applied (Hoots,Slaymaker, 1989; Reneau et al., 1990). Some of the 1- 1931; Davis, 1933; Birkeland, 1972; Lajoie et al., 1979;year data comes from catchments with areas <10 km2 McGill, 1989).and that contain incised valley fills. Clearly, sediment The marine terraces have been recognized since theyields from these basins have a mixed signal and do not 1920s and are well described, correlated and reasonablyreflect primary bedrock erosion (Fig. 9A). These data well dated (Hoots, 1931; Davis, 1933; Birkeland, 1972;therefore provide order-of-magnitude constraints on the Lajoie et al., 1979; McGill, 1989; Weber, 1992; Johnsonrange of probable denudation rates. et al., 1996). The abandoned platforms are correlated

Nine of the estimates are from basins within the Santa with oxygen isotope stages 5e, 7 and 9 (#125, 225–240,Monica Mountains and indicate that erosion rates likely 325–340 kyr, respectively; Imbrie et al., 1984), based onvary between #0.25 and 1.1 mm yr−1 (Fig. 9B). The

uranium-series dating of material deposited on individualpotential range of denudation rate can be narrowed using

platforms (Lajoie et al., 1979; McGill, 1989; Weber,the inferred relationship between relief and denudation1992). Potrero Canyon is a small drainage basin lyingrate (Langbein & Schumm, 1958; Schumm, 1963; Ahnert,entirely within the lowest terrace at Pacific Palisades1970). Regression of denudation rate against basin area(stage 5e age, area=0.7 km2, length=1.7 km, and relieffor some of the southern California data (Fig. 9A)(drainage head to the outlet)=91 m, Fig. 2). Because thesuggested a correlation between sediment yield reliefstream is younger than the terrace, a maximum incision(relief was loosely defined on the basis of topographicrate of 0.5 mm yr−1 for the stream over the past 125 kyrrelief of Taylor, 1981). From this analysis, Taylor (1981)can be calculated at the point of maximum heightdeveloped a simple model relating denudation rate to(#62 m) between the modern stream and the terracebasin area and relief. A mean denudation rate ofsurface (Fig. 10). It is likely that this rate has varied on0.2–0.8 mm yr−1 is revealed when basin area extractedshorter time-scales owing to variations in the rate of mig-from the DEM (Fig. 5) and values of relief betweenration of the drainage head (mean rate of #10.5 mm yr−#100 and <1000 m are used in the model (Fig. 9B).1; Fig. 10) and sea-level oscillations (Merritts et al., 1994;This range of rates captures #66% of the measuredSeidl et al., 1994). Because erosion rate is related torates (6 of 9 basins). Denudation on the time-scale ofdrainage basin size and geometry and climate (Schumm,these data, ≤70 years, appears to be dominated by the1963), this may be a maximum rate given that nearly all50-year-return storm (Ferrell, 1959; Lustig, 1965; Scott

et al., 1968; Campbell, 1975; Scott & Williams, 1978;Taylor, 1981). Shallow landsliding from hillslopes andhollows accounts for a significant fraction of sedimentdelivered from catchments after one such storm cycle inthe winter of 1968–69 (Campbell, 1975). Other significantfactors influencing sediment yields include fire cyclicityand landsliding forced by earthquake shaking, orientationof 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).

Fig. 10. Stream and interfluve (to the east and west of PotreroCanyon) of a drainage incised onto the 125-ka marine terrace atRiver incisionPacific Palisades (Fig. 2). Note that incision is measured

River incision plays a critical role in long-term, range- perpendicular to the stream profile near the canyon mouth atthe point of maximum relief.scale denudation (Schumm, 1963; Koons, 1989;


A. Meigs et al.

Table 1. Time-scale, uplift rate, data, source.

Time-scale Rate (mm yr−1) Data Source

10 kyr 0.1 Offset soils Dolan et al. (1997)100–400 kyr 0.2–0.9 Uplifted marine terraces Lajoie et al. (1979)

McGill (1989)Weber (1992)

800 kyr–1 Myr 0.3–0.4 Minimum fault displacement Hummon et al. (1994)Dolan et al. (1997)Meigs & Oskin (1997)

1–5 Myr 0.5–1.0 Uplifted syntectonic strata Schneider et al. (1996)and pre-tectonic strata Davis et al. (1996)

this studyAverage 0.5±0.4

other significant drainage basins are at least an order of long-term rock uplift rates if age of fold initiation can bemagnitude larger (Fig. 6A). established. Age of formation has been established at

5 Ma (Schneider et al., 1996). Finding a suitable markerin the pre-uplift strata for measuring uplift is notoriouslyUplift ratesdifficult in southern California because pre-uplift strata

Estimates of short-, intermediate- and long-term uplift typically vary in thickness and are time transgressiverates for the Santa Monica Mountains anticlinorium are owing to deposition over pre-existing topographyin general agreement (Table 1) (McGill, 1989; Weber, (Sullwold, 1960; Blake, 1991; Wright, 1991; Schneider1992; Johnson et al., 1996; Dolan et al., 1997; Meigs & et al., 1996). Mohnian-aged strata (the ModeloOskin, 1997). Intermediate-term rates are constrained by Formation, Sullwold, 1960), used to calculate uplift onthe ages and present elevation of the three marine terrace Fig. 12, share many of these traits. A number of uniquelevels (Fig. 11). Terrace age and elevation with respect characteristics of the specific position of our section,to sea level provide a measure of uplift rate after eustatic however, allow reasonable confidence to be placed on thesea-level change is subtracted from terrace elevation uplift estimate using these strata. First, the unit is nearly(Chappell, 1974a). After correction, the Santa Monica continuously exposed from the north flank across theterraces yield an average uplift rate of 0.22 mm yr−1 crest to the south flank of the range (Fig. 2) (Dibblee,since 340 ka: since 125 ka rates have varied from 1991). Its projection into the subsurface of the San0.2 mm yr−1 at Pt. Dume on the west to 0.9 mm yr−1 Fernando Valley north of the range is constrained byat Pacific Palisades on the east (Birkeland, 1972; Lajoie well-data (Dibblee, 1982; Division of Oil & Gas, 1991).et al., 1979; McGill, 1989; Weber, 1992; Johnson et al., On the south, projection into the subsurface is based on1996). These rates are consistent with short- and inter- structural geometry and information from an adjacentmediate-term rates of 0.1–0.4 mm yr−1 inferred from section (Davis & Namson, 1994; Davis et al., 1996).palaeoseismological and structural arguments, respect- Second, detailed stratigraphic analyses in the area of theively (Hummon et al., 1994; Dolan et al., 1997; Meigs & section include mapping of a sand bed exposed con-Oskin, 1997). tinuously above the basal unconformity, mapping of the

Geological cross-sections can be used to determine lateral extent of individual sand beds within the unit,closely spaced measured sections and biostratigraphicconstraints on palaeo-water depths (#900 m)(Sullwold, 1960).

Two key assumptions are necessary in order to useMohnian strata on the line of section to measure struc-tural relief: (1) the same bed or suite of beds can beprojected across the fold; and (2) that those beds weredeposited at approximately the same depth. Whereas thelatter is not well constrained as palaeontological data areonly available for strata on the north limb (Sullwold,1960), the former is justifiable. Bedding on the north

Fig. 11. Profiles at the same position through Point Dume fromflank of the anticlinorium is concordant and dip isthe DEM and mean elevation data sets and the height of therelatively constant (20–25° north) across #900 m of340-ka terrace (Davis, 1933; Birkeland, 1972; Lajoie et al.,section. The base of these beds is continuous with flat-1979; McGill, 1989; Weber, 1992; Johnson et al., 1996). At leastlying strata on the crest to the south (Dibblee, 1991).680 m of topographic relief may have been present by 340 ka

according to this profile. Measured sections indicate that this unit thins by



Fig. 12. Detailed geological cross-section across the Santa MonicaMountains drawn just to the west andparallel to the cross-section of Fig. 3.Uplift is indicated by the base ofMohnian-aged strata (heavy line)exposed on the flanks and crest of therange and the nearby subsurface(structure below the Mohnian stratafrom Davis & Namson, 1994). Datasources include (Hoots, 1931; Sullwold,1960; Dibblee, 1982, 1991; Blake, 1991;Division of Oil & Gas, 1991; Wright,1991; Davis et al., 1996; Schneideret al., 1996).

#600 m eastward (Sullwold, 1960); this thickness vari- Yeats, 1995). Finally, the 1380 m structural relief betweenthe Mohnian in the subsurface of the San Fernandoation provides a generous error estimate for across-strike

changes in thickness. Because the strata are interpreted Valley and Los Angeles basin gives an uplift rate of0.28±0.12 mm yr−1 (Fig. 12).to have been deposited in middle to upper bathyal depths,

600 m also encompasses nearly the entire range of possiblewater depths during deposition. Uplift rates are therefore TOPOGRAPHIC CHANGE:assigned an error of ±0.12 mm yr−1 (0.6 km/5 Myr= IMPLICATIONS OF PRESENT0.12 mm yr−1). It is the unique circumstances of this TOPOGRAPHY, UPLIFT ANDspecific section that justifies the use of the Mohnian as a DENUDATION RATESstrain marker. Mohnian aged strata are not well-suitedas a strain marker regionally, in general, because of One of the most difficult unresolved questions is the

issue of topographic change. An estimate of the timeuncertainties in palaeo-water depths and thickness vari-ations (Sullwold, 1960; Blake, 1991; Wright, 1991). required to create the present landscape is offered by the

uplift and denudation rates. Measured catchment-scaleThe cross-section reveals a complex pattern of upliftof the Santa Monica Mountains anticlinorium with relief varies from #300–900 m (Fig. 6C), potential catch-

ment-scale relief given by drainage spacing and sloperespect to adjacent basins (Fig. 12). Three uplift ratescan be calculated: (1) the crest of the anticlinorium angle varies from #400 to 800 m (Fig. 8), and mean

topographic relief averages between 300 and 600 m (Fig.relative to the Los Angeles basin on the south, (2) thecrest of the anticlinorium relative to the San Fernando 4B). Roughly 0.6–1.6 Myr are required to create between

300 and 900 m of relief if #0.5 mm yr−1 characterizesValley on the north and (3) the San Fernando Valleyrelative to the Los Angeles basin. A mean uplift rate of both the average catchment-scale denudation rate and

fluvial incision rate. Thus the present catchment- and0.52±0.12 mm Myr−1 is given by the #2600 mdifferential relief of the Mohnian between the crest and range-scale relief could have developed by #3.4 Ma

given that the anticlinorium began forming at 5 Ma.the Los Angeles basin (Fig. 12). This rate is consistentwith, although lower than, those indicated on cross- Interestingly, the topographic relief above the stage 9

terrace at Pt. Dume is #680 m (Fig. 11). One interpret-sections by Wright (1991) (0.6 mm yr−1), Schneider et al.(1996) (0.8–1.0 mm yr−1) and Davis et al. (1996) ation of this observation is that the range at the longitude

of Pt. Dume had attained this relief before #400 ka.(1 mm yr−1). These rates imply that uplift across thesouth flank has not varied more than ±0.4 mm Myr−1 Dividing the topographic relief by the lower 0.22 mm yr−

1 end-member uplift rate suggests that as much asabout the long-term mean (Table 1). Differential reliefof the Mohnian across the north flank of the range 2.7 Myr were required to create that relief. This argument

is predicated on the structural model for the range in(1210 m) is smaller, resulting in a lower uplift rate(0.24±0.12 mm yr−1). Different rates of uplift across which uplift of points on the forelimb (the structural

position of the terraces) equals crestal uplift (Davis et al.,the north and south flanks of the anticlinorium arepredicted by the inference that the blind thrust fault-dip 1996). These arguments do not apply to those portions

of the structure that remained below sea level and/or inangle decreases from steep to gentle beneath the moun-tains (Fig. 3A) (Davis & Namson, 1994; Davis et al., the subsurface for the length of structural development

(south end, Fig. 3B), however (Wright, 1991; Schneider1996). Both regions will be uplifted owing to displacementon the fault at depth, but basic geometry requires that et al., 1996). If uplift and denudation rates have been

sustained at approximately the same rates, isostatic upliftthe amount of vertical uplift will be greater on a high-angle segment of the fault than a low-angle segment is expected to be minor and crustal deflection related

to this loading is sustained after #1.6 Myr of upliftgiven the same net horizontal shortening (Huftile &


A. Meigs et al.

(provided rates do not vary on the time-scale of isostatic is the western domain because bedrock landsliding isa threshold response dictated by bedrock strength andresponse).

Are similar uplift and denudation rates a sound basis relief production (Strahler, 1950; Anderson, 1994;Schmidt & Montgomery, 1996; Densmore et al., 1997).for suggesting this landscape has achieved steady-state

(Hack, 1960; Ahnert, 1970)? Rates of denudation and Whether bedrock landsliding plays a central role in long-term hillslope erosion is uncertain, however.uplift are relatively slow and vary about #0.5 mm yr−1

(Figs 9 and 10 and Table 1). It is intriguing that the The magnitude of topographic departures from a meanform depends on response time (Kooi & Beaumont,amount of time required to create either the present

topographic or catchment relief, using either the rate 1996). Regions undergoing persistent, rapid uplift areargued to maintain a steady-state form (or show smallof uplift or denudation, is between 30 and 55%

(1.6–2.7 Myr) of the total duration of folding (5 Myr). fluctuation about a mean form) because bedrock landslid-ing dominates hillslope erosion (Anderson, 1994; SchmidtComparative studies (Ahnert, 1970) and some models of

landscape evolution (Anderson, 1994; Kooi & Beaumont, & Montgomery, 1995, 1996; Burbank et al., 1996;Densmore et al., 1997, 1998). For model landscapes1996) indicate that more than #1.5 Myr is required to

develop steady-state topography. Steady state is reached dominated by bedrock landsliding, but with low upliftand denudation rates comparable with those of the Santain 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., Monica Mountains, landscape response time is #100 kyr(Densmore et al., 1998). Existing data indicate that1998). If so, it is plausible that the topography approxi-

mates a quasi-equilibrium form. Rates of uplift and shallow landsliding plays a central role in hillslope erosionin the Santa Monica Mountains (Campbell, 1975) anddenudation must be steady on time-scales as short as the

landscape response time if this interpretation is valid thus represents a dominant signal in the debris-basinsediment yields (Fig. 9) (Ferrell, 1959; Lustig, 1965;(Hack, 1960; Schumm, 1963; Ahnert, 1970; Kooi &

Beaumont, 1996; Densmore et al., 1998). Scott et al., 1968; Scott & Williams, 1978). Shallowlandslides often involve regolith and regolith-productionDeformation on time-scales up to #tens of kyr is

strongly discontinuous (Dolan et al., 1995, 1997; Dolan rates tend to be lower than bedrock-landslide erosionrates (Anderson & Humphrey, 1989; Rosenbloom && Pratt, 1997). Available data suggest that earthquakes

on the frontal fault system have long recurrence intervals Anderson, 1994). If hillslope erosion rate is set primarilyby shallow landsliding and regolith production rates are(#1 kyr). The time interval between #100 ka and 10 ka

may have been characterized by uplift rates nearly twice less than 0.3 mm yr−1 as suggested by some models(Rosenbloom & Anderson, 1994), landscape responsethe mean (Table 1) (McGill, 1989), succeeded by a drop

to #20% of the mean rate since 10 ka (Dolan et al., time is probably greater than 100 kyr in the Santa MonicaMountains. Despite the inference that the duration of1997). Southern Californian sediment fluxes and river

profiles covary with climatic fluctuations on time-scales uplift has been sufficient for the landscape to approach asteady-state form, the likelihood that both uplift andranging from 101 to 105 years (see Bull, 1991, and

references therein). Thus it is difficult to say how denudation rates have changed significantly over the past100 kyr and that landscape response time is longer thancatchment-scale and fluvial denudation rates have

responded to short-term climate change and the deceler- 100 kyr suggests that the present topography is a depar-ture from a mean form.ation of uplift rate. If denudation rates have persisted at

#0.5 mm yr−1, the present landscape must be inter-preted as a transient form changing in response to a CONCLUSIONSslowing of uplift rate.

Climatic or tectonic-rate changes induce a landscape 1 Bedrock structure varies along strike from a simpleanticline in the west to a compound structural culmi-response and the response time, the time required to

established equilibrium with the new conditions, is dic- nation in the east. The change in geometry is stronglyinfluenced by the change in position of the emergenttated by the rates of erosional processes at and below the

catchment scale (Hack, 1960; Anderson, 1994; Densmore frontal fault system from the forelimb of the anticlinoriumto its crest from west to east, respectively.et al., 1998). Relatively short response times are inferred

for regions characterized by widespread bedrock landslid- 2 Four discrete physiographic domains can be differen-tiated within the Santa Monica Mountains. A range-longing (Anderson, 1994; Schmidt & Montgomery, 1995;

Burbank et al., 1996; Schmidt & Montgomery, 1996; drainage divide separates a northern north-drainingdomain from three south-draining domains. The north-Densmore et al., 1997, 1998). Bedrock landsliding is

related to rock strength and local relief (Strahler, 1950; draining basins are characterized by low differential reliefwith respect to base level in the San Fernando Valley.Schmidt & Montgomery, 1996). Correlation of drainage

spacing, drainage geometry and relief, to a lesser extent, Of the south-draining basins, the Malibu Creek basin isanomalously large (340 km2), is marked by low internalwith bedrock serves as a proxy for bedrock strength

variations in the Santa Monica Mountains. Inferred relief and has low slopes in comparison with either thewestern or the eastern domain. Base level set by Santadifferences in rock strength implies that the eastern

domain may be characterized by a lower threshold than Monica Bay, a mean elevation of 387 m, topographic



relief of #600 m with respect to base level, drainage of Public Works provided the unpublished sediment yielddata that form part of the data set presented in Fig. 9.spacing of 2.4 km, dendritic drainage geometries with

aspect ratios of 351, catchment-scale relief of 300–900 m Doug Burbank is thanked for discussions, suggestionsand a review of this study. Kerry Sieh, Jim Dolan, Dougand bedrock dominated by volcanic rocks characterize

the western domain. In contrast, the eastern domain is Yule and Eric Fielding are thanked for assistance anddiscussions along the way. Peter Talling and two othermarked by base level set by the Los Angeles basin on

the south of the range, a mean elevation of 164 m, anonymous reviewers are thanked for keeping the analysisfocused and honest. This research was supported by thetopographic relief of #300 m with respect to base level,

parallel drainage geometries with 651 aspect ratios, Southern California Earthquake Center. SCEC is fundedby NSF Cooperative Agreement Ear-8920136 and1.6 km drainage spacing, catchment-scale relief of 200–

500 m and bedrock dominated by metasedimentary and USGS Cooperative Agreements 14-08-0001-A0899 and1434-HQ-97ag01718. This is SCEC contribution #415.sedimentary rocks.

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(1999) 11, 59–73 steady, balanced rates of uplift and ...€¦ · santa monica mountains,...

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