JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B04404, doi:10.1029/2003JB002544, 2004
Structural model of the San Bernardino basin, California,
from analysis of gravity, aeromagnetic, and seismicity data
Megan Anderson
Department of Geosciences, University of Arizona, Tucson, Arizona, USA
Jonathan Matti
U.S. Geological Survey, Tucson, Arizona, USA
Robert Jachens
U.S. Geological Survey, Menlo Park, California, USA
Received 16 April 2003; revised 28 November 2003; accepted 28 January 2004; published 6 April 2004.
[1] The San Bernardino basin is an area of Quaternary extension between the San Jacinto
and San Andreas Fault zones in southern California. New gravity data are combined with
aeromagnetic data to produce two- and three-dimensional models of the basin floor. These
models are used to identify specific faults that have normal displacements. In addition,
aeromagnetic maps of the basin constrain strike-slip offset on many faults. Relocated
seismicity, focal mechanisms, and a seismic reflection profile for the basin area support
interpretations of the gravity and magnetic anomalies. The shape of the basin revealed by
our interpretations is different from past interpretations, broadening its areal extent while
confining the deepest parts to an area along the modern San Jacinto fault, west of the city
of San Bernardino. Through these geophysical observations and related geologic
information, we propose a model for the development of the basin. The San Jacinto faultrelated strike-slip displacements started on fault strands in the basin having a stepping
geometry thus forming a pull-apart graben, and finally cut through the graben in a simpler,
bending geometry. In this model, the San Bernardino strand of the San Andreas Fault
has little influence on the formation of the basin. The deep, central part of the basin
resembles classic pull-apart structures and our model describes a high level of detail for
this structure that can be compared to other pull-apart structures as well as analog and
numerical models in order to better understand timing and kinematics of pull-apart basin
INDEX TERMS: 1219 Geodesy and Gravity: Local gravity anomalies and crustal structure;
formation.
7230 Seismology: Seismicity and seismotectonics; 8109 Tectonophysics: Continental tectonics—extensional
(0905); 8150 Tectonophysics: Plate boundary—general (3040); 9350 Information Related to Geographic
Region: North America; KEYWORDS: San Andreas Fault, San Bernardino basin, gravity anomalies
Citation: Anderson, M., J. Matti, and R. Jachens (2004), Structural model of the San Bernardino basin, California, from analysis of
gravity, aeromagnetic, and seismicity data, J. Geophys. Res., 109, B04404, doi:10.1029/2003JB002544.
1. Introduction
[2] The San Bernardino basin of southern California has
been defined as the roughly triangular area centered east to
west between the San Andreas and San Jacinto fault zones
and north to south between Cajon Pass and the Crafton Hills
[Morton and Matti, 1993] (Figure 1). Previous studies of the
San Bernardino basin assume that this triangular shape
extends to depth in a homogeneous zone of extension
[e.g., Youngs et al., 1981] and that this basin accommodates
the transfer of right-lateral slip from the San Jacinto fault
zone to the San Bernardino strand of the San Andreas Fault
[Morton and Matti, 1993]. Because most of the faults in the
basin are wholly concealed by basin fill, their effects on
Copyright 2004 by the American Geophysical Union.
0148-0227/04/2003JB002544$09.00
basin shape and sedimentation patterns are best deduced
from geophysical data.
[3] In this study, we illuminate the structure of the San
Bernardino basin utilizing geologic, gravity, aeromagnetic,
and seismicity data. In addition, we compare our results
with a seismic reflection profile available for part of the
basin. Basin structure is mostly deduced from a threedimensional inversion of the gravity data for basin depth,
as well as two-dimensional modeling of the gravity and
aeromagnetic data. The structural model developed from
these data reveals new information about the dimensions of
many small faults that control the structure of the basin. We
deduce strike-slip offset on faults in the basin utilizing
magnetic features that cross these faults. These offsets are
corroborated in some cases by geologic data, which we feel
makes our interpretations of total strike-slip offset robust
along several faults. However, the available geologic data
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Figure 1. (a) Index map showing study area location within California. (b) Index map showing location
of the study area relative to major faults. (c) Topographic map showing San Bernardino basin and
vicinity. Fault and geographic feature names are given in Table 4. Sources for fault locations are: RCF,
SHFZ, graben bounding faults, and dotted faults in the eastern portion of the basin, this study; DCF/
DUCF, Morton [1976]; all other faults, Matti and Morton [1993]. Profile labeled A-A0 is shown in
Figure 6. Normal faults in the center of the map are inferred in the subsurface, and outline the edges of the
San Bernardino graben shown in Figure 5a.
give hard dates for timing of fault movement in few cases.
From our interpretations of fault offset, we attempt to
reconstruct fault displacements that created the basin, constraining timing in a general sense.
[4] Our geophysical data indicate that basin structure is
more heterogeneous than assumed by previous studies and
that most extension is centered directly over the modern
trace of the San Jacinto fault. This conclusion has implications for the history of basin-bounding faults, and we
suggest that the extensional model for the purported rightstep between the San Jacinto and San Bernardino strand of
the San Andreas Fault zones needs modification.
[5] Our model has a practical application in illuminating
some important factors in estimating seismic hazard for this
particular basin [e.g., Frankel, 1993]. It can be used to
identify rupture segments, estimate moment magnitudes,
and identify areas of high potential ground shaking (e.g.,
the San Bernardino graben). In addition, our model can be
tested through modeling of earthquake waves recorded in
the basin [Magistrale et al., 2000].
[6] More broadly, the geometry in the deepest part of the
basin is notably consistent with documented stepover pullapart basins (e.g., North China Basin, Chen and Nabelek
[1988]) and analog models of pull-apart basins (e.g., Dooley
and McClay [1997]). Most past studies of pull-apart basins
have relied heavily on sediment provenance and dating, a few
seismic reflection lines, and some focal mechanisms. Examination of synthesis papers on pull-apart basins [e.g., Nilsen
and Sylvester, 1999] show that basin structure developed by
these studies has largely been schematic rather than quantitative. In recent years, the driving questions in research about
pull-apart structures center around timing and kinematics.
Some notable studies have synthesized many data sets to
come up with a detailed three- or four-dimensional basin
descriptions [e.g., Kusumoto et al., 1999; Al-Zoubi and ten
Brink, 2002]. Here, we demonstrate the utility of combining
detailed potential field data with other geophysical methods
to create a well-defined and quantified map-view and threedimensional structural model. Such models have the potential
to be directly compared at a fine-scale to numerical and
analog models of pull-apart basins with the aim of answering
questions about timing and kinematics of stress field and fault
development.
2. Geologic Setting
2.1. Major Strike-Slip Faults
[7] Four major fault systems have accommodated tens
to hundreds of kilometers of dextral strike-slip (right-slip)
displacement in the vicinity of the San Bernardino basin:
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the San Andreas Fault zone, the San Jacinto fault zone,
the Banning fault, and the San Gabriel fault (SAFZ,
SJFZ, BF, and SGF, respectively in Figure 1). The
Banning and San Gabriel faults are older strands of the
San Andreas system [Matti and Morton, 1993], not
currently active, which are responsible for the juxtaposition of some basement blocks in the study area. More
complete geological analysis of pre-Neogene and Neogene
fault displacements in southern California are available in
many studies [Crowell, 1981; Dillon and Ehlig, 1993;
Ehlig, 1981; Matti and Morton, 1993; May, 1989; Morton
and Matti, 1993; Nourse, 2002; Powell, 1993; Powell and
Weldon, 1992; Weldon et al., 1993]. We focus on the
Quaternary fault activity because it is integral to the
creation of the San Bernardino basin.
[8] Reactivation of old faults in this area is a common
occurrence in our study area due, in general, to the restraining bend and uplift along the San Andreas Fault to the
north. Faults activate in the path of least resistance through
this area. During the last 1.5 Ma the San Andreas Fault has
been active both south and north of the San Bernardino
strand as well as east on the Mission Creek and Mill Creek
strands (fault nomenclature from Matti and Morton [1993]
and Matti et al. [1992a]). In the same time frame, throughgoing strike-slip displacement within the San Bernardino
basin area appears to have occurred mainly on the San
Jacinto fault zone [Morton and Matti, 1993], with a lesser
amount of slip more recently on the San Bernardino strand
of the San Andreas Fault.
2.2. Basement Rock Provinces
[9] It is important to clarify the geologic and geophysical
character of basement rock provinces as well as their
boundaries; this information is necessary for application in
the development of our two- and three-dimensional models.
The geology of the San Bernardino area can be broadly
divided into four major documented basement provinces
based on rock types, geologic histories, and geophysical
character (Figure 2). The most important information about
each province as well as their areal extents are highlighted
below and a detailed treatment of geophysical and geologic
characteristics, summarized from other reports, is given in
Table 1.
[10] Peninsular Ranges-Type: Peninsular Ranges-type
rock (also known as the Southern California Batholith;
Jahns, 1954) is exposed in the San Bernardino area west
of the San Jacinto fault zone and the Banning fault and
south of the Cucamonga fault zone (Figure 2) [Matti and
Morton, 1993]. The most important detail about this block
is that it is of above-average density for upper continental
crust (Table 1).
[11] San Gabriel Mountains-Type: San Gabriel Mountains-type rock [Matti and Morton, 1993] is divided into
two major mapped units. The lower unit is the Pelona
Schist, which is notably less dense than average upper
continental crust (Table 1). The Pelona schist is bounded
by the Vincent Thrust, a late Cretaceous to early Tertiary,
low-angle thrust fault, which crops out extensively in the
San Gabriel Mountains and in the Crafton Hills (VT in
Figure 2). In the San Bernardino area, the upper-plate rock
consists of prebatholithic crystalline rocks intruded by
Mesozoic plutons [Matti and Morton, 1993] (Table 1).
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[12] San Gabriel Mountains-type rocks are exposed
in three areas surrounding the San Bernardino basin
(Figure 2): 1) in the northern part of the southeastern San
Gabriel Mountains, 2) in Cajon Pass southwest of the San
Andreas Fault, and 3) southeast of the San Bernardino basin
in the San Gorgonio Pass area. These rocks (mostly Pelona
schist) are also interpreted to form the base of the San
Bernardino basin extending from the San Jacinto fault zone
eastward to the San Bernardino strand of the San Andreas
Fault and from Cajon Pass southeastward into the Crafton
Hills. Though the rocks in the basin are mostly covered by
sedimentary fill, the presence of Pelona Schist is substantiated by inselbergs along the San Andreas Fault (Figure 2).
[13] Southeastern San Gabriel Complex: Geologists
debate the affinity of rocks exposed in the southeastern
San Gabriel Mountains (Figure 2), some affiliating them
with the Peninsular Ranges group [Matti et al., 1985; Matti
et al., 1992a; May, 1986], others placing them in a terrane of
uncertain affinity [San Antonio and San Sevaine blocks of
Dibblee, 1982]. We present no evidence to solve this debate,
therefore, we simply refer to these rocks as the Southeastern
San Gabriel Complex. This complex contains a continuous,
east-trending, highly magnetic zone that coincides with the
belt of mylonitic rocks (Figures 2 and 3b) which will be
discussed in our interpretations. The complex is bounded to
the south by the Cucamonga fault zone, to the west by the
San Antonio Canyon fault, and to the north and east by a
complicated zone of faults that Matti and Morton [1993]
correlate with a once throughgoing, connected San GabrielBanning fault (Figure 2).
[14] San Bernardino Mountains-Type: Rocks of San Bernardino Mountains-type are exposed only northeast of and
within the San Andreas Fault zone [Matti and Morton,
1993] (Figure 2).
3. Geophysical Data and Analysis
3.1. Gravity Data
[15] The gravity data consist of measurements that are
spaced approximately 1 km apart throughout the basin, with
tighter spacing (200 to 400 m) along profiles across important features [Anderson et al., 2000] (Figure 4). We reduced
the gravity data from the observed value to an isostatic
residual anomaly (see Anderson et al. [2000]) in order to
enhance signals produced by sources in the upper to middle
crust.
[16] We used maxima on the horizontal gravity gradient
grid calculated from the isostatic gravity anomaly (using the
methods of Blakely and Simpson [1986]) to interpret the
location of steep basement block boundaries and faults
within these blocks. In the case of the San Bernardino
basin, many of these boundaries are substantiated by aeromagnetic and seismic reflection data, as well as twodimensional gravity modeling.
[17] Even with the assistance of an isostatic gravity
reduction, gravitational signals from shallow sedimentary
basins can still be overwhelmed by the higher amplitude,
longer-wavelength signal coming from density contrasts in
the basement rock. In particular, the gravity low over the
San Bernardino basin (Figure 4) merges with a low in the
basement rock signal east of the San Andreas Fault in
the San Bernardino Mountains. Clearly, the basin does not
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Figure 2. Basement rock provinces in the San Bernardino area. Block edges are approximate where not
bounded by faults or outcrop exposure. Fault and geographic feature names are given in Table 4. The
dotted lines at the termination of the Banning Fault suggest 2 possible subsurface trajectories. The San
Gabriel fault in this figure is split up into several faults exposed in the Southeastern San Gabriel
Mountains. Profile labeled A-A0 is shown in Figure 6. Normal faults in the center of the map are inferred
in the subsurface, and outline the edges of the San Bernardino graben as shown in Figure 5a. Geology
assembled from Dibblee [1965, 1968], Matti et al. [1985], Matti et al. [1992b], Miller [1979], Morton
[1976], Morton [1978a], Morton [1978b], Morton [1978c], Morton and Matti [1990], Morton and Matti
[1991], Rogers [1967].
extend into the mountains, therefore, it is necessary to
deconvolve the gravitational signal of the basin sedimentary
fill from the basement rock, which is accomplished by an
inverse technique.
3.2. Basin Depth Inversion
[18] We inverted the isostatic gravity anomaly for basin
depth using an iterative calculation [Jachens and Moring,
1990] controlled by the following data: 1) the isostatically
reduced gravity grid, 2) an isostatic basement gravity grid
interpolated from gravity measurements only on basement
outcrops, 3) wells that penetrate to basement rocks, and 4) a
density-depth function for basin fill (Table 2). The result is
shown in Figure 5a. In addition to the above data, several
control points were utilized in areas where basement was
not exposed nearby. The control points consist of a basement gravity value and are based on our interpretation of the
extent of basement blocks. For instance, the points south-
west of the Rialto-Colton fault (Figure 5a) are interpreted to
be located on Peninsular Ranges rock, based on the exposure of this block just to the south, in the Jurupa Hills. The
basement gravity value for these points is the same as for
the Jurupa Hills. We only used control points to keep the
calculation from showing exposed basement where there is
none and to control the horizontal location of the boundary
between the Peninsular Ranges block and the San Gabriel
Mountains-type rock along the modern strand of the San
Jacinto fault.
[19] There are two main sources of uncertainty in the
calculated depth of the basin: basement rock density uncertainty and the accuracy of the sediment density-depth
function. We have found that moving the location of less
certain basement density boundaries does not affect the
overall shape of features in the basin or the location of
maximum gradients that define faults. Forcing a change in a
basement rock density boundary with a major contrast in
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Table 1. Basement Rock Provinces and Their Geophysical Characteristics Used for Three-Dimensional Inversion and Two-Dimensional
Modeling of Gravity Dataa
Province
Rock Types
No.
Samples
Density
Range,
kg/m3
Density
Average,
kg/m3
Standard
Deviation
Magnetic
Susc.
Range,
siu 103
Magnetic
Susc.
Average,
siu 103
Standard
Deviation
Peninsular
Ranges
Metasediments,
>3
2680 – 2790
2740
64
0.13 – 0.38
0.25
0.13
Jurassic and Cretaceous
granodiorite, quartz diorite,
tonalite, and gabbro
San Gabriel
Proterozoic orthogneiss
8
2580 – 2920
2630
126
0.25 – 5.53
1.38
1.99
Mountains
Anorthosite-syenite
complex, Mesozoic plutons,
Miocene granitoid intrusives
Pelona schist
7
2480 – 2620
2560
51
0.13 – 0.25
0.13
0.05
San Bernardino
Proterozoic orthogneiss
19
2440 – 3040
2670
132
0.13 – 14.07
5.28
4.19
Mountains
Metasediments
Southeastern
Cretaceous granitic rocks,
14
2540 – 2850
2680
104
0.00 – 15.71
5.28
5.55
San Gabriel
migmatite, gneiss,
Complex
Paleozoic metasediments
a
Rock type descriptions are from Matti and Morton [1993], Ehlig [1981], and Ehlig [1982]. Density ranges are from hand sample measurements
[Anderson et al., 2000] and averages were calculated from these data by separating measurements into appropriate basement rock province groups. The
exception to this is the Peninsular Ranges average; the average from the Anderson et al. [2000] study for the Peninsular Ranges is 2720 kg/m3, but because
of the small number of samples (3), this average was in turn averaged with the results of sample measurements from Willingham [1968]. Magnetic
susceptibilities were measured from hand samples [Anderson et al., 2000]; note that the averages shown here may average susceptibilities over areas with
high magnetic variability. The component of greenstone in the Pelona schist, while being very dense and variably magnetic, is never more than 10%
regionally, and is often localized near the Vincent Thrust [Jacobson, 1983]. Thus greenstone was not included in our estimate of bulk density and
magnetization.
rock density by 1 km changes basement depth within 2 or
3 km of the boundary by up to 300 m. The error on layers in
the density-depth function is on the order of ±30 kg/m3, but
is likely to be greater in deeper layers not penetrated by
wells. Changing all sediment layer densities listed in Table 2
by 30 kg/m3 will change shallow areas by less than 100 m,
deep areas (with depth less than 1 km) by 100 m, and very
deep areas (with depth greater than 1 km) by 100 – 300 m.
These uncertainties are evident in the fact that exposed rock
from the inversion and outcrop pattern do not match exactly
(Figure 5a-see ‘‘exposure’’ over the Mentone gravity high);
we interpret basement rock existing just below the sediment
surface in these areas.
[20] We have taken care that features we interpret as
key elements of basin structure are based upon several
corroborative gravity measurements. We consider the
interpretation of features on the basement topography
map greater than 1 square kilometer in area valid in most
regions of the basin, and interpretation of smaller features
(down to 200 m) is possible where tighter station spacing
exists (Figure 4). We used the methods of Blakely and
Simpson [1986] to help define basin-bounding faults from
this map.
3.3. Residual Aeromagnetic Field
[21] Aeromagnetic data for the San Bernardino area
(Figure 3) are of good quality [U.S. Geological Survey,
1996] with north-south flight lines spaced 805 meters
apart in the area of the basin west of longitude
117°300, and east-west lines spaced 536 meters apart
in the other parts of the basin. We used the aeromagnetic
data to identify magnetic zones within the basement rock
blocks underlying the basin that serve as constraints in
two-dimensional modeling and as proxies for geologic
markers. Where linear magnetic bodies cross faults iden-
tified from the gravity data, the amount of strike-slip
offset can be estimated on the basis of the separation
on the edges of these bodies (see Figure 3a for schematic
of the method and Table 3 for a summary of separations
measured). Again, we utilized the methods of Blakely and
Simpson [1986] to assist in defining the edges of these
anomalies. In addition to the original magnetic grid
(Figure 3b), we used a grid filtered for high-frequency
anomalies (upward continued and subtracted from the
original grid; Figure 3c) to distinguish the signal of
smaller zones of magnetization closer to the surface that
otherwise would be overwhelmed by the stronger, deeper,
low-frequency signals in the original grid.
[22] Estimating strike-slip offset from separation on
magnetic body edges is not a fool-proof method. It is
possible that unfaulted bodies coincide with the faults in
such a way as to appear offset (such as coincidental
juxtaposition of two separate bodies, or irregular/dipping
edges on a magnetic body coinciding with the fault).
However, in the absence of other quantitative data on
strike-slip separation, we defer to this method. Where
available, the magnetic anomaly separation estimations
agree with those from geologic maps. At a minimum,
this method provides a testable magnitude of strike-slip
displacement and in some cases (where corroborated by
other data) it provides a robust estimate for strike-slip
displacement.
[23] We consider error in these estimates due to map
resolution to be a maximum of 0.5 km. Dip-slip offset on
these bodies was not calculated from the aeromagnetics
because of the complexity of analysis. We estimate that
500 m of dip-slip separation (such as along the RialtoColton fault) on a non-dipping body could cause an error in
the strike-slip component estimate of around 250 m. However, except for the Rialto-Colton fault, vertical displace-
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Figure 3.
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ment is minimal on the faults along which strike-slip
separations were measured.
3.4. Two-Dimensional Gravity and Magnetic Modeling
[24] Two-dimensional modeling is a good check of the
basin-depth inversion, because both gravity and aeromagnetic data can be modeled simultaneously. Of the several
northeast-trending lines investigated, only one that crosses
the San Bernardino graben is shown here (Figure 6) because
they are all so similar as to be redundant. We pulled gravity
and magnetic profiles from the gravity map and the original
aeromagnetic grid to use in the modeling. Information about
the density and magnetic properties of basement blocks that
are gathered in Table 1 were applied to rock bodies used in
modeling, which was conducted with reference to a gravimetric baseline and the appropriate magnetic inclination (for
details on the forward method see Blakely [1995]).
[25] Magnetic anomalies in the San Bernardino Mountains (not shown in Figure 6) were first modeled to enable
the fit of magnetic data toward the eastern edge of the
profiles. This part of the model includes a broad, flat layer
of magnetic material at shallow depth below the mountains.
Errors in modeling this large-scale anomaly does not affect
the shape and relative depth of small-scale features in the
basin, but they could affect the absolute basin depth. The
exact shape of the Vincent Thrust in these profiles is also
not well constrained and several geometries may be possible, though the general trend and thicknesses of the units are
not likely to differ much from those shown.
[26] The lowest stratigraphic unit shown below the basin
(labeled ‘‘beneath Pelona schist’’ in Figure 6) is not exposed
anywhere in Southern California or documented in the
geologic literature. It is necessary to include this unit
because if Pelona schist is extended to mid-crustal levels,
we cannot fit the gravity data. Based solely on the twodimensional modeling, this unit has the geophysical characteristics of typical upper continental crust. Some small
adjustment in thickness of this layer and its density are
possible while still retaining the fit of the gravity and
magnetic data.
3.5. Seismicity
[27] We used hypocenters of small earthquakes (1975 –
1976 and 1981 – 1998, mostly magnitude <4.0) and a few
focal mechanisms for small- to medium-sized events
(Figure 7) to supplement the potential field data. The
hypocenter data have been relocated by Richards-Dinger
and Shearer [2000]. The majority of the focal mechanisms are from Jones [1988], and the rest are the most
typical focal mechanisms for this area from Hauksson
[2000]. In general, focal mechanisms for larger magnitude
events from the Jones [1988] study, where available, are
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similar to the mechanisms from smaller magnitude events
in the Hauksson [2000] study. The mechanisms show a
clear trend of predominantly normal faulting within the
basin and strike-slip or reverse faulting outside of the
basin. The seismicity does seem to outline several linear
structures not reflected by the gravity or magnetics. We
have left interpretation of these structures for later investigation since the potential field geophysics cannot be
brought to bear on them.
[28] In addition, it should be noted that microseismicity is
not always the best indicator of long-term regional stress or
strain accumulation for a number of reasons. One good
example is that interseismic and aftershock activity does not
necessarily align along major structures, but can instead
cluster in zones of increased Coulomb stress [King et al.,
1994]. Our seismic maps should be considered in the
context of this kind of observation since most of the
seismicity is of smaller magnitude.
3.6. Seismic Reflection
[29] As a part of this study, we acquired a line of gravity
stations along a seismic reflection line in the basin
(Figure 4). This line was projected onto the line A-A0
shown in all figures and was modeled in 2-dimensions as
shown in Figure 6. The details of the reflection line
acquisition and interpretation are discussed by Stephenson
et al. [2002]. The two-dimensional basin depth model
matches very closely the depth along this reflection profile
in its general shape as well as depth in the shallower parts of
the basin, yet was modeled independently of the seismic
data. We believe this somewhat strengthens our interpretation of the basement topography. The base of the graben
differs slightly between the gravity and seismic models,
possibly because of the geometry of the upper-plate of the
Vincent Thrust blocks, which are less well constrained than
other parts of our model.
4. Interpretation of Basin Structure and Fault
Offset from Geophysical and Geologic Data
[30] Below we discuss the geophysical and geologic data
with respect to individual aspects of the basin: basement
block boundaries, which could affect interpretation of some
fault offsets that lie close to these boundaries; the San
Bernardino graben, an obvious area of large-scale subsidence accommodation; the San Andreas and San Jacinto
faults, which are larger throughgoing faults, that set up the
stress conditions in which the small features develop; splays
of the San Jacinto fault zone that largely control basin
structure (see Table 4 for fault name abbreviations); and
finally transfer and accommodation structures within the
basin. These details support a new hypothesis for the
Figure 3. (a) Schematic sketch of how magnetic anomalies were used as geologic markers to gauge total amount of
strike-slip displacement along a fault. (b) Aeromagnetic map for the San Bernardino basin [U.S. Geological Survey, 1996].
Displacements on faults in the southeastern San Gabriel mountains were determined from this map and are summarized in
Table 3. Dotted, unnamed fault shows apparent separation of the magnetic anomaly, but is not associated with a known
fault or substantiated as a fault by other lines of evidence. Box shows area of C. Model along profile labeled A-A0 is shown
in Figure 6. (c) The high-frequency map of the basin floor brings out smaller, shallower magnetic anomalies. Fault and
geographic feature names are given in Table 4. Normal faults in the center of the map are inferred in the subsurface, and
outline the edges of the San Bernardino graben as shown in Figure 5a. See color version of this figure at back of this issue.
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Figure 4. Isostatic gravity anomaly map of the San Bernardino basin. Contour interval, 1 mGal. Fault
and geographic feature names are given in Table 4. Profile labeled A-A0 is shown in Figure 6. Normal
faults in the center of the map are inferred in the subsurface, and outline the edges of the San Bernardino
graben as shown in Figure 5a.
extension of the San Bernardino basin developed in the
discussion section.
4.1. Basement Block Boundaries
[31] While most of the basement block boundaries are
exposed or are easily interpreted from the gravity data, there
are two boundaries of concern that are not well constrained
(the blocks labeled ‘‘unknown affinity’’ in Figure 2). These
are areas of some importance to the geological history of the
San Andreas Fault zone because many faults have cut across
these areas before the Quaternary [Matti and Morton, 1993];
however, we do not feel our gravity map is well-constrained
enough or of high enough resolution to support detailed
interpretation of these boundaries. Through experimentation
with the basement gravity control points mentioned above,
we believe that changes in the location of these boundaries
will have little effect on the shape of basin-bounding
structures.
4.2. San Bernardino Graben
[32] The most prominent feature on the basement rock
topography map for the San Bernardino basin is a classic
rhombohedral-shaped pull-apart graben (Figure 5a). This is
an area that has accommodated much normal faulting in the
basin, and therefore is the key to determining kinematics
related to movement on the larger throughgoing strike-slip
faults. The depth of this graben is corroborated by twodimensional modeling and the seismic reflection profile
(Figure 6). A deeper part of the basin in this area was first
observed from a gravity study by Willingham [1968],
therefore, we adopt Willingham’s terminology here and call
it the San Bernardino graben thus distinguishing it from the
greater San Bernardino basin.
[33] All of the graben-bounding faults (R, S, T, and Q in
Figures 5a and 6) are interpreted as primarily normal faults,
because of their approximately 1-km dip-slip displacement.
Some of the faults may have a right-slip component, but
aeromagnetic anomalies associated with these faults are not
sufficiently coherent to allow for a conclusive interpretation
of strike-slip separation, and it is indeed likely, given they
are involved in a step-over, that there has been some
substantial strike-slip displacement along them.
[34] Small-magnitude seismicity is concentrated in the
basement rock under the San Bernardino graben (Figure 7)
Table 2. Layered Sediment Density-Depth Function Used in the
Basement Topography Calculationsa
Depth, km
Density, kg/m3
0 – 0.3
0.3 – 1.3
1.3 – 2.3
2.3 – 3.3
3.3 –
2120
2240
2310
2370
2440
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a
From Willingham [1968] and adjusted to fit well-control data.
ANDERSON ET AL.: STRUCTURE OF THE SAN BERNARDINO BASIN
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Figure 5.
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ANDERSON ET AL.: STRUCTURE OF THE SAN BERNARDINO BASIN
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Table 3. Summary of the Characteristics of Strike-Slip Faults in the San Bernardino Basina
Fault
Sense of Slip
Age
Constraints on Age
San Jacinto Splays
Contradictory
kinematic indicators
Total Offset
Rialto-Colton-Day
Canyon fault
Right-lateral
older ?
Duncan Canyon fault
Right-lateral
1.5 – 1.3 Ma
San Timoteo deposits
1.5 km
Lytle Creek fault zone,
western strand
Right-lateral
<1.3 Ma-present
3.5 km
Lytle Creek fault zone,
middle strand
Lytle Creek fault zone,
eastern strand
Glen Helen fault
Right-lateral
<1.3 Ma-present
Right-lateral
<1.3 Ma-present
active
seismicity/alluvial
offsets
active seismicity/alluvial
offsets
alluvial offsets
Right-lateral
recent
Shandin Hills
fault zone
Right-lateral
recent
San Jacinto fault,
modern trace
San Antonio
Canyon fault
Right-lateral
recent
active seismicity,
geomorphic features
active seismicity,
trenching
Other Measured Offsets
active seismicity
geomorphic features
-
2 km
1 km
7 – 8 km
? km
5 km
Constraints on Displacement
aeromagnetic
anomalies
(BB/GG)/geologic
mapping
aeromagnetic
anomalies
(CC)/geologic
mapping
aeromagnetic
anomalies (DD)
aeromagnetic
anomalies (EE)
geologic mapping
aeromagnetic
anomalies (FF)
aeromagnetic
anomalies (HH)
Left-lateral
??
4 km
aeromagnetic
anomalies (AA)/geologic
mapping
a
The total of right-slip motion on the Rialto-Colton-Day Canyon, Duncan, all lower Lytle Creek fault strands, the Glen Helen and Shandin Hills faults
plus 5 km of thrusting in the Cucamonga fault zone adds to 25 – 26 km. Letter labels for aeromagnetic anomaly constraints are shown in Figure 3. Data
sources are explained in the text. ‘‘Geomorphic features’’ refers such landforms as sag ponds and offset stream channels.
and extends to 20 km depth [Hill et al., 1990; RichardsDinger and Shearer, 2000; Sanders and Magistrale, 1997],
most of it clustering at mid-crustal levels (Figure 6). Three
depocenters, as deep as 2 km, occur in the northern part of
the graben, flanking the modern strand of the San Jacinto
fault (U, V, and W in Figure 5a). The seismicity under these
depocenters seems to define a westward-dipping structure at
about 15 km depth (Figure 6). This structure extends outside
the bounds of our isostatic gravity model and we feel
significant further analysis is needed to fully understand
it. It could be related to normal faulting and/or right-slip
faulting, which is confirmed by mixed focal mechanisms in
this area (Figure 7).
[35] To the south, there is another depocenter (X in
Figure 5a) that is filled with deposits of the San Timoteo
Badlands which are dated 1.5 Ma – 1.3 Ma [Morton and
Matti, 1993]. These deposits are exposed at the surface and
deformed [Morton, 1978c], therefore we interpret this as an
older, inactive depocenter that is presently being uplifted
due to a restraining bend in the San Jacinto fault geometry
[Kendrick et al., 2002]. Intense seismicity under the south-
4.5 km
ern part of the graben is dominated by normal and obliquenormal mechanisms, so we must call on brief, temporal or
small, spatial variation in stress patterns in order to fit the
seismicity into a restraining bend model.
[36] Regardless of the exact structure defined by the
seismicity, it is clear that much activity in the basin is
confined within the graben. This indicates that the step-over
that creates the graben is at least still partially active.
4.3. San Bernardino Strand of the San Andreas Fault
[37] The San Bernardino strand of the San Andreas Fault
zone bounds the San Bernardino basin along its northeastern margin (SAF in Figures 1 and 5a). This fault was
inactive between 1.5 Ma and 125 Ka and has generated
about 3 km of total right-slip in the past 125 k.y. by
reactivating the old Mission Creek strand [Matti et al.,
1992a; Morton and Matti, 1993]. It would seem logical to
conclude, from the topographic map (Figure 1c) that this
fault should be an integral, controlling structure on the
geometry of the basin, however, our synthesis suggests that
total normal displacement has been small along the fault.
Figure 5. Three-dimensional basin depth inversion. Overall, the basement-surface shape has a horizontal resolution of
1 km in most places and 200 meters where gravity measurements were closely spaced. Depth resolution is discussed in the
text. Model along profile labeled A-A0 is shown in Figure 6. Fault and geographic feature names are given in Table 4. In
addition, several smaller, unnamed faults are shown here: Q, northwestern edge of graben; R, southwestern edge of graben;
S, northeastern edge of graben; T, northeastern edge of southern sub-graben; U, V, W, X, depocenters. Features southeast of
the Crafton Hills are not adequately controlled by data so should be disregarded. (b) Reproduction of cutaway drawing of
Dooley and McClay’s [1997] pull-apart analog model. Colors denote layers of syn-extensional material from oldest
(orange) to youngest (purple). Note the similarity in structure with our model of the San Bernardino graben; specific
structures that are similar are noted on the drawing. See color version of this figure at back of this issue.
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Figure 6. (a) Basin cross section modeled in two dimensions along A-A0 (see Figures 3 – 5 for location).
Gravity profile is from the isostatic gravity map (Figure 4) and the magnetic profile is from the
aeromagnetic map (Figure 3b). The gravity and magnetic data were fit simultaneously. R and S are
graben bounding faults as shown in Figure 5a and their dips are approximate. Dashed lines are uncertain
boundaries. (b) Modeled basin depth matching very closely the depth along a reflection profile shot along
the same transect [Stephenson et al., 2002]. See Figure 7 for area from which seismicity was projected.
[38] The San Bernardino strand forms a series of surface
fault traces and scarps [Matti et al., 1992b; Meisling and
Weldon, 1989; Miller, 1979; Miller and Matti, 2001;
Morton and Matti, 1991; Morton and Miller, 1975]. Some
vertical, down-on-the-south displacement along the fault is
supported by groundwater barriers and faults identified
from reflection seismic surveys that have been reported
parallel to and south of the San Andreas trace (faults K
and L of Dutcher and Garrett [1963]; Bechtel, 1970;
Figure 1). In addition, the strikes of normal faulting
mechanisms nearest to the strand are oblique to the fault
which may be due to transtensional faulting within the
basin in the vicinity of the fault [Jones, 1988] (Figure 7).
However, the offset on these faults is too small to create
distinct gravity gradients, and furthermore, the gravity
inversion and the two-dimensional gravity model show
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ANDERSON ET AL.: STRUCTURE OF THE SAN BERNARDINO BASIN
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Figure 7.
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Table 4. Fault and Geographic Feature Abbreviations
Fault/Geographic Feature Name
Abbreviation
Banning Fault
Cucamonga fault zone
Crafton Hills
Crafton Hills fault zone
Colton
Cajon Pass
Day Canyon fault
Dutcher and Garrett’s (1963) water barriers H, K, L
Duncan Canyon fault
Fontana
Glen Helen fault
Jurupa Hills
lower Lytle Creek, western fault
lower Lytle Creek, middle fault
lower Lytle Creek, eastern fault
Mentone gravity high
Perris Hill
Rialto-Colton fault
Rialto
San Antonio Canyon fault
San Andreas Fault zone
San Bernardino
San Bernardino Mountains
San Gabriel fault
San Gabriel Mountains
San Gorgonio Pass
Shandin Hills fault zone
San Jacinto fault zone
San Timoteo Canyon
Vincent Thrust
BF
CFZ
CH
CHFZ
Colt
CJP
DCF
D-G
DUCF
Font
GHF
JH
LCw
LCm
LCe
MH
PH
RCF
Rial
SACF
SAF
Sbdo
SBM
SGF
SGM
SGP
SHFZ
SJFZ
Stc
VT
that basin depth is very shallow adjacent to the San
Bernardino strand of the San Andreas Fault (Figures 5
and 6). We conclude that the San Bernardino strand of the
San Andreas Fault has played little direct role in largescale basin normal faulting.
4.4. San Jacinto Fault Zone
[39] In contrast to the San Bernardino strand of the
San Andreas Fault, the San Jacinto fault zone (SJFZ in
Figure 1) has generated approximately 25 km of throughgoing right-slip (from general assessments of slip in the
San Andreas Fault system to the south of the San Bernardino basin) within the last 1.5 [Matti and Morton, 1993] to
2 Ma [Sharp, 1967] and 1 km normal displacement has
occurred adjacent to it along the edges of the San Bernardino graben (Figure 5a). Thus we conclude that this fault is
the primary strand of the San Andreas Fault system in the
San Bernardino basin area for the last 1.5 Ma.
[40] Geologic maps of the basin have given little
indication it is a major fault. They have shown only the
modern trace of the San Jacinto fault extending across the
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basin [Matti et al., 1992a; Rogers, 1969]. This trace
forms a series of northwest-trending fault traces and
scarps that are mapped from the San Timoteo Badlands
to about halfway across the San Bernardino basin [Matti
et al., 1992a; Morton, 1978c]. To the northwest, the
modern fault trace is only exposed in local quarry
excavations.
[41] Analysis of the San Timoteo Badlands deposits
indicates that movement on the single strand of the San
Jacinto fault that cuts through the San Timoteo Canyon
area (Stc in Figure 1) started no more than 1.5 Ma
[Morton and Matti, 1993]. Since movement on the San
Jacinto fault zone is an integral part of the basin-forming
process, this provides the lower limit on the age of basin
inception. The youngest deposits of the San Timoteo
formation are dated 1.3 Ma [Morton and Matti, 1993].
This is the upper limit on the age of the basin inception
because these deposits fill the smaller, now uplifting,
southern portion of the San Bernardino graben. It is
possible that some younger deposits have been eroded,
which could slightly extend this limit.
[42] The location of the modern trace of the San Jacinto
fault zone aligns with a prominent, 30 mGal gradient in the
southern part of the basin, but this gradient is more diffuse
in the northern part of the basin (Figure 4). The large
gradient reflects not only the edge of the deepest part
of the basin, but also a large contrast in basement
rock density between dense Peninsular Ranges rocks
(2740 kg/m3) and less dense Pelona schist (2560 kg/m3).
At the north end of the basin, in the vicinity of Cajon Pass,
the diffuse gradient reflects numerous strands of the San
Jacinto fault which probably juxtapose basement slivers
belonging to two or three different basement blocks. This
is supported by groundwater measurements in this area
that define a series of hydrologic barriers that probably
are fault-controlled [hydrologic barriers A – E, F, and
G of Dutcher and Garrett, 1963; Izbicki et al., 1998;
Woolfenden and Kadhim, 1997]. We suggest that these
strands connect to the more distinct, single, ‘‘main’’ trace
of the San Jacinto fault zone that enters the basin from the
south and that these strands have accommodated rightlateral slip as a part of the San Jacinto fault system. We
describe these strands below: the Day Canyon/RialtoColton faults, the Duncan Canyon fault, faults in the Lytle
Creek area, the Glen Helen fault, and the Shandin Hills
fault zone (Figure 1).
[43] A long, linear northwest-striking magnetic body
crosses the modern trace of the San Jacinto fault zone
in the deepest part of the basin. We interpret 4.5 km of
right-slip offset on this trace from separation on this
anomaly (HH in Figure 3c). The apparent disparity
between 25 km of right-slip on the San Jacinto fault zone
Figure 7. Seismicity (1975 – 1976 and 1981 – 1998) [Richards-Dinger and Shearer, 2000] and focal mechanisms
[Hauksson, 2000; Jones, 1988] for the San Bernardino basin. Gray contours are the basement topography from Figure 5a.
Gray zones show the location of bands of seismicity cited as evidence of rotation of blocks within the San Bernardino basin
[Nicholson et al., 1986a]. Fault and geographic feature names are given in Table 4. Profile labeled A-A0 is shown in Figure
6; projected seismicity is enclosed within the box. Normal faults in the center of the map are inferred in the subsurface, and
outline the edges of the San Bernardino graben as shown in Figure 5a.
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south of the San Bernardino basin and 4.5 km of rightslip on the modern strand in the basin is due to the fact
that right-slip on other strands as well as some thrust
faulting in the southeastern San Gabriel Mountains have
accommodated much of this displacement.
4.5. Splays of the San Jacinto Fault
4.5.1. Rialto-Colton//Day Canyon Fault
[44] A fault in the Rialto and Colton areas has long been
inferred to coincide with a known water barrier (barrier H of
Dutcher and Garrett [1963]; [Woolfenden and Kadhim,
1997]; D– G – H in Figure 1). A steep gravity gradient is
associated with this water barrier (Figure 4) which diverges
from the southwestern edge of the San Bernardino graben in
a slight left-stepping geometry in the Rialto/Colton area and
trends northwestward toward the southeastern San Gabriel
Mountains. This gradient is modeled in 2-dimensions as the
position of the Rialto-Colton fault, with normal displacement of the hanging wall (northeast side) 400 – 700 m
(Figure 5a).
[45] Right-lateral strike slip displacement is supported
on this fault by 2 km of right-lateral, left-stepping
separation in a linear magnetic body that crosses the
Rialto-Colton fault (which actually consists of three
strands at this point; GG in Figure 3c). This offset is
also supported by a high in the basement topography
along this segment (Figures 4 and 5) which fits with the
uplift expected from a left-step in a right-lateral system.
We interpret the Rialto-Colton fault, based on these
observations, as a strand of the San Jacinto fault zone,
accommodating 2 km of dextral slip.
[46] The Rialto-Colton fault, as we have mapped it, is
coextensive with the Day Canyon fault in the southeastern
San Gabriel Mountains (DCF in Figure 1). The Day Canyon
fault displaces a mylonite zone by about 2 km in a rightlateral sense (Figure 2) [Morton and Matti, 1987] and a
magnetic body which correlates in map view with the
mylonite zone is also apparently offset about 2 km (BB in
Figure 3b). Because of the alignment of these faults and
their equivalent strike-slip offsets, we propose that they are
parts of the same structure, which we call the Rialto-Colton/
Day Canyon fault.
[47] Indicators of current activity on the Rialto-Colton/
Day Canyon fault are mixed. First, there has clearly been
normal movement along the fault in the basin. However,
near the Cucamonga fault zone (CFZ in Figure 1), a belt of
late Quaternary thrust and reverse faults, trenching has
revealed reverse faulting in Quaternary sediments dated
13 Ka (J. Treiman, California Geological Survey, 2000,
personal communication; Figure 2), the strike of which is
consistent with the strike of the Rialto-Colton and Day
Canyon faults. In addition, there is a mixture of focal
mechanisms for recent earthquakes along the fault including
normal, oblique reverse, and strike-slip (Figure 7), as well
as a lack of seismicity along the Day Canyon segment and
along the Rialto-Colton fault near the left-step.
[48] Because of the inconsistencies between recent
kinematic indicators, we conjecture that the Rialto-Colton/Day Canyon fault is an older right-lateral fault, a
weak zone currently being exploited in the present stress
regimes in the basin area: as a normal fault with in the
basin, as a thrust fault where it crosses the Cucamonga
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fault zone, and inactive or locked in the southeastern San
Gabriel Mountains.
4.5.2. Duncan Canyon Fault
[49] The Duncan Canyon fault is another northwesttrending structure in the southeastern San Gabriel Mountains (DUCF in Figure 1) that shows evidence of right-slip
displacement. It cuts the same mylonite zone as the Day
Canyon fault, and in this location, there is 1.5 km of rightslip separation on it (Figure 2) as well as 1.5 km right-slip
separation of the associated magnetic anomaly (CC in
Figure 3b). The gravity map shows no distinct anomaly
associated with this fault where it would project into the
basin at its southern end. Though this is uncertain, if the
fault does not change strike, it should merge southeastward
into the zone of faults in the lower Lytle Creek area. Diffuse
seismicity in the vicinity of this fault shows it may be
presently active with focal mechanisms showing nearly pure
strike-slip motion in a right-lateral sense (Figure 7).
4.5.3. Fault Strands in the Lower Lytle Creek Area
[50] Several faults associated with the San Jacinto fault
zone trend northwest up Lytle Creek in the San Gabriel
Mountains (LCw, LCm, and LCe in Figure 1). The
western and middle strands show total separations on
magnetic anomalies of 3.5 and 1 km, respectively (DD
and EE in Figure 3b), although the separation on the
middle strand is not well defined (separation on edges do
not agree). No magnetic anomaly crosses the eastern
strand. From pluton offsets, Morton [1975] indicates an
aggregate right-lateral displacement of 15– 16 km on all
strands of the San Jacinto fault zone in the southeastern
San Gabriel Mountains, including the Glen Helen fault,
San Jacinto strands in Lytle Creek Canyon, and the
Duncan Canyon and Day Canyon faults (corroborated
by Nourse [2002]). Since we cannot constrain the offset
on the eastern Lytle Creek strand, we defer to the figures
for total offset from the geology, which, after accounting
for the slip already constrained on the other strands,
leaves approximately 11 – 12 total offset on the Lytle
Creek strands (see Table 3 for breakdown of offset on
each strand).
[51] Focal mechanisms and seismicity in the vicinity of
all of these fault strands show quite a bit of current activity,
with right-slip to right-reverse oblique and reverse slip
motions (Figure 7). However, youthful primary fault features are associated with the westernmost Lytle Creek strand
[Mezger and Weldon, 1983; Morton and Matti, 1987].
4.5.4. Glen Helen Fault
[52] Youthful scarps and sag ponds in lower Cajon Pass
[Morton and Matti, 1987] show that the Glen Helen fault is
the most recently active strand of the San Jacinto fault zone
in the vicinity of Cajon Pass (GH in Figure 1). A further
indication of current activity is the abundant microseismicity which aligns with the Glen Helen fault.
[53] We further interpret activity southward along this
fault past the end of its mappable surficial trace. Here, a
subtle step in basement topography parallel to and east
of the modern trace of the San Jacinto fault lies along
the northern flank of the graben (dashed fault line in
Figure 5a). Projection of this feature northward is coextensive with the Glen Helen fault. Intensive seismicity is
concentrated under depocenter U (Figure 5) between the
modern San Jacinto fault and this inferred southern exten-
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ANDERSON ET AL.: STRUCTURE OF THE SAN BERNARDINO BASIN
sion of the Glen Helen fault. We interpret this as a small
pull-apart graben, which is corroborated by normal faulting
mechanisms in this area (Figure 7).
4.5.5. Shandin Hills Fault Zone
[54] We apply the name ‘‘Shandin Hills fault zone’’ to
northwest-trending faults near Cajon Pass (SHFZ in
Figure 1). The gravity map shows a NW-SE-trending lineation of gravity contours (B on Figure 4) on the southwest side
of the Shandin Hills which coincides with the edge of a
shallow (200 m deep) part of the basin (Figure 5a). Photolineaments and discrete faults identified in trenches (that
break alluvial deposits thought to be between 5000 and
7000 years old) located within the trend of the gravity
gradient also supports recent normal faulting in this zone
[Schell, 2000; B. A. Schell, written communication to J. C.
Matti, 2002; California Department of Water Resources,
1969, 1975]. The strikes of the many normal focal mechanisms in the vicinity of this zone are oblique to the trend of the
inferred faults (Figure 7) and indicate transtensional faulting
and perhaps the formation of a new graben structure.
[55] In the same vicinity, magnetic anomalies appear to be
displaced 4 to 5 km along two distinct right-lateral steps (FF
in Figure 3c). We use the gravity and the offset magnetic
anomalies to define two faults. The interpretation of the
southeastern fault strand we feel is strong because the
inferred position from the magnetic map, the gravity gradient, and seismicity are aligned along this segment. The
interpretation of a fault to the northwest is more tenuous
because, while the seismicity extends that far north, it is not
as well aligned (Figure 7) and there is no distinct gravity
gradient associated with this fault. The strike of the fault is
based entirely on the magnetic anomaly interpretation. We
interpret these two faults as strands that each accommodate
right-slip in a step over from the modern San Jacinto fault or
the proposed extension of the Glen Helen fault to the San
Bernardino strand of the San Andreas Fault.
4.6. Accommodation Structures and Transfer Faults
4.6.1. San Antonio Canyon Fault
[56] The San Antonio Canyon fault also enters the San
Gabriel Mountains in the northwestern part of our study
area (SACF in Figure 2), but it offsets the magnetic high
in the Southeastern San Gabriel Mountains in a left-lateral
sense by about 4 km (AA in Figure 3b). In agreement with
the magnetic data, displacement of older faults that cross
the San Antonio Canyon fault show a similar left-lateral
offset of approximately 3 –5 km [Matti and Morton, 1993;
Nourse, 2002]. This 4 km offset indicates a northward
movement of the Southeastern San Gabriel Mountains
block relative to the rest of the San Gabriel Mountains.
4.6.2. Cucamonga Fault Zone
[57] The Cucamonga fault zone is located along the
southern edge of the San Gabriel Mountains (CFZ in
Figure 1) and has generated thrust slip in the late Quaternary
[Morton and Matti, 1987]. This is due, in general, to the big
bend in the San Andreas Fault north of the Transverse
Ranges causing compression [Weldon and Humphreys,
1986]. We can tell little about thrust movement along this
fault zone from the gravity, but Morton and Matti [1993]
estimate a convergence rate of 3 – 5 mm/yr across this fault
zone based on faulting in Pleistocene and younger sediment.
If this rate has been consistent for the 1.2 ma the more
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eastern strands of the San Jacinto fault zone have been
active, 5 km of San Jacinto strike-slip offset could be
transferred into Cucamonga fault zone thrusting.
4.6.3. Crafton Hills Fault Zone
[58] The Crafton Hills form the southeastern boundary of
the San Bernardino basin (CHFZ in Figures 1 and 2) and are
defined clearly by geophysical and topographic data. The
topographic high associated with the Crafton Hills extends
westward into the subsurface of the San Bernardino basin
(Figures 4 and 5). A similar, roughly east striking gravity
high (which we name the Mentone high; Figure 6) is
centered about 6 km northwest of the Crafton Hills adjacent
to the San Bernardino strand of the San Andreas Fault
(Figures 4 and 5). Magnetic highs correspond with both of
these features (Figures 3b and 3c) and they both are
modeled in two dimensions with rock of similar density
and magnetic susceptibility corresponding to upper-plate
rocks of the Vincent Thrust.
[59] The Crafton Hills fault zone consists of normal to
oblique-normal and strike-slip faults based on geomorphic
features and seismicity. Normal displacement has in the past
been interpreted to be due to transtension between the San
Jacinto and San Andreas Fault zones [Matti et al., 1985;
Matti et al., 1992a; Matti et al., 1992b]. However, the
Crafton Hills magnetic high could also be interpreted as
offset in a left-lateral sense (II in Figure 3c) and numerous
strike-slip mechanisms in the Crafton Hills could be interpreted as left-lateral (Figure 7). This would be consistent
with interpretations of east-west striking faults in the basin
as zones bounding rotating blocks [Nicholson et al., 1986a;
Seeber and Armbruster, 1995]. However, there are no leftlateral geomorphic features observed in the Crafton Hills
fault zone and the separation on the magnetic anomaly
could be due to tear faults in the Vincent Thrust.
[60] Another linear, east-trending gradient in the basement topography occurs just south of Perris Hill (Figure 5a).
The gradient shows a basement offset of 100– 200 m (down
to the south); this relief is at the limit of data resolution, but
because it persists throughout several gravity measurements, we believe it represents a real geologic feature. This
trend parallels the Crafton Hills and the Mentone High and
suggests a general E-W pattern for fault- and/or foldcontrolled structures in this part of the San Bernardino
basin. Extension may be utilizing pre-existing structure,
which explains the east-striking structures’ misorientation
with respect to stress fields that would be created by the
pull-apart basin.
5. Discussion
[61] We use the general structure and individual fault
descriptions to propose a developmental model of the San
Bernardino basin. In our model, the Rialto-Colton/Day
Canyon fault, Duncan Canyon fault, all lower Lytle Creek
area fault strands, the Glen Helen fault, and the Shandin
Hills fault zone are all splays of the main trace of the San
Jacinto fault; these strands are all involved in the genesis of
the San Bernardino graben, which has been subsequently
cut through by the modern trace of the San Jacinto fault.
The right-lateral strike-slip displacement on these faults
along with estimated thrusting on the Cucamonga fault
zone totals that estimated for the San Jacinto fault zone to
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ANDERSON ET AL.: STRUCTURE OF THE SAN BERNARDINO BASIN
Figure 8. Palinspastic reconstruction of progressive evolution of the San Bernardino basin. Magnetic
anomalies and faults used in the reconstruction are shown. Faults are dotted where location is
approximate and names are given in Table 4. (a) Configuration prior to movement on main trace of the
San Jacinto fault, (1.5 Ma). The first two faults break through: the Rialto-Colton/Day Canyon fault and
the Duncan Canyon fault. (b) Configuration between 1.5– 1.3 Ma, after 2 km right-slip displacement on
the Rialto-Colton/Day Canyon fault and 1.5 km right-slip displacement on the Duncan Canyon fault. At
this point in time, the western and middle fault strands in the lower Lytle Creek area become active.
Development of the southern subgraben has started. (c) Configuration after 3.5 km right-slip and 1 km
right-slip on the western and middle Lytle Creek strands, respectively. At this time, the northern
subgraben is forming and strike-slip movement is concentrated on the eastern Lytle Creek fault strand.
(d) After 8 km displacement on the eastern Lytle Creek fault strand. The modern strand of the San Jacinto
fault zone is breaking through the center of the graben, and broader extension has begun to the east and
west of the graben. (e) Present-day, after 4.5 km displacement on the modern strand of the San Jacinto
fault and the Shandin fault, as well as 5 km thrusting in the Cucamonga fault zone. Strike-slip movement
is concentrated on the modern strand of the San Jacinto fault and moves to the Glen Helen and Shandin
faults in the northern part of the basin. Our model predicts that extension is developing between the
modern strand of the San Jacinto and the Shandin Hills fault zone. Note that the San Antonio Canyon
magnetic anomaly described in the text and in Figure 3b is not accurately reconstructed. This is probably
because the zones of compression in the Southeastern San Gabriel Mountains utilized for the
reconstruction are only approximate and not based on mapping of thrust structures.
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the south of the San Bernardino basin. Our gravity data
indicate that basement block rotation is probably minor
within the basin and so it is not included in our model.
5.1. Development of the Basin
[62] Individual fault age and interpretation information is
summarized in Table 3 and a palinspastic basin development summary is given in Figure 8. Where geophysical,
geologic, and geomorphic data are lacking (note that many
constraints on fault age in Table 3 lack precision and
accuracy), we invoke a step-to-bend type of interpretation
for the development of fault strands in the basin; this is an
interpretation proposed for other pull-apart basins and fault
systems [Wesnousky, 1989; Zhang et al., 1989; Ben-Zion
and Sammis, 2003] in which a jog that starts as parallel,
stepping faults will cut through by a throughgoing, bending,
simpler fault after a threshold amount of displacement. This
framework of interpretation creates a developmental model
that generally fits the geometry and retro-deformation of
identified right-slip on faults in the basin, the distribution
of the deepest parts of the basin, the present pattern of
seismicity, the general younging of fault-related geomorphic
features in the Southeast San Gabriel Mountains from west
to east, and the few available dates.
5.1.1. Basin Inception
[63] We have already inferred that the Rialto-Colton/Day
Canyon fault is likely an older strand of the San Jacinto
system and that it has generated about 2 km of right-slip
displacement (Figure 8a). It could not have caused much
extension when it was active as a throughgoing strike-slip
fault because it is coextensive with the main trace of the San
Jacinto fault (except for the small left-step). The leftstepping compressional dynamic in the middle of the
Rialto-Colton fault could have inhibited more than 2 km
of movement.
[64] After the Rialto-Colton/Day Canyon Fault was abandoned, right-slip on the San Jacinto fault zone likely stepped
east to the Duncan Canyon strand. Geophysical measurements presented here do not show a distinct, connecting
fault strand in the basin between the Duncan Canyon fault
and the northeastern edge of the southern part of the graben.
Subsequent normal and strike-slip displacement has likely
obscured this connection. However, palinspastic reconstruction of fault configuration at the time the Duncan Canyon
fault was first active indicate that it would be in a favorable
position to initiate a step in the San Jacinto fault zone, thus
starting the southern part of the San Bernardino graben
(Figure 8b); as suggested by the age of the San Timoteo
deposits within this part of the graben, this right-step
occurred after 1.5 Ma and before 1.3 Ma.
5.1.2. Period of Major Normal Fault Development
[65] After 1.5 km movement on the Duncan Canyon fault,
strands developed eastward in the lower Lytle Creek area.
These splays of the San Jacinto fault incurred the greatest
amount of slip (11 – 12 km); therefore it is likely that the
most normal displacement in the graben developed by
distributed extension in the step between the main trace of
the San Jacinto fault and these splays (Figure 8b, 8c, and
8d). It is also likely that the Cucamonga fault zone would
have been active at this time (between Figures 8d and 8e)
accommodating another 5 km of slip from the main trace of
the San Jacinto fault.
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[66] Toward the end of this time frame, during slip on the
eastern strand in the Lytle Creek area, the San Jacinto fault
zone cut through its own graben in the distinct, rightbending modern trace of the San Jacinto fault (Figure 8d).
The fault has been in this configuration for approximately
4.5 km of displacement. During this later part of the graben
development, far-field normal stresses developed beyond
the bounds of the graben and were accommodated by
normal faulting along the Rialto-Colton/Day Canyon fault
in the western part of the basin and perhaps the Crafton
Hills/Mentone/Perris Hill structures in the eastern part of the
basin.
5.1.3. Current Activity
[67] Most recent fault activity at the north end of the
basin has been concentrated on the Glen Helen fault and
the Shandin Hills fault zone (Figure 8e). The path of the
step of right-slip displacement to the Glen Helen fault
from the modern trace of the San Jacinto fault is probably
located within the graben in the vicinity of depocenter U
(Figure 5a). The Shandin Hills fault zone is also currently
accommodating normal faulting in a stepover from the
Glen Helen fault and could possibly transfer slip onto the
San Andreas Fault.
5.2. Path of Right Slip Fault Displacement
[68] Table 3 shows that the total right-slip displacement
for splays of the San Jacinto fault zone entering Cajon
Pass and the Southeast San Gabriel Mountains is around
20– 21 km. This subtotal does not count displacement on
the modern trace of the San Jacinto fault, which connects
to the Lytle Creek area faults. If one adds in the 5 km of
San Jacinto fault zone strike-slip displacement that we
have estimated is taken up by Cucamonga fault zone
thrust and reverse faulting, this figure becomes 25– 26 km.
This is surprisingly close to estimates of total right-slip
displacement on the San Jacinto fault zone south of the
basin which range from 24– 28 km [Morton and Matti,
1993; Powell, 1993; Sharp, 1967]. If these estimates are
correct, then most right-slip displacement on the San
Jacinto fault zone exits the San Bernardino basin at its
north end on several distinct strands. This implies that all
of the right-slip passes through or near the modern San
Jacinto fault strand rather than stepping across the basin
to directly link into the San Bernardino strand of the San
Andreas Fault. In other words, we must use other mechanisms to explain extension on faults such as those in the
Crafton Hills.
[69] In reconstructions of the whole San Andreas Fault
system, various authors call on the San Bernardino basin
to act as a locus for the transfer of 25 km of displacement on the San Jacinto fault zone to the Mojave strand
of the San Andreas Fault to the north [Matti and Morton,
1993; Powell, 1993]. Our reconstruction runs contrary to
these interpretations because, with the exception of the
Shandin Hills fault zone (which can only account for a
maximum of 5 km transfer of strike-slip onto the San
Andreas Fault), none of the faults utilized in our reconstruction connect through mapped faults in the Southeastern San Gabriel Mountains to the San Andreas Fault.
Since about 5 km strike-slip movement has been taken up
in documented thrusting in the Cucamonga fault zone,
there is still approximately 15 km of right-slip displace-
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Figure 9. Location of uplift/basin formation according to the block rotation model proposed by
Nicholson et al. [1986a]. Grey triangles show the location of zones of deformation and arrows indicate
material moving in/out of the zones. Heavy black lines extending between the modern trace of the San
Jacinto fault zone and the San Bernardino strand of the San Andreas Fault show the location of bands of
seismicity cited as evidence of rotation of blocks within the San Bernardino basin [Nicholson et al.,
1986a] (Figure 6). Fault and geographic feature names are given in Table 4; not all faults are shown.
ment on the San Jacinto fault zone that has no connecting
fault to the San Andreas Fault zone exposed at the
surface [Matti et al., 1985; Morton, 1975; Morton and
Matti, 1993] and is not taken up by documented thrusting
or folding in the Southeastern San Gabriel Mountains.
Therefore unless unmapped thrusting and folding exist,
other mechanisms are required to accommodate slip north
of the termination of the San Jacinto fault zone.
[70] Meisling and Weldon [1989] suggested the existence
of low-angle, deep-seated detachments under the San Bernardino Mountains as well as a northward warp in the
deeper part of the San Andreas Fault zone. Perhaps this sort
of structure could accommodate slip in such a way as to not
be readily visible in the right-slip history of faults in and
north of Cajon Pass. However, deep-seated, blind thrust slip
accommodation should be complemented by faulting or
folding of strata at a higher level somewhere in the
Southeastern San Gabriel or San Bernardino Mountains.
5.3. Rotation in the San Bernardino Basin
[71] Rotation of crustal blocks throughout the Transverse Ranges has been documented by many authors (see
Luyendyk [1991] for review) and it has been suggested
that the San Bernardino basin also has been a locus for
rotation of crustal blocks [Nicholson et al., 1986a; Seeber
and Armbruster, 1995]. In our model, strike-slip displacement on the San Jacinto fault zone can be accounted for
on distinct fault strands using a simple right-lateral strikeslip model perhaps with secondary extension in other
parts of the basin; rotation is not required in the interpretation of our data.
[72] Nicholson et al. [1986a, 1986b] denoted bands of
seismicity in the basin and used them to delineate
locations of northeast striking left-lateral faults used to
explain rotation. While east-striking faults have been
outlined from the gravity inversion, the east-strike of
these faults does not coincide well with the bands of
seismicity (Figure 7). In addition, some of the left-slip
mechanisms cited by Nicholson et al. fall near the
Shandin Hills fault zone and the San Bernardino strand
of the San Andreas Fault and could be interpreted, in light
of our model, as right-slip (Figure 7). Jones [1988] and
Hauksson [2000] also show that most mechanisms in
these bands are normal rather than strike-slip. There is a
possible left-lateral separation on a magnetic anomaly
associated with the Crafton Hills fault zone, but as
discussed above, this is not a solid interpretation and
other alternatives exist.
[73] Large amounts of rotation would create alternating
basins and ‘‘pop-ups’’ at the corners of the rotated blocks
where they meet the master faults along the edges of the
basin [Nicholson et al., 1986a]. It would be difficult to
see this along the modern San Jacinto fault, for those
features would be overprinted by the graben structure.
Adjacent to the San Bernardino strand of the San Andreas
Fault, however, these features should be prominent because of the shallow, level basin floor (Figure 5a). There
is no clear evidence in the basement topography for these
structures in places where they would be predicted
according to published rotation models (Figure 9). However, these uplift/normal faulting zones may not exist if
the rotating blocks are not rigid. Our conclusion is that
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block rotation has not played a major role in the development of the San Bernardino basin structure.
5.4. Comparison to an Analog Model
[74] Our basement topography more closely resembles a
classic stepover basin [sensu Nilsen and Sylvester, 1999]
than a broadly extended triangular basin [e.g., Nilsen and
McLaughlin, 1985]. Figure 5b presents a cutaway of an
analog model [Dooley and McClay, 1997] which shows
one-to-one similarity with our basin structure. The analog
model mimics the asymmetry of our basin even on a faultby-fault basis. Such structures as the Shandin Hills fault
zone and the modern strand of the San Jacinto fault are
predicted by the analog model. With further study, such
comparisons can improve our understanding of timing and
kinematics of the development of pull-apart basins.
6. Conclusions
[75] We have demonstrated that potential field data, in
conjunction with other data such as seismicity and reflection
seismic, can be used to describe strike-slip basins with a
high level of detail. Our study shows that the structure of the
basin is significantly different than proposed in previous
geologic and geomorphic studies, indicating that discrete,
basin-related normal faults extend well west of the modern
San Jacinto fault to the Rialto-Colton fault. Our model also
features the San Bernardino graben more distinctly than
previous gravity studies [e.g., Willingham, 1968], bounded
by well-defined faults that create a classic pull-apart basin
shape. This feature illustrates that significant extension is
centered directly over the San Jacinto fault rather than
distributed evenly between the San Jacinto and San Andreas
Faults. Therefore we see the role of the San Andreas Fault
in the last 1.5 Ma as minor in the development of the basin,
and instead, steps within the throughgoing San Jacinto fault
zone created most of the basin-related extension.
[76] We estimate a total of about 20 –21 km of rightlateral strike-slip displacement on splays of the San Jacinto
fault zone in the basin and 5 km thrusting along the
Cucamonga fault zone. Added together, this is similar to
the 24– 28 km of strike-slip displacement on the San Jacinto
fault zone to the south of the basin estimated by other
studies [Morton and Matti, 1993; Powell, 1993; Sharp,
1967]. While many authors have argued that the basin is a
center for transfer of slip from the San Jacinto fault zone to
the San Andreas Fault, our interpretations require most
displacement to enter the Southeastern San Gabriel Mountains on several different fault strands. Approximately 15 km
of this right-slip displacement cannot be traced north of the
Southeastern San Gabriel Mountains through faults exposed
at the surface. However, there is no mechanism for accommodation of slip that we see as most plausible. Our model
provides a new framework which can guide future investigation of such structures.
[77] Through reconstruction of offset on San Jacinto fault
splays, we outline a scenario of development of the basin in
which the San Bernardino graben first develops within steps
in the San Jacinto fault zone and then is cut through by the
single, modern strand of the San Jacinto fault. The resulting
detailed, map-view and three-dimensional structural model
can be directly compared to numerical and analog models at a
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high level of detail. We hope that further work with such
comparisons will help to better illuminate the timing and
kinematics of the development of pull-apart structures that
are not easily addressed by only geologic or geophysical data.
[78] Acknowledgments. We would like to thank Linda Woolfenden
of the USGS Water Resources Division, the San Bernardino Valley
Municipal Water District, the City of Colton, and the City of Rialto for
supporting this project. Thanks to Carter Roberts, Jeff Davidson, and Geoff
Phelps for their extensive help gathering field data and Sally McGill, Jerry
Treiman, and Mike Rymer for their tips on geologic and seismic data.
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Woolfenden, L. R., and D. Kadhim (1997), Geohydrology and water chemistry in the Rialto-Colton basin, San Bernardino County, California, U.S.
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Youngs, L. G., S. P. Bezore, R. H. Chapman, and G. W. Chase (1981),
Resource investigation and low- and moderate-temperature geothermal
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M. Anderson, Department of Geosciences, University of Arizona,
Tucson, AZ 85716, USA. (anderson@geo.arizona.edu)
R. Jachens, U.S. Geological Survey, Menlo Park, CA 94025, USA.
J. Matti, U.S. Geological Survey, Tucson, AZ 85719, USA.
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Figure 3.
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Figure 3. (a) Schematic sketch of how magnetic anomalies were used as geologic markers to gauge total amount of strikeslip displacement along a fault. (b) Aeromagnetic map for the San Bernardino basin [U.S. Geological Survey, 1996].
Displacements on faults in the southeastern San Gabriel mountains were determined from this map and are summarized in
Table 3. Dotted, unnamed fault shows apparent separation of the magnetic anomaly, but is not associated with a known fault
or substantiated as a fault by other lines of evidence. Box shows area of C. Model along profile labeled A-A0 is shown in
Figure 6. (c) The high-frequency map of the basin floor brings out smaller, shallower magnetic anomalies. Fault and
geographic feature names are given in Table 4. Normal faults in the center of the map are inferred in the subsurface, and
outline the edges of the San Bernardino graben as shown in Figure 5a.
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Figure 5.
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Figure 5. Three-dimensional basin depth inversion. Overall, the basement-surface shape has a horizontal resolution of
1 km in most places and 200 meters where gravity measurements were closely spaced. Depth resolution is discussed in the
text. Model along profile labeled A-A0 is shown in Figure 6. Fault and geographic feature names are given in Table 4. In
addition, several smaller, unnamed faults are shown here: Q, northwestern edge of graben; R, southwestern edge of graben;
S, northeastern edge of graben; T, northeastern edge of southern sub-graben; U, V, W, X, depocenters. Features southeast of
the Crafton Hills are not adequately controlled by data so should be disregarded. (b) Reproduction of cutaway drawing of
Dooley and McClay’s [1997] pull-apart analog model. Colors denote layers of syn-extensional material from oldest
(orange) to youngest (purple). Note the similarity in structure with our model of the San Bernardino graben; specific
structures that are similar are noted on the drawing.
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