Download Steady, balanced rates of uplift and erosion of the Santa Monica

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
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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.
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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
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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
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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
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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.
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© 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
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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.
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