Structural model of the San Bernardino basin, California, from analysis of gravity, aeromagnetic, and seismicity data

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /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 [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 formation. INDEX TERMS: 1219 Geodesy and Gravity: Local gravity anomalies and crustal structure; 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,, doi: /2003jb 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 /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 1of20

2 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: 2 of 20

3 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 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). [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 Gabriel- Banning 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 3of20

4 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-A 0 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 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 southwest 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 4of20

5 Table 1. Basement Rock Provinces and Their Geophysical Characteristics Used for Three-Dimensional Inversion and Two-Dimensional Modeling of Gravity Data a Province Peninsular Ranges San Gabriel Mountains Rock Types No. Samples Density Range, kg/m 3 Density Average, kg/m 3 Standard Deviation Magnetic Susc. Range, siu 10 3 Magnetic Susc. Average, siu 10 3 Standard Deviation Metasediments, Jurassic and Cretaceous granodiorite, quartz diorite, tonalite, and gabbro > Proterozoic orthogneiss Anorthosite-syenite complex, Mesozoic plutons, Miocene granitoid intrusives Pelona schist Proterozoic orthogneiss Metasediments Cretaceous granitic rocks, migmatite, gneiss, Paleozoic metasediments San Bernardino Mountains Southeastern San Gabriel Complex 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/m 3, 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/m 3, 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/m 3 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 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 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 , 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 identified 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 Rialto- Colton 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- 5of20

6 Figure 3. 6of20

7 ment is minimal on the faults along which strike-slip separations were measured 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 Seismicity [27] We used hypocenters of small earthquakes ( and , 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 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 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-A 0 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-A 0 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. 7of20

8 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-A 0 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 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 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 Calculations a Depth, km Density, kg/m a From Willingham [1968] and adjusted to fit well-control data. 8of20

9 Figure 5. 9of20

10 Table 3. Summary of the Characteristics of Strike-Slip Faults in the San Bernardino Basin a Fault Sense of Slip Age Constraints on Age Total Offset Constraints on Displacement San Jacinto Splays Right-lateral older? Contradictory kinematic indicators Rialto-Colton-Day Canyon fault 2 km aeromagnetic anomalies (BB/GG)/geologic mapping Duncan Canyon fault Right-lateral Ma San Timoteo deposits 1.5 km aeromagnetic anomalies (CC)/geologic mapping Lytle Creek fault zone, western strand Right-lateral <1.3 Ma-present active seismicity/alluvial offsets 3.5 km aeromagnetic anomalies (DD) Lytle Creek fault zone, middle strand Right-lateral <1.3 Ma-present active seismicity/alluvial offsets 1 km aeromagnetic anomalies (EE) Lytle Creek fault zone, Right-lateral <1.3 Ma-present alluvial offsets 7 8 km geologic mapping eastern strand Glen Helen fault Right-lateral recent active seismicity,?km - geomorphic features Shandin Hills fault zone Right-lateral recent active seismicity, trenching 5 km aeromagnetic anomalies (FF) Other Measured Offsets San Jacinto fault, modern trace Right-lateral recent active seismicity geomorphic features 4.5 km aeromagnetic anomalies (HH) San Antonio Canyon fault 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 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; Richards- Dinger 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 southern 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 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-A 0 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. 10 of 20

11 Figure 6. (a) Basin cross section modeled in two dimensions along A-A 0 (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 11 of 20

12 Figure of 20

13 Table 4. Fault and Geographic Feature Abbreviations Fault/Geographic Feature Name 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 Abbreviation 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 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 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/m 3 ) and less dense Pelona schist (2560 kg/m 3 ). 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/Rialto- Colton 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 ( and ) [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-A 0 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. 13 of 20

14 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 Splays of the San Jacinto Fault 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) 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 fault zone, and inactive or locked in the southeastern San Gabriel Mountains 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) 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 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 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] 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- 14 of 20

15 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) 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 Accommodation Structures and Transfer Faults 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 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 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 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 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 15 of 20

16 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 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. 16 of 20

17 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 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 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 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. [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 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 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 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 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 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- 17 of 20

18 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 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 18 of 20

19 block rotation has not played a major role in the development of the San Bernardino basin structure 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 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 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 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. References Al-Zoubi, A., and U. ten Brink (2002), Lower crustal flow and the role of shear in basin subsidence; an example from the Dead Sea basin, Earth Planet. Sci. Lett., 199, Anderson, M. L., C. W. Roberts, and R. C. Jachens (2000), Principal facts for gravity stations in the vicinity of San Bernardino, southern California, Open File Rep , U.S. Geol. Surv., 32 pp. Bechtel, Inc. (1970), Water transmission project: Engineering feasibility report, Appendix A, Geology, Consult. Rep., San Bernardino Valley Muni. Water Dist., 41 pp., San Bernardino, Calif. Ben-Zion, Y., and C. G. Sammis (2003), Characterization of fault zones, Pure Appl. Geophys., 160, Blakely, R. J. (1995), Potential Theory in Gravity and Magnetic Applications, 441 pp., Cambridge Univ. Press, New York. Blakely, R. J., and R. W. Simpson (1986), Approximating edges of source bodies from magnetic or gravity anomalies, Geophysics, 51, California Department of Water Resources (1969), Geology and construction materials data: Santa Ana Valley pipeline, Devil Canyon power plant to Mill Street, Proj. Geol. Rep. C-91, plates 1 3, 11 pp., State Water Facil. Calif. Aqueduct, Santa Ana Div., San Bernardino, Calif. California Department of Water Resources (1975), Final geologic report, Santa Ana Valley pipeline, Devil Canyon power plant to Mill Street: State Water Facilities, California Aqueduct, Santa Ana Division, San Bernardino County, California, Project Geol. Rep. C-91, 11 pp., plates 1 3, Calif. Dep. of Water Res., San Bernardino, Calif. Chen, W. P., and J. Nabelek (1988), Seismogenic strike-slip faulting and the development of the North China Basin, Tectonics, 7, Crowell, J. C. (1981), An outline of the tectonic history of southeastern California, in The Geotectonic Development of California, vol. 1, edited by W. G. Ernst, pp , Prentice-Hall, Old Tappan, N. J. Dibblee, T. W. (1965), Geologic map of the Cajon 7 1/2 minute quadrangle, San Bernardino County, California, U.S. Geol. Surv. Open File Rep Dibblee, T. W. (1968), Geologic map of the Yucaipa Quadrangle, San Bernardino County, California, U.S. Geol. Surv. Open File Rep Dibblee, T. W. (1982), Geologic map of the Perris (15 minute) quadrangle, California, South Coast Geol. Soc. Dillon, J. T., and P. L. Ehlig (1993), Displacement on the southern San Andreas Fault, Geol. Soc. Am. Mem., 178, Dooley, T., and K. McClay (1997), Analog modeling of pull-apart basins, AAPG Bull., 81, Dutcher, L. C., and A. A. Garrett (1963), Geologic and hydrologic features of the San Bernardino area, California-with special reference to underflow across the San Jacinto fault, U.S. Geol. Surv. Water Suppl. Pap. 1419, 114 pp. Ehlig, P. L. (1981), Origin and tectonic history of the basement terrane of the San Gabriel Mountains, central Transverse Ranges, in The Geotectonic Development of California, vol. 1, edited by W. G. Ernst, pp , Prentice-Hall, Old Tappan, N. J. Ehlig, P. L. (1982), The Vincent thrust: Its nature, paleogeographic reconstruction across the San Andreas Fault, and bearing on the evolution of the Transverse Ranges, in Geology and Mineral Wealth of the California Transverse Ranges: South Coast Geological Society Guidebook no. 10, edited by D. L. Fife and J. A. Minch, Frankel, A. (1993), Three dimensional simulations of ground motions in the San Bernardino Valley, California, for hypothetical earthquakes on the San Andreas Fault, Bull. Seismol. Soc. Am., 83, Hauksson, E. (2000), Crustal structure and seismicity distribution adjacent to the Pacific and North America plate boundary in southern California, J. Geophys. Res., 105, 13,875 13,903. Hill, D. P., J. P. Eaton, and L. M. Jones (1990), Seismicity, , U.S. Geol. Surv. Prof. Pap., 1515, Izbicki, J. A., W. R. Danskin, and G. O. Mendez (1998), Chemistry and isotopic composition of ground water along a section near the Newmark area, San Bernardino County, California, Water Res. Invest. Rep of 20

20 Jachens, R. C. and B. C. Moring (1990), Maps of the thickness of Cenozoic deposits and the isostatic residual gravity over basement for Nevada, U.S. Geol. Surv. Open File Rep , 15 pp. Jacobson, C. E. (1983), Relationship of deformation and metamorphism of the Pelona Schist to movement on the Vincent thrust, San Gabriel Mountains, southern California, Am. J. Sci., 283, Jahns, R. H. (1954), Geology of the Peninsular Range Province, southern California and Baja California (Mexico), in Geology of southern California, edited by R. H. Jahns, pp , Calif. Dept. of Natl. Resour. Div. of Mines Bull. Jones, L. M. (1988), Focal mechanisms and the state of stress on the San Andreas Fault in southern California, J. Geophys. Res., 93, Kendrick, K. J., D. M. Morton, S. G. Wells, and R. W. Simpson (2002), Spatial and temporal deformation along the northern San Jacinto Fault zone, southern California: Implications for slip rates, Bull. Seismol. Soc. Am., 92, King, G., R. Stein, and J. Lin (1994), Static stress changes and the triggering of earthquakes, Bull. Seismol. Soc. Am., 84, Kusumoto, S., K. Takemura, Y. Fukuda, and S. Takemoto (1999), Restoration of the depression structure at the eastern part of central Kyushu, Japan by means of dislocation modeling, Tectonophysics, 302, Luyendyk, B. P. (1991), A model for Neogene crustal rotations, transtension, and transpression in Southern California, Geol. Soc. Am. Bull., 103, Magistrale, H., S. Day, R. Clayton, and R. Graves (2000), The SCEC southern California reference three-dimensional seismic velocity model version 2, Bull. Seismol. Soc. Am., 90, S65 S76. Matti, J. C., and D. M. Morton (1993), Paleogeographic evolution of the San Andreas Fault in southern California: A reconstruction based on a new cross-fault correlation, Geol. Soc. Am. Mem., 178, Matti, J. C., D. M. Morton, and B. F. Cox (1985), Distribution and geologic relations of fault systems in the vicinity of the Central Transverse Ranges, southern California, U.S. Geol. Surv. Open File Rep Matti, J. C., D. M. Morton, and B. F. Cox (1992a), The San Andreas Fault system in the vicinity of the central transverse ranges province, southern California, U.S. Geol. Surv. Open File Rep Matti, J. C., D. M. Morton, B. F. Cox, S. E. Carson, and T. J. Yetter (1992b), Geologic setting of the Yucaipa quadrangle, San Bernardino and Riverside Counties, California: Summary to accompany geologic map of the Yucaipa quadrangle, U.S. Geol. Surv. Open File Rep May, D. J. (1986), Amalgamation of metamorphic terranes in the southeastern San Gabriel Mountains, California, Ph.D. thesis, Univ. of California, Santa Barbara, Calif. May, D. J. (1989), Late Cretaceous intra-arc thrusting in southern California, Tectonics, 8, Meisling, K. E., and R. J. Weldon (1989), Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California, Geol. Soc. Am. Bull., 101, Mezger, L., and R. J. Weldon (1983), Tectonic implications of the Quaternary history of the Lower Lytle Creek, Southeast San Gabriel Mountains, Geol. Soc. Am. Abst. Prog., 15, 418. Miller, F. K. (1979), Geologic map of the San Bernardino North Quadrangle, San Bernardino County, California, U.S. Geol. Surv. Open File Rep Miller, F. K., and J. C. Matti (2001), Geologic map of the San Bernardino North quadrangle, San Bernardino County, California, version 1.0, scale 1:24,000, U.S. Geol. Surv. Open File Rep Morton, D. M. (1975), Synopsis of the geology of the eastern San Gabriel Mountains, southern California, Spec. Rep Calif. Div. Mines Geol., 118, Morton, D. M. (1976), Geologic map of the Cucamonga fault zone between San Antonio Canyon and Cajon Creek, southern California, U.S. Geol. Surv. Open File Rep Morton, D. M. (1978a), Geologic map of the Fontana quadrangle, San Bernardino and Riverside Counties, U.S. Geol. Surv. Open File Rep Morton, D. M. (1978b), Geologic map of the Redlands quadrangle, San Bernardino and Riverside Counties, California, U.S. Geol. Surv. Open File Rep Morton, D. M. (1978c), Geologic map of the San Bernardino South quadrangle, California, Open File Rep Morton, D. M., and J. C. Matti (1987), The Cucamonga fault zone: Geologic setting and Quaternary history, U.S. Geol. Surv. Prof. Pap., 1339, Morton, D. M., and J. C. Matti (1990), Geologic map of the Cucamonga Peak Quadrangle, California, U.S. Geol. Surv. Open File Rep Morton, D. M., and J. C. Matti (1991), Geologic map of the Devore Quadrangle, California, U.S. Geol. Surv. Open File Rep Morton, D. M., and J. C. Matti (1993), Extension and contraction within an evolving divergent strike-slip fault complex: The San Andreas and San Jacinto fault zones at their convergence in southern California, Geol. Soc. Am. Mem., 178, Morton, D. M., and F. K. Miller (1975), Geology of the San Andreas Fault zone north of San Bernardino between Cajon Canyon and Santa Ana Wash, Calif. Div. Mines Geol. Spec. Rep., 118, Nicholson, C., L. Seeber, P. Williams, and L. R. Sykes (1986a), Seismic evidence for conjugate slip and block rotation within the San Andreas Fault system, southern California, Tectonics, 5(4), Nicholson, C., L. Seeber, P. Williams, and L. R. Sykes (1986b), Seismicity and fault kinematics through the eastern Transverse Ranges, California: Block rotation, strike-slip faulting and low-angle thrusts, J. Geophys. Res., 91, Nilsen, H. T., and R. J. McLaughlin (1985), Comparison of tectonic framework and depositional patterns of the Hornlen strike-slip basin of Norway and the Ridge and Little Sulphur Creek strike-slip basins of California, Spec. Pub. Soc. Econ. Paleo. Min., 37, Nilsen, T. H., and A. G. Sylvester (1999), Strike-slip basins, part 2, Leading Edge, 18, Nourse, J. A. (2002), Middle Miocene reconstruction of the central and eastern San Gabriel Mountains, southern California, with implications for evolution of the San Gabriel Fault and the Los Angeles basin, Geol. Soc. Am. Spec. Pap., 365, Powell, R. E. (1993), Balanced palinspastic reconstruction of pre-late Cenozoic paleogeography, southern California: Geologic and kinematic constraints on evolution of the San Andreas Fault system, Geol. Soc. Am, Mem., 178, Powell, R. E., and R. J. Weldon II (1992), Evolution of the San Andreas Fault, Annu. Rev. Earth Planet. Sci., 20, Richards-Dinger, K. B., and P. M. Shearer (2000), Earthquake locations in southern California obtained using source-specific station terms, J. Geophys. Res., 105, 10,939 10,960. Rogers, T. H. (1967), Geologic atlas of California: San Bernardino Sheet, Calif. Div. of Mines and Geol. Rogers, T. H. (1969), Geologic atlas of California: Santa Ana Sheet, Calif. Div. of Mines and Geol., Sacramento, Calif. Sanders, C., and H. Magistrale (1997), Segmentation of the northern San Jacinto fault zone, Southern California, J. Geophys. Res., 102, 27,453 27,467. Schell, B. A. (2000), Holocene faulting between San Jacinto and San Andreas Faults, San Bernardino area, San Bernardino County, California, Geol. Soc. Am. Abst. Prog., 33, 78. Seeber, L., and J. G. Armbruster (2002), The San Andreas Fault system through the Transverse Ranges as illuminated by earthquakes, J. Geophys. Res., 100, Sharp, R. V. (1967), San Jacinto fault zone in the Peninsular Ranges of Southern California, Geol. Soc. Am. Bull., 78, Stephenson,W.J.,J.K.Odum,R.A.Williams,andM.L.Anderson (1995), Delineation of faulting and basin geometry beneath urbanized San Bernardino Valley, California, from seismic reflection and gravity data, Bull. Seismol. Soc. Am., 92, U.S. Geological Survey (1996), Aeromagnetic map of parts of Los Angeles, San Bernardino, Bakersfield, Long Beach, and Santa Ana 1 degrees 2 degrees quadrangles, California, Open U.S. Geol. Surv. File Rep Weldon, R. J., II, and E. Humphreys (1986), A kinematic model of southern California, Tectonics, 5, Weldon, R. J., II, K. E. Meisling, and J. Alexander (1993), A speculative history of the San Andreas Fault in the central Transverse Ranges, California, Geol. Soc. of Am. Mem., 178, Wesnousky, S. G. (1989), Seismological and structural evolution of strikeslip faults, Nature, 335, Willingham, C. R. (1968), A gravity survey of the San Bernardino valley, southern California, M.A. thesis, Univ. of California, Riverside, Calif. Woolfenden, L. R., and D. Kadhim (1997), Geohydrology and water chemistry in the Rialto-Colton basin, San Bernardino County, California, U.S. Geol. Surv. Water Res. Invest. Rep Youngs, L. G., S. P. Bezore, R. H. Chapman, and G. W. Chase (1981), Resource investigation and low- and moderate-temperature geothermal area in San Bernardino, California, U.S. Geol. Surv. Water Res. Invest. Rep , 242 pp. Zhang, P., B. C. Burchfiel, S. Chen, and Q. Deng (1989), Extinction of pullapart basins, Geology, 17, 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. 20 of 20

21 Figure 3. 6of20

22 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-A 0 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. 7of20

23 Figure 5. 9of20