Topographic effects of the Eastern California Shear Zone ...web.gps.caltech.edu/~jstock/Ge136-2012/DokkaReS... · block of California were used to test the topographic implications

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. 12, PAGES 30625-30644, DECEMBER 10, 2001

Topographic effects of the Eastern California Shear Zone in the Mojave Desert

Roy K. Dokka Center for GeoInformafics and Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana, USA

Kristine Y. Macaluso

Minerals Management Service, New Orleans, Louisiana, USA

Abstract. Digital elevation data were used to evaluate the topographic implications of the late Neogene Eastern California Shear Zone (ECSZ). Analysis shows that the ECSZ has directly affected >18% of the surface of the Mojave Desert block (MDB); uncertainties regarding the distribution of strain in basins suggest that the area of effect may be larger by perhaps a factor of 2. Remaining areas are likely due to Quaternary erosion or are inherited from pre-ECSZ times. Major drainages and basins of the MDB are developed along transtensional depressions within the ECSZ. These depressions established the regional and local base levels that have governed erosion by running water. Local topography is closely tied to the style of strain. Highlands of the MDB are spatially associated with (1) areas that have or are currently undergoing contractional strain associated with transpression within the ECSZ and (2) areas that lie adjacent to uplifted transpressional belts that bound the Mojave Desert (San Bernardino and San Gabriel Mountains). In contrast, lowland areas are spatially associated only with transtensional areas within the ECSZ. Examination of the topography in the transtensional areas also reveals two, distinct populations of elevation. Spatial analysis indicates that areas that have undergone the greatest local transtension associated with dextral shear are also the lowest topographically. These areas formed during the earliest phase of movement of the ECSZ (late Miocene to early Pleistocene). The second population is coextensive with recently formed transtensional basins that continue to be active.

1. Introduction

It is well understood that the surface of the Earth is the

consequence of the complex and dynamic interplay of tectonism and erosion carried out over time [e.g., Bloom, 1995]. Tectonism directly changes the face of the Earth through disruption and displacement by faulting, by tilting associated with folding, by loading, by material emplacement at depth, and by construction due to volcanism. Tectonism also results in a redistribution of mass

and heat within the lithosphere that brings about density and thermally driven changes to the interior and, in turn, is reflected at the surface. Erosion by wind, water, and mass wasting more gradually sculpts the surface and reduces the local relief created by tectonism by transporting matehals denuded at the surface to adjacent areas. Climatic factors, such as temperature and degree of aridity, govern the rates of denudation and debris distribution. Thus, with the exception of end-member cases, determination of the specific roles played by tectonism and erosion, as well as the modifying influences of climate, in the creation of a given landscape is a complex task. This task is made even more difficult because the surface of the Earth in many areas is composed of groups of landscape elements that are inherited from the past when morphogenic conditions were different.

One such end-member landscape that is often cited is that of the Mojave Desert of North America [e.g., Cooke et al., 1993]. This arid landscape of high tectonic activity is thought to be dominated by earth movements which have given rise to fault-scarp-bounded mountains, disrupted and tilted surfaces, and structurally controlled

Copyright 2001 by the American Geophysical Union.

Paper number 2000JB000017. 0148-0227/01/2000JB000017509.00

drainage basins, etc. [e.g., Cooke et al., 1993; Blackwelder, 1954]. Tectonism is thought to dominate because the region's high aridity has limited the effects of running water, H20-related chemical reactions (hydration, acid attack reactions, certain oxidation-reduc- tion reactions, etc.), and physical processes involving water (frost and root wedging). To be sure, a large portion of the Mojave Desert is an active tectonic environment [e.g., Dokka and Travis, 1990a], but evidence linking tectonic movements to specific landscape changes is anecdotal. The critical step in establishing this linkage for the entire Mojave Desert block, namely, an analysis to determine how much of the topography can be explained by tectonism, has yet to be accomplished.

The failure to establish this critical relationship has been in large part due to the lack of specific tectonic models upon which to base such a test and due to a lack of appropriate topographic data to quantitatively evaluate the test. Recent advances in our understanding of the tectonic evolution of the Mojave Desert, coupled with the availability of detailed digital topographic data and analytical tools, now make it possible to more directly evaluate the effects of tectonism on the landscape. This paper is a case study of how digital elevation data of the Mojave Desert block of California were used to test the topographic implications of the prevailing model for the late Neogene tectonics of the region.

2. Geology and Topography of the Mojave Desert Block

2.1. Geologic Setting

The Mojave Desert block (MDB) of southern California is a wedge-shaped region delimited by the Garlock fault to the north, the San Andreas fault system to the west and southwest, and the

30625

30626 DOKKA AND MACALUSO: TOPOGRAPHIC EFFECTS OF ECSZ IN MOJAVE DESERT

Figure 1. Tectonic elements of the Pacific-North American transform boundary in the western United States. The location of the Eastern California Shear Zone is indicated by the shaded area. Solid lines denote faults; and shaded lines are state boundaries. GF, Garlock fault.

southern Death Valley fault zone, Granite Mountains, and Packard Well faults to the east [Dokka, 1983] (Figure 1). The Eastern California Shear Zone (ECSZ) is the most recent tectonic system active in the MDB, developing sometime between 6 and 13 Ma and continuing to the present [Dokka and Travis, 1990a]. Prior to the ECSZ, two other Neogene tectonic regimes influenced the tectonic-thermal evolution of the Mojave region. These included (1) the Mojave extensional belt of ,-•24-21 Ma [Dokka, 1986, 1989] and (2) the Trans Mojave Sierran shear zone, active between 21 and 18 Ma [Dokka and Ross, 1995; Dokka et al., 1998]. These regimes are thought to be tectonically related to major intraplate, gravitationally driven transtension of the southwestern part of the North American plate in early Miocene time [Dokka and Ross, 1995; Dokka et al., 1998].

Although often considered part of the adjacent Basin and Range province by physiographers, the MDB has a distinctive landscape all its own (Plate 1) [Hewerr, 1954a]. To illustrate the character of the regional topography, we constructed topographic profiles of the Mojave Desert block as well as for adjacent environs (Figure 2). These profiles illustrate that the MDB lacks the alternating approximately N-S trending basins and ranges characteristic of the Basin and Range province. Figures 2b and 2c also nicely showthe mark of the ECSZ in the MDB and illustrate that it is composed of several small subregions of unique topographic expression. Dokka and Travis [1990a] showed that these subregions are generally coextensive with late Neogene structural domains of the ECSZ. Each of these domains is composed of a family of similar structures that facilitate local simple shear deformation. Most domains are dominated by right-slip faulting, whereas others are controlled by sinistral faults. These later domains are also associated with large (300-80 ø ) clockwise vertical axis rotations of encapsulated fault blocks. The boundaries between structural domains are faults that accommodate differential development. Together, the concerted motions of this network of kinematically linked late Neogene structures are the expression of 65-85 km of distributed right shear [Dokka and Travis, 1990a, 1990b; Dokka et al., 1998].

The Eastern California Shear Zone is a regionally significant structural feature because it has accommodated, and continues to accommodate, 9-23% of the motion between the Pacific and North American plates (Figure 1) [Dokka and Travis, 1990a, 1990b; Savage et al., 1990; Sauber et al., 1986, 1994; Miller et al., 2001 ]. The physical-kinematic linkage between the ECSZ and

the Pacific-North American plate boundary was first demonstrated by Dokka and Travis [1990a, 1990b], who showed that the active faults of the ECSZ can be traced from the Death Valley area into the San Andreas fault system beginning near the Salton Sea and continuing to western Sonora, Mexico. In the Mojave Desert block, the ECSZ is a broad, •80-km-wide band of dextral shear that strikes NNW-SSE (Figure 1) [Dokka and Travis, 1990a, 1990b; Dokka, 1993; Dokka et al., 1998; Miller et al., 2001]. The ECSZ continues north in the region between the Sierra Nevada and the California-Nevada border into eastern Oregon, possibly connecting with the Cascade Range [e.g., Pezzopane and Weldon, 1993].

Most models on the structural development of the Mojave Desert block have emphasized broad, conceptual themes such as the simple shear deformational pattern of the southern subregions caused by strike-slip faults [Gaffunkel, 1974] or the role of vertical axis rotations in accommodating regional shear [Luyen- dyk et al., 1980; Carter et al., 1987]. Although critical for the development of our conceptual understanding of the Mojave, these early models lacked the geological and geophysical specificity to successfully explain or correctly predict the behavior of regional and local faults, the distribution and mechanisms of strain, the rotations about vertical axes, and the patterns of landscape development. In contrast to these previous models, Dokka and Travis [1990a] integrated detailed structural data and other geological and geophysical constraints into a geometric- kinematic model of the ECSZ and proposed that the regional strain in the Mojave Desert block is actually heterogeneous. Their model explained all fault slips and vertical axis rotations published on the area and argued that regional strain in the Mojave Desert block is partitioned into six domains of local strain [Dokka and Travis, 1990a]. Revised and improved ECSZ models have been presented by Dokka [1993] and most recently by Dokka et al. [1998]. The details of this model are provided in section 3.

2.2. Tectonic Controls on the Landscape

Hewett [1954a] was the first to propose that the current physiography of the Mojave Desert is related to the current tectonic regime; he considered that this regime was dominated by vertical movements along steep, NW striking faults. Dibblee [1961] concurred with Hewett's notions regarding the tectonic

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Figure 3. Tectonic model of Dokka et at. [1998] for the evolution of the Eastern California Shear Zone in the Mojave Desert block. The three stages in the evolution of the Mojave desert region are shown: (a) present-day, (b) � 1 Ma, and (c) a time just before inception of the ECSZ (-.. ':6 Ma)_ Methods and data used in the reconstruction are provided by Dokka et at. [1998]. The ruled line pattern and solid areas represent regions of contraction and extension associated with the ECSZ, respectively_ Solid lines are active faults, and shaded lines are inactive or future faults_ BCF, Blackwater-Calico fault; BCF, Blue Cut fault; BMF, Bristol Mountains fault; CCF, Coyote Canyon fault; CdF, Cady fault; CF, Calico fault; CoF, Chiraco fault; CRF, Camp Rock fault; DKF, Desert King Spring fault; GF, Garlock fault; GHF, Gravel Hills fault; GMF, Granite Mountains fault; GSF, Goldstone Lake fault; HF, Helendale fault; LF, Lenwood fault; M-ACF, Manix-Afton Canyon fault; PF, Paradise fault; PMF, Pinto Mountain fault; PVF, Panamint Valley fault; PWF, Packard Well fault; R-PF, Rodman-Pisgah fault; SDVFZ, Southern Death Valley fault zone; SBT, San Bernardino Mountain thrust system; ShF, Sheep Hole fault

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Figure 4. Topographic subdivisions of the Mojave Desert block implied by the tectonic model of Dokka et al. [1998]. Transtension is thought to have produced lowlands, whereas transpression is considered to have resulted in highland areas. Areas not strained by the Eastern California Shear Zone are reckoned to have not been changed topographically. Abbreviations are from Plate 1.


Digital Elevation Model of the Mojave Desert block

Mask of Transtensional Areas of the Eastern California shear zone

Extracted Digital Elevation Model of the Transtensional Areas of the

Eastern California shear zone

Figure 5. General method used in the extraction of hypsography of tectonic subdivisions of the Eastem Califomia Shear Zone (ECSZ) in the Mojave Desert block according to the model ofDokka et al. [1998]. The example presented is for transtensional areas, but the method is the same for all subdivisions. Extraction was accomplished by multiplying the elevation value for each pixel in the original DEM by the value in a corresponding image mask. The mask is a 1-bit image constructed from Figure 4. Pixels in the transtensional area are assigned a value of one, whereas pixels outside the area are given a value of zero. Multiplication of one image by the other has the following effect: pixels lying within a tectonic subdivision retain their original value because they are multiplied by 1, whereas pixels located outside have a value of zero because their multiplier is 0.

controls on the landscape but speculated that the current tectonic setting was actually dominated by NW striking right-slip faults. This idea was proved by Dokka [1983], who determined the kinematics and net slip values for faults of the south central Mojave Desert. Dokka and Travis [1990a, 1990b], Dokka [1993], and, most recently, Dokka et al. [1998] hypothesized that the tectonic controls on the landscape were even more profound. Their models predict that major late Neogene topographic features such as depressions and uplifts have formed and continue to form as the result of strain brought on by differences in the velocity field within the ECSZ. Dokka and Travis [1990a] reasoned that transtension thins the crust and results in lowering of the Earth's surface, whereas transpression thickens the crust, which leads to isostatically driven uplift and positive relief. To test this basic idea for the Mojave Desert block, Dokka and Travis [1990a] used geophysical relations from the Soda Lake region (Plate 1), the regions largest topographic depression and a site of major transtension. They argued that the Soda Lake depression was created by transtension and that it is underlain by crust that is •6 km thinner than surrounding areas [Fuis, 1981].

3. Model Predictions and Limitations

The Dokka et al. [1998] model was developed to better understand the tectonics of the Mojave Desert block by explaining the lateral (x, y) translations of fault blocks, vertical axis-rotations of the blocks, and strains within the blocks and

along major faults (Figure 3). The Dokka et al. [1998] and earlier models used the positions of previously mapped faults to define the vast majority of boundaries of fault blocks. Where the boundaries of fault blocks were not previously mapped but were suspected, their surface traces were inferred from linear patterns of earthquakes and/or fault scarps. The boundaries of the few remaining unmapped blocks were defined on the basis of the geometry constraints imposed by adjacent fault blocks. For example, because formerly adjacent fault blocks were created with a common boundary, determination of the geometry of a boundary of one block also defines that of its counterpart. Finally, the model simplifies all fault block boundaries to a single fault to facilitate model- ing. Translations are defined by net slip values, whereas vertical axis rotations of blocks are constrained by declination anomalies inferred from paleomagnetism studies. We emphasize that topography was not an input for the creation of the Dokka et al. [1998] model.

Although the Dokka et al. [1998] model is two-dimensional, it does have implications for the vertical dimension in that it predicts the location, amount, and general type of strain in the plane of the Earth's surface (transtension versus transpression versus unstrained), as well as the associated topographic effect (highland versus lowland versus tectonically unchanged). The model is somewhat limited, however, in that it does not predict the specific style of deformation, its extent, or the distribution of strain within the area of effect. For example, the model does predict the location and amount of transtensional lengthening along a NW line, but it does not predict how that amount of


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lengthening is distributed over that line (i.e., percent extension). The model depicts areas of deformation such as the transtensional domains as being the sites of concentrated strain instead of what is most likely the case, distributed strain. The model was configured in this manner because of the lack of specific subsurface data for most areas of strain in the MDB. The important topographic implication of this is that because the percent extension is not likely to be locally infinite as implied by the model, the actual area of strain and associated topographic effect should be expected to be larger. This feature of the model will be expanded in section 5.

4. Analysis 4.1. Goal and Rationale

This study evaluates the relationship between the ECSZ and topography of the Mojave Desert block. Establishment of this

relationship would be highly significant in helping to decide how much of the Mojave Desert block landscape is the product of tectonism, erosion, or inheritance. Our approach compares the hypsographic predictions of the model of Dokka et al. [1998] with the current surface. Using this approach, we can address the following questions:

1. How is the ECSZ expressed in the topography of the Mojave Desert block?

2. How well correlated spatially and thematically are strain domains and their predicted topographic features, i.e., lowlands and highlands, and unstrained areas? In other words, does the model correctly predict the general location, actual extent, and type of topographic effect?

The model predicts that tectonic motions within the ECSZ have resulted in three different subregions of topography: (I) lowland areas produced by transtension (clUstal thinning), (2) highland areas produced by transpression (clUstal thickening), and (3) unstrained areas are unchanged by the ECSZ. These areas have a

Table 1. Hypsometry of the Surface of the Mojave Desert Blocka

Area, Percent Mean Range of km2 ofMDB Elevation In SD In Elevations m

Transtensional 4,688 II 480 237 91-1105 Transpressional 2,845 7 1601 520 619-3432 Unstrained 33,813 82 809 281 92-2234

Mojave Block Desert 41,346 100 825 384 91-3432

"Mojave Desert block, MDB. Data are from U.S. Geological Survey digital elevation model. Values are organized to include the entire MOB and strain-related subregions predicted by the Eastern California Shear Zone model of Dokka el 01. [1998].


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topographic expression that is intermediate to subregions 1 and 2 (Figure 4).

4.2. Methods

To test the hypothesis that tectonism associated with the late Neogene Eastern California Shear Zone has been an important factor in the development of the landscape of Mojave Desert block (MDB), we compared the model predictions of Dokka et al. [1998] with a digital elevation model (DEM) of the area. A DEM is a digital representation of a topographic surface that consists of regularly spaced x, y, and z data. DEMs used in this study were obtained from the U.S. Geological Survey (USGS) and have an x, y sample spacing of 30 m and a vertical resolution of � 10m. DEM data were manipulated within a Geographical Information System (GIS) using ERDAS Imagine 8.4 software and its implementation of the ESRI, Inc. ArclInfo data model. The spatial distribution of elevation data values, i.e., hypsography, was evaluated within each of the transtensional, transpressional, and unstrained areas, as predicted by the ECSZ model relative to the entire Mojave Desert block.

Eighty, individual 7.5-min topographic DEMs were merged to create a single, composite data set of the Mojave Desert region (Plate I). This data set contains over 60 million hypsographic elements. The composite DEM was then reprojected into universal transverse Mercator (UTM) coordinates (datum, WGS84). Editing was required because the original data contained production errors such as "hot" pixels and edge matching drop out lines created during mosaicking. Hot pixels, or single pixels that were misrepresented, were corrected by using an averaging filter. This filter averaged the values of the pixels immediately surrounding

the hot pixel and assigned it a new value based upon the average. Edge match errors consisted of narrow (one to two pixels wide), north-south or east-west bands of contiguous zero value pixels. Zero values were replaced with data derived from the original maps of the area (i.e., USGS 7.50 quadrangles). All changes were verified using the appropriate USGS 7.50 topographic quadrangle map.

To separately evaluate the hypsography of the transtensional areas, transpressional areas, and unstrained areas, it was first necessary to extract the topography of each category according to the predictions of the ECSZ model of Dokka et at. [1998]. To accomplish this data extraction, we created, in effect, a digital "cookie cutter"; an illustration of the extraction process is shown in Figure 5. In this process, a mask was first constructed by drawing polygons around each subregion (transtensional, transpressional, and unstrained) as indicated by the model (Figure 5). Each mask was then digitally combined with the DEM of the region, resulting in a topographic image that contained only the data covered by the mask, i.e., the topography of the individual category (Figure 5).

4.3. Data and Results

Figure 6 is a composite frequency distribution of DEM values and shows the contribution of each strain subregion defined by the tectonic model. The mean and standard deviation for each strain subregion were also calculated and are presented in Table I. The frequency distribution of elevation values for the entire Mojave Desert block (MDB) is unimodal and has a mean elevation of 825 m, with a standard deviation (I sigma) of 383 m (Figure 6). The frequency distribution of the unstrained subregion resembles that of the Mojave Desert block as a whole,


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possessing a similar mean of 809 m and standard deviation of 281 m (956 feet). This is not surprising considering that •82% of the area of the MDB is predicted by the model to have not changed due to ECSZ tectonism (Table 1). In contrast, the transtensional and transpressional subregions occupy •11% and •7% of the MDB, respectively (Table 1). The elevation characteristics and frequency distributions of the transtensional and transpressional subregions are also markedly different. The average elevation of the transtensional subregion is 480 m with a standard deviation of 237 m and a range of elevations from 40 to 1107 m (Table 1). The mean elevation of the transpressional subregion is 1601 m, standard deviation is 502 m (see Figure 8b), and a range of elevations from 619 to 3432 m (Table 1).

Figure 7 illustrates the relative contributions to the topography made by each of the subregions predicted by the model and provides two important insights into topographic-tectonic relations in the MDB. First, it illustrates that the highest areas of the MDB are overwhelmingly coextensive with transpressional areas within the ECSZ or with transpressional areas that lie adjacent to the MDB. Second, transtensional areas occur only in areas with elevations less than the mean for the entire MDB. Only •1/3 of lowland areas are coextensive, however, with predicted areas of transtension. Explanations for these observations are offered in section 5.

5. Discussion

5.1. General Discussion

Analysis of the DEM of the Mojave Desert block (MDB) shows that the distribution of basic topographic elements (i.e.,

mountains and basins) is generally consistent with the landscape predictions of the tectonic model of Dokka et al. [1998]. The data show that areas that have experienced high degrees of lateral strain within the ECSZ in late Neogene time are generally coextensive with lowlands and highlands of the Mojave Desert block. Transtensional areas occur mainly in the lowlands, whereas transpressional areas are restricted to highland areas. Regions within the Mojave Desert block that have not undergone strain occupy intermediate elevations. The topographic data also provide several unexpected clues regarding the details of the tectonic development of the region. Our analysis also high- lights known shortcomings of the tectonic model but provides insights into its improvement. Finally, this paper provides a new model that explains the origin of the surface of the entire Mojave Desert block.

5.2. Underestimation of the Areal Effect of Transtension

Although the Dokka et al. [1998] model correctly predicts the general location of lowland areas of the MDB, it can only explain the spatial distribution of •1/3 of the lowland topography (Figure 7). We consider three possibilities to explain this apparent anomaly: (1) the model is unable to correctly account for the distribution of extensional strain within the

transtensional areas; (2) the lowlands are dominantly the product of other processes; and (3) much of the lowland topography is inherited and unrelated to the ECSZ. We contend that much of this anomaly is most likely the result of the first and second possibilities (because of the previously noted geo- metrical imprecision of the tectonic model) and erosion. The model predicts only the central location and total extensional

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created by transtension

Figure 10. Map showing extent of lowland areas of the Mojave Desert block (MDB). Predictions of the Dokka et al. [1998] model (shown as dark shading) are inadequate to explain the distribution of all lowland areas (all shading). We consider this to be the result of (1) the underestimation by the model of the nature of strain beneath transtensional basins (Figure 8) and (2) subsequent backwearing. We propose that tectonism created the initial transtensional basin underlying the depression and that subsequent erosional processes have expanded the areal extent through backwearing. This idea is supported by the observation that much of the lowland areas not originally explained correctly actually lie adjacent to the predicted sites (light shading). This also implies that all lowlands of the MDB have been created directly or indirectly by the Eastern California Shear Zone. See text for discussion.

lengthening of transtensional areas and not the distribution and style of strain. Instead of the infinite extension indicated by the model, we propose that lowland areas have extended in a much more distributed fashion. Figure 8 illustrates this important point and suggests that the actual region of effect and surface expression may be several times the amount implied by Dokka et al. [1998]. Figures 9b and 9e illustrate how the model predictions of surface effect across transtensional areas compare with actual topography in the MDB. This reinterpre- tation is supported by the observation that much of the lowland areas that are not explained correctly actually lie adjacent to the predicted sites (Figure 10). It is our view that tectonism created the initial transtensional basin underlying the depression and that subsequent erosional processes have expanded the areal extent through backwearing (Figures 9a, 9b, 9c, 9d, and 9e). Expanding the areal limits of the transtensional areas

along the extension direction by a factor of 2 or 3 would significantly reduce the amount of unexplained lowland. Detailed subsurface investigations of these areas to determine the actual structure of the transtensional basins are needed to test this idea.

5.3. Two Groups of Transtensional Basins

As shown in Figure 6, the distribution of elevations of predicted transtensional areas of the Eastern California Shear Zone is

bimodal. We propose that this bimodal character reflects two distinct groups of extended areas with different histories and that the relative elevation of each is a function of the amount of

extensional lengthening. A specific relationship between present- day elevation and transtension cannot be proposed, however, because we do not know (1) the initial configuration of the surface


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Figure 11. Location of "high" and "low" transtensional basins within the Eastern California Shear Zone in the Mojave Desert block. These basins correspond to the two groups of low elevations identified on the bimodal histogram of elevations for transtensional areas. Because the DEM is geospatial, we are also able to observe how elevations are distributed spatially. As shown here, low basins and high basins are spatially distinct, with the former being associated with early (before � 1 Ma) strike-slip faults and associated transtensional areas of the ECSZ, whereas the latter are associated with currently active areas, See text for discussion,

of the MDB before the ECSZ, (2) the subsurface architecture of transtension below basins, and (3) the degree of basin infilling by sediments.

Figure 11 illustrates the spatial distribution of each population of basin elevations; the population with the lowest elevations is referred to here as the "low basins," whereas the popUlation with the highest elevations is referred to as the "high basins." Low basins and high basins are spatially distinct, with the former being associated with early (before � 1 Ma) strike-slip faults and associated transtensional areas of the ECSZ, whereas the latter are associated with areas currently active (Figures 2b, 3a, 3b, 9a, and 9b). It is also observed that the low basins are also the sites where the greatest amounts of extension have occurred. Greater magnitudes of extension are associated with the basins along the now inactive faults of the eastern part of the ECSZ (Figure 10); lengthening across these transtensional basins is equal to the net slip along their associated strike-slip faults (Granite Mountain fault, 21.5 km, and the Bristol Mountains fault (16 km) [Dokka et al., 1998]. This is in contrast to the smaller transtensional basins of the south central Mojave Desert block that are associated with the currently active faults (Helendale, Lenwood, Camp Rock-Emerson, Calico, RodmanPisgah, and Ludlow). These faults have smaller net slips (2.0, 3.0, 1.6, 14.0, and 1.0 km, respectively [Dokka, 1983]) and thus smaller degrees of transtensional lengthening. Therefore it is concluded that the elevations of basins in the transtensional areas reflect that relative amount of strain accumulated compared to other basins of the ECSZ.

5.4. Topographic Expression of Transpressional Areas Within the ECSZ

As shown on Figure 6, areas of transpression in the ECSZ are spatially associated with the highest areas of the Mojave Desert block. Areas of transpression yield a bimodal distribution of elevations with each mode likely representing a spatially and temporally different tectonic regime (Figure 6). The lower-cle-

vation popUlation includes areas in the northeastern and north central parts of the Mojave Desert block, whereas the higherelevation population is restricted to the areas of the southern Mojave Desert block directly adjacent to the San Bernardino and San Gabriel Mountains (Plate 1). The two elevation populations can be attributed to two separate tectonic influences: (1) local transpression within the ECSZ (the low-elevation population) and (2) transpression in areas adjacent to the MDB caused by tectonic thickening processes external to the MDB (the high-elevation population).

The low-elevation population is generally coextensive with the areas of transpression predicted by the Dokka et al. [1998] model and includes the Calico Mountains-Mud Hills area of north central MDB, the Avawatz Mountains in the northeast comer of the MDB (Figure 4). Transpression in the Calico Mountains-Mud Hills is thought to be due to a local change in fault strike along the Calico-Blackwater fault from NW to WNW [Dokka and Travis, 1990a], The presence of large, approximately E-W folds in intimate association with active right-slip faults in the Mud Hills ("Barstow Syncline") and Calico Mountains [Dibblee and Bassett, 1966] supports the idea of localized transpression [Dokka, 1989]. In addition, synchronous uplift of rocks in these transpressional areas is indicated by dissection of folded older Quaternary sediment deposits [Dokka, 1989], The second area of transpression predicted by the model is the Avawatz Mountains where the evidence of late Neogene transpression is abundant [Troxel, 1970; Spencer, 1981, 1990; Brady, 1984; Brady and Verosub, 1984], Extensive mapping shows that this area, nestled at the intersection of the eastern end of the Garlock and southern Death Valley faults, is the site of late Neogene-Holocene WNW to E-W folding, NW to north directed thrust faulting, and related NW right-slip faults, as well as approximately E-W left-slip faults; the eastern part of the Garlock fault is not considered related to this deformation because it is apparently no longer active [Miller et al., 2001],

The high-elevation population is part of a sediment debris apron that was shed from areas outside of the MDB province


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Plate 2. Geology and topography of the south central Mojave Desert block (MDB) highlighting the Cajon Basin area. This is an example of an area of the MDB where local topography is the result of factors external to the Mojave Desert block. The Cajon Basin is a large accumulation the alluvial debris derived from the San Gabriel and San Bernardino Mountains. These ranges are areas of late Neogene uplift within the Transverse Ranges formed by transpression within the San Andreas fault system [Dibblee, 1967; Meisling and Weldon, 1989]. It is interesting to note that although the Cajon Basin is structurally separated from its source by the San Andreas fault, it still continues to receive sediments. See text for discussion.


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Plate 3. Composite map of seismicity and ancient low-relief landforms of the Mojave Desert. The boundary of the Mojave Desert block is shown as dashed line. The broad swath of seismicity through the central Mojave Desert defines the general limits of present-day tectonism. Areas outside the Eastern California Shear Zone, both east and west, contain areas of low-relief bedrock plain-pediment systems that have been shown locally to be of great antiquity (Apple Valley by Oberlander [1974], and Cima Dome and environs by Dohrenwend et al. [1987] and Miller [1995]). Seismicity obtained from the USGS covers the period from 1981 to mid-1999. Landform and geologic data are derived from unpublished mapping by Dokka et al. [2000a]. See text for discussion.

(Figure 9 and Plates 1 and 2). This area of the Mojave is referred to as the Cajon Basin [Dibblee, 1967] and lies across the San Andreas fault adjacent to the San Gabriel and San Bernardino Mountains. Uplift of these mountains is thought to be due to tectonic thickening facilitated by •2 Ma and younger thrusting on south dipping faults of rocks of the Transverse Ranges province over the Mojave Desert block [Meisling and Weldon, 1989].

5.5. Topographic Expression of Unstrained Areas Within the MDB

In contrast to the substantial vertical changes predicted for the strain domains of the ECSZ, the model indicates that the general elevation of remaining areas of the MDB should not have changed significantly. The only tectonically induced changes to the topography that we expect to observe are minor and centered along traces of faults of the ECSZ and along boundaries with other areas. The effects of the former include the prominent NW physiographic grain first observed by Hewett [1954a, 1954b].

Deposition of debris shed from surrounding regions is associated with the latter.

Approximately 82% of the MDB is predicted by the model to have escaped strain-induced vertical changes by the ECSZ, although the true amount is likely less due to the suspected broader extent of transtensional areas. The model indicates that the mean

elevation of the unstrained subregion should lie between the means of the transtensional and the transpressional areas. Table 1 shows that this is indeed the case. The areas affected include the entire

western Mojave flanking the ECSZ as well as local areas inside the ECSZ. We are left, however, with an essential question. If tectonism has played only a limited role in shaping this subregion, what then is its primary origin? Previous geomorphic work in the Mojave Desert area, as well as relations set forth in this paper, leads us to propose that the current surface is also the product of landscape inheritance and erosion.

The dominant arid conditions that have prevailed over much of the late Cenozoic previously led some to propose that portions of the surface of the Mojave Desert are inherited and of great antiquity. Studies by Oberlander [1974] in the western Mojave


Figure 12. Photograph of the Williams plain of the north central Mojave Desert block. The camera looks to the northwest. The landform in the foreground is a bedrock plain located north of Coyote Lake. The plain consists of deeply weathered Mesozoic granite and diorite. The background includes the Paradise Range, which consists of slightly metamorphosed Paleozoic, and Mesozoic sedimentary and volcanic rocks that have been intruded by Mesozoic granite and diorite.

near Apple Valley (Figure 9e) and by Dohrenwend et al. [1984, 1987] and Miller [1995] in the eastern Mojave Desert (Figure 9e) present a convincing body of geomorphic, geochemical, and geochronologic evidence that the surfaces in their respective areas were inherited from pre-late Miocene when the climate was semiarid and tectonic activity was low. Relations presented here suggest that these ancient surfaces may be much more widespread.

Observations of the topography and the geomorphology of areas of the MDB both outside and inside the ECSZ are

consistent with the notion of landscape inheritance and antiquity. For example, the DEM of the MDB (Plate 1 and Figure 2), as well as previous mapping of the landforms and surface materials [Dibblee, 1967; Dokka et al., 1998, 2000a, 2000b] (also see GIS and Web pages to the Mojave Desert Ecosystem Project: http:// www. geol. lsu. edu/mdei2000.html or http://www. mojavedata.gov/ mdep/geomorphic/glindex.html), shows that the western Mojave consists of a very low relief landscape dominated by a few expansive bedrock plain-pediment-alluvial plains systems (Plate 3). Away from its high-relief boundaries (San Andreas and Garlock faults; Plate 1 and Figure 2), the western Mojave has slopes that average •2.6 ø [Cooke, 1970]. The ancient Apple Valley pediment described by Oberlander [1974] is part of this landform system.

Aspects of the structure and topography of the ECSZ show striking similarities and contrasts with the western Mojave and the areas of the eastern Mojave Desert. For example, several broad areas between active faults are, in fact, plateaus with deeply weathered granite surfaces that are similar to the western Mojave; these include the Williams plain north of Barstow described by Dibblee [1967] (Figures 9d and Figure 12) and the high plain of Joshua Tree National Park. In contrast, the DEM shows that the surface of the south central MDB occupied by

the ECSZ is markedly rougher than areas to the west and north (Plate I and Figures 9c, 9d, and 13); summits across the area, however, are generally accordant (Figure 9). This area is the most tectonically active area of the ECSZ in the MDB [e.g., Dokka and Travis, 1990a; Peterson and Wesnousky, 1994; Rockwell et al., 1993; Rubin and Sieh, 1997; Kanamori et al., 1992] and consists of numerous, areally small mountains and inselbergs fringed by relatively small alluvial aprons (Figure 13 and Plate 4). Most aprons are highly incised and studded with the remnants of multiple generations of alluvial fans, perhaps reflecting ongoing relief creation associated with faulting (Figure 13) [Zebker et al., 1994]. Although the area consists of narrow (10-20 km) right-slip bounded fault blocks [Garfunkel, 1974; Dokka and Travis, 1990a], earthquake patterns and landform mapping in the area suggest that these blocks are not entirely intact but are instead segmented locally (Figure 13 and Plate 4). This segmentation or mountain range- scale comminution of the landscape by the ECSZ is also reflected in the current drainage pattern (Figure 13). Thus the actions of both tectonic and erosional processes have led to the disruption and dissection of the surface of the south central Mojave Desert.

The interplay between tectonism and erosional processes in the creation of the current surface within the ECSZ is also

evident at the individual mountain range scale. Examination of the DEM (Plate 1) shows that the margins of many mountains and inselbergs of this subregion have complex surface rough- ness patterns that consist of both low sinuosity (sublinear) and highly sinuous components. Mountain ranges of the western and eastern Mojave Desert lack sublinear margins and consist only of highly sinuous bedrock-debris apron contacts (Plate 1 and Figure 6). Within the ECSZ, low-sinuousity margins are generally coextensive with mapped faults, whereas fronts that

DOKKA AND MACALUSO: TOPOGRAPHIC EFFECTS OF ECSZ 1N MOJAVE DESERT 30641

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Tertiary Metamorphic Rock (Pre cambrian) Lower Pleistocene

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Plate 4. Geologic map of the area shown in Figure 13 showing faults, earth materials, and geomorphic landforms. Earthquake patterns (Figure 13) and landform mapping in the area suggest that narrow (10-20 km) right-slip bounded fault blocks are not entirely intact but are instead segmented locally. See text for discussion.

are more sinuous are associated with inactive faults or are The effect of this erosion is to expand the extent of lowlands unassociated with known structures (Figure 13 and Plate 4). beyond the region of transtension. The latter, more sinuous margins have embayments that are We propose that the plateaus and accordant summits within associated with drainage thalwegs and have patterns that are the ECSZ are all that remains of an old erosion surface(s) maintained at several scales of observation. They are thus inherited from pre-late Miocene time. We also speculate that fractal by definition [Mandelbrot, 1983]. In contrast, active this surface may been part of a single or series of Mojave-wide faulting tends to produce landform shapes that are not statisti- erosion surface(s) that included the areas of the western Mojave cally distributed, as are fractals, but are instead stochastic, Desert block documented by Oberlander [1974] and of the controlled by the processes that formed them and the intrinsic eastern Mojave Desert by Dohrenwend et al. [1987] and Miller properties of the affected rocks and/or soil. Thus faulting [1995]. Figures 9b and 10 suggest that the elevation of this results in the creation of simple, polygonal-shaped objects surface was originally near 1000 m. Subsequent erosion of this with sublinear boundaries. If relief is produced along the surface is considered to be the product of downwearing and boundary (fault scarp), debris eroded from the uplifted side backwearing due to (1) regional base level changes in adjacent will be deposited on the downthrown block. As long as the strain areas of the ECSZ, (2) local base level changes due to fault remains active, the contact between the rock and its local faulting, and (3) climate change. Finally, the lateral extent debris will remain sublinear, mimicking the simple geometry of the anomalous topography at Ord Mountain is coextensive of the fault. Inactivity, however, allows erosional processes to with the area of coseismic uplift detected at the northern end of dominate by backwearing and downwearing of the upthrown the rupture associated with the 1992 Landers earthquake [Mas- block. The contact will then migrate upslope over time, sonnet et al., 1993; Zebker et al., 1994]. The anomalous resulting ultimately in the burial of the mountain in its own topography suggests that the coseismic uplift may be part of debris [e.g., Bloom, 1995]. This effect is most pronounced an incremental buildup of permanent strain and mountain along the margins of the transtensional areas of the ECSZ. building within fault blocks. The topographic analysis presented

DOKKA AND MACALUSO: TOPOGRAPHIC EFFECTS OF ECSZ 1N MOJAVE DESERT 30643

here could be substantially improved by using a tectonic model that could account for permanent strain within fault-bounded blocks.

6. Conclusions

The following conclusions regarding topography and tectonics of the Mojave Desert block of southern California were reached during this study:

1. The Eastern California Shear Zone (ECSZ) is strongly expressed in the topography of the Mojave Desert block (MDB) and has been a major factor in the evolution of the landscape since its inception in late Miocene time. Major drainages and basins are developed squarely along depressions created by transtensional processes within the ECSZ. Tectonism, through the creation of relief, has focused erosion along the boundaries of strained areas of the ECSZ. Soda Lake, created by transtension, is the lowest point in the MDB and is thus the regional base level that governs erosion regionally by running water. Highland areas within the Mojave are highly correlated with areas of transpressional strain. These general tectonic-topographic relationships are consonant with predictions of the geometric-kinematic model of Dokka et al. [1998] and earlier versions [Dokka and Travis, 1990a; Dokka, 1993].

2. There is a strong relationship between topography and the strain domains associated with the ECSZ. Areas of high elevation in the Mojave Desert block are spatially associated with (1) areas that have or are currently undergoing transpression within the broad, distributed zone of right shear of the ECSZ and (2) areas that lie adjacent to uplifted transpressional belts that bound the Mojave Desert (San Bernardino Mountains). In contrast, transtension within the ECSZ is predicted to have produced the lowland areas. Although the model correctly predicts the general location and total strain of most lowland areas, it apparently underestimates the actual areal effect of transtension. This underestimation is

attributed to the tectonic model's lack of specificity regarding the style and distribution of local strain within transtensional areas. Future improvement of our knowledge of the subsurface structure beneath these basins will also provide a better understanding of the topography.

3. Examination of the topography in the transtensional areas reveals two distinct populations of elevation. Areas that have undergone the greatest local transtension associated with dextral shear within the ECSZ are also the lowest topographically. These areas are associated with the earliest phase of movements (bracketed between ,-•13 and 1 Ma) of the ECSZ. The second population is coextensive with transtensional basins that are currently active.

4. Although the ECSZ has had a major impact on the surface of the MDB, tectonism can only directly account for a portion of the topography. We propose that the remaining areas of the MDB owe their origin to Quaternary erosion and landscape inheritance. Modem erosion is focused in high-relief areas produced by the ECSZ. This is most apparent (1) at the interface between the unstrained and transtensional areas of the

ECSZ and (2) along active faults. Erosion of the former has resulted in the expansion of the lowland areas originally created by tectonism, whereas the latter has resulted in the development of the prominent NW physiographic grain of the south central MDB. Furthermore, we propose that the accordant summits and deeply weathered bedrock plateaus within the ECSZ are all that remains of a Mojave-wide erosion surface. This surface may be correlative with the pre-late Miocene surfaces documented in the western MDB and eastern Mojave Desert by Oberlander [1974] and by Dohrenwend et al. [1987] and Miller [1995], respectively. This surface was and continues to be significantly disrupted by faulting and strain associated within the Eastern California Shear Zone.

5. The method defined in this paper was successful in its demonstration of how topographic data can be used to test and refine tectonic models. It certainly could be applied in other similar terranes. The efficacy of the method could be greatly enhanced if more details were known regarding vertical and lateral strains.

Acknowledgments. Our efforts have been generously supported by grants and contracts from the National Science Foundation, the Jet Propulsion Laboratory, and the U.S. Army Corps of Engineers (Sacramento District). Careful formal reviews by J. Freymuller, D. Holm, and J. Oldow greatly improved the paper. Discussions and reviews of early versions of the paper were provided by J. Bosch, C. Breed, C. Christensen, B. Horton, J. MacCauley, J. Rinker, A. Taylor, J. Watts, and M. Woodbume. Finally, we express our thanks to Larry Mayer for his wise counsel, insights, and friendship during this project.

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Brady, R. H., III, and K. Verosub, Intersecting wrench faults, northeastern Mojave terrane, and symmetrical terminations of the Garlock fault, southern California, Geol. Soc. Am. Abstr. Programs, 16, 453, 1984.

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Dibblee, T. W., Jr., Areal geology of the western Mojave Desert, California, U.S. Geol. Surv. Prof Pap., 522, 153 pp., 1967.

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R. K. Dokka, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA. ([email protected])

K. Y. Macaluso, Minerals Management Service, 1201 Elmwood Park Blvd., New Orleans, LA 70123-2394, USA.

(Received October 17, 2000; revised June 17, 2001; accepted June 25,2001; published Month XX, 2001.)

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Topographic effects of the Eastern California Shear Zone ...web.gps.caltech.edu/~jstock/Ge136-2012/DokkaReS... · block of California were used to test the topographic implications