Late Cenozoic Tectonics of the Northwestern San Bernardino Mountains, Southern California

Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California

KRISTIAN E. MEISLING ARCO Oil and Gas Company, 2300 West Piano Parkway, Piano, Texas 75075 RAY J. WELDON Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403

ABSTRACT which punctuated the deposition of three distinctive stratigraphic packages The late Cenozoic structural and stratigraphic history of the of Miocene, Pliocene, and Pleistocene age. Structural styles associated with northwestern San Bernardino Mountains supports two distinct epi- the uplift events suggest that regional low-angle detachments have played a sodes of uplift, in late Miocene to earliest Pliocene and Quaternary much more important role in the late Cenozoic evolution of the central time, that we hypothesize are related to movements on low-angle Transverse Ranges than is generally recognized. Our conclusions have structures beneath the range. In this paper, we document the nature, interesting structural implications for the development of the central distribution, and timing of late Cenozoic deformation and deposition Transverse Ranges province and its relationship to the evolution of the San in the northwestern San Bernardino Mountains, and we illustrate the Andreas fault in southern California. neotectonic evolution of the area in a series of interpretive paleotec- The study area includes the northern range front of the San Bernar- tonic block diagrams. dino Mountains between its junction with the San Andreas fault and In the first episode of deformation, late Miocene to earliest Plio- Lucerne Valley (Fig. 1). We present new data on the distribution, charac- cene motion on the south-southwest-directed Squaw Peak thrust sys- ter, and age of Late Cenozoic strata and structures, derived from our tem disrupted drainage in pre-existing Miocene nonmarine basins and mapping of more than 500 km2 at a scale of 1:24,000 or greater. We also uplifted the western third of the present range to form the ancestral summarize important new age control from magnetostratigraphy and mi- San Bernardino Mountains. Crystalline rocks of the San Bernardino crovertebrate paleontology that motivates significant revisions in the geo- Mountains were thrust southward across the present site of the San logic history of the area. We have reconciled these new geologic data with Andreas fault between 9.5 and 4.1 Ma, at a time when the San Gabriel previous work to establish a tectonostratigraphic framework for the late fault was the active strand of the San Andreas transform system. We Cenozoic evolution of the northwestern San Bernardino Mountains. Our speculate that the Liebre Mountain crystalline block at the northern regional synthesis should serve as a useful context for studies in progress margin of the Ridge Basin may be the missing upper plate of the and a foundation for future work aimed at resolving many remaining Squaw Peak thrust, now offset along the San Andreas fault. problems. The second episode of deformation began with uplift of the northern plateau of the modern San Bernardino Mountains on north- Setting and Previous Work directed, range-front thrusts in early Pleistocene time, between 2.0 and 1.5 Ma. Synchronous uplift of the northern plateau, recorded in The Transverse Ranges province trends obliquely across the predom- early Pleistocene fanglomerates on the northwestern margin of the inant northwesterly strike of the tectonic grain in southern California, range, is interpreted to be the result of movement of a relatively including the trace of the San Andreas fault. Many models have been coherent crustal block northward up a south-dipping detachment proposed for the structural evolution of the central Transverse Ranges ramp beneath the central range. In middle Pleistocene time, activity on province and its relation to the San Andreas fault. Although it is widely the northern range front began to wane, and the locus of uplift shifted agreed that topographic relief and thrusting in the San Bernardino Moun- to a narrow zone of arching and northward tilting adjacent to the San tains are the result of local convergence along the San Andreas fault, Andreas fault, which subsequently migrated rapidly northwestward the cause of this convergence is the subject of continued debate. Compres- along the San Andreas fault from the western San Bernardino Moun- sion in the San Bernardino Mountains has been variously attributed to tains into the northeastern San Gabriel Mountains. We attribute this (1) different movement histories on the San Andreas fault north and south pattern of deformation to the passage of a bulge or strike-slip ramp of the Transverse Ranges (Chinnery, 1965; Baird and others, 1974; Horna- attached to the southwest side of the San Andreas fault at depth. fius, 1985), (2) changes in trend of the San Andreas fault through the Transverse Ranges region (Allen, 1957, 1968; Crowell, 1981, 1982; Hill, INTRODUCTION 1982; Weldon and Humphreys, 1986), (3) clockwise rotation of the Transverse Ranges in a regional right-lateral shear couple (Garfunkel, In this paper, we document geologic constraints on the nature and 1974; Luyendyk and others, 1980,1985; Kamerling and Luyendyk, 1979, timing of late Cenozoic deformation, deposition, and uplift in the north- 1985), (4) incipient subduction of continental lithosphere (Bird and western San Bernardino Mountains, and we illustrate neotectonic Rosenstock, 1984; Sheffels and NcNutt, 1986), and (5) movement on evolution of the area in a series of interpretive paleotectonic block dia- deep-seated detachments in the crust or mantle (Hadley and Kana- grams for selected time intervals. We develop evidence for two distinct mori, 1977; Webb and Kanamori, 1985). In most models, timing of uplift uplift events, in late Miocene to earliest Pliocene and Quaternary time, in the Transverse Ranges is closely tied to timing of activity on the San

Geological Society of America Bulletin, v. 101, p. 106-128, 15 figs., January 1989.

106

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Figure 1. Index map of the San Bernardino Mountains and their relationship to the central Transverse Ranges in southern California, showing the location of major physiographic and cultural features, as well as principal regional faults and exposures of late Cenozoic sedimentary rocks (open circle pattern). See Figure 2 for geologic map of study area. Line of seismicity cross section of Figure 9 is longitude 117°W. LV: Lucerne Valley; AV: Apple Valley; WSBA: Western San Bernardino arch; CC: Crowder Canyon; SL: Silverwood Lake; SPF: Squaw Peak fault.

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Andreas fault and convergence must be consistent in magnitude with slip ernmost part of the range in conjunction with reconnaissance studies of the on lateral faults to the north and south. San Andreas fault zone. Mapping of pre-Tertiary rocks and structures The San Bernardino Mountains are the principal topographic expres- along the northern range front substantiated the local importance of late sion of the Transverse Ranges province east of the San Andreas fault Cenozoic deformation (Woodford and Harriss, 1928; Gillou, 1953; Rich- (Fig. 1). The northern San Bernardino Mountains are capped by a broad mond, 1960; MacColl, 1964). Allen (1957) and Shreve (1959) showed plateau of mature geomorphic landforms that stands 2 km above sea level, that a useful late Tertiary and Quaternary stratigraphy existed along the and 1 km above the floor of the Mojave Desert to the north. A narrow margins of the range with which to develop a detailed history of uplift and western extension of the range links the San Bernardino Mountains with deformation. Geologic quadrangle mapping at 1:62,000 by Dibblee the San Gabriel Mountains across the trace of the San Andreas fault. The (1964a, 1964b, 1965a, 1965b, 1967,1974) established a regional basis for southern San Bernardino Mountains are underlain by the San Gorgonio future studies and revealed the importance of late Quaternary uplift in the massif, which includes some of the highest peaks in southern California western range (Dibblee, 1975a). Late Cenozoic sedimentary rocks in the that stand more than a kilometer above the northern plateau and 2 km Cajon Pass area were described and mapped in detail by Yerkes (1951), above San Gorgonio Pass to the south. All along the margins of the central Woodburne and Golz (1972), Morton and Miller (1975), and Foster and western range, steep canyons contrast sharply with the reduced mature (1980, 1982). Detailed studies by Sadler (1981), Weldon and others landforms of the flat range crest, underscoring both the recency of uplift (1981), Meisling (1984), Weldon (1986), and Miller (1987) contributed and the vigorous activity of structures marginal to the range. Existing neotectonic mapping at 1:24,000 along the northwestern front of the San tectonic models do not satisfactorily address the manner in which conver- Bernardino Mountains. Shreve (1968), Sadler (1982b, 1985), and Strat- gence along the San Andreas fault is translated into uplift in the central house (1983) described late Cenozoic deposits in the northern and central Transverse Ranges, nor do they adequately address the timing of this uplift part of the range. Late Cenozoic age control was established by microver- as reflected in the rock record. tebrate studies of Woodburne and Golz (1972), May and Repenning The geologic literature provides a great deal of very useful data on the (1982), and Reynolds (1984, 1985) and was refined by magnetostrati- nature and timing of tectonic events in the northwestern San Bernardino graphic studies of Meisling (1984), Weldon and others (1984), Winston Mountains. Mendenhall (1905) first noted the geomorphic discordance (1985), and Weldon (1986). between the extensive upland erosion surface and steep northern escarp- A regional context for these geologic data must be developed if we ment of the San Bernardino Mountains as evidence of the extreme youth- are to use them to discriminate between the wide variety of tectonic fulness of the range. The first reconnaissance mapping and description of models that have been proposed for the structural origin of the central Cenozoic strata and structures along the north-central range front and Transverse Ranges and its relationship to the evolution of the San Andreas northern plateau was done by Vaughan (1922). Noble (1932,1954b) and fault. A synthesis seems particularly timely in light of the resurgence of Wallace (1949) first described the stratigraphy and structure of the west- interest in the tectonics of the northwestern San Bernardino Mountains

Figure 2. Simplified geologic map of the study area showing locations of regional cross sections of Figure 3, stratigraphic columns of Figure 4, and detailed geologic maps of Figures 5 through 8. See Figure 4 for key to geologic unit symbols. P: Phelan Peak column; C: Crowder Canyon column; S: Silverwood Lake column; D: Deep Creek column; A: Arrastre Canyon column; B: Blackhawk Canyon column (see Fig. 1); VF: Victorville fan; SV: Summit Valley.

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A SHOEMAKER CANYON

Figure 3. Regional cross sections trending northeast from San Andreas fault between Lake Arrowhead and Valyermo. See Figure 2 for location of lines of cross section, and Figure 8 for geologic unit symbols. San Andreas fault is at left edge of each section. Cedar Springs and Cleghorn faults are interpreted to flatten out into Squaw Peak thrust or deeper low-angle structures beneath the western San Bernardino Mountains. The Western San Bernardino arch (WSBA) deforms basement, late Cenozoic cover, and Squaw Peak thrust plane just northeast of the San Andreas fault. The northwestward migration of uplift associated with the WSBA is recorded in the Victorville fan, which was shed northeastward from the San Gabriel Mountains on the southwest side of the San Andreas fault.

recently sparked by siting of the first DOSECC deep scientific drillhole in Mountains are underlain by Mesozoic batholithic rocks with Precambrian Cajon Pass, and choice of the western San Bernardino Mountains for a to Paleozoic metasedimentary roof pendants of Cordilleran miogeoclinal CALCRUST deep crustal seismic reflection profile. Integration of new affinity (Noble, 1932; Dibblee, 1967; Stewart and Poole, 1975; Miller and data just now emerging from these studies, however, is far beyond the Morton, 1975, 1980; Cameron, 1981). Near the San Andreas fault be- scope of this work. The data and conclusions that follow are drawn simply tween Cajon Creek and Devore, this crystalline complex is depositionally from field observations. overlain by the San Francisquito Formation (Dibblee, 1967), a marine conglomerate, sandstone, and siltstone unit of problematic Late Creta- STRATIGRAPHY ceous or Paleocene age (Kooser, 1980). The relationship between San Francisquito Formation and overlying late Cenozoic strata is nowhere Late Cenozoic stratigraphy constrains the timing of tectonic events observed. and provides clues to the extent and nature of deformation in the western Between the Squaw Peak and Cajon Valley faults, a basement com- San Bernardino Mountains (Figs. 2 and 3). Late Cenozoic strata in the plex of unknown affinity underlies late Cenozoic strata (Figs. 4 and 5). In study area can be divided into three tectonostratigraphic packages of Mio- this block, the Cajon Formation rests on fractured and weathered intru- cene, Pliocene, and Quaternary age (Fig. 4). These three depositional sives and gneisses that consist predominantly of granodiorite of probable intervals bracket two separate episodes of deformation and uplift in the Cretaceous age. This basement complex is locally overlain by marine western San Bernardino Mountains, one during the late Miocene to earli- conglomerates and sandstones of the Vaqueros Formation of early Mio- est Pliocene and one during Quaternary time. cene age (Noble, 1932; Dibblee, 1967; Woodburne and Golz, 1972). The Pre-Miocene rocks are not considered in detail in this study. They can Vaqueros Formation occurs in small fault-bounded slices locally preserved be loosely subdivided into basement suites which underlie structural beneath the basal unconformity of the Cajon Formation, which has been blocks bounded by the San Andreas, Cajon Valley, and Squaw Peak attributed to subaerial erosion (Woodburne and Golz, 1972; Fig. 5). Be- faults. Northeast of the Squaw Peak fault, the western San Bernardino tween Cajon Creek and Devore, granodiorites and quartz monzonites of

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Figure 4. Stratigraphie correlation chart for the late Ceno- zoic sedimentary rocks of the northwestern San Bernardino Mountains. See Figure 2 for locations of columns. Sources of age data include (M) magnetostratigraphy, (F) vertebrate fossils, and (R) fission-track data on ash. Numbered units in Crowder For- mation from Foster (1980). Columns are composite, and thick- nesses shown are maximum values. All units are highly variable in thickness.

unknown age and affinity underlie unnamed and undated Tertiary deposits including latites, tuffs, and agglomerates of dacitic to andesitic composition (Fig. 6). These rocks may be an extension of the structural block of the and highly variable color, texture, and abundance; and sedimentary rocks, Cajon Formation, or they may be a separate fault slice within the San including red sandstones of uncertain origin (Yerkes, 1951; Woodburne Andreas fault zone. and Golz, 1972; this study). In contrast, the Punchbowl Formation has a Between the San Andreas and Cajon Valley faults, basement consists distinctive set of clasts that includes unique rock types such as Pelona of marble and gneiss of unknown affinity, intruded by granodiorite of Schist, "polka-dot" granite, blue quartz granite, and marine sandstones probable Cretaceous age (Yerkes, 1951; Dibblee, 1965a; Woodburne and (Noble, 1953; Woodburne and Golz, 1972; Barrows and Barrows, 1975; Golz, 1972; Ross, 1972). These crystalline rocks are exposed along the Barrows, 1979, 1985) that could not have been easily overlooked in the north side of the San Andreas fault as far northwest as Valyermo (Fig. 2). Cajon Formation. In the western San Bernardino Mountains, crystalline basement is The lithologic variations in the Cajon Formation, represented by its capped by a deeply weathered erosional surface of low relief that is charac- subdivision into six members (Woodburne and Golz, 1972), generally terized by residual boulders embedded in as much as 5 m of grus encompass the entire outcrop area and suggest deposition in a broad, (Vaughan, 1922; Blackwelder, 1925; Meisling and Weldon, 1982b). The homogeneous basin. Distinctive members, such as the hogback-forming bedrock-soil interface underlies the Crowder Formation and is therefore Member 2, red-bed Member 3, variegated Member 5, and the unusual pre-middle Miocene in age. Woodburne and Golz (1972) and Foster volcanic and red sandstone clast suite of Member 6 (Fig. 5), are laterally (1980) correlated this weathered surface with a surface developed on top extensive and probably represent basin-wide variations in sediment source of the Cajon Formation. New data on the age of the Crowder Formation and depositional environment. Only Unit 5a (as defined by Woodburne preclude this interpretation, however. The surface has also been correlated and Golz, 1972) and Member 4 are not ubiquitous, and may be marginal with similar weathering surfaces widely exhumed throughout the San facies of the widespread Member 5. This is in contrast to the type Punch- Bernardino Mountains and southern Mojave Desert (Oberlander, 1972). bowl Formation where marked lithologic and clast-type differences repre- These surfaces have, in turn, been correlated with pre-late Miocene sur- sent lateral variation along a narrow trough, perhaps a rift zone in the early faces that underlie the Santa Ana Sandstone (Sadler and Reeder, 1983; San Andreas system (Weldon, 1986). Sadler, 1985) and late Miocene basalt flows in the eastern San Bernardino The Miocene Crowder Formation (Dibblee, 1967) overlies the Mountains. Thin gravel veneers, locally preserved on these surfaces, con- weathered erosion surface in the western San Bernardino Mountains (Figs. tain both locally derived quartzites and rounded volcanic clasts that pre- 2 and 3). In the type area at Crowder Canyon, it consists of 980 m of sumably had their source in the Mojave Desert to the north (Sadler and nonmarine arkosic sandstone and conglomerate (Fig. 4). Fluvial deposi- Reeder, 1983), suggesting that the San Bernardino Mountains did not exist tion by braided streams is indicated by trough cross-stratification, channel as a topographic feature at the time. Sadler and Reeder (1983) favor a scour, and paleosols (Foster, 1980). Cobble imbrication and cross-bedding possible correlation of the gravel veneer with Unit 3 of the Crowder record southward transport (Foster, 1980; Winston, 1985), consistent with Formation, which is particularly rich in quartzite and volcanic clasts. documented northern source areas for distinctive metamorphic, volcanic, Alternatively, the veneer could be correlative with the pre-middle Mio- and plutonic conglomerate clasts (Dibblee, 1967; Woodburne and Golz, cene weathering surface that underlies the Crowder Formation. 1972; Foster, 1980; this study). Common clast types in the Crowder Formation include (1) coarse- to fine-grained, generally leucocratic, plu- Miocene Units tonic rocks; (2) metasedimentary rocks of Paleozoic miogeoclinal affinity, including fine-grained buff to pink quartzites referred to the Oro Grande The Miocene Cajon Formation (the Cajon facies of the Punchbowl Series and epidotized quartz/granite-pebble conglomerate referred to the Formation of Noble, 1932, 1954b; Dibblee, 1967; Sharp, 1972; Wood- Fairview Valley Formation exposed to the northeast near Victorville; and burne and Golz, 1972; Foster, 1980) is widely exposed in Cajon Valley (3) volcanic rocks, including extrusives, tuffs, shallow intrusives, and ag- (Fig. 5) and consists of as much as 2,440 m of nonmarine arkosic sand- glomerates attributed to the Jurassic Sidewinder Volcanics and early Mio- stone and conglomerate (Fig. 4). The Cajon Formation has been corre- cene volcanic suite of the central Mojave Desert. lated with the Punchbowl Formation on the south side of the San Andreas The Crowder Formation was deposited in late early to early late fault near Valyermo (Noble, 1932,1954b), and the Punchbowl name has Miocene time, between 17 and 9.5 Ma, as determined by microvertebrate been used by many workers, despite the fact that they have not accepted faunal assemblages (Reynolds, 1984,1985) and magnetostratigraphic po- the correlation (Dibblee, 1967; Woodburne and Golz, 1972; Foster, larity zonation (Weldon and others, 1984; Weldon, 1984,1986; Winston, 1980). We use the name "Cajon Formation" for the Cajon Valley expo- 1985). Pliocene strata, originally interpreted as western and eastern facies sures to emphasize important differences in both lithology and fossil age equivalents of the upper Crowder Formation (Foster, 1980; Meisling and from the Punchbowl Formation. The Cajon Formation overlies the early Weldon, 1982b; Meisling, 1984) are informally renamed the "Phelan Peak Miocene Vaqueros Formation in angular unconformity and is assigned an deposits" (Weldon, 1984), to distinguish them from the Crowder Forma- age of late early to late middle Miocene, 18 to less than 13.5 Ma, based on tion, and they are discussed separately below. mammalian fauna (Reynolds, 1985; Woodburne and Golz, 1972). The Although the original geometry and extent of the Crowder basin are age of the uppermost unit of the Cajon Formation (Member 6 of Wood- not known, lithologic similarity of exposures throughout the western San burne and Golz, 1972) is poorly constrained, and so the top may be Bernardino Mountains (Figs. 2 and 7) argues for a relatively homogeneous somewhat younger. In contrast, the type Punchbowl Formation is late depositional system several tens of kilometers in width. We interpret the Miocene in age (Woodburne and Golz, 1972). westward thickening and abrupt truncation of fine-grained facies of the Trough cross-stratification and cobble imbrication indicate deposi- Crowder Formation (Fig. 5; Units 2 and 4 of Foster, 1980; Weldon, 1984) tion of the Cajon Formation by southwestward-flowing streams (Wood- to suggest that the center of the Crowder basin lay to the west of the burne and Golz, 1972). Subrounded to well-rounded clasts in the Cajon Squaw Peak fault and that it has since been removed. Formation are mainly leucocratic plutonic and foliated gneissic rocks of The Miocene to Quaternary(?) Santa Ana Sandstone is exposed dis- quartz-monzonitic to granodioritic composition; dark grayish-green, fine- continuously from south of Lake Arrowhead to Pipes Wash (Fig. 1; grained metamorphic rocks including schist and hornfels; volcanic rocks, Vaughan, 1922; Dibblee, 1975a; Sadler, 1985; Sadler and Demirer, 1986).

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Figure 5. Geologic map of the Cajon area. See Figure 2 for the Phelan Peak basin appears to lie beneath the Victorville fan deposits location and Figure 8 for key to geologic unit symbols. Range and and may be reflected in the northwest-trending gravity low that passes Township Section lines are shown for local geographic reference. through the town of Phelan (Dibblee, 1975b). Numbered contacts within Cajon and Crowder Formations designate The Phelan Peak deposits unconformably overlie both the Crowder base of members as defined by Woodburne and Golz (1972) and and Cajon Formations and are overlain by the Harold Formation (Fig. 4; Foster (1980), respectively. Note that Squaw Peak fault (SPF) con- Weldon, 1984). The age of an ash deposit from Summit Valley has been tinues south of Cleghorn fault to San Andreas fault. Western San determined to be 3.8 ± 0.4 m.y. (C. F. Naeser, 1982, written commun.). Bernardino arch (WSBA) lies just north of, and parallel to, the San At Phelan Peak, these sediments were deposited between 4.1 and 1.5 Ma, Andreas fault. Part of line of section C-C' (Fig. 3) shown for on the basis of correlation of magnetostratigraphic polarity zonation with reference. age constraints provided by the age of the ash and fossils in these and overlying units (Weldon, 1984, 1986). The Phelan Peak deposits were originally correlated with the upper Crowder Formation (Foster, 1980; Meisling and Weldon, 1982b; Meisling, 1984); however, fossil and magnetostratigraphic studies (Reynolds, 1984, 1985; Weldon, 1984, 1986; It is composed of as much as 1,500 m of conglomerate, sandstone, shale, Winston, 1985) have since shown the upper Crowder Formation to be and basalt (Sadler, 1985). On the basis of lithology and clast populations, much older. Sadler (1985) recognized distinctive western, central, southeast, and The Pliocene Old Woman Sandstone is exposed south and east of northeast facies, implying multiple source areas. Clast types in the western, Lucerne Valley (Fig. 1; Vaughan, 1922; Shreve, 1968; Sadler, 1981, northeastern, and southeastern facies are compatible with sources within 1982a). It consists of up to 260 m of conglomeratic arkose, conglomerate, the San Bernardino Mountains or Mojave Desert to the north, but an siltstone, and claystone (Fig. 4). The Old Woman Sandstone is divided exotic clast suite in the central facies that includes Pelona-type schist and into central, western, and eastern facies on the basis of lithology (Sadler, anorthosite clearly indicates a source in the San Gabriel Mountains across 1982b). The central facies (ol of Sadler, 1981) is predominantly mudstone the San Andreas fault to the south (Sadler, 1985; Sadler and Demirer, and conglomeratic arkose, containing scattered clasts of vesicular andesite, 1986). gneiss, quartzite, schist, vein quartz, rhyolite, and basalt (Shreve, 1959), The northeastern facies of the Santa Ana Sandstone is late Miocene in largely attributed to sources in the Mojave Desert to the north. Some age, on the basis of K-Ar dates on interbedded basalts (Woodburne, depositional units in the lower central facies fine and thin northward and 1975). The southeastern facies may be younger (Strathouse, 1982,1983), contain north-dipping cross-beds indicative of north-directed transport possibly Pleistocene (P. M. Sadler, 1985, written commun.). No age data (Shreve, 1959). The central facies grades up into a coarse fanglomerate exist for the central facies; however, the younger beds of this facies contain made up of angular clasts of marble shed northward from Paleozoic rocks basalt clasts, possibly derived from the eastern facies, indicating an age of exposed during latest Pliocene to early Pleistocene uplift of the modern late Miocene or younger. The lower portion, containing the exotic clasts, is San Bernardino Mountains to the south (Cushenbury Formation of believed by Sadler and Demirer (1986) to be younger than the eastern Shreve, 1968, the name we use here; o2 of Sadler, 1981). In the western facies. Microvertebrate fauna at the base of the western facies tentatively facies, basal beds correlated with the central facies are succeeded by clay- suggest an age of early to middle Miocene (Hemingfordian/Barstovian; stone and siltstone and are capped by concretionary sandstone and Carlton, cited in Sadler, 1985). Sadler (1985), however, favors a Plio- carbonate (Sadler, 1982b). Exposures of siltstone and claystone along the Pleistocene age for most of the western facies based on relationships with Tunnel Ridge fault may be a western correlative of the Old Woman the central facies to the east. The westernmost outcrops of the western Sandstone (Rock Springs Road deposits of Meisling, 1984). In the eastern facies of the Santa Ana Sandstone are in proximity to the easternmost facies, sandstone and shale are capped by a fanglomerate with well- outcrops of the Crowder Formation at Mud Flats (Fig. 1) with which they rounded clasts of quartzite, basalt, and other volcanic rocks, probably bear strong similarities in terms of (1) early to middle Miocene fossils derived from underlying gravel veneers (Vaughan, 1922; Sadler, 1982b). at base of section, (2) stratigraphic position above deeply weathered Vertebrate faunas from the central and western facies of the Old basement, (3) clast composition, (4) structural position relative to the Woman Sandstone indicate ages of 2.0 to 3.2 m.y. and 2.5 to 3.0 m.y., east-west-trending Waterman Canyon and Santa Ana faults, and respectively (May and Repenning, 1982). The eastern facies of the Old (5) unconformable relationship with overlying undeformed middle Woman Sandstone overlies basalts of probable late Miocene age (Neville Pleistocene deposits. We therefore favor the interpretation that the western and Chambers, 1982) and is inferred to be the same age as the other two facies of the Santa Ana Sandstone is Miocene to Pliocene in age. Unfortu- facies. nately, complex facies relationships and sparse age data still allow a wide range in interpretations for age of the western outcrops of the Santa Ana Quaternary Units Sandstone. Massive, coarse fanglomerates of early to middle Pleistocene age Pliocene Units generally overlie fine-grained Pliocene units along the northwestern margin of the San Bernardino Mountains and northeastern margin of the San The Pliocene Phelan Peak deposits (Weldon, 1984) are exposed Gabriel Mountains. These fanglomerates include the Victorville fan depos- between Summit Valley and Valyermo (Figs. 2 and 3). They consist of as its that unconformably overlie the Phelan Peak deposits at Summit Valley much as 500 m of interbedded siltstone, swelling claystone, siliceous ash, and Valyermo, and the Cushenbury Formation which conformably over- sandstone, and conglomerate (Fig. 4). Clasts include plutonic, gneissic, lies the Old Woman Sandstone at Lucerne Valley, as well as scattered volcanic, and metamorphic rock types, apparently derived from underly- remnants of unnamed and undated deposits overlying basement along the ing basement and recycled from Miocene units. Locally exposures contain range front in between (for example, Qf3 of Meisling, 1984). These fan-

numerous argillic paleosols, CaC03-cemented layers, and finely laminated, glomerates were shed from the northern escarpment of the San Bernardino gastropod-bearing siltstones, suggesting intermittent lacustrine conditions. Mountains during rapid uplift of the north-central plateau, and from the Clast imbrication records both northward and southward transport direc- northeastern escarpment of the San Gabriel Mountains as they moved into tions (Foster, 1980), yet lithologic character of the Phelan Peak deposits is position opposite the Mojave Desert block by right-lateral slip on the San consistent over 50 km of outcrop (Foster, 1980; Weldon, 1984). The axis of Andreas fault.

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Figure 6. Geologic map of the Devore area. See Figure 2 for location and Figure 8 for key to geologic unit symbols. Range and Township Section lines are shown for local geographic reference. Postulated southeastern extension of the Squaw Peak thrust (SPF) lies just north of the San Andreas fault, placing San Bernardino Mountains crystalline basement and San Francisquito Formation over unnamed Tertiary sandstone and crystalline basement of unknown affinity. (Note overlap in center.)

The Victorville fan, which covers the area from Summit Valley to plutonic rocks, Pelona Schist, and volcanics from the San Gabriel Moun- Valyermo and from Cajon Pass to Victorville (Fig. 2), is underlain by tains near Palmdale. Noble (1932, 1954a) correlated the Harold Forma- deposits of lithic to arkosic sandstone, siltstone, and conglomerate of early tion with similar rocks across the San Andreas fault in the Devil's to middle Pleistocene age. The Victorville fan deposits are divided into Punchbowl and Palmdale areas (Fig. 1). The Harold Formation is overlain several formal and informal units, including the Harold Formation, Shoe- by the Shoemaker Gravel, which is as much as 120 m thick in Cajon maker Gravel, Ord River deposits of Meisling (1984), and older alluvium Valley and extends northwest to Valyermo and east to Summit Valley. The of Noble (1954a; Fig. 4), which are distinguished primarily on the basis of Shoemaker Gravel contains clasts of leucocratic plutonic rocks, Mt. Lowe clast suites. The name "Victorville fan deposits" encompasses the now Granodiorite, San Francisquito sandstone, and cobbles recycled from the obsolete Inface Bluffs gravels of Noble (1954a; Yerkes, 1951) and is Punchbowl Formation, all derived from the San Gabriel Mountains near chosen to emphasize that, despite detailed differences in lithology and clast Valyermo (Noble, 1954b; Foster, 1980). East of Cajon Pass, distinction composition, the constituent units were all deposited as part of a great between units of the Victorville fan becomes difficult due to changes in coalesced alluvial apron shed off the northeast flank of the San Gabriel clast source types and the time-transgressive nature of the units. The Ord Mountains and the north flank of the western San Bernardino Mountains. River deposits in Summit Valley, however, which contain predominantly The Harold Formation, which reaches 76 m in thickness, uncon- quartzite and quartz-monzonite clasts recycled from the Crowder Forma- formably overlies the Crowder, Cajon, and Phelan Peak deposits in nearly tion in the western San Bernardino Mountains (Meisling, 1984), can be continuous exposure from Cajon Pass to Palmdale (Noble, 1954a; Foster, correlated with both the Harold Formation and Shoemaker Gravel on the 1980; Weldon, 1986; Figs. 1, 2, and 3). It contains clasts of leucocratic basis of magnetostratigraphic polarity zonation (Meisling, 1984; Weldon,

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1986). The Shoemaker Gravel and Ord River deposits are overlain by 10 drainage basin and coalesce to form a bajada. Although up to three genera- to 15 m of older alluvium (Noble, 1954a), which underlies the relict tions of alluvial fan surfaces can be recognized in almost every major surface of the Victorville fan. The older alluvium in the Cajon Pass area drainage, correlation is difficult due to differences in composition of detri- consists predominantly of clasts of albite-epidote-chlorite schist character- tus associated with each small drainage basin. The oldest fan deposits, istic of the Pelona schist of Blue Ridge, near Wrightwood. An angular which are preserved as remnants with deep soil profiles (Qf3 of Meisling, unconformity separates the Shoemaker Gravel and older alluvium within 1984), are tentatively correlated with the Shoemaker Gravel and Cushen- a few kilometers of the San Andreas fault in the northeast San Gabriel bury Springs Formation (Figs. 4 and 8) and considered middle Pleistocene Mountains and Cajon Pass area; but they are conformable to the north and in age. All younger fans are tentatively considered late Pleistocene in age east of the Cajon Pass area. and presented as one unit for the purposes of this paper (Figs. 4 and 8). The formations of the Victorville fan deposits have been dated by a Although latest Pleistocene and Holocene alluvial fans and terraces are combination of fossils and magnetostratigraphic techniques (Fig. 4). Mag- present throughout the study area (Foster, 1980; Meisling, 1984; Weldon, netostratigraphic polarity zonation within the Victorville fan deposits 1986), a detailed discussion of them is beyond the scope of this paper. shows that units systematically decrease in age to the northwest along the San Andreas fault (Weldon, 1984, 1986). Assuming uniform deposition STRUCTURE rates, the base of the Harold Formation ranges in age from 1.6 m.y. in Crowder Canyon to less than 0.7 m.y. at Valyermo, and the top of the Late Cenozoic structures in the western San Bernardino Mountains Shoemaker Gravel must decrease in age from 1.0 m.y. to less than 0.4 m.y. can be grouped by timing, vergence, and geometry into two distinct de- over the same distance (Weldon, 1986). formational systems of late Miocene and Quaternary age. Structures of late The range front east of Summit Valley is characterized by an alluvial Miocene to earliest Pliocene age can be related to south-directed thrusting, apron, made up of fan surfaces that emerge at the foot of each range-front which uplifted the central and western parts of the range to form the

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Figure 7. Geologic map of the Silverwood Lake area. See Figure 2 for location and Figure 8 for key to geologic unit symbols. Range and Township Section lines are shown for local geographic reference. Left-lateral Cleghorn fault (CF) displays strong geometric similarities with arcuate faults of the Cedar Springs fault system, from which it is interpreted to have evolved. Part of line of section D-D' (Fig. 3) shown for reference. Western San Bernardino arch (WSBA) deforms foliation in basement just northeast of San Andreas fault (SAF).

ancestral San Bernardino Mountains. Structures of latest Pliocene and Formation and the Cajon Formation on opposite sides of the Squaw Peak Pleistocene age were associated with northward thrusting, resulting in a fault (Fig. 2). Although there is no exposure of the Phelan Peak deposits second period of uplift in the northern San Bernardino Mountains, which resting directly on the Squaw Peak fault, Phelan Peak beds dip consistently shifted southward and westward to expand the western limit of the range 25°-30° to the northeast on both sides of the structure, with no evidence of along the San Andreas fault. deformation across the projected trace of the fault. Furthermore, the Phe- lan Peak deposits truncate small-scale structures in underlying units which Late Miocene Structures are associated with the Squaw Peak deformational event. We therefore Late Miocene to earliest Pliocene deformation in the western San conclude that motion on the Squaw Peak fault ceased by 4.1 Ma, prior to Bernardino Mountains occurred on predominantly south-southwest-ver- deposition of the basal Phelan Peak deposits. gent compressional structures. Structural elements of this system, which The westernmost San Bernardino Mountains are cut by a system of we have named the "Squaw Peak thrust system," include the Squaw Peak northwest- to east/west-trending, arcuate, south-block-down reverse fault, the Cedar Springs reverse fault zone, and other related faults and faults herein named the "Cedar Springs reverse fault zone" for exposures folds. east of Cedar Springs Dam (Fig. 7). The Cedar Springs reverse fault zone The Squaw Peak fault is here defined as that structure which sepa- terminates at the Squaw Peak and San Andreas faults on the west and rates basement of San Bernardino Mountains and overlying Crowder south and extends east to the Tunnel Ridge fault. Similarly of trend, style, Formation from basement of unknown affinity and overlying Cajon vergence, and our interpreted timing of the Waterman Canyon and Santa Formation (Fig. 5). At Squaw Peak, the main trace of the fault dips 70° Ana fault zones suggest to us that all of these east/west-trending fault to the east, placing San Bernardino Mountains granodiorite over the Cajon zones are related (Fig. 1). Formation. A secondary trace dips steeply west, placing the granodiorite The earliest-formed structures in the Cedar Springs system are a set of over the Crowder Formation. North of Squaw Peak where the fault trends north-plunging asymmetric anticlines and synclines which locally deform north and separates the Crowder and Cajon Formations, its dip is variable, the Crowder Formation in the western San Bernardino Mountains (Figs. 5 but its generally straight surface trace suggests a relatively high-angle fault and 7). These folds are also expressed topographically in the underlying (Fig. 5). In the region where the Squaw Peak fault changes trend from weathered basement surface where the Crowder Formation has been re- north to northwest, railway cuts reveal numerous low-angle faults and moved by erosion (Fig. 7). The folds range in amplitude from a few tens of recumbent isoclinal folds in the Cajon and Crowder Formations which meters to more than 100 m. The anticlines and synclines occur in asym- obscure the primary dip of the fault. Exposures of the fault along its metric pairs, sharing a common steep, east-dipping limb; the folds are northwest-trending segment show that it parallels bedding in both the monoclinal steps with an east-block-down sense of displacement that have Crowder and Cajon Formations with a northeasterly dip of 25° to 45°. been subsequently tilted northeast during Quaternary time. One such fold Exposures of the Cajon Formation continue for ~7 mi to the northwest pair is associated with the north-south-trending segment of the Squaw (Fig. 2), implying that the Squaw Peak fault trends parallel the San An- Peak fault north of the Cleghorn fault (Fig. 5). The asymmetric geometry dreas fault beneath the Phelan Peak and Victorville fan deposits. of this Cedar Springs fold and its parallelism with the Squaw Peak fault The Squaw Peak fault was tilted 25° northeastward during the Qua- suggests a genetic relationship. Other folds in the Cedar Springs system are ternary uplift event. When tilting is removed, the northwest-trending seg- therefore inferred to have developed above structures analogous to the ment of the Squaw Peak fault is restored to a horizontal to gently north-trending segment of the Squaw Peak fault in the subsurface. northeast-dipping attitude along with bedding in much of the Crowder and Arcuate, south-block-down, high-angle reverse faults of the Cedar Cajon Formations. The north-trending segment restores to a steep east- Springs reverse-fault system cut the north-plunging folds (Fig. 7). These dipping attitude, projecting beneath the Crowder Formation. The north- reverse faults generally dip 75° north to vertical. When bedding in the trending segment is interpreted as a high-angle tear fault in the otherwise Crowder Formation and younger units of the western San Bernardino northwest-southeast-trending, low-angle thrust system. Mountains is restored to horizontal by unfolding the Pleistocene Western The north-trending segment of the Squaw Peak fault is offset left- San Bernardino arch, reverse faults of the Cedar Springs zone dip about laterally on the Cleghorn fault, and continues southward to form the 50° to 60° northward. Displacement on these reverse faults varies from a contact between the Cajon Formation and San Bernardino Mountains few tens of meters to over 500 m across larger members of the system, such basement (Fig. 5). A high-angle zone of intense shearing characterizes this as the Cleghorn and Waterman Canyon faults. Offsets of north-plunging southern extension of the Squaw Peak fault. Although the trace of the fold axes suggest that the Cedar Springs faults have pure north-side-up, Squaw Peak fault becomes disrupted by younger cross faults near the San dip-slip movement across east-west-trending segments. Offset Cedar Andreas fault zone, discontinuous outcrops of a thrust in the Devore area Springs fault patterns suggest right-oblique slip on northwest-trending suggest that the Squaw Peak fault returns to a subhorizontal attitude and segments. continues southeast just north of the San Andreas fault (Fig. 6). In this Crosscutting relationships between faults in the Cedar Springs zone area, gneissic basement of San Bernardino Mountains affinity, deposition- are complicated by Pleistocene reactivation of some members. Most nota- ally overlain by San Francisquito Formation, is in low-angle thrust contact bly, the Cleghorn and Waterman Canyon faults (Fig. 2) display evidence above red quartz monzonite of unknown affinity, depositionally overlain of late Pleistocene motion (Weldon and others, 1981; Meisling and Wel- by "unnamed Tertiary gravels" of Morton and Miller (1975). We interpret don, 1982a; Weldon, 1986), but are inferred to have originated as part of this thrust zone as the eastern continuation of the Squaw Peak fault. the Cedar Springs reverse fault zone because of their geometric similarity The Squaw Peak fault cuts the upper Crowder Formation, and is to members of the system that have not been reactivated. therefore younger than 9.5 m.y. Foster (1980) proposed that the Squaw East-west-trending sets of paired anticlines and synclines are ex- Peak fault was active during deposition of the Crowder Formation based pressed in the Crowder Formation in the Cajon Pass and Silverwood Lake on the incorrect correlation of the Crowder and Phelan Peak formations. areas (Figs. 5 and 7). These folds resemble steps with a south-block-down We see no change in the character of the Crowder Formation near the sense of displacement and match the Cedar Springs reverse faults in trend, Squaw Peak fault to suggest that faulting and deposition were contempo- arcuate shape, sense of offset, and magnitude of displacement. We con- raneous, and we conclude that movement on the Squaw Peak fault clude that they are developed over Cedar Springs reverse faults in the completely postdates deposition of the Crowder Formation. The Phelan subsurface. These fold pairs are truncated by the unconformity at the base Peak deposits in Cajon Valley rest unconformably on both the Crowder of the Phelan Peak deposits.

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Figure 8. Geologic map of the North Frontal Thrust area. See Figure 2 for location. Range and Township Section lines are shown for local geographic reference. Sky High Ranch fault offsets North Frontal thrust system where it joins Tunnel Ridge and Deep Creek faults. These faults link the North Frontal thrust system with the Cleghorn fault to the southwest (see Fig. 2). (Note overlap in center.)

Springs reverse-fault system, which suggests to us that they may be related. Although Sadler has proposed that the Santa Ana thrust is a Pleistocene structure, we note that motion is only absolutely bracketed by truncated late Miocene basalts and undisturbed middle Pleistocene gravels. We favor an alternative hypothesis in which motion on the Santa Ana thrust occurred as part of the Cedar Springs deformational event in the western San Bernardino Mountains. Because motion on the thrust postdates deposition Cedar Springs structures that have not been reactivated deform the of the Santa Ana Sandstone, which is, in part, younger than the Crowder upper Crowder Formation and are truncated by the unconformity at the Formation, our interpretation implies that the Cedar Springs and Squaw base of the Phelan Peak deposits in the Cajon area (Fig. 5). This makes Peak deformation events occurred during the latter part of the 9.5 to 4.1 them active between 9.5 and 4.1 Ma, which is the same time interval that Ma time window defined by the relationships at Cajon Pass, brackets motion on the Squaw Peak fault. North-plunging anticline- syncline pairs are clearly cut by reverse faults and associated east- Quaternary Structures west-trending forced folds in the Silverwood Lake area (Fig. 7). In contrast, east-west-trending forced folds are truncated by the Squaw Peak Quaternary structures in the northwestern San Bernardino Mountains fault and deformed by the related north-plunging syncline in the Cajon can be grouped into two distinct deformational systems according to ver- area (Fig. 5). Such contradictory crosscutting relationships can be ex- gence, style, timing, and distribution. In early to middle Pleistocene time, plained simply if motion on all of the Cedar Springs structures was con- uplift of the broad north-central plateau was associated with deformation temporaneous with motion on the Squaw Peak fault. on the North Frontal thrust system along the northern margin of the range. The Waterman Canyon reverse fault and Santa Ana thrust have Starting in middle Pleistocene time, uplift shifted to the southern and strong similarities in trend, style, and timing with members of the Cedar western margin of the range. Middle to late Pleistocene uplift of the

CRYSTALLINE SAN FRANCISQUITO FU CROWOER FM PHELAN PEAK DEPOSITS YOUNGER ALLUVIUM BASEMENT OF SAN BERNARDINO MTNS BEDDING ATTITUDE SQUAW PEAK FAULT

OLDER ALLUVIUM 3 OLD WOMAN SANDSTONE TP^JJ. r '•>' ,0 FOLIATION ATTITUDE CRYSTALLINE ilili- UNNAMED TERTIARY LU CAJON FM CONTACT; BASEMENT BETWEEN SHOEMAKER GRAVEL CONCEALED; INFERRED SOUAW PEAK FAULT SANTA ANA SANDSTONE SCAJON VALLEY OR SAN ANDREAS FAULT ftrffîi VAQUEROS FM HAROLD FORMATION FAULT; DIP; SLIP; THRUST \CAJON VALLEY FAULT | \SAN ANDRÉAS FAULT | CONCEALED; INFERRED -V'.v'; -4 HAROLD FM/ X i 1 ' SHOEMAKEH GRAVEL ^P ' , > UNDIFFERENTIATED ANTICLINE; SYNCLINE CRYSTALLINE CRYSTALLINE « EQUIVALENTS CONCEALED; INFERRED BASEMENT BETWEEN BASEMENT OF CAJON VALLEY FAULT SAN GABRIEL MTNS & SAN ANDREAS FAULT

westernmost San Bernardino Mountains is expressed in a northwestwardly Frontal thrust zone, the Deep Creek fault zone, the Tunnel Ridge fault, migrating locus of arching and tilting along the San Andreas fault called and the Cleghorn fault (Fig. 8). the "Western San Bernardino arch." A system of Pleistocene thrust faults extends along the northern mar- Early Quaternary Structures. Early to middle Pleistocene structures gin of the San Bernardino Mountains from Summit Valley to Old Woman in the northwestern San Bernardino Mountains exhibit predominantly Springs (Fig. 8; Santa Fe and Voorheis thrusts of Woodford and Harriss, northward vergence. They are grouped together in this section to empha- 1928; Grapevine thrust of Shreve, 1968; White Mountain thrust of Sadler, size their collective role in the rapid early to middle Pleistocene uplift of 1981, and Meisling, 1984). Displacement on these thrusts appears to the north-central plateau of the modern San Bernardino Mountains. This decrease away from the central range front on the basis of a systematic uplift is interpreted to have occurred on a deep-seated, north-directed decrease in range-front relief westward from Lucerne Valley. Total short- thrust ramp and associated structures, herein named the "North Frontal ening on the range-front thrust system is probably not more than a few thrust system." The North Frontal thrust system is made up of the North kilometers (2-3 km, Baird and others, 1974; 4,000+ ft, Shreve, 1968, PI.

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1). Where exposed, thrust planes dip gently both south and north (Sadler, and 8). The Tunnel Ridge fault branches off from the North Frontal thrust 1981). zone and ends or merges with the Cleghorn fault west of Lake Arrowhead Thrusting on the northern range front began in early Pleistocene time (Fig. 1). The straight trace of the Tunnel Ridge fault suggests a high-angle (Vaughan, 1922, Woodford and Harriss, 1928; Shreve, 1968; Sadler, geometry, and an inconsistent sense of throw on the Miocene weathering 1981, 1982b), recorded by the dramatic change in provenance, texture, surface hints at strike-slip or scissors motion. The close match of basement and sorting from the Plio-Pleistocene fluvial-lacustrine Old Woman Sand- rock types (MacColl, 1964) and continuity of the weathered erosion sur- stone to early Pleistocene marble fanglomerates of the Cushenbury Springs face across the structure, however, preclude large late Cenozoic lateral or Formation (Shreve, 1968), both of which are overridden by the range- vertical offset. Nevertheless, parallelism of the Tunnel Ridge fault and front thrusts (Richmond, 1960; Sadler, 1981). A maximum age for the Deep Creek fault zone suggests a genetic relationship; perhaps they follow onset of thrusting at the range front is provided by the maximum possible a pre-existing structural trend. age of the uppermost beds of the Old Woman Sandstone beneath the The Tunnel Ridge fault cuts and deforms both the Rock Springs Cushenbury Springs Formation (Fig. 4), dated at 3.2-2.0 Ma (May and Road deposits (Fig. 4), which are paleomagnetically reversed and there- Repenning, 1982) based on mammalian fauna. fore older than 0.7 m.y., and overlying terrace gravels, which are of normal Monoclinal warps are associated with range-front thrusting. South- polarity and presumably younger than 0.7 m.y. (Meisling, 1984). The east of Lucerne Valley, thrusts are deformed into a northeast-block-down, Rock Springs Road deposits are of probable Plio-Pleistocene age, based on northwest-trending monocline, with dips reaching 85° northeast in the Old lithologic similarity to the Old Woman Sandstone and Phelan Peak depos- Woman Sandstone (Shreve, 1968). Although thrusts generally dip south its (Fig. 1). The terrace gravels are associated with geomorphic surfaces along the range front (Shreve, 1968, PL 1), surfaces are commonly ob- that parallel modern drainage paths, and were probably deposited in late served to roll over and dip northward (Sadler, 1981; Meisling, 1984). Pleistocene time (Meisling, 1984). Motion on the Tunnel Ridge fault is Large landslides are common where thrusts have been warped and are therefore considered to be Pleistocene in age. dipping parallel to range-front slope (Shreve, 1959, 1968). Monoclinal The Deep Creek fault zone trends south from Apple Valley to Sum- warps are also commonly expressed in the early Miocene weathering mit Valley, where it takes another abrupt 90° westward bend and dies out surface along the range front (Meisling, 1984). The size of these range- in Summit Valley (Fig. 8). Range-front relief along the Deep Creek fault front warps suggests folding of foliated basement rocks above deep-seated zone decreases from about 400 m at the Ord Mountains to a few tens of structures. meters as the fault system turns west in Summit Valley. Some relief may Microseismicity data provide some insight into the possible nature of predate faulting, due to the erosional resistance of metamorphic rock types structures beneath the San Bernardino Mountains. The San Bernardino exposed in the Ord Mountains. Mountains are characterized by a diffuse zone of microseismicity which The Phelan Peak deposits, Ord River deposits, and late Pleistocene roughly coincides with the area of the physiographic range (Corbett, alluvial fans are all cut or deformed by the Deep Creek fault zone (Fig. 8). 1984). The lower limit of this seismicity beneath the range defines a Prominent scarps displace late Pleistocene fan surfaces up to 50 m along south-dipping plane descending from about 5 km beneath the Mojave the west flank of the Ord Mountains. Faceted spurs, steeply westward- Desert to about 12 km beneath the San Gorgonio massif, a distance of dipping fanglomerate-basement contacts, and sudden changes in fault about 40 km (Fig. 9; Corbett, 1984). The seismicity "floor" suggests a 10° strike along the western margin of the Ord Mountains suggest moderately south-dipping zone of decoupling or contrast in mechanical behavior of to steeply dipping dip-slip faults. Clast types in fanglomerates rule out any the crust beneath the San Bernardino Mountains. The scale of this struc- significant lateral component of motion on range-front faults (Meisling, ture is such that 6 km of northward motion on it would result in 1 km of 1984). At Deep Creek, a major reverse fault dips 45° to the east and places uplift across the entire northern plateau of the San Bernardino Mountains. quartz-monzonite over Phelan Peak deposits (U.S. Army Corps of Engi- We use this seismicity floor as a basis for locating our proposed deep neers, 1982). No north-block-down, range-front faults are evident west of crustal detachment in the discussion of tectonostratigraphic evolution that Silverwood Lake. follows. Most of the relief on the Deep Creek fault zone appears to have At their western end, thrusts of the North Frontal thrust zone turn developed in middle Pleistocene time, suggesting that uplift may have southwest and joint two prominent northeast-trending faults, herein propagated westward from the central range front area. Clasts from the named the "Tunnel Ridge fault" and "Deep Creek fault zone" (Figs. 1 Ord Mountains metamorphic pendant are not found in either the Pliocene

SOUTH NORTH

o -100.00 -80.00 -60.00 -40.00 -20.00 0.00 20.00 40.00 60.00 80.00 100. DIST (KM) N OF 34 N, @ 117 +-23 KM

Figure 9. Microseismicity beneath the central San Bernardino Mountains. Cross section shows depth of seismic events of quality A from October 1981 to August 1983 projected to longitude 117° (the center line of the northern-central plateau; see Fig. 1) from the region between 116°45' and 117°15'. SA and SJ indicate the location of the San Andreas and San Jacinto faults, respectively. NFT indicates the location of the North Frontal thrust system. Seismic events do not occur below a "floor" (dashed line) which rises from about 12 km beneath San Gorgonio Pass to less than 5 km beneath the Mojave Desert. Our proposed early Pleistocene detachment ramp coincides with the seismicity "floor," which is interpreted to reflect a contrast in mechanical behavior or decoupling in the upper crust. The seismicity ramp would project to the surface 20 to 30 km north of the north frontal thrusts; we postulate that it actually flattens and merges with a horizontal detachment plane at about 5 km beneath the Mojave block (figure adapted from Corbett, 1984).

Phelan Peak deposits or the early(?) Pleistocene Ord River deposits. At the 1984). Bedrock scarps (Weldon and others, 1981) suggest late Quaternary foot of the Ord Mountains, the Ord River deposits consist of fluvial sand motion on the Cleghorn fault. Scarp geometry is best explained by a and conglomerate containing well-rounded clasts of gneiss, quartz- combination of left-lateral and normal (north-block-down) motions. Al- monzonite, and occasional volcanic porphyries derived from the west- though evidence for left-lateral offset is restricted to the Cleghorn fault, central San Bernardino Mountains (Meisling, 1984). The Ord River many other faults in the western San Bernardino Mountains have similar deposits are conformably overlain by middle Pleistocene fanglomerates back-facing scarps, suggesting possible late Pleistocene extensions of the that are highly angular and composed almost exclusively of metamorphic area (Weldon, 1986). quartzite clasts derived from the immediately adjacent Ord Mountains Dip of foliation in the western San Bernardino Mountains defines a escarpment (Meisling, 1984). Thus the north-central plateau of the San broad northwest-plunging antiform, that we call the Western San Bernar- Bernardino Mountains was in place by middle Pleistocene time, and the dino arch (Figs. 3, 5, and 7). Bedding in the Crowder Formation dips prominent northwestern escarpments occupied much the same position consistently northward 25° to 30° along the north flank of the western San then as they do today. Bernardino Mountains. Foliation and cataclastic fabric in the underlying Late Quaternary Structures. Following the rapid early to middle batholithic rocks are subparallel to Crowder bedding, and generally dip Pleistocene uplift of the north-central plateau, the locus of uplift shifted to 25°-30° northward. South of the topographic crest of the western San south-vergent structures near the San Andreas fault in the westernmost and Bernardino Mountains, the foliation in the batholithic rocks dips generally south-central San Bernardino Mountains. This latest phase of uplift is southwestward. The strata of the Cajon Formation in Cajon Valley also interpreted to have occurred in response to deformation of the San An- define a broad arch (Fig. 5). The Western San Bernardino arch can be dreas fault itself. The younger system of middle to late Pleistocene struc- followed northwestward to Valyermo, expressed in progressively younger tures resulted in extremely rapid uplift in a narrow zone along the San strata to the northwest (Figs. 2 and 3). Andreas fault in the westernmost San Bernardino Mountains and San Uplift on the Western San Bernardino arch took place in middle to Gorgonio area (Fig. 1), producing the present form of the range. late Pleistocene time (Meisling and Weldon, 1982a; Weldon and Meisling, The Cleghorn fault is the principal fault in the westernmost San 1982; Weldon, 1986). The time of antiformal growth on the Western San Bernardino Mountains. It extends east from the Cajon Valley (Fig. 5) Bernardino arch is recorded in an angular unconformity on the north flank through the Silverwood Lake area (Fig. 7) to the Tunnel Ridge fault west of the structure (Fig. 3). At Cajon Pass, for example, both the Harold of Lake Arrowhead. It was named by Noble (1932) for exposures in Formation and Shoemaker Gravel dip 20° to 30° northward, but the older Cleghorn Valley. The 25-km-long trace of the Cleghorn fault defines two alluvium dips only about 5° northward. arcuate segments separated by a central cusp (Fig. 7). This shape suggests Arching began west of Lake Arrowhead between 2.0 and 1.5 Ma, as that it evolved out of the coalesced traces of two or more Cedar Springs recorded in the angular unconformity between the Phelan Peak deposits reverse faults (Weldon and others, 1981). Over most of its length, the and the Ord River deposits (Weldon, 1986). At Cajon Pass, the uncon- Cleghorn fault dips between 85° north and vertical (Fig. 3). When north- formity occurs between the Shoemaker Gravel and the older alluvium ward dip due to the Pleistocene Western San Bernardino arch is removed, (Fig. 3). The top of the Shoemaker Gravel is dated at 1.0 m.y., whereas the the Cleghorn fault dips about 60° north, consistent with its inferred origin older alluvium is younger than 0.7 m.y., indicating that arching occurred as a large member of the Cedar Springs reverse fault zone. The Cleghorn between about 1.0 and 0.7 Ma. Uplift rates for the Western San Bernar- fault cannot be traced west of the Squaw Peak fault but may extend an dino arch are high, perhaps exceeding 5 mm/yr (Weldon, 1986). Magne- unknown distance beneath the alluvium of Cajon Valley. At its east end, tostratigraphic studies show conclusively that the unconformity marginal to the Cleghorn fault merges with the northeast-trending Tunnel Ridge fault. the Western San Bernardino arch becomes progressively younger along Three short northeast-trending faults that diverge from the Cleghorn fault the arch to the northwest as far as Valyermo (Fig. 3), where arching has in the Silverwood Lake area (Fig. 7) show evidence of Pleistocene move- just begun. The angular unconformity within the Victorville fan deposits ment and are considered minor splays of the Cleghorn fault (California thus represents a moving locus of nondeposition or erosion, and uplift of Department of Water Resources, 1968a, 1968b). These splays may the Western San Bernardino arch must also have migrated northwestward transfer slip to the Deep Creek fault zone. with time. Unlike most members of the Cedar Springs zone, the Cleghorn fault While uplift along the San Andreas fault was increasing in intensity, exhibits abundant evidence of late Pleistocene motion, such as disturbed late Pleistocene deformation on the northern range front was waning and terraces and alluvial fans (Meisling and Weldon, 1982a). The Cleghorn manifesting a new diversity of structural style. Scarps and seismicity along fault truncates all structures of the Cedar Springs zone, with the notable the northern range front suggest a complex pattern of normal, thrust, and exception of the above-mentioned splays which show geomorphic evi- strike-slip motions (Meisling, 1984; Sadler, 1981). This pattern may indi- dence of minor Pleistocene motion (Weldon and others, 1981). cate a dominance of motion on northwest-southeast structural trends of the Total vertical separation on the Cleghorn fault is about 300 m, south Mojave Desert block over the east-west trends of the North Frontal thrust block down. Between 3.5 and 4.0 km of cumulative left-lateral motion system. Deep embayment of the northern range front, coupled with the on the Cleghorn fault is proposed on the basis of (1) the offset eastern limit discontinuous and varied nature of low scarps in the Pleistocene alluvial of the Cajon Formation and western limit of the Crowder Formation, fans, also supports the conclusion that patterns of deformation have (2) offset traces of north-plunging monoclines in the Crowder Formation changed from those of the peak period of uplift of the northern plateau. and Miocene weathering surface, and (3) the restoration of the pre-existing Waning deformation on the north frontal thrust systems seems an Cedar Springs reverse fault system (Meisling and Weldon, 1982a). Addi- unavoidable conclusion, in light of the modification of the range-front tional evidence for left-lateral offset is found in late Quaternary terrace thrusts by high-angle faulting. The northern range-front thrusts both cut deposits, stream courses, and a bedrock scarp (Weldon and others, 1981; (Shreve, 1968, p. 19; Sadler, 1981, p. 22) and are cut by (Sadler, 1981, Meisling and Weldon, 1982b). About 1 km of left-lateral motion on the p. 22, Fawnskin Quadrangle; Meisling, 1984, p. 209) northwest-trending, Cleghorn fault has occurred since ~0.5 Ma, and as much as 200 m has high-angle faults that characterize the Mojave Desert structural block. The occurred since about 50,000 to 100,000 yr ago, based on correlation of Sky High Ranch fault (Fig. 8) is such a northwest-trending, right-lateral terraces in Cleghorn Valley, Miller Canyon, and Cajon Creek (Weldon fault which cuts both range-front thrusts and Pleistocene fans west of and others, 1981; Meisling and Weldon, 1982a). These approximate Lucerne Valley. Clast types in the range-front fans show about 0.5 km of offsets yield an average slip rate for the Cleghorn fault of 2 to 3 mm/yr right-lateral displacement, with little or no evidence of vertical displace- (Weldon and others, 1981; Meisling and Weldon, 1982a, 1982b; Meisling, ment (Meisling, 1984). Both the Ord Mountains frontal fault zone and the

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Figure 10. Paleotectonic block diagram of the study area during Weathered erosion Lower Santa Ana development of the Crowder and Cajon basins in late early to early surface Sandstone (?) late Miocene time (18 to 10 Ma). The Crowder Formation was depos- Crowder ited on a weathered basement surface of low relief in a southward- Area of moderate Formation relief flowing drainage. Weathered basement was subaerially exposed Future site of throughout the San Bernardino Mountains and Mojave regions. San Andreas fault Crowder sediment was derived from volcanic, metamorphic, and plutonic source areas to the northeast. The Cajon Formation was deposited in a similar, yet separate, basin an unknown distance to the west. The oldest part of the Santa Ana Sandstone was probably deposited at this time, as well. A: Arrowhead; C: Cajon; H: Hesperia; P: Phelan.

18 to 10 Ma View southeast

Tunnel Ridge fault appear to merge with, or be cut by, the Sky High Ranch fault at its northwestern end. The Sky High Ranch fault appears to Inferred shoreline turn east to parallel the range front at its southeast end. It resembles the Helendale and Old Woman Springs faults in trend and sense of displace- Future site of ment (Sadler, 1981), and it truncates Pleistocene range-front alluvial fans. San Andreas fault It is therefore tentatively considered late Pleistocene in age. The ~l,000-m-high escarpment in Lucerne Valley has developed since the end of deposition of the Old Woman Sandstone less than 2 m.y. series of paleotectonic block models that illustrate the major tectonostrati- ago, which requires an uplift rate of at least 0.5 mm/yr (1,000 m in a total graphic phases in the late Cenozoic evolution of the northwestern San of 2 m.y.). Scarps in alluvial surfaces that are tentatively considered ~0.5 Bernardino Mountains (Figs. 10 through 15). These models span selected m.y. in age, based on correlation with the older alluvium surface of the periods of geologic time that have been chosen to reflect major tectonic Victorville fan (Meisling, 1984), are generally less than 25 m in height. breaks evident in the regional stratigraphy. Features on the block diagrams Taking the 25 m displacement of these ~0.5-m.y.-old alluvial surfaces into are shown in their present geographic location because data are inadequate account, early to middle Pleistocene range-front uplift rates would be to allow palinspastic restoration of most structural offsets. With the excep- greater than 0.65 mm/yr (975 m in ~1.5 m.y.) in contrast with late tion of the San Andreas and perhaps the Squaw Peak faults, however, the Pleistocene rates of 0.05 mm/yr (25 m in -0.5 m.y.). These admittedly effects are minor at the scale presented. crude estimates suggest a late Pleistocene decrease in rates of at least an order of magnitude, leading us to the conclusion that a pulse of uplift in Crowder and Cajon Basins early to middle Pleistocene time was followed by waning deformation on the northern range front. The Crowder and Cajon Formations were both deposited in middle Pleistocene uplift rates on the west flank of the Ord Mountains, Miocene continental basins fed by sediments from the north or northeast crudely estimated from scarp heights in alluvial fans and the distribution of (Fig. 10). Lithology of the Crowder Formation is consistent over 35 km early Quaternary deposits, also suggest a dramatic decrease in deforma- across the western San Bernardino Mountains in strata ranging in age from tion in late Pleistocene time. The 400-m-high western escarpment of the 17 to 9.5 m.y. The Cajon Formation is similarly uniform across 20 km of Ord Mountains largely developed since deposition of the Ord River depos- exposure in Cajon Valley within strata that range in age from 18 to less its, which contains few clasts of Ord Mountain affinity, at about 1.5 Ma. than 13.5 m.y. Although the two basins are now physically adjacent, This suggests an uplift rate of nearly 0.3 mm/yr (400 m in 1.5 m.y.). important differences between them indicate that they were separate sys- Scarps averaging about 50 m in height displace fans graded to the level of tems at the time of deposition (Dibblee, 1967; Woodburne and Golz, the older alluvium and estimated to be -0.7 m.y. in age, yielding a late 1972; Foster, 1980; Weidon, 1984; Reynolds, 1985). Clast suites in the Pleistocene uplift rate of 0.07 mm/yr (50 m in 0.7 m.y.). If 50 m of uplift Cajon and Crowder Formations are different. Members of Cajon Forma- since 0.7 Ma is removed, middle Pleistocene uplift rates on the Ord tion are more laterally continuous, better sorted, and better indurated than Mountains range front exceed 0.4 mm/yr (350 m in 0.8 m.y.). Thus a those of the Crowder Formation. The Cajon Formation is also far more change in style and decrease in rate of activity on the North Frontal thrust lithologically heterogeneous than the Crowder Formation, containing system accompanied the middle to late Pleistocene shift in the locus of thick red-bed sequences, lignites, limestones, and other rock types not uplift to structures nearer the San Andreas fault. found in the Crowder Formation. Although the Crowder Formation spans more time than the Cajon Formation, the Crowder Formation is less than TECTONOSTRATIGRAPHIC EVOLUTION half as thick, and the timing of coarse- versus fine-grained sedimentation is different in the two basins. The two basins are therefore shown as sepa- We have documented two major pulses of late Cenozoic deformation rated by an unknown distance in Figure 10. and uplift in the western San Bernardino Mountains in late Miocene to The Squaw Peak Thrust System earliest Pliocene and Quaternary time, bracketed by three distinctive tectonostratigraphic packages, of Miocene, Pliocene, and Pleistocene age. In Late Miocene uplift of the ancestral San Bernardino Mountains is this section, we integrate our structural and stratigraphic field data into a attributed to a major episode of deformation along the Squaw Peak and

Cedar Springs fault zones of the Squaw Peak thrust system (Fig. 11). We angle nature of the fault in westernmost (Fig. 5) and easteinmost (Fig. 6) speculate that this thrust system may have extended to the east to include exposures is more suggestive of thrust-style deformation. We interpret the parts of the Santa Ana thrust, and to the south to include the Liebre Squaw Peak fault as a major northwest-southeast-trending thrust, and Mountain fault system, now displaced 160 km to the northwest on the consider the north-south-trending, high-angle segment to be a right-lateral opposite side of the San Andreas fault. In our interpretation, detritus from tear or lateral ramp in an otherwise south-southwest-directed compres- the northwestern San Bernardino Mountains were shed south, perhaps sional system. feeding the late Miocene Ridge Basin. The San Gabriel fault zone was the This interpretation is strongly supported by the geometry of the active member of the San Andreas transform system at this time (Crowell Cedar Springs reverse-fault zone. The arcuate shape and offset constraints and Link, 1982). on faults in the Cedar Springs zone suggest that they are scoop-shaped and Contrasts in stratigraphy, geologic history, and structural style across flatten northward within a few kilometers of the surface (Figs. 3 and 7; the Squaw Peak fault suggest that displacement on it must have been at Weldon and others, 1981). We therefore interpret the Cedar Springs faults least several kilometers, and perhaps as great as several tens of kilometers. as upper-plate imbricates of the Squaw Peak thrust and infer that they Juxtaposition of the separate Crowder and Cajon Basins requires signifi- flatten to join the master fault at depth (Fig. 3). The trend of the north- cant displacement on the Squaw Peak fault. If deposited in their current plunging folds matches the trend of the high-angle segment of the Squaw configuration, streams feeding the Cajon Basin must have crossed the Peak fault in the Cajon area, and we infer that this set of folds developed coeval Crowder Basin, which does not seem possible given the differences over subsurface tear faults in the Squaw Peak thrust itself. Because motion in sediment composition and stratigraphy. Major displacement on the on both east-west- and north-south-trending fold sets would be expected Squaw Peak thrust is also suggested by the contrast between early Miocene to have continued throughout motion on the Squaw Peak thrust, this deposition, downfaulting, and erosion of the marine Vaqueros Formation interpretation can also explain the observed crosscutting relationships. beneath the Cajon Formation and the relative tectonic stability implied by The Squaw Peak fault can be traced to the San Andreas fault (Fig. 6), preserved early Miocene subaerial weathering surfaces in the western San and similar deformation patterns can be extended throughout the south- Bernardino Mountains. Late Miocene faults and folds in the Cajon Forma- western San Bernardino Mountains. If the observed deformation is indeed tion (Woodburne and Golz, 1972) differ in both style, size, and trend from characteristic of the upper plate of the Squaw Peak thrust, its distribution those in the Crowder Formation. Finally, large displacements are sup- implies that the upper plate of the thrust extended several tens of kilome- ported by intense deformation along the Squaw Peak thrust, including ters southeastward, across and subparallel to the current trace of the San isoclinal folding along the fault north of Squaw Peak and a broad zone of Andreas fault. shearing south of the Cleghorn fault. Evidence for large displacement and irregular geometry on the The Phelan Peak Basin Squaw Peak fault could be consistent with either an oblique strike-slip or thrust origin for the structure. Strike-slip deformation was first proposed to During Pliocene time, the Phelan Peak deposits accumulated in a explain the apparent right-lateral drag folding of beds in the Cajon and long, narrow trough that developed just north of the present range front in Crowder Formations adjacent to the Squaw Peak fault (Fig. 5; Weldon, the northwestern San Bernardino Mountains (Fig. 12). Deposition of the 1984). Although the surface expression of fold geometries in the Crowder Phelan Peak beds spanned the period of time during which drainage in the Formation suggests right-lateral drag along the Squaw Peak fault, magne- western San Bernardino Mountains underwent a complete reversal in tostratigraphic data from the Crowder Formation suggest little folding direction from southerly flow in the Crowder and Cajon Formations to about vertical axes (Weldon and others, 1984; Winston, 1985). The low- northerly flow in the Victorville fan deposits. Coarse conglomerates in the

Figure 11. Paleotectonic block diagram Cedar Springs of the study area during deformation and fault system uplift associated with development of the Santa Ana Squaw Peak thrust system in early late Mio- Sandstone cene to early Pliocene time (10 to 5 Ma). The Squaw Peak Cajon and Crowder basins are juxtaposed, thrust Erosion surface and the weathered basement surface and Future site of of low relief overlying basins have been uplifted to form San Andreas fault the ancestral San Bernardino Mountains with Liebre thrust steep south-facing relief. Uplift and deforma- Ridge Basin tion are attributed to the south-directed Squaw Peak thrust system, made up of a San Gabriel fault major detachment (Squaw Peak fault), a series of upper-plate imbricates (Cedar Springs fault zone), and possibly deeper thrusts. The Squaw Peak thrust system probably crossed the site of the modern San Andreas fault, while transform motion was concentrated on the San Gabriel fault. We show San Bernar- dino Mountains rock types on the northeast flank of the Ridge Basin at Liebre Mountain. A: Arrowhead; C: Cajon; H: Hesperia; P: 10 to S Ma Phelan. View southeast Future San Andreas Cajon Valley fault (?) Cajon Formation

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Phelan Peak deposits are evidence of local relief. Fine-grained lithology accompanied uplift of the plateau to its present height of 1 km above the and lacustrine facies of the Phelan Peak deposits suggest drainage diversion Mojave Desert. Because the range-front thrusts do not appear to have more and sediment ponding. Lithologic similarity of the Phelan Peak deposits than about 3 km of displacement on them (Woodford and Harriss, 1928; over 50 km of exposure, coupled with the evidence for local relief, suggests Shreve, 1968; Baird and others, 1974), we speculate that the remaining deposition in a narrow west-northwest-trending structural trough. The displacement may be accommodated on a subhorizontal surface within the northwesterly trend of this trough, subparallel to the modern trace of the Mojave Desert block. Range-front warping would then be a geometric San Andreas fault, suggests a genetic relationship. consequence of northward movement of the San Bernardino Mountains There is a strong similarity in age and depositional setting between block over the change in dip of the detachment surface where it flattens to the Phelan Peak deposits and the Old Woman Sandstone. Transport direc- join zones of weakness beneath the Mojave Desert block. tions suggest that the Old Woman Sandstone was deposited in a narrow, In our interpretation, the Deep Creek fault zone and Tunnel Ridge downwarped "moat" peripheral to the northern San Bernardino Moun- fault are the surface expression of a lateral ramp or tear that defines the tains (Sadler, 1982b). The trough occupied by the Old Woman Sandstone westward limit of the detachment plane. Left-lateral motion on the Cleg- may have been connected with the Phelan Peak Basin (Sadler, 1982b). horn fault was probably contemporaneous with motion on the North Evidence of this link may be found in the fine-grained Rock Springs Road Frontal thrust system; our offset estimate of 4 km at 2 to 3 mm/yr implies deposits (Fig. 8; Meisling, 1984). that motion began between 1.3 and 2.0 Ma. Although early Pleistocene reverse/left-lateral motion on the Cleghorn fault is consistent with ob- The North Frontal Thrust System and Plateau Uplift served offsets, the precise relationship of the Cleghorn fault to the western termination of the North Frontal thrust system is difficult to determine The first phase of Quaternary deformation in the northwestern San because of its earlier history as part of the Cedar Springs system and its late Bernardino Mountains began in early Pleistocene time with uplift of the Pleistocene reactivation. north-central plateau along the North Frontal thrust system (Fig. 13). As the northern escarpment developed, it began to shed coarse alluvial debris The Western San Bernardino Arch northward onto the floor of the Mojave Desert. Fine-grained deposition continued in the Phelan Peak Basin, however, indicating that uplift had not The second phase of Quaternary deformation began in middle Pleisto- yet begun in the westernmost part of the range. The North Frontal thrust cene time with uplift of the western "wing" of the San Bernardino Moun- system extended westward to include the Deep Creek and Tunnel Ridge tains along the Western San Bernardino arch (Figs. 14 and 15). Activity fault zones, and probably terminated westward into the Cleghorn fault. on the North Frontal thrust system began to wane in middle Pleistocene We propose a model in which the northern plateau of the San Ber- time, as the locus of uplift shifted to a narrow zone along the San Andreas nardino Mountains was uplifted by northward motion on a deep crustal fault. Attitudes of foliation and strata in the western wing of the San detachment ramp which parallels or coincides with the south-dipping Bernardino Mountains and northeastern San Gabriel Mountains define a plane defined by the seismicity floor (Fig. 9). Northward motion on this narrow panel of uniform northward dip about 5 km wide and several tens detachment ramp offers a means of uplifting the entire San Bernardino of kilometers long (Fig. 3; and Dibblee, 1964b). Rapid northwestward Mountains plateau as an intact block, which is required by the observed migration of the locus of uplift in the western wing suggests that the lack of significant deformation in the interior of the plateau. Part of the causative agent in this deformation is moving with the rocks on the displacement on this ramp reached the surface as the North Frontal thrust southwestern side of the San Andreas fault. We hypothesize that the cause zone, and part of the displacement is inferred to have occurred on "blind" of uplift is actually the surface of the San Andreas fault itself, which we detachment(s) which extended beneath the Mojave Desert (Fig. 13). infer to step or bulge northward at a depth of several kilometers (Fig. 14). If the south-dipping seismicity floor represents a detachment ramp, its Motion of the Mojave Desert block over this bulge in the San Andreas gentle 10° dip suggests that about 6 km of northward motion would have fault would account for the rapid, migratory uplift of the western wing and explain the pattern of northward dips. The depth at which the San Andreas fault might step northeastward is not known, but we speculate Rock Springs that it may coincide with the intersection of the Cucamonga thrust and the Road deposits Phelan Peak

Figure 12. Paleotectonic block diagram of the study area during development of the Phelan Peak basin in Pliocene time (5 to 2 Ma). A marked change in depositional patterns throughout the western San Bernardino Mountains is reflected in the rocks of the Phelan Peak deposits, Old Woman Sandstone, and Santa Ana Sandstone. Deposi- tion appears to have been confined to a narrow east-west- to northwest-southeast-trending trough. Strata deposited in the Pliocene trough are characterized by distinctive fine-grained volcanogenic and lacustrine sequences with rapid facies and/or thickness variations. The existence of a region of uplift confining the trough on the south is supported by the occurrence of clasts of local affinity in the Phelan Peak deposits and Old Woman Sandstone. Uplift may have been an early expression of north-directed thrusting, or residual relief related to the Squaw Peak thrust system. The San Andreas fault was active at the western edge of the study area at this time. A: Arrowhead; fault (inactive) C: Cajon; H: Hesperia; P: Phelan.

Figure 13. Paleotectonic block diagram of the study Tunnel Ridge area during deformation and uplift associated with devel- fault San Andreas opment of the North Frontal thrust system in early Pleisto- fault cene time (2.0 to 1.5 Ma). Most of the relief on the North Frontal thrust system prominent north frontal escarpment of the San Bernardino Mountains developed by thrusting and warping during this Cushenbury Phelan Peak time. North-directed thrusts overrode the Old Woman Springs Formation deposits Sandstone and uplifted the broad central plateau of the San Bernardino Mountains. Early to middle Pleistocene Central San uplift of the plateau without disruption of internal drainage Gabriel Mountains is modeled by northward movement of the San Bernardino Mountains block up a detachment ramp at a depth of 5-12 Low relief km beneath the range. We interpret range-front warping as a geometric consequence of northward shallowing of the deep-seated master thrust system where it merges with the detachment horizon beneath the Mojave Desert. The Ord Mountains frontal faults and Tunnel Ridge fault are the surface expression of a lateral ramp in the detachment surface. West of these structures, deformation appears to have been taken up by predominantly left-lateral motion on the Cleghorn fault. A: Arrowhead; C: Cajón; H: Hespe- ria; P: Phelan.

Squaw Peak fault (inactive) San Andreas surface. A northeastward step in the San Andreas fault surface at depth would help to align the deeper trace of the fault in the the San Andreas fault, uplift of both the San Gorgonio block and the southern San Bernardino Mountains with its surface trace south of San western wing may be explained by oblique motion on the steep north- Gorgonio Pass (Fig. 1). dipping surface of the San Andreas fault itself. There are, in fact, some strong geometric similarities between the late Pleistocene deformation of the western wing and that of the San Gorgonio Implications for Uplift in the Central Transverse Ranges massif along the San Andreas fault to the southeast. Northward dip of the early Miocene weathering surface and overlying Santa Ana Sandstone Sadler and Reeder (1983; Sadler, 1981, 1982b) proposed that con- characterizes the San Gorgonio massif (Dibblee, 1964b, 1975a). Late vergence in the San Bernardino Mountains is the surface expression of a Pleistocene uplift rates for San Gorgonio Mountain have been estimated to basement-involved flower structure, drawing on analogous upwardly di- be 18 to 36 mm/yr (Morton and Herd, 1980). Furthermore, late Pleisto- verging fault geometries observed in both experimental and naturally oc- cene thrusting on the central segment of the Banning fault suggests that the San Gorgonio massif is being uplifted on north-dipping oblique-slip re- Western San verse faults (Matti and others, 1985). If these faults are considered a part of Bernardino arch

Victorville fan San Andreas fault Sky High Ranch fault San Jacinto fault Helendale Figure 14. Paleotectonic block diagram of the Cucamonga thrust study area during deformation and uplift associated with early development of the Western San Bernar- Eastern San Gabriel Mountains dino arch system in middle Pleistocene time (1.5 to 0.7 Ma). A decrease in rates of uplift on the northern range front accompanied a shift of the locus of uplift to the south and west side of the range. The San Gabriel Mountains crystalline terrane, which lies to the southwest of the San Andreas fault, was brought into position by strike-slip movement to be- come the dominant sediment source for the Victor- ville fan deposits. Late Pleistocene uplift of the westernmost San Bernardino Mountains ("western 1.5 to 0.7 Ma wing") is interpreted as a response to motion on a View southeast steep north-dipping ramp which may coincide, in part, with the San Andreas fault itself. A: Arrow- Squaw Peak head; C: Cajon; H: Hesperia; P: Phelan. fault (inactive)

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San Andreas fault

Older alluvium San Jacinto fault Sky High Ranch fault Figure 15. Paleotectonic block diagram Helendale of the study area during deformation and up- fault lift associated with migration of the Western Western San Bernardino arch San Bernardino arch system in late Pleisto- cene time (0.7 Ma to present). Deformation associated with the western wing has mi- Cucamonga thrust grated northwestward along the San Andreas fault to the vicinity of Valyermo. The north- Eastern San Gabriel Mountains western range front has been modified by a complex system of high-angle lateral, reverse, and normal faults which have followed pre- existing weaknesses. Although motion continues on the northern range front, rates are approximately an order of magnitude less than in the early to middle Pleistocene. A: Arrowhead; C: Cajon; H: Hesperia; 0.7 Ma to present P: Phelan. View southeast

the bend developed in response to left-lateral motion on the Pinto Moun- tain fault (Matti and others, 1985). It appears that the onset of rapid slip on curring strike-slip systems (Wilcox and others, 1973; Bartlett and others, the San Jacinto fault and the deflection of the smooth trace of the San 1981). They cite (1) en echelon depositional trends of the Santa Ana Andreas fault by the Pinto Mountains fault are related. In our models, Sandstone and Old Woman Sandstone, (2) symmetry of opposing frontal uplift of the north-central plateau was the first response to rapid conver- thrusts north and south of the range, and (3) the oblique orientation of the gence across the bend in the San Andreas fault, taking advantage of long axis of uplift relative to the trend of the San Andreas fault, as evidence pre-existing low-angle structures within the San Bernardino Mountains consistent with the relationship predicted for compressional structures and the Mojave Desert blocks. As the topographic load of the plateau along a major wrench zone (Wilcox and others, 1973). If basins and became an impediment to further uplift, a shift in the locus of deformation structures developed in en echelon systems during growth of a flower to the southern margin of the range was favored. In time, the San Jacinto structure, however, they should be roughly contemporaneous (Harland, fault, which lies south of the San Andreas fault, and the Pinto Mountain 1971; Lowell, 1972; Wilcox and others, 1973). The wide range in age of fault, which lies north, became separated by motion on the San Andreas depositional units and structural elements in the San Bernardino Moun- fault, localizing uplift in San Gorgonio Pass and the western wing at the tains seems inconsistent with the symmetrical evolution of a simple flower northern terminus of the San Jacinto fault. structure. Furthermore, if south-directed thrusting on the Santa Ana thrust Thus, the hypothesized northeastward bulge of the lower San were a late Miocene or early Pliocene event contemporaneous with mo- Andreas fault near the San Jacinto-Cucamonga-San Andreas junction tion on the Squaw Peak fault system, it may be unrelated to the Quater- may be the present-day subsurface expression of the left step that origi- nary uplift of the mountains. nated in San Gorgonio Pass due to motion on the Pinto Mountain fault. Several authors have proposed deep-seated, low-angle faults beneath The Banning thrust would be the surface expression of the step, and the the Mojave Desert and San Bernardino Mountains to explain observed deep step is northwest of the San Jacinto termination. If one imagines patterns of deformation in the eastern Transverse Ranges (Yeats, 1980, rocks underlying the Mojave Desert moving southeast along the San 1981; Powell, 1981; Silver, 1982). Hadley and Kanamori (1977) ad- Andreas fault, they would first be lifted up as they slid past the deep mani- vanced a hypothesis in which the plate boundary in the upper mantle lies festation of the step under the western wing, then slide undeformed until east of the crustal trace of the San Andreas fault in the region of the they were uplifted again at the surface manifestation of the step near San east-central Transverse Ranges and western Mojave Desert. Seismic first- Gorgonio Pass. motion studies (Webb and Kanamori, 1985) appear to confirm the presence of detachment-style earthquake events over a broad area, in- Implications for Offset on the Modern San Andreas Fault cluding the San Bernardino Mountains. If such detachment zones exist, it seems reasonable to assume that they might have interacted with the San Matti and others (1985) and Frizzell and others (1986) have noted the Andreas fault during the uplift of the San Bernardino Mountains. similarity between rocks in the Liebre Mountain area (Fig. 1) and the The migrating Quaternary pattern of deformation documented on the south-central San Bernardino Mountains. They proposed that the two Western San Bernardino arch suggests that steps, ramps, or bulges in the masses have been offset by the San Andreas fault about 160 km. The San Andreas fault at depth may be as effective as surface restraining-bends strong similarity of the arcuate style and east-west trend of fault patterns in in producing secondary compression. The observed pattern of migratory the Liebre Mountains crystalline block with those in the western San uplift implies that the causative bulge at depth is located near the north end Bernardino Mountains also support the correlation. The Liebre Mountain of the San Jacinto fault, at its junction with the San Andreas fault. When thrust system, which bounds the Liebre Mountain crystalline block to the the uplift of the San Bernardino Mountains began at 2 Ma, the north end south, could be the offset portion of the Squaw Peak thrust. Movement on of the San Jacinto fault was just southwest of the major restraining bend in the Squaw Peak thrust system is constrained to have occurred between 9.5 the San Andreas fault in San Gorgonio Pass (see Fig. 1; assuming 50 km of and 4 Ma. The timing of movement on the Liebre Movement thrust offset on the San Andreas fault at 2.5 cm/yr). It has been suggested that system between 8 and 4 Ma (Ensley and Verosub, 1982) is consistent with

motion on the Squaw Peak system. Although published cross sections of northern plateau and (2) late Pleistocene uplift along the San Andreas the Ridge Basin show the Liebre thrusts steepening with depth (for exam- fault. Early Pleistocene uplift is expressed in the broad north-central ple, Crowell and Link, 1982), we speculate that this low-angle structure plateau bordered on the north by thrusts and large monoclinal warps may project all the way to the San Andreas fault, and that the crystalline interpreted as the result of north-directed motion up a gently south-dipping rocks of Liebre Mountain may be an allochthonous sheet from the San detachment ramp beneath the range. Late Pleistocene uplift in the western- Bernardino Mountains. most part of the range is characterized by arching and northward tilting of Our hypothesis not only explains the origin of Mio-Pliocene relief in a narrow zone along the San Andreas fault which has migrated rapidly and the ancestral San Bernardino Mountains, but also places this relief north- steadily northwestward, as documented by dated unconformities in the east of the Ridge Basin. This provides a ready source for Ridge Basin Plio-Pleistocene stratigraphic record. The location of the latest uplift sug- sediments which contain clast types attributed to sources in the San gests that the structure responsible is the San Andreas fault itself, which is Bernardino Mountains and are inferred to have come from highlands to inferred to have a step or bulge in it at depth where it intersects the the northeast across the site of the San Andreas fault (Crowell, 1982). Cucamonga and San Jacinto faults. We speculate that this structural inter- Ramirez (1983) has proposed that the late Miocene to Pliocene Hungry section originated with, and has been offset from, the left step in the San Valley Formation of the Ridge Basin was derived from the eastern San Andreas fault at San Gorgonio Pass. Bernardino Mountains. We suggest that a more likely source for the sediment in the Hungry Valley Formation is the western San Bernardino ACKNOWLEDGMENTS Mountains and Cajon Pass area, because the eastern part of the range was not uplifted until the Quaternary time. Uplift of the ancestral San We would like to thank Jon Matti, Peter Sadler, Don Ross, Jay Bernardino Mountains in late Miocene to Pliocene time provides the Namson, Grant Cushing, Art Sylvester, and Travis Hudson for their criti- needed source, supplying debris derived from both the Miocene basins and cal reviews of this manuscript in its many stages of preparation. We underlying basement southward into the Ridge Basin. gratefully acknowledge cooperation, assistance, and input from Dick Correlation of the Liebre Mountain fault with the Squaw Peak thrust, Crook, Jack Marlette, John Foster, Joe Kirschvink, Bob Powell, Jon which supports the proposed match of basement rocks and total offset of Matti, Doug Morton, Fred Miller, Bob Reynolds, Lee Silver, Mike 160 km on the modern San Andreas fault (Matti and others, 1985), opens Woodburne, and Doug Winston. This paper is based on thesis work done up new possibilities for reconstruction of the San Andreas fault which by the writers at the Department of Earth and Planetary Sciences, Califor- incorporates significant horizontal displacements on low-angle structures. nia Institute of Technology; we thank Clarence Allen and Kerry Sieh for The difference between the observed offset of 160 km on the modern San their support and advice, and the Division staff for their generous help. We Andreas fault and the widely cited offset of up to 270 km for the entire San gratefully acknowledge support by the U.S. Department of the Interior, Andreas system (Crowell, 1982, p. 30) requires that about 100 km of right U.S. Geological Survey, Earthquake Hazard Reduction Program under slip be taken up on earlier structures (Powell, 1981). If any of these Contracts 14-08-0001-19754, 16774, 18285, 19756, and 21275, and by structures passed through the region of Squaw Peak thrusting, they may the California Department of Water Resources under Agreement B- have been displaced and obscured by low-angle deformation. 53653. Finally we would like to thank ARCO Oil and Gas Company for support and assistance. CONCLUSIONS REFERENCES CITED

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