Abstract

The southern San Andreas fault system (California, USA) provides an excellent natural laboratory for studying the controls on vertical crustal motions related to strike-slip deformation. Here we present geologic, geomorphic, and gravity data that provide evidence for active northeastward tilting of the Santa Rosa Mountains and southern Coachella Valley about a horizontal axis oriented parallel to the San Jacinto and San Andreas faults. The Santa Rosa fault, a strand of the San Jacinto fault zone, is a large southwest-dipping normal fault on the west flank of the Santa Rosa Mountains that displays well-developed triangular facets, narrow footwall canyons, and steep hanging-wall alluvial fans. Geologic and geomorphic data reveal ongoing footwall uplift in the southern Santa Rosa Mountains, and gravity data suggest total vertical separation of ∼5.0–6.5 km from the range crest to the base of the Clark Valley basin. The northeast side of the Santa Rosa Mountains has a gentler topographic gradient, large alluvial fans, no major active faults, and tilted inactive late Pleistocene fan surfaces that are deeply incised by modern upper fan channels. Sediments beneath the Coachella Valley thicken gradually northeast to a depth of ∼4–5 km at an abrupt boundary at the San Andreas fault. These features all record crustal-scale tilting to the northeast that likely started when the San Jacinto fault zone initiated ca. 1.2 Ma. Tilting appears to be driven by oblique shortening and loading across a northeast-dipping southern San Andreas fault, consistent with the results of a recent boundary-element modeling study.

INTRODUCTION

The structural architecture and displacement histories of strike-slip fault zones provide insights into accumulation of three-dimensional (3-D) strain through time at transform plate boundaries. Rapid vertical motions of narrow (∼0.5–5 km wide) fault slivers commonly are driven by translation of crust through small-scale geometric complexities in strike-slip fault zones (Wilcox et al., 1973; Aksu et al., 2000; McClay and Bonora, 2001; Spotila et al., 2001, 2002; Mann, 2007; Gueguen et al., 2010; Cooke et al., 2013). Vertical motions also result from tilting of larger (∼10–100 km wide) strike-slip fault-bounded crustal blocks. Large-scale tilting toward strike-slip faults has been proposed to result from transform-normal extension due to rotation of principal stresses on weak strike-slip faults (Ben Avraham and Zoback, 1992). Horizontal axis tilting in transtensional strike-slip basins can occur as a result of plate motions reoriented by continental collision and escape tectonics (Mann, 1997), and translation of crust through large bends in a master strike-slip fault (Cormier et al., 2006; Sorichetta et al., 2010). In general, the causes of large-scale crustal tilting in strike-slip fault zones remain little-studied and poorly understood.

The San Andreas fault (SAF) system in southern California provides an excellent natural laboratory for the study of vertical crustal motions and tilt patterns associated with active strike-slip faulting. The geologic history is well studied, rocks and fault zones are well exposed, and the area contains a rich sedimentary archive that records changes in fault kinematics and vertical displacements through time. The Salton block (Meade and Hager, 2005; Fay and Humphreys, 2005, 2006) is a relatively intact crustal block bounded by active strands of the SAF on the northeast and the San Jacinto fault zone (SJFZ) on the southwest (Fig. 1). It occupies a structural transition from zones of active transpression along strike to the northwest in San Gorgonio Pass, to oblique dilation in the Salton Sea to the southeast (Fig. 1). Modern motions of fault-bounded crustal blocks as measured with global positioning system (GPS) are generally parallel to major strike-slip faults with a small angle of oblique convergence across the SAF in Coachella Valley (Bennett et al., 1996; Plattner et al., 2007; DeMets et al., 2010; Spinler et al., 2010). Geomorphic features and eroding Pleistocene sediments provide evidence for active uplift and erosion. The age of the SJFZ, and thus the Salton block, is known from multiple stratigraphic and structural analyses to be ca. 1.3–1.1 Ma (Morton and Matti, 1993; Matti and Morton, 1993; Lutz et al., 2006; Kirby et al., 2007; Steely et al., 2009; Janecke et al., 2010).

Recent debate has focused on the dip of the southern SAF in the Coachella Valley and Salton Trough. Historically, geophysical models have assumed a vertical dip on the fault (e.g., Meade and Hager, 2005; Smith-Konter and Sandwell, 2009; Loveless and Meade, 2011; Herbert and Cooke, 2012; Luo and Liu, 2012; Nicholson et al., 2013). Other studies suggest that the active Coachella segment of the SAF dips 60°–70° northeast based on seismicity patterns, aeromagnetic data, and modern strain documented with GPS and interferometric synthetic aperture radar (Lin et al., 2007; Fuis et al., 2012a; Lin, 2013; Lindsey and Fialko, 2013). The hypothesis for a steeply northeast-dipping SAF in this area is supported by a recent 3-D boundary-element modeling study that tested alternative fault geometries using comparison of model results to geologic evidence for vertical surface displacements around the Coachella Valley (Fattaruso et al., 2014). Debate over the dip on the southern SAF highlights the need for improved understanding of vertical motions in strike-slip fault zones over geologic time scales (∼1–2 m.y.), which offers a useful tool for testing alternative models and improving seismic hazard predictions.

This paper presents a study of crustal structure and late Quaternary to modern vertical deformation in the Santa Rosa Mountains and southern Coachella Valley, in the central part of the Salton block (Fig. 1). We integrate geologic, geomorphic, and geophysical data to interpret the geometry and timing of mountain range uplift and basin subsidence associated with strike-slip deformation. We find that since initiation of the SJFZ ca. 1.2 Ma, the central Salton block has undergone progressive northeastward tilting between the San Jacinto and southern San Andreas faults. We consider several mechanisms that could drive this pattern of crustal tilting, and find that it is best explained by a small angle of oblique convergence and crustal loading across a steeply northeast-dipping southern SAF.

GEOLOGIC AND TECTONIC SETTING

The southern SAF system (Fig. 1) is a network of dextral and oblique-slip faults that record a complex history of late Cenozoic transtensional and transpressional deformation (e.g., Crowell, 1974, 1981; Ehlig, 1981; Matti and Morton, 1993; Powell, 1993; Weldon et al., 1993; Darin and Dorsey, 2013). The SAF in Coachella Valley consists of a single main strand in the southeast that splits into the Banning and Mission Creek faults in the northwest (Fig. 1). The fault zone contains abundant geomorphic signs of recent and ongoing deformation, including dextrally deflected and beheaded streams, shutter ridges, sag ponds, fault scarps, and displaced Quaternary and Holocene alluvial fans (e.g., Keller et al., 1982; Sieh and Williams, 1990; Shifflett et al., 2002; Behr et al., 2010; Gray et al., 2014). Modeling of GPS data suggests that slip rates decrease northward from ∼23 mm/yr in the southernmost Coachella Valley to ∼14–18 mm/yr in the northern Coachella Valley (Spinler et al., 2010; Behr et al., 2010). Geologic structures and stratigraphy in the Mecca Hills record Pleistocene to modern transpression and fault-normal shortening in a narrow belt northeast of the SAF (Fig. 1; Sylvester and Smith, 1976, 1987; Rymer, 1991, 1994; Sheridan and Weldon, 1994; McNabb, 2013). The restraining bend in San Gorgonio Pass (Fig. 1) is a zone of active transpressional deformation, crustal thickening, and high topography (Matti et al., 1985, 1992; Harden and Matti, 1989; Spotila and Sieh, 2000; Yule and Sieh, 2003). The SAF zone on the north side of San Gorgonio Pass is a network of dextral-reverse faults with an overall north dip revealed by seismology (Yule and Sieh, 2003; Carena et al., 2004) and gravity and magnetic data (Langenheim et al., 2004, 2005), a geometry consistent with the results of 3-D numerical modeling (Dair and Cooke, 2009; Cooke and Dair, 2011; Herbert and Cooke, 2012).

The late Cenozoic tectonic history of the region is recorded in sedimentary rocks and structures exposed in zones of recent uplift around the margins of the modern valleys. Basin subsidence and deposition in the Fish Creek–Vallecito basin (western Salton Trough) began ca. 8 ± 0.5 Ma in response to regional extension and transtension associated with initiation of the southern SAF system (Dorsey et al., 2011). Late Miocene to Pleistocene crustal extension and thinning produced deep subsidence in the Salton basin and accumulation of thick nonmarine and marine deposits that are now exposed west of the Salton Sea (Winker, 1987; Winker and Kidwell, 1986, 1996; Dorsey et al., 2011). The Imperial Formation records widespread marine incursion ca. 6.3–6.5 Ma along a narrow fault-bounded marine seaway at the north end of the Gulf of California (Dibblee, 1954; Allen, 1957; Matti et al., 1992; McDougall et al., 1999; Dorsey et al., 2007). Latest Miocene to Pleistocene transtension was accommodated by combined strike-slip offset on the SAF and top-to-the-east offset on the low-angle West Salton detachment fault (WSDF), which accommodated ∼8–10 km of Pliocene to early Pleistocene east-west extension and related subsidence at the western margin of the Salton Trough (Axen and Fletcher, 1998; Shirvell et al., 2009; Dorsey et al., 2011). The WSDF is a regional structure bounding a large supradetachment basin that is well mapped where the basin has been inverted by young uplift, and is inferred to be present beneath deep modern basins in the western Salton Trough and Coachella Valley where inversion has not occurred (Axen and Fletcher, 1998; Janecke et al., 2010). Regional extension and lithospheric rupture across the plate boundary have continued to the present day, resulting in rapid subsidence and accumulation of Colorado River–derived sediment in basins as much as 10–12 km deep (Fuis et al., 1984; Elders and Sass, 1988; Herzig et al., 1988; Schmitt and Vazquez, 2006; Schmitt and Hulen, 2008; Dorsey, 2010).

The SJFZ cuts obliquely across the northern Peninsular Ranges in a network of strike-slip and oblique-slip fault strands that truncate, postdate, and offset the WSDF (Fig. 1). The main strands of the SJFZ west of the Salton Sea are the Coyote Creek, Clark, and Santa Rosa faults. The Santa Rosa fault was originally identified by Dibblee (1954) and confirmed by Janecke et al. (2010). Geologic studies show that the SJFZ and related faults initiated ca. 1.3–1.1 Ma in the western Salton Trough (Lutz et al., 2006; Kirby et al., 2007; Steely et al., 2009; Janecke et al., 2010), overlapping with an age of 1.3–1.5 Ma in the northern SJFZ (Morton and Matti, 1993; Matti and Morton, 1993) and 1.2 ± 0.1 Ma for the Elsinore fault at the western edge of the Salton Trough (Dorsey et al., 2012). A slightly older age of 1.8 ± 0.5 Ma was suggested for the SJFZ based on late Pleistocene slip rates and total offset (Blisniuk et al., 2010). The younger age (1.1–1.3 Ma) is well supported by multiple lines of geologic and stratigraphic data in diverse locations (e.g., Lutz et al., 2006; Kirby et al., 2007; Steely et al., 2009). The discrepancy between published initiation ages suggests that slip rates may have slowed since fault initiation (Bennett et al., 2004), or late Pleistocene estimates may have missed some off-fault strain that is pervasive in this area (Janecke et al., 2010).

METHODS

This study integrates geologic, geomorphic, and geophysical data to interpret crustal structure, depth of basins, and patterns of Pleistocene to modern deformation. Methods include compilation of previously published and unpublished geologic maps, mapping and observations of fault-scarp morphology, measurement of fault-zone fabrics, quantitative geomorphic analysis in ArcMap (http://www.esri.com/software/arcgis) using 30 m digital elevation models (DEMs), and measurement of alluvial fan and corresponding catchment areas on DEMs and satellite imagery. Gravity data (Biehler et al., 1992; Langenheim et al., 2005, 2007; Martin et al., 1997; Langenheim, 2008, personal data) were used to examine basin geometry using two different methods. Depth of subsurface basins throughout the region of Figure 1 was modeled through 3-D inversion of gravity data using the method of Jachens and Moring (1990) and parameters in Langenheim et al. (2005; see following). Forward modeling (2-D) was used to examine basin and fault geometry along a profile across the central Salton block (red line in Fig. 2), assuming a density contrast of –400 kg/m3 between the basement and basin fill. The geology at the surface places a critical constraint on both modeling methods. The gravity model does not inform on fault geometry below the basin-basement interface.

The gravity inversion and 2-D forward model take advantage of the significant density contrast between dense crystalline basement rocks and Cenozoic lower density sedimentary rocks and deposits. The method of Jachens and Moring (1990), modified to include drill hole data and other geophysical data as constraints, separates the isostatic gravity field into the component produced by variations in basement density and that caused by thick basin deposits, which is then inverted for thickness. The basement gravity field is allowed to vary laterally and is estimated by passing a surface through those values measured on basement outcrops. This field is subtracted from the isostatic gravity field to produce an initial estimate of the basin gravity. Because the basement gravity values near the basin edge are affected by the gravity effect of the low-density basin fill and thus underestimated, a forward calculation of the basin gravity field is used to correct the basement gravity measurements. This process is repeated until further steps do not result in significant changes to the modeled thickness of the basin deposits, usually in five or six steps. The basin gravity field is converted to basin thickness using an assumed density contrast that varied with depth between the basin and underlying basement (Table 1). The density-depth relationship produces basin depths and shapes that are generally consistent with those obtained by recent seismic profiling in Coachella Valley (Langenheim et al., 2012, 2013).

RESULTS

Regional Gravity and Structure

The geologic map (Fig. 1) and isostatic gravity map (Fig. 2) show the distribution of fault-bounded sediment-filled basins (gravity lows) and mountain range uplifts (gravity highs) in the southern SAF system. Total depth of the southern Coachella Valley increases gradually from the southwest, where sediments onlap crystalline and sedimentary rocks of the Santa Rosa Mountains, to the northeast where the basin is abruptly truncated at the SAF (Figs. 1 and 2). Along strike to the southeast, high gravity values at the south end of the Salton Sea indicate the presence of mafic intrusions ∼12–20 km beneath a deep sediment-filled basin in the Salton Trough (Fig. 2; Fuis et al., 1984; Schmitt and Vazquez, 2006; Han et al., 2013). The gravity high at the south end of the Salton Sea is enhanced by increased density of sedimentary fill (Kasameyer and Hearst, 1988) that results from hydrothermal alteration, metamorphism, and emplacement of basaltic sills, dikes, and flows into the thick sedimentary fill. In this area, basin depth contours are not shown because of the absence of a gravity low associated with the basin.

Deep fault-bounded basins are also present within the SJFZ beneath Borrego and Clark Valleys (Figs. 1 and 2). The 3-D inversion of gravity data indicates that the subsurface Clark basin is as much as 4 km deep ∼5 km southeast of Clark Lake (Fig. 3). Along a southwest-northeast cross section across the Santa Rosa Mountains and southern Coachella Valley (Figs. 1 and 2), the 3-D inversion estimates a basin thickness of ∼3 km, whereas our 2-D forward model using a constant density contrast of –400 kg/m3 indicates a depth of 4 km. The northeast margin of the basin is marked by a steep gravity gradient. The location of the Clark fault relative to the gravity gradient indicates a nearly vertical dip in Clark Valley, east and northeast of Clark Lake (Fig. 3). The Clark fault curves into a more east trend southeast of the Santa Rosa Mountains where it splays into multiple strands that define an overall restraining bend in a zone of complex active transpressional deformation (Fig. 1; Kirby et al., 2007; Belgarde, 2007; Janecke et al., 2010; Thornock, 2013).

Geology of the Santa Rosa Mountains and Clark Valley

The geologic map in Figure 3 shows major faults, sedimentary rocks, and basement units of the Santa Rosa Mountains and adjacent area. The SJFZ in this area includes the Coyote Creek, Clark, and Santa Rosa faults. The Clark fault in Clark Valley consists of two main strands: (1) a southwestern strand with a prominent surface trace that cuts the Ocotillo Formation and marks the eastern margin of Clark dry lake; and (2) a northeastern strand that is mostly covered by Quaternary alluvium but marks the eastern faulted margin of the basin at the base of steep alluvial fans (Fig. 3). A distinctive belt of Cretaceous mylonite has dextral separation of ∼15–18 km across the Clark fault (Sharp, 1967; Bartholomew, 1970; Todd et al., 1988; Janecke et al., 2010). The Santa Rosa fault is a large active normal fault that bounds the steep southwestern margin of the Santa Rosa Mountains (Fig. 3; Dibblee, 1954, 1984; Janecke et al., 2010). The fault appears to be interrupted by a large Quaternary landslide in the footwall (possibly an eastward reentrant of the main fault) that coincides with a ∼40° change in the trend of the range crest. The trace of the Santa Rosa fault, however, cuts the landslide, as revealed by smaller discontinuous northwest-striking fault scarps that record young offset of the inferred landslide mass. Other young faults associated with the SJFZ in this area include the northwest-striking Buck Ridge and Coyote Creek faults, unnamed normal faults on the east and west sides of Coyote Mountain, and the east-northeast–striking Travertine fault (Fig. 3).

In the Santa Rosa Mountains, the WSDF (Zosel fault of Matti et al., 2006, 2007) juxtaposes Pliocene–Pleistocene sedimentary rocks in the hanging wall against Cretaceous plutonic rocks and mylonite in the footwall (Fig. 3). This fault is reported to dip ∼20° east near the range crest, steepening to >65° at lower elevations along the eastern range front (Matti et al., 2002, 2006, 2007), although our estimates based on 3-point constructions yield dips of 13°–24° (most are <20°) for the WSDF in the southern Santa Rosa Mountains. At the south end of the range (Fig. 3), a dip estimate of 17° using this method is consistent with a dip of 15° that was measured directly on the fault plane, lending support for generally low dips on the WSDF in the southern Santa Rosa Mountains. The WSDF is truncated and offset dextrally and vertically by strands of the postdetachment SJFZ, including the Clark and Santa Rosa faults (Figs. 1 and 3). These relationships show that the buried trace of the WSDF on the west side of Clark Valley was originally a southern continuation of the WSDF in the Santa Rosa Mountains, prior to dextral offset on the Clark fault.

Sediments and sedimentary rocks in the study area consist of (1) the late Miocene to early Pliocene Imperial group (nomenclature of Winker and Kidwell, 1996); (2) Pliocene-Pleistocene Palm Spring group including the Canebrake Conglomerate and Olla, Diablo, and Ocotillo formations; and (3) late Pleistocene to modern alluvial-fan and stream deposits (Fig. 3). Marine deposits and conglomerate of the Imperial group onlap onto erosional paleotopography in Cretaceous plutonic rocks on the north flank of Travertine Ridge in the southeastern Santa Rosa Mountains (Cox et al., 2002; King et al., 2002; Powell, 2008). Northwest of there, the Imperial group is overlain by a gently west-dipping, coarsening-upward sequence from fine-grained marine claystone through interbedded fossiliferous sandstone and mudstone to upward-coarsening Canebrake Conglomerate exposed at elevations to ∼1400 m in the high Santa Rosa Mountains (Cox et al., 2002; King et al., 2002). King et al. (2002) reported marine deposits exposed at elevations as much as 625 m above sea level. Our map compilation indicates a slightly lower maximum elevation of ∼550–600 m for marine deposits in the southeastern Santa Rosa Mountains (Fig. 3). At the south end of the range, coarse boulder Canebrake Conglomerate passes laterally to the southeast into pebbly sandstone and interbedded sandstone and mudstone of the age-equivalent Olla formation (Fig. 3).

Geomorphology

The topography of the Santa Rosa Mountains is asymmetrical and segmented into linear-trending southeast and northwest sectors (Fig. 4). In the southeast, the range crest trends N25°W and the precipitous western slope drops from 1600 m at the crest to 500 m elevation at the heads of steep alluvial fans over a distance of ∼2.5 km, at an average slope of ∼24°. In contrast, a similar elevation drop on the east side of the range spans a distance of ∼6.3 km at an average slope of ∼10° (Fig. 4A). Pronounced topographic asymmetry is also seen in the slope map (Fig. 4B), which reveals overall steeper slopes on the west side of the range and gentler slopes on the east, and topographic profiles in Figure 5. Local ridges and embayments on the northeast side of the range are produced by uplift, erosion, and cannibalization of older rocks and alluvial fan deposits. The northwest sector of the Santa Rosa Mountains trends ∼N60°W and the range crest reaches elevations of >2000 m (Fig. 4). The high mountains are flanked on the southwest by the northern continuation of the Santa Rosa fault, defined by a sharp break in topography, and a high perched valley separated from Clark Lake by an east-trending ridge of Cretaceous plutonic rock (Figs. 3 and 4). Topographic asymmetry seen in the southeast is not as pronounced in the northwest sector due to the presence of the perched valley.

The southern Santa Rosa Mountains are flanked by a steep fault-bounded range front on the southwest and a lower gradient, unfaulted northeast side. The Santa Rosa fault displays abundant geomorphic features typical of active range-bounding normal faults, including steep landslide-dominated catchments in the footwall that deliver coarse sediment to small steep alluvial fans in the hanging wall (Figs. 3 and 6A). Well-developed triangular fault facets range in height from ∼250 to 600 m, suggestive of active slip in the range of 0.5–2.0 mm/yr (dePolo and Anderson, 2000). Steep boulder-rich alluvial fans are faulted against pre-Tertiary crystalline rocks along a steeply southwest-dipping fault zone characterized by brittle gouge and microfaults exposed along the range front. The Santa Rosa fault exposed at the mouth of one steep canyon (inset, Fig. 6A) has a measured strike of 335° and dip of 67°SW.

The northeastern Santa Rosa Mountains display lower gradient slopes with larger, lower gradient alluvial fans compared to the southwest side (Figs. 4 and 6B). Sedimentary units in this area are divided into (1) syndetachment deposits of the Pliocene to early Pleistocene Imperial and Palm Spring groups exposed in the hanging wall of the inactive WSDF; and (2) postdetachment late Quaternary to Holocene alluvial fan deposits (Fig. 3). Marine deposits of the Imperial Formation are exposed at elevations up to 625 m above sea level (King et al., 2002), and overlying alluvial fan deposits of the Canebrake Conglomerate are exposed at elevations to 1420 m.

Postdetachment alluvial fan deposits are erosionally inset into older syndetachment deposits and display systematic age-dependent variations in morphology. Late Quaternary fans have inactive, desert-varnish surfaces and are ∼0.8°–1.5° steeper than modern alluvial fans and upper fan channels (Fig. 7). Elevations of late Quaternary and modern fan surfaces diverge in the upslope direction, such that modern upper fan channels are incised as much as 70–80 m into the older, more steeply inclined late Quaternary fan surfaces. These relationships provide clear evidence for incision of older alluvial fans due to progressive northeastward tilting around a horizontal axis or fulcrum (Figs. 7 and 8). These geomorphic features are commonly observed on hanging-wall dip slopes associated with other active normal faults, and are similarly interpreted to record progressive crustal tilting (e.g., Hooke, 1972; Gawthorpe and Leeder, 2000).

Fan Catchment Analysis

Fan catchment analysis is motivated by previous studies showing that the rate of basin subsidence exerts a primary control on the ratio of alluvial fan area (Af) to catchment area (Ac) (Whipple and Trayler, 1996; Allen and Hovius, 1998; Allen and Densmore, 2000; Dade and Verdeyen, 2007). Fast subsidence causes a greater percent of sediment to accumulate in the vertical dimension, reducing fan area for a given catchment area and generating a smaller ratio Af/Ac. Similarly, if other variables such as rock lithology and climate are held constant, slower subsidence generates a larger fan area for a given catchment size, and larger ratio Af/Ac.

Figure 8 is a shaded 30 m DEM showing modern alluvial fans and corresponding catchments in the southern Santa Rosa Mountains that were analyzed for this study. Alluvial fans with areas <∼0.4 km2 are omitted because uncertainties become large relative to measured areas. Fan areas on the southwest side of the range vary between ∼0.5 and 8 km2, and the mean Af/Ac is 0.64 ± 0.08 (1 standard error) (Table 2; Fig. 9). In contrast, modern fans on the northeast side of the range have areas of ∼3–29 km2 with an average ratio Af/Ac of 1.19 ± 0.14 (Table 2; Fig. 9). The lower Af/Ac ratios on the southwest side of the range reflect the influence of active fault offset, basin subsidence, and trapping of coarse deposits close to the Santa Rosa normal fault. In contrast, tilting about a horizontal axis (fulcrum) on the unfaulted northeast side produces relatively slow subsidence, resulting in larger alluvial fans and larger average Af/Ac (Fig. 9). These results are consistent with other evidence presented here (Figs. 6 and 7) that the southern Santa Rosa Mountains occupy a relatively intact, actively tilting crustal block bounded on its southwest by an active fault zone with significant vertical displacement.

Structural Cross Section

The cross section in Figure 10 shows interpreted crustal structure along a transect from the SJFZ in the southwest to the SAF in the northeast, constrained by surface geology, gravity data, and field observations summarized here. Splaying active strands of the SJFZ (Clark and Santa Rosa faults) define a transtensional negative flower structure that bounds a sediment-filled basin 3–4 km deep in Clark Valley. This style of oblique offset is common in transtensional strike-slip fault zones, where basins subside in response to active fault-controlled oblique dilation (e.g., Aksu et al., 2000; Mann, 2007). We infer that the sediment-basement contact beneath Clark Valley is a truncated segment of the inactive WSDF based on the mapped trace of the WSDF along the western edge of Clark Valley (Figs. 1 and 3; Axen and Fletcher, 1998; Janecke et al., 2010). The amount of extension on the WSDF is not known from the Santa Rosa Mountains, but is estimated to be ∼8–10 km in the Pinyon Ridge area southwest of Clark Valley (Shirvell et al., 2009). We infer similar extension in this area. A step in the gravity profile suggests the presence of a steep normal fault and large basement rider block buried beneath sediment in the upper plate of the WSDF. The lack of surface expression of this fault suggests that it is not currently active or has a very slow slip rate.

In addition to dextral offset of ∼17 km on the Clark fault (Janecke et al., 2010), map relations in the southern Santa Rosa Mountains and the large gravity low in Clark Valley suggest significant vertical displacement on splaying fault strands in Clark Valley (Figs. 3 and 10). The WSDF in the Santa Rosa Mountains projects to a height of ∼2–2.5 km above Clark Lake. Adding this to the depth of the detachment fault inferred beneath Clark Valley (3–4 km) indicates ∼5.0–6.5 km of combined vertical separation across the Santa Rosa and Clark faults (Fig. 10). The true vertical offset could be, and likely is, considerably less than this if structural markers that started at different elevations have been juxtaposed by strike-slip motion. A conservative estimate of vertical offset is obtained by adding the height of the highest triangular facets on the Santa Rosa fault (∼0.6 km), the drop in elevation down steep alluvial fans from the fault trace to the valley floor (∼0.3 km), and the thickness of postdetachment sediments in the subsurface which is not known. Because Clark Valley has been an actively subsiding transtensional depocenter for the past ∼1.2 m.y. (Lutz et al., 2006), we infer that postdetachment sediments are at least 0.5–1.0 km thick in Clark Valley, suggesting combined vertical displacement of at least 1.5–2.0 km across the Santa Rosa and Clark faults.

Subsurface sediment in the southern Coachella Valley thickens gradually to the northeast from a tapered edge on the southwest side of Coachella Valley to a depth of ∼5 km, where it is abruptly truncated at the SAF (Fig. 10). Although the depth to the contact between syndetachment and postdetachment sediments is not known, we infer that postdetachment deposits thicken to the northeast across the southern Coachella Valley based on the presence of northeastward-fanning dips in the upper 2–3 km of sediment beneath the northern Salton Sea (Fig. 1, Line 7; Kell et al., 2012; Fuis et al., 2012b, 2012c; see following discussion). A steep northeast dip on the SAF is suggested by seismic, magnetic, geodetic, and modeling studies (Lin et al., 2007; Fuis et al., 2012a; Langenheim et al., 2012; Lindsey and Fialko, 2013; Fattaruso et al., 2014) and is used in our cross section (Fig. 10).

DISCUSSION

Summary of Observations

This study documents new geomorphic, geologic, and geophysical evidence for active northeastward tilting of the Santa Rosa Mountains and southern Coachella Valley between the San Jacinto and San Andreas faults (Fig. 1). Data that support crustal-scale northeast tilting include: (1) asymmetric crustal structure with large vertical offsets on the San Jacinto fault and SAF zones (Fig. 10); (2) contrasting morphology across the Santa Rosa Mountains, with a steep active faulted range front on the southwest and large low-gradient, unfaulted alluvial fans on the northeast (Figs. 4–6 and 8); (3) geologic evidence for substantial postdetachment uplift in the footwall of the Santa Rosa fault (Fig. 3); (4) the presence of tilted and incised late Pleistocene alluvial fans in the northeast Santa Rosa Mountains (Fig. 7); (5) lower ratios of fan area to catchment area (Af/Ac) on the southwest side of the range and higher ratios on the northeast (Fig. 9); and (6) northeastward thickening of sediment beneath the Coachella Valley to an abrupt margin at the SAF (Figs. 1, 2, and 10). The proposed tilt direction is consistent with a receiver function study showing that the base of the crust is deeper southwest of the San Jacinto fault and shallower northeast of the San Jacinto fault (Miller et al., 2014).

The depth to the contact between syndetachment and postdetachment sediments beneath the southern Coachella Valley is not known due to a lack of seismic reflection data and deep wells in the valley. However, late Quaternary to Holocene postdetachment fan deposits overlie syndetachment deposits along a northeast-dipping unconformity at the irregular eastern edge of the eastern Santa Rosa Mountains (Fig. 3). This contact projects northeast into the subsurface of the southern Coachella Valley, and is present at unknown depth where the total sediment thickness is 4–5 km close to the SAF (Fig. 10). If the accumulation rate is similar to that documented nearby in rapidly subsiding depocenters of the Salton Trough (1–2 mm/yr; Herzig et al., 1988; Schmitt and Hulen, 2008; Dorsey et al., 2011; McNabb, 2013), we would predict a depth of ∼1.2–2.4 km to the contact between predetachment and postdetachment deposits in the subsurface. Combining the asymmetric morphology of the Santa Rosa Mountains, wedge-shaped basin geometry beneath the southern Coachella Valley, and possible range of depth to the base of postdetachment deposits (Fig. 10), we estimate that the central Salton block has tilted ∼5°–10° to the northeast since ca. 1.2 Ma (age of the SJFZ).

Although our estimate of northeast tilting is hindered by limited subsurface data, a similar geometry and amount of tilting was documented in a recent seismic-reflection study of the northern Salton Sea (L-7 in Fig. 1), where sediments in the upper 3–4 km display northeastward-fanning dips toward the SAF at dip values to ∼8° (Kell et al., 2012; Fuis et al., 2012b, 2012c). In the southern Salton Sea, the presence of the 770 ka Bishop ash interbedded with lake deposits at a depth of 1.7 km and 430 ka extrusive rhyolites buried to depths of 2 km nearby record subsidence and accumulation at a rate of ∼2–4 mm/yr (Herzig et al., 1988; Schmitt and Hulen, 2008). At these rates, fanning dips in the upper 3–4 km of sediment in line 7 (Kell et al., 2012; Fuis et al., 2012b, 2012c) record tilting over the past ∼1–2 m.y. Systematic northeast dips of 5°–10° in subsurface sediments are also imaged in a nearby seismic reflection study of the San Felipe Badlands southwest of the Salton Sea (S in Fig. 1; Severson, 1987). Thus, while it is difficult to quantify the amount of postdetachment tilt, our conclusion of ∼5°–10° in the past ∼1.2 m.y. is supported by young sediments beneath the northern Salton Sea that display a style, scale, geometry, and timing of northeast tilting that are strikingly similar to the tilt geometry and timing estimated in this study for the Santa Rosa Mountains and southern Coachella Valley.

Spatial Extent of Crustal Tilting

The lateral extent of the crustal block involved in northeastward tilting can be inferred from regional geology, geomorphology, and gravity. Gravity data reveal an along-strike change from a northeast-dipping basin floor in the southeast Coachella Valley to a shallower irregular basin geometry in the northwestern Coachella Valley (Fig. 1). This change coincides with a prominent northeast-trending ridge that protrudes northeast into the Coachella Valley across from the junction of the Banning and Mission Creek faults (Ridge in Fig. 2). These features suggest the presence of a structural boundary between a more intact, northeast-tilted crustal block in the southern Coachella Valley and a segmented crustal structure in the northern Coachella Valley (Fig. 1). North of this boundary, the San Jacinto Mountains lack the pronounced asymmetric morphology seen in the Santa Rosa Mountains, suggesting the possible absence of northeast tilting in the San Jacinto Mountains and northern Coachella Valley. The implied structural boundary may be controlled by a poorly understood tear fault at the northwestern boundary of a relatively intact northeast-dipping tilt block.

Structural disruption and segmentation are also evident southeast of the Santa Rosa Mountains in a zone of intense active transpressional deformation related to active dextral faulting and rotation in the SJFZ (Brothers et al., 2009; Janecke et al., 2010; Thornock, 2013). Extreme structural complexity in this area is related to interaction of the Clark and Extra fault zones, and may reflect ongoing breakup of a corner of the Salton block due to the local effects of clockwise crustal rotation. Despite these complexities, seismic reflection studies provide evidence for systematic northeastward tilting in the subsurface of the central to northern Salton Sea and adjacent parts of the western Salton Trough (Fig. 1; Severson, 1987; Kell et al., 2012; Fuis et al., 2012b, 2012c). The distribution of faults suggests that the zone of northeastward crustal tilting may continue southeast to the Extra fault zone in the Salton Sea (Brothers et al., 2009); if so, the northeast-tilting fault block would be ∼35 km wide and 50–60 km long, with significant structural complexity and disruption in the southeast.

Controls on Crustal Tilting

We consider several processes that could drive northeastward tilting of an ∼35-km-wide crustal block between the San Jacinto and San Andreas faults about a horizontal axis during the past ∼1.2 m.y.: (1) oblique convergence and loading across a northeast-dipping SAF in Coachella Valley; (2) dextral translation of crust through a releasing bend in the Santa Rosa fault; (3) vertical loading by mafic intrusions beneath the southern Salton Sea; and (4) short-wavelength upper mantle convection related to lithospheric drips or delamination.

1. Although the southern SAF in Coachella Valley is widely considered to be vertical (e.g., Meade and Hager, 2005; Loveless and Meade, 2011; Herbert and Cooke, 2012; Luo and Liu, 2012; Nicholson et al., 2013), recent seismic, geophysical, and modeling studies suggest that it dips steeply northeast (Lin et al., 2007; Fuis et al., 2012a; Lindsey and Fialko, 2013; Fattaruso et al., 2014). A small component of convergence across a northeast-dipping SAF can produce a vertical load that may drive northeast tilting of the Salton block (Fig. 11). GPS data appear to support this hypothesis. The SAF in the Coachella Valley strikes N46°W (Fig. 1). An average of GPS-based Pacific–North America plate motions calculated from 8 studies in California is 49 mm/yr toward N38°W (DeMets et al., 2010). GPS sites on the Salton block show that it is moving along an azimuth of N37°W and N43°W relative to stable North America, with an average direction of N40°W at rates varying from ∼10 to 23 mm/yr (Spinler et al., 2010). The average motion of the Salton block relative to North America thus is oriented ∼6°–8° clockwise from the strike of the SAF in Coachella Valley, suggesting a small component of convergence across the Coachella Valley strand of the SAF (Fig. 11). This inference is supported by geologic structures in the Mecca Hills, northeast of the SAF in the southern Coachella Valley, that record active transpression and shortening adjacent to the SAF in this area (Fig. 1; Sylvester and Smith, 1976, 1987; Rymer, 1991, 1994; Sheridan and Weldon, 1994; McNabb, 2013).

The 3-D boundary element models show that a small angle of convergence across a northeast-dipping SAF produces significant northeast tilting that includes subsidence in the Coachella Valley and uplift in the Santa Rosa Mountains (Fattaruso et al., 2014). The pattern of progressive tilting predicted by the model agrees closely with the pattern of northeast tilting documented in this study (Fig. 10); models that assume a vertical southern SAF do not replicate the observed tilt pattern (Fattaruso et al., 2014). According to this hypothesis, uplift of the Santa Rosa Mountains results from (1) vertical loading at the northeast side of the Salton block as a result of oblique convergence across the SAF; (2) conservation of mass: as the northeast side is displaced downward, the rest of the block must also be displaced; (3) force imbalance due to the presence of a downward load in the northeast and absence of a similar load in the southwest; (4) uniform crustal buoyancy, and (5) subvertical SJFZ, which provides a break in the crust to accommodate vertical displacement (Fig. 11). The crust responds by rigid tilting across the full width of the Salton block because the effective elastic thickness in this area is ∼15 km (Fay and Humphreys, 2005). This corresponds to a flexural wavelength of at least 60–80 km, much greater than the across-strike width of the Salton block (30–35 km).

As the Salton block tilts, the faults bounding it may also tilt through time in a manner analogous to progressive rotation of normal faults in extended terranes (e.g., Jackson and McKenzie, 1983; Sharp et al., 2000). This would imply that the SAF originally had a shallower northeast dip, and the SJFZ may have started with a steep northeast dip. Alternatively, block tilting may occur without significant reorientation of bounding faults if the fault zones can crush and redistribute a sufficient volume of rock through time. Rotation of the faults, if active, likely is a minor effect since the total amount of tilting is <10°.

2. Crustal tilting in strike-slip fault zones may occur as a response to torques applied by translation of crust through large bends in a master strike-slip fault (e.g., Mann, 1997; Aksu et al., 2000; Cormier et al., 2006; Sorichetta et al., 2010). The discontinuity between the southern and northern Coachella Valley and lack of evidence for tilting in the San Jacinto Mountains suggest that northeast tilting in the Santa Rosa Mountains and southern Coachella Valley is not directly related to translation through the large restraining bend in San Gorgonio Pass (Fig. 1). However, if the northwest-trending northwest segment of the Santa Rosa fault has significant dextral slip, as suggested by the preponderance of northwest-striking dextral faults in the SJFZ (Fig. 1), a releasing bend would be inferred where the fault strike changes from northwest in the north to north-northwest in the south (Fig. 3). Translation through this releasing bend could drive footwall uplift in the southern Santa Rosa Mountains and contribute to partitioning of transtensional strain on neighboring strike-slip and normal faults. While this mechanism is plausible, we consider it unlikely because there is no known evidence for major dextral offset on the northwest segment of the Santa Rosa fault, and the scale and angle of the implied releasing bend are small relative to the Salton block.

3. Dense mafic intrusions are another possible driver of subsidence. Buried basaltic intrusive bodies are indicated by the presence of a large gravity high in the southern Salton Sea (Fig. 2) in the middle of the deep sediment-filled Salton Trough basin (e.g., Fuis et al., 1984; Herzig et al., 1988; Han et al., 2013). However, the center of the gravity high is located slightly east of the Brawley seismic zone (Fig. 2), indicating that the main intrusion is not today entirely in the Salton block. Brothers et al. (2009) suggested that the releasing stepover geometry of the modern Brawley seismic zone may have initiated as recently as ca. 0.5 Ma, and prior to that time the main SAF continued to the southeast along the northeast margin of the Salton Trough. If correct, this would imply that the mafic intrusions were within the Salton block until the inferred reorganization ca. 0.5 Ma. Nonetheless, high heat flow should weaken the crust around intrusions and limit the effects of loading to a small area. Thus we conclude that vertical loading by mafic intrusions beneath the southern Salton Sea probably does not play a major role in northeastward crustal tilting documented in this study.

4. Vertical foundering of a dense eclogitic root into the upper mantle has been inferred as the driving force for uplift and tilting in the Sierra Nevada Mountains (Zandt et al., 2004; Boyd et al., 2004; Saleeby and Foster, 2004; Maheo et al., 2009) and Wallowa Mountains in Oregon (Hales et al., 2005). Numerical models show that vertical drips of dense lithospheric bodies can produce Rayleigh-Taylor instabilities that generate vertical motions of the crust and Earth’s surface at a wide range of spatial scales, including the length scale of tilting documented here (Le Pourhiet et al., 2006; Gogus and Pysklywec, 2008; Harig et al., 2008). Receiver function studies show that high topography of the eastern Peninsular Ranges lacks an Airy crustal root and is supported by upper mantle buoyancy (Lewis et al., 2000, 2001; Persaud et al., 2007). The geometry, distribution, and post-Pliocene timing of uplift in the Peninsular Ranges suggest that removal of mantle lithosphere is related to the modern phase of transform tectonics (Mueller et al., 2009). Numerical modeling based on surface-wave tomography shows that small-scale upper mantle convection plays an important role in driving transpressional crustal deformation in the Transverse Ranges (Fay et al., 2008). If loading from below were applied unevenly, it could produce tilting geometries difficult to predict in detail. Thus we consider short-wavelength upper mantle convection to be a plausible but untested control on tilting of the Salton block.

CONCLUSIONS

Geologic, geomorphic, and geophysical data indicate that the central Salton block is undergoing systematic northeastward tilting between the San Jacinto and San Andreas faults. This conclusion is supported by (1) geologic and gravity evidence for large vertical offsets on the San Jacinto and SAF zones; (2) a steep fault-controlled range front at the southwest margin of the southern Santa Rosa Mountains and large low-gradient alluvial fans on the northeast side; (3) lower ratios of fan area to catchment area (Af/Ac) in the southwest Santa Rosa Mountains compared to higher ratios on the northeast side; (4) evidence for substantial postdetachment uplift in the footwall of the Santa Rosa fault; (5) the presence of tilted and incised late Pleistocene alluvial fans in the northeast Santa Rosa Mountains; and (6) progressive northeastward thickening of sediment beneath the Coachella Valley to an abrupt margin at the SAF (Figs. 1 and 10). Northeastward tilting likely began ca. 1.2 Ma when a regional tectonic reorganization initiated the SJFZ.

Processes that may drive crustal tilting include (1) oblique convergence and vertical loading across a northeast-dipping SAF in Coachella Valley; (2) dextral translation of crust through a hypothesized releasing bend in the Santa Rosa fault; (3) loading by buried mafic intrusions beneath the southern Salton Sea; and (4) short-wavelength upper mantle convection. Hypothesis 1, oblique convergence across a northeast-dipping San Andreas fault, is supported by a recent boundary-element modeling study (Fattaruso et al., 2014) and appears to be a likely driver of crustal tilting. We consider hypotheses 2 and 3 to be unlikely, and suggest hypothesis 4 as a plausible but as-yet untested control on active tilting of the central Salton block.

Support for this study was provided by National Science Foundation grant EAR-1144946 (Dorsey), and by the National Cooperative Geologic Mapping and Earthquake Hazards programs of the U.S. Geological Survey (Langenheim). This study benefited from discussions with Michele Cooke, Laura Fattaruso, Cody Mason, and Charlie Ogle. Reviews by Robert Powell, Keith Howard, Nathan Niemi, Andy Barth, Terry Pavlis, Francesco Mazzarini, and three anonymous reviewers helped us improve the quality and clarity of the paper.