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Southern California Geology

Major Faults of Southern California
Inland Empire Region

Text from USGS Open-File Report 92-354

Banning Fault


index map of faults and basement terranes Click on this thumbnail to view an index map of faults and basement terranes in the Inland Empire region of San Bernardino and Riverside Counties. The file is a 135Kb pdf graphic that requires a portable-document file reader to examine and read. Link to Adobe Acrobat Reader to download a cost-free version of a pdf reader. Depending on your browser's setup, you may read the file now or save it to your disk.

Table of Contents for the San Andreas Fault Zone (click on any item to jump to that section

Previous Investigations
Distribution and Geologic Setting
Western segment
Central segment
Eastern segment
Geologic History
Late Cenozoic right-lateral history
Quaternary History
Regional Correlation
The Banning-San Gabriel Connection via Neogene Faults in the Southeastern San Gabriel Mountains
Provincial Affinities
Correlation of Faults
Summary

Previous Investigations

Vaughan (1922) first mapped faults later referred to the Banning fault; his map shows these as unnamed faults that extend west from their juncture with the San Andreas fault in the east part of San Gorgonio Pass. It is clear from Vaughan's text (1922, p. 399-401) that he viewed the San Andreas fault as the dominant structure in San Gorgonio Pass; he attached no particular significance to the unnamed faults that he recognized to the west. Hill (1928) reinterpreted fault relations in San Gorgonio Pass and introduced the name "Banning fault" for the fault segments that Vaughan (1922) first identified. Although Hill (1928, plate II) did not specifically designate the Banning fault on his map, he evidently applied the name to a fault he showed extending from the east part of San Gorgonio Pass west to the San Jacinto fault and beyond; he did not indicate an identifiable extension of the Banning fault eastward into the Coachella Valley. Hill (1928, p. 142) indicated that the fault Vaughan had called the San Andreas in the east part of San Gorgonio Pass did not extend to the northwest as Vaughan believed, but instead continued west to the fault segment that Vaughan had not named or evaluated. Allen (1957) clarified many of the geologic and nomenclatural problems associated with the Banning fault zone, and his report has formed the basis for all later discussions of the zone. Allen recognized that the Banning fault not only is an important zone of crustal convergence, as indicated by the zone of thrust and reverse faults associated with the fault in San Gorgonio Pass, but also is an important strike-slip fault with as much as 11 to 19 km of right-lateral offset. Reexamination of the Banning fault by Matti and Morton (1982) enlarged on Allen's studies by refining the geologic history and tectonic role of the fault zone.

Distribution and geologic setting

The Banning fault can be identified or inferred over a distance of about 100 km between the Indio Hills and the San Jacinto fault. The fault zone today consists of western, central, and eastern segments, each having a unique geologic and geomorphic setting and each recording a distinctive depositional and tectonic history during Quaternary time. These Quaternary events have obscured the distribution and history of an ancestral strike-slip fault that originally formed a single continuous trace throughout the three geographic segments.

Western segment .--The western segment of the Banning fault extends from the San Jacinto fault east to the Calimesa area. This segment has no surface expression because it is covered by late Pliocene and Quaternary sediments, and the position of the ancestral strand can only be inferred on the basis of gravity data (Willingham, 1971, 1981) and indirect geologic evidence.

Central segment .--The central, or San Gorgonio Pass, segment of the Banning fault extends from Calimesa to the vicinity of Whitewater Canyon. This segment largely is obscured by Quaternary sedimentary deposits, and has been modified by Quaternary reverse, thrust, and wrench faults of the San Gorgonio Pass fault zone. Where it crops out east of Calimesa and north of Banning, the Banning fault dips steeply to the north and juxtaposes crystalline rocks of San Gabriel Mountains-type against late Cenozoic sedimentary rocks; these exposures probably represent the ancestral trace of the fault. East of Banning, the ancestral trace is enmeshed in the San Gorgonio Pass fault zone and has been reactivated and obscured by Quaternary reverse and thrust faults of that system. For example, between Cabazon and Whitewater canyon, crystalline sheets have been thrust southward over the ancestral Banning fault and over Tertiary and Quaternary sedimentary deposits along low-angle fault surfaces that locally are flat or even south-dipping (Allen, 1957; D.M. Morton and J.C. Matti, unpublished data).

Eastern segment .--The eastern, or Coachella Valley, segment of the Banning fault extends from the vicinity of Whitewater canyon southeastward to the southern Indio Hills, where it merges with the San Andreas fault. The trace of the segment is well defined by conspicuous linear vegetation traces (Allen, 1957, fig. 1 of pl. 6) and forms degraded scarps in alluvial units that are late Pleistocene and Holocene in age. No published studies address the Quaternary history of this segment of the fault.

Oblique aerial photograph looking east Oblique aerial photograph looking east at the Coachella Valley trace of the Banning Fault in the northern Coachella Valley, southern California. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

Geologic History

Late Cenozoic right-lateral history .--Evidence for at least 16 to 25 km of late Cenozoic right slip on the Banning fault is provided by geologic relations among Tertiary sedimentary rocks in the San Gorgonio Pass region. There, uppermost Miocene strata of the marine Imperial Formation south of the Banning fault have been displaced at least 11 km in a right-lateral sense from Imperial beds in the Whitewater area on the north side of the fault (Allen, 1957, p. 329). The 11-km separation is a minimum displacement, however, because the two sequences are not exact cross-fault counterparts: they differ in details of their physical stratigraphy and in their relations with underlying rocks. Moreover, their benthic foraminiferal assemblages suggest different paleogeographic settings for the two sequences (K. A. McDougall and J. C. Matti, unpublished data): Imperial beds south of the Banning fault represent a more offshore facies than beds north of the fault in the Whitewater area, which suggests that the southern sequence should be restored to a palinspastic position at least several kilometers farther offshore (southeast) from the more onshore Whitewater section.

In San Gorgonio Pass, the Painted Hill and Hathaway Formations (of Allen, 1957) provide additional evidence for right slip on the Banning fault. South of the fault, many conglomeratic beds in these units contain north-derived volcanic, plutonic, and gneissic clasts that could not have been derived from bedrock sources presently cropping out north of the fault (J. C. Matti and D. M. Morton, unpublished data). After about 140 km of right slip is restored on the Coachella Valley segment of the San Andreas fault (described above), restoration of 16 to 25 km of right slip on the Banning fault positions the Hathaway and Painted Hill Formations south of the southern Chocolate Mountains, where bedrock sources for some of their volcanic, plutonic, and metamorphic clasts can be found. However, the Hathaway Formation must have been deposited in a two-sided depositional basin having both northern and southern source areas because many of the unit's conglomeratic beds contain clasts of Peninsular Ranges-type from both lower and upper plates of the Eastern Peninsular Ranges mylonite belt (Matti and Morton, 1992).

Available evidence restricts right slip on the Banning fault to the late Miocene and earliest Pliocene. Displacements occurred after about 8 Ma because sedimentary and volcanic rocks displaced by the fault in San Gorgonio Pass are that age and younger (D. M. Morton, J. C. Matti, and J. L. Morton, unpublished data). Right slip occurred after about 7 Ma because the Banning fault has displaced the Imperial Formation 16 to 25 km and the unit appears to be about 7 m.y. old (Table 2). The late Miocene displacement episode may have extended into earliest Pliocene time, but recognition of Pliocene right slip on the Banning fault (if any) must await improved age control and facies analysis in the San Timoteo and Painted Hill Formations.

Quaternary history .--After the Banning fault was abandoned as a right-lateral strike-slip fault in earliest Pliocene time, the three segments of the fault had different geologic histories.

(1) The western segment did not generate ground ruptures of any kind during the Quaternary.

(2) The San Gorgonio Pass segment has been overprinted by compressional tectonism that created the San Gorgonio Pass fault complex--a later deformation that has been superimposed along the trend of the ancestral Banning fault but is not related kinematically to it.

(3) Unlike the western and central segments, the Coachella Valley segment has generated right-lateral displacements during Quaternary time. Paleocurrent directions and cobble compositions in Quaternary gravels of the eastern San Gorgonio Pass area indicate deposition by an ancestral Whitewater River; the clasts include a variety of bedrock stones of San Gabriel Mountains and San Bernardino Mountains type, and have southeasterly paleocurrents (J.C. Matti and B.F. Cox, unpubl. data). These data suggest that during late Quaternary time the gravels have been displaced about 2 to 3 km into San Gorgonio Pass by right slip on the Coachella Valley segment of the Banning fault (Sheet 2, F-F'). This figure probably represents the total amount of right slip on the segment since its Quaternary reactivation. Additional evidence for late Quaternary displacements on the Coachella Valley segment include right-laterally offset stream gullies cut into Pleistocene gravels between Whitewater Canyon and U.S. Highway 92 (Clark, 1984) and late Pleistocene gravel deposits in the Coachella Valley that may have been offset by the fault (Sheet 2, G-G'). The neotectonic trace of the segment probably corresponds to the position occupied by the ancestral Banning/San Gabriel fault in late Miocene and early Pliocene time.

Although the age span for late Quaternary displacements has not been established, the Coachella Valley segment of the Banning fault probably has generated Holocene as well as late Pleistocene displacements. The fault appears to have been the source for the M=5.9 1986 North Palm Springs earthquake. However, although secondary ground failures developed in the vicinity of the surface trace of the Banning fault (Morton and others, 1989), the earthquake did not generate tectonic ground ruptures. The hypocenter for the 1986 event was located approximately halfway between Banning fault and the Coachella Valley segment of the San Andreas fault, and focal-plane solutions indicate reverse dip-slip as well as right-slip movements (Jones and others, 1988). If the 1986 earthquake originated on the Banning fault, the structure must dip moderately to the north beneath the Coachella Valley and may join the Coachella Valley segment of the San Andreas at depth.

Regional correlation .--Its late Miocene right-lateral history indicates that the ancestral Banning fault was a strand of the San Andreas transform system during the same period when the San Gabriel fault was an active component of the system. Many workers have proposed that the west end of the Banning fault has been displaced by the San Jacinto fault, and that its offset counterpart must continue to the west. Allen (1957, p. 339) may have been the first to suggest that the Banning is "the offset segment of one of the prominent east-west faults of the San Gabriel Mountains", but he did not cite the San Gabriel fault by name. Sharp (1967, p. 726) suggested that the Sierra Madre-Cucamonga fault may be the offset counterpart of the Banning fault, a view shared by Dibblee (1975a, p. 134).

We conclude that the San Gabriel and Banning faults originally formed a single throughgoing right-lateral fault, and suggest that the 22 km of right-lateral displacement suggested by Ehlig (l968a; 1975, p. 184) for the north branch of the San Gabriel fault corresponds to the 16 to 25 km of displacement we recognize for the Banning fault between 7.5 and 4 or 5 Ma. This right-slip activity would have coincided with latest Miocene displacements on the San Gabriel fault in Ridge Basin, where major right-lateral activity on the San Gabriel fault ceased about 5 m.y. ago (Crowell, 1982, p. 35, fig. 12). In the earliest Pliocene, the San Gabriel-Banning strand was bypassed by the San Andreas system as the San Andreas fault (sensu stricto) evolved to the east.


The Banning-San Gabriel connection via Neogene faults in the southeastern San Gabriel Mountains

Our conclusion that the Banning and San Gabriel faults once were continuous does not follow obviously from what is known about the two faults because (1) the north and south branches of the San Gabriel fault cannot be traced easily through and beyond the eastern San Gabriel Mountains, (2) the Banning fault cannot be traced easily west of the San Gorgonio Pass region, and (3) Quaternary right slip on the San Jacinto fault has rearranged and obscured any throughgoing connection between the two older faults.

Rocks and structures in the southeastern San Gabriel Mountains provide insight into possible relations between the San Gabriel and Banning faults. In the southeasternmost part of the range, two distinct suites of crystalline rock occur (fig. 3; Dibblee, 1968a, 1982a, figs. 1, 2; Morton, 1975a, fig. 1; especially see Ehlig, 1975, fig. 1, and 1981, fig. 10-2): (1) Pelona Schist and structurally overlying granitoid and gneissic rocks that respectively form lower and upper plates of the Vincent thrust, and (2) a suite of granitoid rocks and prebatholithic metasedimentary rocks of uncertain provincial affiliation. Lower- and upper-plate rocks of the Vincent thrust are typical of those elsewhere in the San Gabriel Mountains (Ehlig, 1975, fig. 1; 1981, fig. 10-2). However, granitoid and metasedimentary rocks in the southeasternmost part of the range have enigmatic provincial affinities and are structurally isolated from typical rocks of San Gabriel Mountains-type. The two suites everywhere are separated by high-angle faults (fig. 3): (1) on the east, the two suites are separated by northwest-trending right-lateral faults traditionally assigned to the San Jacinto fault zone; (2) on the north, the two suites are separated by an east-trending zone of strike-slip faults referred to as the Icehouse Canyon fault; (3) on the west, the two suites are separated by the poorly studied Evey Canyon and San Antonio faults that traverse San Antonio Canyon (Ehlig, 1975, fig. 1; Morton, 1975a, figs. 1, 2; Dibblee, 1982a, fig. 1). We propose that these three fault zones and the granitic and metasedimentary terrane they enclose can be used to establish a connection between the San Gabriel and Banning faults.

Our correlation of the San Gabriel and Banning faults depends heavily on two inferences: (1) the enigmatic terrane of granitoid and metasedimentary rock in the southeastern San Gabriel Mountains is Peninsular Ranges-type rock that has been juxtaposed against San Gabriel Mountains-type rock (Matti and others, 1985; Matti and Morton, 1992), and (2) right-lateral displacement on a throughgoing San Gabriel-Banning fault contributed to this juxtaposition. In order to defend the San Gabriel-Banning connection we must defend these two propositions.

Provincial affinities. --Crystalline rocks enclosed by the "San Jacinto", Icehouse Canyon, and Evey Canyon faults are broadly similar to rocks of Peninsular Ranges-type. The granitoid rocks consist mainly of foliated biotite-hornblende quartz diorite and tonalite (Ehlig, 1975, 1981; Morton, 1975a; Evans, 1982; Morton and others, 1983; Morton and Matti, 1987, Plate 12.1, rock units Kqd and Kd; May and Walker, 1989) that locally are intruded by small bodies of monzogranite and garnetiferous muscovite-bearing monzogranite (Morton and Matti, 1987, Plate 12.1, unit Kqm). These granitoids probably all are Cretaceous, although emplacement ages of about 87 Ma (May and Walker, 1989) have been moderately to strongly reset by an early Tertiary thermal event (Miller and Morton, 1980). The quartz dioritic and tonalitic rocks are progressively more deformed southward toward the San Gabriel Mountain front, culminating in an east-trending zone of mylonite and mylonitic quartz diorite first studied by Alf (1948) and Hsu (1955) and later mapped by Morton (1975a, 1976; Morton and others, 1983; Morton and Matti, 1987, Plate 12.1). At the mountain front, crystalline rocks structurally beneath the mylonite belt consist of multiply-deformed gneiss, quartz diorite, and garnetiferous hypersthene-bearing retrograded granulite rocks. Most workers assign a Precambrian age to these deformed rocks based on their unique structural and metamorphic history, but Ehlig (l975, fig. 1; 1981, fig. 10-2) believed them to be younger and grouped them with other granitoid rocks in this part of the southeastern San Gabriel Mountains that he inferred to be Mesozoic in age. Granulite-facies metamorphism appears to be early Cretaceous in age on the basis of a 108-Ma U-Pb age obtained by May and Walker (1989) from a late-stage syntectonic pyroxene-plagioclase pegmatite associated with the retrograded granulite rocks.

Prebatholithic metasedimentary rocks intruded by the granitoids consist of amphibolite-grade marble, metaquartzite, and pelitic gneiss and schist that locally is graphitic. North of the main mylonite belt the metasedimentary rocks occur as large bodies and pendants; south of the mylonite belt the metasedimentary rocks occur only as thin septa and xenoliths. Most workers assign a late Proterozoic or early Paleozoic age to the sedimentary protoliths based on their general similarities to rocks known to be that age elsewhere in southern California.

Matti and others (1985, p. 3) speculated that the crystalline terrane enclosed by the "San Jacinto", Icehouse Canyon, and Evey Canyon faults is part of the Peninsular Ranges block. We reiterate this proposal here, and support it with several lines of evidence:

(1) Quartz diorite and tonalite in the southeasternmost San Gabriel Mountains are similar to quartz diorite and tonalite in the northeastern Peninsular Ranges block: the granitoids appear to have similar intrusive ages that have been reset by a younger thermal event (Miller and Morton, 1980, and unpublished K/Ar data), and they have similar major and minor element compositions (Baird and others, 1974, 1979).

(2) The mylonitic belt of ductile deformation that separates foliated granitoid rocks from highly deformed retrograded granulites in the San Gabriel Mountains is similar to the Eastern Peninsular Ranges mylonite zone of Sharp (1979), except that mylonitic lineations are oriented down-dip in most parts of the Eastern Peninsular Ranges mylonite zone (Erskine, 1985) but are oriented parallel to strike in the San Gabriel Mountains (Morton and Matti, 1987; May and Walker, 1989). This discrepancy could reflect either spatial differences in the orientation of the syntectonic strain field within the regionwide mylonite belt (May and Walker, 1989) or localized post-tectonic rotation of the mylonite fabrics around vertical and/or horizontal axes. Other workers have pointed out similarities between the two mylonite belts (Hsu, 1955; Ehlig, 1975, p. 183; Erskine, 1985).

(3) Bodies of garnetiferous muscovite monzogranite structurally overlying the mylonite belt in the southeastern San Gabriel Mountains (Morton and Matti, 1987, Plate 12.1, units Kg and Kgc) are comparable to similar bodies structurally above and beneath the Eastern Peninsular Ranges Mylonite Belt in the Santa Rosa Mountains (Matti and others, 1983b, unit Mzlm).

(4) Prebatholithic metasedimentary rocks in the southeasternmost San Gabriel Mountains are broadly similar to metasedimentary rocks structurally above and beneath the mylonite belt in the San Jacinto and Santa Rosa Mountains (Powell, 1982a; Matti and others, 1983b, units gsc, mq, and mc; Erskine, 1985). Powell (1982a) has grouped these and similar prebatholithic metasedimentary rocks into his Placerita terrane.

Correlation of faults .--If the enigmatic granitoid and metasedimentary rocks in the southeastern San Gabriel Mountains are rocks of Peninsular Ranges-type, then their present tectonic juxtaposition against rocks of San Gabriel Mountains-type must be explained by identifiable structures--ductile, brittle, or both. May and Walker (1989) proposed that the enigmatic suite (their Cucamonga and San Antonio terranes) was juxtaposed against rocks of San Gabriel Mountains-type in late Cretaceous or early Paleogene time by ductile oblique left-lateral convergence within a mylonite zone locally preserved along the boundary between the two terranes (May, 1990). Regional telescoping along Cretaceous or Paleogene ductile zones may well account for the primary structural geometry between the two crystalline terranes. However, we propose that their present juxtaposition in the San Gabriel Mountains resulted from brittle displacements on throughgoing Neogene faults whose dismembered segments now are represented by the "San Jacinto", Icehouse Canyon, and Evey Canyon faults.

If these three faults are Neogene, and once were continuous with other Neogene strike-slip faults in southern California, then their regional counterparts should be nearby and should have a similar relationship to rocks of San Gabriel Mountains-type and Peninsular Ranges-type. One candidate is the Banning fault, which in the San Gorgonio Pass region intervenes between rocks of San Gabriel Mountains-type to the north and rocks of Peninsular Ranges-type to the south. Likewise, in the southeastern San Gabriel Mountains, the "San Jacinto" and Icehouse Canyon faults intervene between rocks of San Gabriel Mountains-type to the north and east and rocks we identify as Peninsular Ranges-type to the south (fig. 3). The structural position occupied by the "San Jacinto" and Icehouse Canyon faults thus is similar to that of the Banning fault (fig. 3, map sheets 1 and 2). We use this relation as a primary basis for our proposal that the Banning, "San Jacinto", and Icehouse Canyon faults once were continuous and shared common movement histories.

We complete the San Gabriel-Banning connection by projecting the Banning-"San Jacinto"-Icehouse Canyon trend westward beyond San Antonio Canyon. A likely candidate for this continuation is the east-oriented north branch of the San Gabriel fault--a structure that has about 22 km of late Miocene right slip (Ehlig, 1973, 1975, 1981) that compares well with 16 to 25 km of late Miocene right slip on the Banning fault in San Gorgonio Pass. Similarities in their movement histories suggest that the faults are related, and we propose that they once formed a single throughgoing right-lateral trend that since has been disrupted by about 25 km of Quaternary right slip on the "San Jacinto fault" and about 3 km of Quaternary left slip on the San Antonio fault. The former has displaced the east end of the Icehouse Canyon fault from the Banning fault; the latter has displaced the west end of the Icehouse Canyon fault from the north branch of the San Gabriel fault (fig. 3).

This scenario provides a connection between the north branch of the San Gabriel fault and the Banning fault but leaves unresolved the connection (if any) between the south branch of the San Gabriel fault and the Banning. Most workers follow Crowell (1975a,b, 1981, 1982) and Ehlig (1975, 1981) who convey 38 km of right slip on the south branch eastward toward the San Andreas or Banning faults by way of connecting faults in the southeastern San Gabriel Mountains (Dibblee, 1968a, fig. 1; Ehlig, 1973, fig. 1 and 1981, fig. 10-2; Crowell, 1975a, fig. 1, 1975c, p. 208-209, 1982a, fig. 1). To these scenarios we add one in which the San Gabriel fault (south branch) traverses the south front of the San Gabriel Mountains, is displaced about 3 km left laterally by the San Antonio fault, and continues east to the "San Jacinto fault" by way of the Stoddard Canyon fault and thence to the Banning fault (fig. 3).

As developed to this point, our model for the San Gabriel-Banning connection accommodates three concerns: (1) it accounts for two of the three Neogene faults in the southeastern San Gabriel Mountains that separate Peninsular Ranges-type rock from San Gabriel Mountains-type rock; (2) it ties together similar right-slip histories on the San Gabriel fault (north branch) and the Banning fault; and (3) it provides a testable hypothesis for a connection between the San Gabriel fault (south branch) and the Banning fault. This model also provides a testable solution for two other problems: the amount of right slip on the San Gabriel fault (south branch) and the regional distribution of the left-lateral Malibu Coast fault zone. We propose that a solution to both problems is provided by the Evey Canyon fault--the third Neogene fault in the southeastern San Gabriel Mountains that juxtaposes rocks of Peninsular Ranges-type and San Gabriel Mountains-type.

Pivotal to this analysis is the regional distribution and tectonic role of the Malibu Coast-Santa Monica-Raymond fault zone--a major left-lateral fault that trends easterly from the California coast to the south-frontal fault zone of the San Gabriel Mountains. Barbat (1958, fig. 2, p. 64) originally suggested that the Santa Monica segment of the zone generated about 13 km of left slip. Subsequently, Yeats (1968), Yerkes and Campbell (1971), Jahns (1973), Campbell and Yerkes (1976), and Truex (1976) presented evidence for 60 to 90 km of left slip on the Malibu Coast-Santa Monica-Raymond trend during the middle Miocene (about 16 Ma to 12 Ma). Based on the premise that a left-lateral fault of this scale must be of regional extent, these workers extended the Malibu Coast-Raymond system eastward through the Cucamonga fault zone (Barbat, 1958, fig. 1; Jahns, 1973, fig. 5; Campbell and Yerkes, 1976, fig. 1) and ultimately through San Gorgonio Pass by way of the Banning fault (Jahns, 1973, fig. 6-9, p. 166).

We accept the premise that the Malibu Coast-Santa Monica-Raymond fault is a major left-lateral structure that should be recognizable east of the frontal-fault zone of the San Gabriel Mountains. However, rather than merge the Malibu Coast system with the frontal fault zone and extend it east through the Cucamonga fault, we suggest that the Malibu Coast system may have been truncated by the south branch of the San Gabriel fault and displaced right laterally from a cross-fault counterpart located to the east. We propose that the Evey Canyon fault is the displaced continuation of the Malibu Coast-Santa Monica-Raymond trend, which requires that the Evey Canyon is a left-lateral fault that juxtaposed Peninsular Ranges-type rocks against San Gabriel Mountains-type rocks (fig. 3; Matti and others, 1985). The once continuous middle Miocene Malibu Coast-Santa Monica-Raymond-Evey Canyon fault was truncated in late Miocene time by the San Gabriel fault (south branch), which has displaced the southwest end of the Evey Canyon fault about 22 km from the east end of the Raymond fault. This forms the basis for our displacement estimate of 22 km for the south branch, and underlies our conclusion that the San Gabriel fault (north and south branches) has no more than 44 km of right slip (22 km + 22 km).

Our conclusion that the Evey Canyon fault is a left-lateral structure that once was part of the Malibu Coast system has implications for the Icehouse Canyon, "San Jacinto", and Banning faults. The northeast end of the Evey Canyon fault terminates against the north-trending San Antonio fault, a structure that has displaced the north and south branches of the San Gabriel fault about 3 km from their inferred cross-fault counterparts (the Icehouse Canyon and Stoddard Canyon faults, respectively; fig. 3). Oblique convergence between the Evey Canyon and San Antonio faults precludes accurate determination of their geometric relations, and it is likely that left slip on the north-trending San Antonio fault has occurred by reactivation of north-trending segments of the Evey Canyon fault. Despite these complications, we propose that restoration of 3 km of left slip on the San Antonio fault would align the northeast end of the Evey Canyon fault with the Icehouse Canyon fault, which thereby is potentially continuous with the left-lateral Malibu Coast-Evey Canyon trend.

This implication appears to be at odds with our proposal that the Icehouse Canyon fault is a right-lateral component of the San Gabriel-Banning connection--a contradiction that becomes more profound if large left-lateral displacements proposed for the Malibu Coast-Raymond system are projected east along an Evey Canyon-Icehouse Canyon-"San Jacinto" trend that ultimately includes the Banning fault. This apparent contradiction can be resolved in two ways:

 

First , fault zones established initially by middle Miocene left slip within a throughgoing Malibu Coast-Santa Monica-Raymond-Evey Canyon-Icehouse Canyon-"San Jacinto"-Banning trend could have been reactivated as zones of late Miocene right slip when the Banning-"San Jacinto"-Icehouse Canyon segment of the Malibu Coast system was incorporated into the San Gabriel-Banning system (a concept suggested to us generically by T.H. McCulloh).

Second , May and Walker (1989, p. 1262-1263) suggested that kinematic models for tectonic rotations in southern California (Luyendyk and others, 1980; Hornafius and others, 1986) require that left slip on the Malibu Coast-Raymond system should decrease to the east, thereby eliminating the need to extend large left-lateral displacements throughout the entire length of the Malibu Coast system. May and Walker (1989) adopted our earlier suggestion (Matti and others, 1985) that the Evey Canyon fault is part of the Malibu Coast system, but suggest that large left-lateral displacements (60 to 90 km) proposed for western segments of the Malibu Coast-Evey Canyon system diminish to about 20 km in the southeastern San Gabriel Mountains. By this interpretation, left slip may not be significant on the Icehouse Canyon-"San Jacinto"-Banning segment of the Malibu Coast system, and may not even have extended all the way east on the Banning fault. This would obviate the need to extend large displacements on the Malibu Coast system east through San Gorgonio Pass (Jahns, 1973), and would fit the lack of evidence for Neogene left slip on the Banning fault.

We cannot resolve uncertainties involving the regional distribution of the middle Miocene Malibu Coast system. However, rather than arbitrarily ending the Malibu Coast-Santa Monica-Raymond system at the northeast end of the Evey Canyon fault, we incorporate modest left slip on the Evey Canyon-Icehouse Canyon-Banning segment of the Malibu Coast system. We estimate this left slip to be about 25 km based on our speculation that the Eastern Peninsular Ranges mylonite zone in the southeastern San Gabriel Mountains has been displaced about 25 km from a possible cross-fault counterpart buried beneath Neogene and Quaternary sedimentary rocks between the Santa Monica Mountains and Verdugo Hills.

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Summary

In the southeastern San Gabriel Mountains and San Gorgonio Pass region, several strike-slip faults traverse the boundary zone between rocks of Peninsular Ranges-type and San Gabriel Mountains-type. Our interpretation of movement histories for these faults requires that they are local segments of regionwide strike-slip fault trends that include (1) the middle Miocene Malibu Coast-Santa Monica-Raymond-Evey Canyon-Icehouse Canyon-"San Jacinto"-Banning left-lateral system and (2) the late Miocene San Gabriel-Icehouse Canyon-Stoddard Canyon-"San Jacinto"-Banning right-lateral system. The Malibu Coast-Banning system produced about 25 km of left slip in middle Miocene time (14 to about 12 Ma). In late Miocene time (about 10 Ma), the Malibu Coast-Banning left-lateral system was dismembered and locally reactivated by displacements within the right-lateral San Gabriel-Banning system.

During late Miocene time the San Gabriel-Banning fault formed a single, throughgoing right-lateral structure that was part of the San Andreas transform system. The middle segment of this throughgoing structure, now located in the San Gabriel Mountains, developed two discrete strands that probably formed sequentially. To the northwest, these two strands coalesced to form a single strand (the San Gabriel fault) that traverses the west margin of Ridge Basin; to the southeast, the two strands coalesced to form a single strand (the Banning fault) that traverses the San Bernardino Valley, San Gorgonio Pass, and Salton Trough. Within the southeastern San Gabriel Mountains, remnants of the two San Gabriel-Banning fault strands are represented by the Stoddard Canyon, Icehouse Canyon, and "San Jacinto" faults.

The throughgoing San Gabriel-Banning system generated about 44 km of right-lateral displacement in late Miocene to earliest Pliocene time (10 to 4 or 5 Ma). The south branch of the San Gabriel fault probably is the older fault strand, and we propose that from about 10 Ma to about 7.5 Ma it generated about 22 km of displacement that displaced the Evey Canyon fault from the Raymond fault. This displacement must have extended along the Banning fault, but rocks of this age that would record right-lateral movements from 10 Ma to 8 Ma are not exposed in San Gorgonio Pass. If the San Gabriel-Banning fault gradually was bowed into a convex-west arc during this early history, the curvature ultimately could have become too extreme to have accommodated efficient right-slip displacements. We propose that this happened about 7.5 m.y. ago, at which time the south branch was abandoned and right slip stepped inboard (east) to the north branch. From about 7.5 Ma to about 4 or 5 Ma the north branch generated 22 km of displacement (Ehlig, 1973, p. 174)--a displacement recorded by upper Miocene sedimentary and volcanic rocks that are traversed by the Banning fault in San Gorgonio Pass. According to this model, total right slip on the combined north and south branches of the San Gabriel-Banning fault is about 44 km--16 km short of the 60 km proposed by Crowell (1982a) and by Ehlig and Crowell (1982). Major right-lateral activity on the throughgoing fault ceased about 4 or 5 m.y. ago (Crowell, 1982a, p. 35, fig. 12) as the San Gabriel-Banning strand was bypassed by the San Andreas system and as strands of the San Andreas fault proper evolved to the east.

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