Late Cenozoic Tectonics of the East Ventura Basin,Transverse Ranges, California1Robert S. Yeats, Gary J. Huftile, and Leonard T. Stitt2

ABSTRACTThe east Ventura basin originated in the middleMiocene as a rift system bounded on one side by theOak RidgeSimi Hills structural shelf and on theother side by a granitic ridge parallel to the SanGabriel fault. This fault began accumulating rightslip 1012 m.y. ago at a rate of 4.59 mm/yr(depending on whether total slip is 45 or 60 km),slowing to about 1 mm/yr in the Quaternary. Northof the Santa Clara River, rifting ended prior to deposition of the uppermost Miocenelower PlioceneTowsley Formation. South of the Santa Clara River,the rift axis shifted southwest toward the OakRidgeSimi Hills shelf as the Towsley Formationaccumulated against a normal-fault ancestor of theSanta Susana fault. A change to contractile tectonicsoccurred in the Pliocene with deposition of the Fernando Formation, when the Newhall-Potrero anticline developed as a monocline above a blindreverse fault; the Pico anticline to the southeast andthe Temescal and Hopper RanchModelo anticlinesto the northwest may have a similar origin. Tectonicinversion and displacement on the southwest-verging Santa Susana fault began about 0.5 Ma based on

Copyright 1994. The American Association of Petroleum Geologists. Allrights reserved.1Manuscript received, February 10, 1992; revised manuscript received,March 1, 1994; final acceptance, March 4, 1994.2Department of Geosciences, Oregon State University, Corvallis, Oregon97331.This study was supported by contracts 14-08-0001-G1372 and 14-080001-G1798, which built on earlier work supported by contracts 14-08-000115886, 14-08-0001-16747, 14-08-0001-19138, and 14-08-0001-21279 fromthe Earthquake Hazards Reduction Program of the U. S. Geological Survey.Additional support was provided by petroleum-industry grants to the SouthernCalifornia Fault Studies program at Oregon State University. Leonard Stittreceived salary support from Arco Exploration and from Conoco, Inc. for partsof the study. Discussions with Bill Cotton, John Crowell, Tom Dibblee, PerryEhlig, Ed Hall, Tom Hopps, Shaul Levi, Dick Saul, Jerry Treiman, HaroldWeber, and Jack West helped sharpen our understanding, although HaroldWeber was never able to convince us that the San Gabriel fault was not amajor strike-slip fault. John Crowell, Tom Dibblee, and Jerry Treiman provided us with unpublished geologic maps, and Crowell, in addition, provided adetailed review of the paper. Margaret Mumford prepared the illustrations. Weespecially acknowledge the detailed field and subsurface work of Alton Albin,Mark Butler, Ibrahim emen, Kevin Lant, Lu Huafu, Kevin Lant, JimMcDougall, Fred Nelligan, Rick Ricketts, Jill Schlaefer, Kermit Shields, andKeith Whaley, which was the foundation on which this study was built.

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appearance of locally-derived clasts in the upperSaugus Formation and its equivalents, and continuestoday, along with the southwest-verging SanCayetano fault farther west. Also active are northeast-verging backthrusts occurring in the east Ventura basin fold belt, and a segment of the San Gabrielfault which now acts as a northeast-dipping obliqueslip reverse fault.Northeast-trending discontinuities and structuresdivide the present deformation zone into four segments. In the Hopper Canyon segment at the westend of the area in the west Ventura basin, the SanCayetano fault places Miocene Modelo Formationover Pliocene-Pleistocene strata more than 5 kmthick, largely at maximum burial. To the southeast,the Newhall-Potrero segment is characterized bynorth-vergent backthrusts within the east Venturafold belt and by southward thrusting of the basinsequence (tectonic inversion) over the structuralshelf on the Santa Susana fault. Farther southeast,the Placerita segment is marked by reverse faultingon both the Santa Susana fault and the San Gabrielfault. Southeast of the basin in the San Fernando Valley, the Sylmar segment contains a thick PliocenePleistocene sequence overridden by basement rocksof the San Gabriel Mountains as well as the southverging Mission HillsGranada Hills and NorthridgeHills fault zones.INTRODUCTIONThe Transverse Ranges of California cut across thenorthwest grain of the Coast Ranges to the north andthe Peninsular Ranges to the south (Figure 1, inset).In the center of the western Transverse Ranges lies amajor oil-producing province: the Ventura basin.The basin is divided into the west Ventura basin,which produces oil mainly from Pliocene and lowerPleistocene strata, and the east Ventura basin, whereoil is produced mainly from upper Miocene andlower Pliocene strata.In the west Ventura basin, the valley of the SantaClara River and its extension offshore in the SantaBarbara Channel contain one of the thickest sectionsAAPG Bulletin, V. 78, No. 7 (July 1994), P. 10401074.

Figure 1Index map of the east Ventura basin and San Gabriel fault. Base map from Jennings (1975). Basement geology of San Gabriel Mountains fromEhlig (1975). Abbreviations: AM, Alamo Mountain; CF, Canton fault; CR, Caliente Range; F, Fillmore; FM, Frazier Mountain; MC, Modelo Canyon; MF,Morales fault; MG, Mission HillsGranada Hills fault; N, Newhall; NH, Northridge Hills fault; P, Piru; S, Saugus; SB, Sylmar basin; SGF, San Gabriel fault.

Yeats et al.1041

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East Ventura Basin

of Pliocene-Pleistocene strata in the world. This section is bounded by active reverse faults: the RedMountain fault and San Cayetano fault on the north,and the Oak Ridge fault on the south. The RedMountain, San Cayetano, and Santa Susana faults arepart of a south-verging zone of reverse faults extending from the western Santa Barbara Channel to theSan Andreas fault near San Bernardino (Figure 1). Inthe west Ventura basin, this Quaternary fault systemcoincides with the northern edge of the thickPliocene-Pleistocene trough.In the east Ventura basin east of the town of Piru(Figures 1, 2), the south-verging Santa Susana fault issouth of the trough, not north of it, and this faultmarks a zone in which the trough sequence is thrustsouthward over its own structural shelf (Yeats,1979), a process known as tectonic inversion. Thetrough sequence and its northern structural shelf aredeformed in a fold belt. Young reverse faults cutthese folds, but except for two (the Holser and DelValle faults), displacements are relatively small.The east Ventura basin is crossed by the SanGabriel right-slip fault, a former strand of the SanAndreas system. Upper Miocene strata of the east Ventura basin cannot be correlated across the San Gabrielfault because coeval strata have been displaced bystrike slip. In contrast, the Pliocene and Quaternarybeds are correlated without difficulty across the fault,indicating that most strike slip preceded deposition ofthese strata. Their deformation is principally byreverse faulting and folding. The east Ventura basin,therefore, contains evidence critical to the timing ofthe shift from strike slip to contractile deformation asthe Transverse Ranges were formed.This paper summarizes the surface and subsurfacegeology of the east Ventura basin. A preliminary summary of all but the western part of the basin was published by Stitt (1986). We compiled the surface geology at 1:24,000 scale from published literature andunpublished student theses, remapping the geologywhere it was incompatible with subsurface well data.A simplified version of the surface geology is shownas Figure 2. For our 1:24,000 maps and subsurfacestudies, see Ricketts and Whaley (1975), emen(1977), Lant (1977), Shields (1977), Yeats et al. (1977,1985), Nelligan (1978), Stitt (1980), Huftile (1988),and Huftile and Yeats (in press). We relied on Mefferd(1965) and Cordova (1966) for the subsurface geology of the Newhall-Potrero and Castaic Junction oilfields, respectively. Surface geologic mapping of thesoutheastern Ventura basin (Winterer and Durham,1962), the northern margin of the east Ventura basin(J. Crowell, unpublished map), the northwest part ofthe Fillmore quadrangle (Dibblee, 1990), the SanGabriel fault zone (Weber, 1982), the Newhall quadrangle (Smith, 1984; Treiman, 1986), the Mint Canyonquadrangle (Saul and Wootton, 1983), and the DevilsHeart Peak and Cobblestone Mountain quadrangles

(T. W. Dibblee, Jr., unpublished maps), was of particular value to the study. We based our subsurface geology on data from more than 2000 wells obtained fromthe California Division of Oil and Gas and from oiloperators. We did not do our own biostratigraphy butinstead relied on industry biostratigraphic zonation,which is based on benthic foraminiferal zones ofKleinpell (1938) and Natland (1952) supplementedby planktonic microfossils. For an up-to-date discussion of microfossil zonation and the ages of benthicforaminiferal zones, see Blake (1991). Seismic linesavailable to us were not of high enough quality to contribute significantly to the study.STRATIGRAPHYThe east Ventura basin formed at least in part oncrystalline rocks correlated to exposures in the SanGabriel Mountains to the east and the Alamo Mountain region to the north. Unmetamorphosed Paleogene strata unconformably overlie the crystallinerocks, but these were deposited in a basin framework unrelated to the east Ventura basin. The eastVentura basin itself began with deposition of middleMiocene strata in a trough extending southeast fromthe Topatopa Mountains across the Santa SusanaMountains and ending west of the San Fernando Valley. The Miocene and earliest Pliocene basin ends atthe San Gabriel fault, whereas younger strata are preserved on both sides of the fault.Basement RocksGneiss crops out in a narrow sliver between theSan Gabriel fault on the east and the Canton fault onthe west; gneiss is also found in the Conoco Alexander well to the southeast (Figure 3, well 3; Table 1).The oldest strata deposited on the gneiss are ModeloFormation of late Miocene (Mohnian) age. The gneissis correlated with the Mendenhall Gneiss of the western San Gabriel Mountains (Oakeshott, 1958; Ehlig,1981) and the Mendenhall Gneiss of the AlamoMountain area west of the Ridge basin (Ehlig andCrowell, 1982). Zircons from layered gneiss in thewestern San Gabriel Mountains adjacent to theSoledad basin are dated as 1715 30 Ma and 1670 20Ma (Silver, 1966), and correlative granodioritic augengneiss west of the Ridge basin is dated as about 1655Ma (L. T. Silver, in Frizzell and Powell, 1982).West of the Canton fault, the basement consists ofthe Whitaker Granodiorite of Shepherd (1960), whichis in part thrust over Paleogene strata along theWhitaker thrust. Well data document a subsurfaceridge of granodiorite and granite close to and southwest of the San Gabriel fault overlain by Mohnian strata (Figure 3). The southeastern part of this ridge

Towsley Fm.

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Fernando Fm.Rincon Fm.

Marine EoceneBasement

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KE

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Figure 2Geologic map of the east Ventura basin, compiled from 1:24,000-scale mapping cited in the text. Location of map is on Figure 1.Abbreviations: CF, Canton fault; DCF, Devil Canyon fault; SSF, Santa Felicia fault; WCF, Whitney Canyon fault.

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Yeats et al.1043

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East Ventura Basin

Figure 3Paleogeologic map of the base of Mohnian strata, approximately equivalent to the base of the upperMiocene, east Ventura basin. There are two options for a pre-upper Miocene (pre-Mohnian) fault bounding thebasement ridge on the southwest. Option (A) curves the fault to the east, separating basement rocks on the northfrom a thick Eocene and Sespe(?) sequence to the south. Option (B) continues the fault southeast between theArgosy-Lassalle and Celeron-Towsley wells and to the Texaco 1-A Evans well, containing breccia possibly derivedfrom a pre-Mohnian fault scarp. Formation symbols: gn, gneiss; gr, granite; K, Upper Cretaceous marine strata; Te,Paleocene and Eocene marine strata; Ts, Sespe Formation; Tv, Vaqueros Formation; Tr, Rincon Shale; Ttp, TopangaFormation; Tmm, Modelo Formation with middle Miocene (Relizian and Luisian) microfossils. In footwall block ofSanta Susana fault, the paleogeologic map is at the base of the middle Miocene (Luisian) Modelo Formation. CF,Canton fault; DCF, Devil Canyon fault; WCF, Whitney Canyon fault. Exxon D-1 NL&F and Texaco A-1 Newhall wellscontain breccia in the upper Mint Canyon Formation east of the San Gabriel fault.

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Table 1. Wells Penetrating Basement, East Ventura BasinMapNo.1234567891011121314151617

Well Name

Rock Description

Conoco Vier Kenny*Union Alexander*Conoco Alexander*

biotite granite and biotite granodiorite, cataclastically deformedbiotite granite, partially altered to chlorite and clays, followed by cataclasishornblende gneiss, sphene-bearing hornblende-biotite gneiss, epidotehornblende gneiss, cataclastic textureConoco 2 Rynne Fisherbiotite granodiorite partially altered to epidote and chloriteTexaco A-38 Honorgranodiorite, slickensides and fractures, associated with contorted andRancho (NCT-2)metamorphosed sedimentsTexaco 20 Waysidegranite, pink, coarse grained, somewhat gneissic; associated ultramafic dikeTexaco 10 Waysidebiotite granite, coarse grained, some schistosity, slightly metamorphosedThomas B-3 NL&Fbiotite schistUnocal 2 NL&F*muscovite granite and granodiorite with subsidiary hornblende; cataclastictexture (original hole). Granite gneiss, biotite streaks (redrill)Texaco E-1 Newhallbiotite graniteSuperior 14-23 Bonelli*biotite-muscovite schist; andalusite-sillimanite schist; granite, shearedConoco Georgina Swanson biotite granite, whiteMobil H&M*chlorite schist, biotite-actinolite schistMobil J-2 Circle*biotite granodiorite, some alteration to chlorite and epidote, fracturedMobil Bermite*biotite granodiorite, some alteration to chlorite, epidote, and calcite;cataclastically deformedConoco Phillipsschist, dark green to black with thin beds of recrystallized limestone;fractures and slickensidesMobil Macson Missionhornblende-plagioclase-quartz gneiss with subordinate amounts of marble

*Wells where basement rocks were examined in thin section.

includes biotite-muscovite schist, andalusite-sillimaniteschist, chlorite schist, and biotite-actinolite schist inaddition to granitic rocks (Table 1), suggesting a correlation with the Placerita Formation of Oakeshott(1958) in the western San Gabriel Mountains south ofthe San Gabriel fault. Schist with recrystallized limestone in the Conoco Phillips well (Figure 3, well 16)and gneiss with marble in the Mobil Macson Missionwell (Figure 3, well 17) are also correlated to thePlacerita Formation. The basement rocks in the Macson Mission well are unconformably overlain by middle Miocene Topanga Formation. The basement in thePhillips well is found beneath probable Paleogene strata, but the contact may be the Whitney Canyon fault.Southwest of the subsurface basement ridge, thebasement beneath the east Ventura basin is notexposed and is not reached by wells. The oldestknown rocks southwest of the Santa Susana fault areof Late Cretaceous age; these can be correlated withstrata in the Santa Monica Mountains that unconformably overlie granitic plutons that intrude theJurassic Santa Monica Formation.Sub-Middle Miocene StrataPre-basinal rocks consist of four roughly coevalsequences which are shown as separate stratigraphic columns in Figure 4.

The sequence south of the Santa Ynez fault can betraced westward through the Topatopa Mountainsand Santa Ynez Mountains (Figure 1). Upper Cretaceous deep-water strata are overlain disconformablyby an Eocene sequence 2500 m thick including theJuncal Formation, Matilija Sandstone, Cozy DellShale, and Coldwater Sandstone (Vedder et al., 1969;Dibblee, 1982) (Figure 4). In the Sespe Creek area,the marine Eocene sequence is overlain by nonmarine Sespe Formation of Eocene-Oligocene age, shallow-marine Vaqueros Sandstone of Oligocene age,and Rincon Shale of early Miocene (Saucesian) age(emen, 1977, 1989).On the north side of the basin in the Piru Creekarea (Figure 1), upper Paleocene to upper Eoceneconglomerate, sandstone, and shale rest unconformably on granitic basement (Kriz, 1947; Nilsenand Clarke, 1975). At the east end of the outcropbelt, middle and upper Eocene strata (Shepherd,1960; Squires, 1977) are in fault contact withgranitic basement along the Whitaker fault, whichShepherd (1960) and J. C. Crowell (unpublishedmapping) regarded as an overturned, shearedunconformity. In Canton Canyon, east of Piru Creek,the marine Eocene strata are overlain by nonmarineSespe Formation (Figure 4), which contains in itslower part conglomerate and breccia derived in partfrom a distinctive source terrain in the western SanGabriel Mountains (Crowell, 1954; Bohannon, 1975,

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East Ventura Basin

1976). The Sespe is overlain by the Vaqueros Sandstone, which is more conglomeratic toward the eastern end of its outcrop belt, and by Rincon Shale (Figure 4), a brown silty mudstone with a lens of pebblysandstone (Yeats et al., 1985). North of the SantaClara Valley, formations older than the Sespe are notencountered in wells. The Sespe, Vaqueros, and Rincon formations are found in the Union 1 Moran well,and the Rincon is found in the Gulf 1 Hathaway well(both located on Figure 3).The well-known sedimentary section of the SimiHills and Simi Valley has been traced to the footwallof the Santa Susana fault (Yeats, 1979; Seedorf,1983). A thickness of 2000 m of Upper Cretaceousdeep-water strata, largely turbidites, is found in theSimi Hills, with the base not exposed. The nonmarine Paleocene Calabasas Formation of the westernSimi Valley is overlapped eastward by the PaleoceneEocene Santa Susana Formation, which is predominantly deep-water mudstone with a basal conglomerate (Yeats, 1987b). This formation is overlaindisconformably by the marine Eocene Llajas Formation, which also contains a basal conglomerate, bythe Sespe Formation of Eocene-Oligocene age, andby the Vaqueros Sandstone. The Rincon Shale of thenorthern two sections appears to grade laterally intothe Vaqueros in the Santa Susana footwall block.North of the Santa Susana fault, the Continental 1Phillips well in the Placerita oil field (Figure 3, well16), penetrated approximately 1700 m of Paleoceneand Eocene marine strata in apparent fault contactwith metamorphic rocks (Stitt, 1986). Seedorf(1983) correlated the lower part of this sequence, aconglomerate overlain by claystone, siltstone, andminor sandstone, with the Santa Susana Formation,and the upper part, a conglomerate overlain by siltstone and sandstone, with the Llajas Formation.Between the Aliso Canyon and Placerita oil fields,upper Miocene marine strata are underlain unconformably by nonmarine beds known only from thesubsurface; these rest on marine Eocene rocks (Winterer and Durham, 1962; Nelligan, 1978) (for mapextent, see Figure 3). The sequence consists of gray,red, green, and bluish shale, claystone, siltstone,sandstone, and conglomerate, with local interbedsof blue to blue-gray bentonite. The sequence is 260m thick in the Morton & Dolley 5 Needham well(Figure 3), where it rests on marine Eocene rock.Elsewhere, where the marine Eocene is not reached,the nonmarine sequence is at least 610 m thick inthe British American 1 Edwina well and possibly atleast 1100 m thick in the Basenberg 1 Hamilton well(both located on Figure 3), where, however, part ofthe section may be repeated by a fault.Winterer and Durham (1962) questionably assignedthe sequence to the Sespe Formation based on its stratigraphic position. The Sespe of the Santa Susana footwall block is overlapped by the Modelo Formation

1013 km west of these strata (Figure 3) and is not laterally continuous with the nonmarine sequence northof the Santa Susana fault. The Topanga Formation ofthe San Fernando Valley to the east (Oakeshott, 1958;Shields, 1977) and Oat Mountain anticline to the south(Saul, 1975; this paper) is much coarser grained and isinterbedded with basalt. The middle Miocene stratabeneath the Pico anticline as well as those closer to theSanta Susana fault are conformable with the overlyingModelo Formation, whereas the unnamed nonmarinesequence underlies the Modelo with angular unconformity. We conclude that these nonmarine strata aremost likely the same age as the Sespe Formation or theVasquez Formation of the Soledad basin, and they mayhave been deposited in a separate, fault-bounded basinsimilar to others described by Bohannon (1975).Middle Miocene SequenceThe east-dipping strata overlying the Rincon Shalein the hills between Sespe Creek and Piru Creekinclude a lower shale member, a lower sandstonemember, a middle shale member, an upper sandstone member, and an upper shale member (Figures5; 6A, B). Eldridge and Arnold (1907) named theModelo Formation for a sequence including the twosandstone members and the upper two shale members in Modelo Canyon. They included the lowermost shale member in the Vaqueros Formation. Kew(1924) described as Modelo Formation a thicksequence of sandstone and shale between the Vaqueros Sandstone and the overlying Pico Formation ofhis Fernando Group. Kew (1924) included the Rincon Shale in the lower shale member of his ModeloFormation. Hudson and Craig (1929) also includedthe Rincon Formation in the lower shale member,but they preferred to call this shale member and theoverlying sandstone member and the middle shalemember the Topanga Formation, restricting the termModelo to the upper sandstone member and theuppermost shale member. Bramlette (1946) and Dibblee (1989) referred to the Miocene strata above theRincon Shale as the Monterey Formation. We followemen (1977, 1989) in restricting the Modelo ofEldridge and Arnold (1907) to those strata betweenthe Rincon Shale and the overlying Towsley Formation of Winterer and Durham (1962) of latestMioceneearliest Pliocene age, discussed below. Thisresults in a five-part subdivision of the Modelo Formation: Tm1, shale; Tm2, sandstone; Tm3, shale; Tm4,sandstone; Tm5, shale (Figure 6A, B).emen (1977) showed that the lower Modeloshale, as restricted by Bramlette (1946), and thelower sandstone contain benthic foraminifera of theRelizian and Luisian stages of Kleinpell (1938),which range in age from 17.4 to 13.9 Ma (Yeats etal., 1989; Blake, 1991). The lower shale is in part

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Figure 4Stratigraphic columns of the submiddle Miocene Tertiary strata in the eastVentura basin region.These strata weredeposited in a Paleogene basin unrelatedto the Neogene eastVentura basin. ForNeogene stratigraphiccolumns, see Figure 5.

dark gray to brown, thin bedded to laminated, cherty to porcelaneous, and in part calcareous todiatomaceous; these are lithologies characteristic ofthe Monterey Formation to the west (Dibblee,1989). The lower sandstone is locally interbeddedwith dark brown siltstone and claystone. This twopart subdivision of middle Miocene strata is recognized throughout the Piru 712' quadrangle (emen,1977; Huftile and Yeats, (in press), although thelower sandstone lenses out southward 5 km northnortheast of Fillmore (emen, 1977, 1989; Dibblee,1990) (Figure 2). To the north, in the CobblestoneMountain quadrangle, the lower sandstone is muchthicker in the high country between the Sespe Creekand Piru Creek drainages, and is thinner again closeto the Pine Mountain fault (Dibblee, 1989; T. W. Dibblee, Jr., unpublished map) (Figure 2). The outcroppattern (Figure 2; cf. Figure 3 of Dibblee, 1989) cutsacross the axis of a depocenter south of the PineMountain fault.The five-part subdivision of the Modelo Formation

cannot be mapped in the subsurface east of PiruCreek (Figure 6CE). Strata with Luisian and Relizianforaminifera are 1480 m thick in the Gulf 1 Hathawaywell (Figure 6C). South of the Santa Clara River in thehanging-wall block of the Santa Susana fault, sandstone and conglomerate overlain by an amygdaloidalbasalt flow 3060 m thick were called Topanga(?) Formation by Winterer and Durham (1962) (Figure 6E,F), who included overlying shale containing Luisianmicrofossils in the Modelo Formation. Saul (1975)mapped Topanga Formation in the core of the OatMountain anticline at Aliso Canyon oil field, placingthe contact with the overlying Modelo Formation(Figure 5) at the top of the predominantly sandstoneunit with intercalated shale containing Luisian microfossils. The sandstone is interbedded with conglomerate and with greenish-gray siltstone, suggesting thatthe Topanga there is grading to shallow marine. Thebasalt in the Topanga Formation is dated by the potassium-argon method as 15.1 Ma (Turner, 1970, revisedby Weigand, 1982). West of Oat Mountain anticline,

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East Ventura Basin

Figure 5Neogenestratigraphiccolumns, east Ventura basin. Left column: local biostratigraphic unitscorrelated to timescale. Right threecolumns show representative Neogenesections; thicknessesare highly variable.For Paleogene stratigraphic columns,see Figure 4.

the middle Miocene consists of greenish-gray, micaceous, poorly sorted sandstone interbedded withblack to dark-brown siltstone. The base of the middleMiocene is cut out by the Santa Susana fault (Figure6DF); minimum thicknesses are 180 m in Oakridgeoil field and 762 m in Aliso Canyon oil field (bothfields located on Figure 7). Only the Mobil 1 MacsonMission well (Figure 3, well 17) penetrates the base ofthe middle Miocene; there, the sequence consists of335 m of biotitic, locally carbonaceous gray siltstoneand sandstone resting on sheared gneiss.Down dip from the Santa Susana fault, the middleMiocene sequence is at least 1125 m thick in theCeleron-Chevron 1 Towsley well, a deep test in thePico anticline (Figures 3, 6E). Two additional deeptests, the Exxon 78 NLF well at Castaic Junction oilfield and the Sun A-1 RSF well at Newhall-Potrero oilfield (Figure 6D), may have reached the middleMiocene, but faunal evidence is not available. In allthree wells, the lower part of the section is dominated by sandstone, as is the Topanga at Oat Mountainanticline, and sub-Miocene rocks were not reached.Farther northeast, upper Miocene strata overlap themiddle Miocene and rest directly on basement rocks.

The Santa Susana fault dies out at the surface nearthe Oakridge oil field, and it is possible to correlateelectric-log markers in the Modelo across the fault(Ricketts and Whaley, 1975; Yeats 1987a). On thefootwall side of the fault, the Modelo includes bothLuisian and Mohnian microfaunas. The Luisian Modelo consists of gray to brown, fine- to very finegrained sandstone interbedded with gray to brownsiltstone and shale, and it is overlain by light brownto black silty shale and sandy siltstone. At AlisoCanyon oil field, the Sesnon producing zone of theModelo Formation consists of very fine-grained tofine-grained sandstone with worm impressions,fish scales, and uncommon impressions of megafossils. In contrast to the thick middle Miocenesequence north of the Santa Susana fault, the Modelosouth of the fault is relatively thin, comprising astructural shelf. The Luisian Modelo rests with angular unconformity on the Vaqueros at the northwestend of the Santa Susana fault and on Paleocene andCretaceous strata at the southeast end of the fault(Figure 3). The contact between Luisian and Mohnian strata is conformable south of the fault and probably north of the fault as well.

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(A)Modelo lobe and east end of west Ventura basin (Hopper Canyon segment).

(B)

East end of Oak Ridge and San Cayetano faults near Piru Creek (Hopper Canyon segment).

Figure 6Cross sections, east Ventura basin, no vertical exaggeration. Well numbers are identified in theAppendix. Symbols: gn, gneiss; gr, granite; K, Cretaceous marine strata; Te, Paleocene and Eocene marine strata; Ts,Sespe Formation; Tv, Vaqueros Formation; Tr, Rincon Shale; Ttp, Topanga Formation; Tm, Modelo Formation, subdivided in (A) and (6) into Tm1 (lower shale), Tm2 (lower sandstone), Tm3 (middle shale), Tm4 (upper sandstone),and Tm5 (upper shale); Tmc, Mint Canyon Formation; Tc, Castaic Formation; Tvb, Violin Breccia; Tt, Towsley Formation; Tf, Fernando Formation; Qs, Saugus Formation; Qp, Pacoima Formation. Shading shows oil-producing strata. Bedding dips are shown by short lines at the surface and by lines extending from the well course (dipmeter).Core dips are shown as bidirectional from the well course because dip direction is not known. Lines of section arelocated on Figure 7.

Figure 6Continued.

Santa Susana fault to San Gabriel fault through Newhall-Potrero and Castaic Junction oil fields (Newhall-Potrero segment).

(D)

East Ventura basin fold and thrust belt (Newhall-Potrero segment).

(C)

1050East Ventura Basin

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(E)Santa Susana fault to San Gabriel fault through the Pico anticline (Placerita segment).

(F)Eastern end of east Ventura basin (Placerita segment).Figure 6Continued.

Upper Miocene Modelo FormationWe include the middle shale, upper sandstone,and upper shale of the Modelo Canyon section in theMohnian (upper Miocene) part of the Modelo Formation (Figure 5). The Mohnian is 6.5 0.3 to 13.9Ma in age in the Los Angeles basin (Blake, 1991);Yeats et al. (1989) considered the base of the Mohni-

an to be 13.8 Ma in the Cuyama basin in the southern Coast Ranges. Dibblee (1989, 1990, 1991) correlated the upper shale to the Sisquoc Formation ofpredominantly Delmontian age based on its silty orclay shale lithology, similar to Sisquoc exposureswest of Sespe Creek and different from the LuisianMohnian siliceous shale of his Monterey Formation.However, the upper shale contains Mohnian micro-

1052

East Ventura Basin

11845'WSA

WHITAKERPEAK

N

COBBLESTONEMTN.

11830'W

VAL VERDE

HR

LT

TA

CH

TFAUL

U

PI

NO

SA

T

UL

FA

PI

YE

HL

RA

DV

CJ

LT

.

6DOR

LC

OF

TRSS

5 KM

NP

G

TN

FI

EU

DW

FS

ET

PC

WH

ELTW

ST

WIRC

SANTA

OM

SU

SA

NA

SANTASUSANA

PLTO

TU

FIG

TY

3422.5'N

FAU

FIG. 6E

CA

OAK RIDGE

MINT CANYON

WA

FA

HS

HP

NEWHALL

TC

TE

HOLSER

SAN

6C

G.

FI

OC

. 6F

FIG. 6B

FIG. 6A

PIRU

ELRI

3430'N

ABG

CC

AC

OAT MTN.

fossils, not Delmontian, as does the Sisquoc Formation. In addition, in the Santa Felicia and Holser synclines west of Piru Creek (Figure 6B), the uppershale is overlain by the Hasley conglomerate, thebasal member of the Towsley Formation, which isitself characterized by Delmontian microfossils.Isopachs of the Mohnian part of the Modelo Formation (Figure 8) outline a linear, northwest-deepening depocenter beneath the Pico anticline northeast of the Santa Susana fault. In the center of thistrough, the Mohnian Modelo is 2550 m thick in theCeleron Towsley well and 1750 m thick in the MobilMacson Mission well farther southeast. The Mohniandecreases in thickness southwestward to less than700 m at the outcrop in the hanging-wall block ofthe Santa Susana fault as it approaches the structuralshelf in the footwall block (Figure 6E, F). Northeastof the Pico anticline, Mohnian thickness decreasesto zero, with abrupt decreases at coeval normalfaults (Figure 8). North of the Santa Clara River, theMohnian is more than 3000 m thick in the Gulf Hathaway well and adjacent outcrop. Thickness decreases eastward to 1350 m in the Oak Canyon oil field(located on Figure 7), part of a broad structural shelfextending east almost to Honor Rancho oil field.Thickness decreases abruptly across the DevilCanyon fault and gradually southeastward to

FAULT

Figure 7Oil fieldsof the east Venturabasin. Black indicates middle andupper Miocene Modelo production; darkshading indicatesModelo (upperMiocene) andTowsley production;light shading indicates Fernando production. Other symbols are identifiedon Figure 2 and inTable 2. Unpatterned oil fields produce from the OakRidgeSimi Hillsblock beneath theSanta Susana fault.AC, Aliso Canyon;OM, Oat Mountain;OR, Oakridge; SS,Santa Susana; ST,South Tapo Canyon;TY, Torrey Canyon.Lines of cross sections are shown forFigure 6.

Temescal oil field. Thicknesses decrease as the SanGabriel fault is approached, but isopachs are generally not parallel to the fault.Winterer and Durham (1962) pointed out thatsandstone in the Modelo is characterized by repeated graded bedding and was deposited by turbiditycurrents. The sandstones are increasingly conglomeratic toward the San Gabriel fault to the northeastand were deposited as submarine fans (Dibblee,1989). Submarine fans characterize most of the Modelo except for the southern and eastern margins ofthe basin, where the Modelo is largely characterizedby shale. The facies boundary trends west-northwest, parallel to the direction of thinning of theModelo toward the structural shelf in the footwallblock of the Santa Susana fault. Farther east, thefacies boundary trends north, parallel to Modeloisopachs west of the Placerita oil field.Towsley FormationA sequence of light-colored sandstone, conglomerate, and brown-weathering mudstone on the northslope of the Santa Susana Mountains (located on Figure 9), previously mapped as Modelo by Kew (1924),was named the Towsley Formation by Winterer and

Yeats et al.

1053

Figure 8Isopachs of the upper Miocene Mohnian Modelo Formation (in meters). Dots show subsurface well control. Shaded pattern shows Mohnian Modelo exposures. Syndepositional normal faults are shown with tick markson downthrown side. DCF, Devil Canyon fault; HR, Honor Rancho oil field; PA, Pico anticline.

Durham (1954, 1962). They mapped the top of theTowsley at the base of the first sandstone and conglomerate underlying the lowest concretion-bearingolive-gray siltstone of the Fernando Formation. Thesandstone contains large, rounded brown concretions in which bedding can be recognized (Wintererand Durham, 1962). Winterer and Durham (1962)pointed out the difficulty of mapping the base andtop of the Towsley away from the type locality inTowsley Canyon (located on Figure 9), and theynoted that these contacts, as defined on lithology, aretime transgressive. However, north of the Santa ClaraRiver, the base of the Towsley is marked by the deepwater Hasley conglomerate, which facilitates map-

ping the Towsley-Modelo contact there.The Towsley at its type locality is characterized bybenthic foraminifera of the Delmontian Stage ofKleinpell (1938). The Delmontian is not a true stage;Delmontian faunas in southern California are nottime equivalent to the fauna at the type Delmontiansection in the central Coast Range (Pierce, 1972; Barron, 1976). The age of the local Delmontian ofsouthern California is estimated as 4.98 0.15 to 6.5 0.3 Ma (Blake, 1991). We map the base of theTowsley at the base of the Hasley conglomerate,although some samples from the Hasley yield Mohnian faunas. The top of the Towsley is the faunalboundary between Delmontian and Repettian micro-

1054

East Ventura Basin

Figure 9Isopachs of the Towsley Formation (in meters). Symbols are the same as in Figure 8. TC, TowsleyCanyon; WCF, Whitney Canyon fault.

fossils, which generally corresponds to the colorchange between brown mudstone below and olivegray mudstone above.The repeated graded bedding, displaced foraminiferal faunas, broken megafossils, and abrupt facieschanges in Towsley sandstone indicate deposition byturbidity currents (Winterer and Durham, 1962). Thesand bodies comprise large submarine fans that thicken and become more conglomeratic toward the SanGabriel fault (Crowell, 1954). Some of the pebblysandstones are lenticular and channelized, and aresimilar to the suprafan lobe facies of Walker (1978,his Figure 18).Near the Placerita oil field, the Towsley overlapsthe Modelo and rests on the Sespe(?) Formation,Eocene strata, and basement rocks (Figures 3, 6F).The strata change facies to shallow-marine deposits

that are difficult to distinguish from the overlyingFernando Formation, leading Nelligan (1978) to mapthem as a single unit.Isopachs of the Towsley Formation (Figure 9)delineate three zones of reduced thickness trendingnortheast, normal to the San Gabriel fault. In detail,isopachs within two of these zones and a zone ofthicker deposition between them trend east, suggesting that thickness changes are related to coevaldisplacement on the San Gabriel fault. Farther southwest, Towsley thickness increases toward the outcrop belt in the Santa Susana Mountains (Figure 9),indicating that the Towsley depocenter, close to thepresent surface trace of the Santa Susana fault, has inpart been eroded away. However, there is noTowsley south of the fault; the Fernando Formationrests directly on Modelo Formation.

Yeats et al.

Miocene Strata East of San Gabriel FaultMiocene rocks in the Soledad basin east of the SanGabriel fault, including strata mapped as TowsleyFormation by Winterer and Durham (1962), differgreatly from the Modelo and Towsley formationswest of the fault. Nonmarine units include theVasquez Formation of Oligocene(?) to earlyMiocene(?) age (Muehlberger, 1958; Bohannon,1975; Woodburne, 1975), the Tick Canyon Formation of late early to early middle Miocene age (Jahns,1940), and the Mint Canyon Formation of middle tolate Miocene age (Winterer and Durham, 1962; Ehliget al., 1975; Ehlert, 1982) (Figure 5). Vertebrate fossils discussed by Winterer and Durham (1962) andradiometrically-dated tuff beds (Terres andLuyendyk, 1985) indicate that the Mint Canyon iscoeval with much of the Modelo Formation, but onedoes not grade into the other.The Mint Canyon Formation is overlain by andlocally interbedded with the marine Castaic Formation (Crowell, 1954), which comprises the lowestformation of the Ridge basin (Crowell, 1975; Crowell and Link, 1982; Stitt, 1982). The Castaic containsmicrofossils of the Mohnian and Delmontian stages,and is thus the same age as parts of the Modelo andTowsley formations. However, the microfossils(Skolnick and Arnal, 1959; Schlaefer, 1978) and themegafossils (Stanton, 1966, 1982) are differentacross the San Gabriel fault. A possible connectionbetween the Castaic Formation and the east Venturabasin is discussed by Paschall and Off (1961), Schlaefer (1978), McDougall (1982), and Stanton (1982).The Castaic Formation grades abruptly southwestward into the Violin Breccia, which accumulated asa talus at the base of the active San Gabriel fault, indicating that the San Gabriel fault marked the southwestern edge of the late Miocene Ridge basin. TheViolin Breccia is traced at least 1340 m southeastward into the subsurface to the Conoco 1 Alexanderwell (Figure 3, well 3), but farther southeast, theCastaic Formation abuts the San Gabriel fault without the intervening Violin Breccia.Fernando FormationEldridge and Arnold (1907) described all stratayounger than the Modelo Formation and older thanterrace deposits as the Fernando Formation. Kew(1924) raised the Fernando Formation to the Fernando Group, which consisted of a lower unit, the PicoFormation, and an upper unit, the Saugus Formation.Winterer and Durham (1962) did not use the termFernando, and their Pico Formation included stratawhich Kew had earlier assigned to the Saugus. In theeastern part of the area, Oakeshott (1950) used theterm Repetto Formation, which elsewhere refers to

1055

strata with microfauna assigned to the RepettianStage of Natland (1952). In this paper, we follow thecurrent practice of the U. S. Geological Survey (cf.stratigraphic nomenclature discussion in Jenningsand Strand, 1969) in assigning all strata between theTowsley and Saugus formations to the Fernando Formation. This corresponds to the Pico Formation ofWinterer and Durham (1962). In the western part ofthe east Ventura basin, the Fernando Formation consists of a lower member with Repettian microfossilsand an upper member with shallower water upperPliocene or Pico microfossils not easily correlatedto the Venturian or Wheelerian stages of Natland(1952) in the west Ventura basin (Figure 5). The topof the Fernando Formation near Newhall is about 2Ma, based on magnetic stratigraphy (Levi et al.,1986; Levi and Yeats, 1993), but the top is youngerwestward and older eastward due to a facies changeto Saugus Formation from west to east (Figure 5).In the western part of the basin, the Fernandoconsists of olive-gray and medium bluish-gray siltstone with reddish-brown concretions interbeddedin its lower part with sandstone and conglomeratecharacterized by repeated graded bedding (Wintererand Durham, 1962; Yeats et al., 1985). North of theHolser fault, the Fernando consists of pebbly sandstone interbedded with bioturbated, sandy siltstonedeposited in relatively shallow water; the faciesboundary trends approximately east (Figure 10). TheFernando also grades to shallow-water deposits inthe eastern part of the basin, where it is commonlydifficult to separate from the underlying TowsleyFormation, especially in the subsurface (Nelligan,1978). This facies boundary trends north to northwest (Figure 10). The facies boundary changes trendat the San Gabriel fault southeast of the Honor Rancho oil field in an area characterized by a westwardthickening depocenter (Figure 10).The Fernando is the oldest formation easily correlated on both sides of the San Gabriel fault, indicating that most strike slip preceded Fernando deposition. East of the fault, the Fernando is tiltedsouthwestward, eroded, and overlapped by theSaugus Formation in such a way that the isopachs ofthe Fernando are in part parallel to the San Gabrielfault (Figure 10).Farther away from the San Gabriel fault, the thickness of the Fernando increases markedly toward theSanta Susana Mountains and the Oak Ridge fault (Figure 10). Part of the thickness increase is due to thelateral change southwest from the Saugus to the Fernando, documented in surface exposures and inwells (Winterer and Durham, 1962; Yeats et al.,1985). Farther southwest, the thickness increasesfrom 900 to 2500 m due to coeval development of asouthwest-verging monocline at the site of theyounger Newhall-Potrero anticline (Figures 6D, 10;see following discussion).

1056

East Ventura Basin

Figure 10Isopachs of the Fernando Formation (in meters). Symbols are the same as in Figure 8. Hachured linesmarks facies boundary between the shallow-water and the deep-water Fernando Formation. N-P anticline, NewhallPotrero anticline; WCF, Whitney Canyon fault.

Fernando thickness is also 2500 m in the SantaClara Valley, where it is preserved between the SanCayetano and Oak Ridge faults (Figure 6A, B). Thisthickness is representative of that in the west Ventura basin (Yeats, 1983, 1988).Saugus FormationThe Fernando Formation grades upward and laterally to shallow-marine to nonmarine strata whichcomprise the uppermost major formation of the eastVentura basin. Hershey (1902) referred to these strata as the Saugus division, and Kew (1924) assignedthe Saugus-Formation to the upper part of his Fernando Group. The base of the Saugus consists ofgreen-gray and gray pebbly sandstone and green-graysiltstone which may contain marine to brackishwater fossils (Winterer and Durham, 1962). In manyareas, the Saugus-Fernando contact is placed at thetop of the fossiliferous sequence, but in the Santa

Clara Valley between the Oak Ridge and SanCayetano faults, the contact is placed between finegrained Fernando strata and the fossiliferous sandstone and conglomerate of the Saugus Formation(Figure 6A). Conglomerate predominates in morenortherly exposures west of the San Gabriel fault.Northeast of the Holser fault, Weber (1979) subdivided the Saugus on the basis of clast composition ofconglomerate: clasts of Paleocene gray-brown sandstone vs. clasts of schist. The Saugus also occurssouth of the Santa Susana fault (Yeats, 1979, 1987a).The uppermost deformed nonmarine strata in theNewhall area north of the Santa Susana Mountains(between wells 18 and 22 on Figure 6D) (Treiman,1986) and in the Horse Flats area near Aliso Canyonoil field (left edge of Figure 6F) south of the SantaSusana fault (Saul, 1975) show an abrupt changefrom clasts of basement rocks upward to clasts locally derived from the Modelo and Towsley formationsof the Santa Susana Mountains. Near Newhall, thelocally-derived strata have been mapped by Treiman

Yeats et al.

(1986) as the Pacoima Formation of Oakeshott(1958) because of their fanglomeratic nature. Thesestrata were mapped as terrace deposits by Wintererand Durham (1962), whose mapping indicated thatlittle if any of the section has been removed by erosion. South of the Santa Susana fault, similar stratawere mapped by Saul (1975) as the upper part of theSaugus Formation. These locally derived depositsreflect the uplift and erosion of the Santa SusanaMountains (Saul, 1975; Levi and Yeats, 1993).Winterer and Durham (1962) described bouldersof crystalline rocks more than 6 m in diameter resting on fine-grained Modelo Formation at an altitudeof 870 m on ridge crests on the north side of theSanta Susana Mountains near the Aliso Canyon oilfield (located to the right of well 12 on Figure 6F).We found crystalline boulders and cobbles overlyingModelo rocks at altitudes of 825 to 975 m in thissame area. Winterer and Durham (1962) concludedthat these are terrace deposits, but this is unlikely.The uppermost Saugus is dominated by clasts fromthe Santa Susana Mountains (Saul, 1975), indicatingthat the Santa Susana Mountains were upliftedbefore the end of Saugus deposition. How, then,could the area receive gravels of crystalline provenance unless the Santa Susana Mountains first lostmost of their topographic relief to receive the clasts,then were uplifted again? The clasts are more likely alag gravel of basal Saugus, which implies that theTowsley and Fernando formations were eroded offthe top of the Santa Susana Mountains before theSaugus was deposited. We consider it likely that thisoccurred during a pre-Saugus stage of movement onthe Santa Susana fault. The Torrey fault in the footwall block underwent reverse displacement prior toSaugus deposition (Yeats, 1979, 1987a) (Figure 6D),and we view the Santa Susana fault as undergoingdisplacement at the same time. This pre-Saugusreverse faulting apparently did not lead to enoughuplift to shed locally derived stones into the basalSaugus.The age of the Saugus is Pleistocene, based onseveral magnetostratigraphic sections described byLevi et al., (1986) and Levi and Yeats (1993). One ofthe sections contains the Bishop or Friant ash (Leviet al., 1986; Levi and Yeats, 1993). The Bishop ash isdated as 0.759 0.003 Ma (Pringle et al., 1992), andthe Friant ash is dated as 0.62 Ma (Sarna-Wojcicki etal., 1991). The youngest paleomagnetically datedstrata were deposited during the Brunhes Chron andare from 0.6 to 0.5 Ma.Post-Saugus and Post-Pacoima DepositsAlluvium, landslide deposits, and other undeformed or locally-deformed sediments are widespreadin the area, but are not described in this paper.

1057

STRUCTUREThe structure is discussed in four parts: (1) deposition of a rifted basin in the middle and late Miocene,(2) displacement on the San Gabriel fault, (3) faultingand folding during deposition of the Pliocene-Pleistocene sequence, and (4) post-Saugus deformationwhich, for the most part, still continues today.Miocene Rifted BasinA thick sequence of middle and upper Miocenestrata occupies a southeast-trending basin extendingfrom outcrops in the Cobblestone Mountain and Piruquadrangles (Figure 2) into the subsurface northeastof the Santa Susana fault (Figure 3). The great thickening of strata in outcrop does not appear to involve thelowest shale member (Tm1) of the Relizian-Luisian(middle Miocene) Modelo Formation, but it clearlyinvolves the overlying sandstone member (Tm 2),which forms prominent ledges in the western part ofthe Cobblestone Mountain quadrangle (Figure 2). Allmicrofossils in the thick middle Miocene TopangaFormation southeast of the Santa Clara River areLuisian, and a basalt flow interbedded with the Topanga at Aliso Canyon is dated within the age range of theLuisian (Turner, 1970; Blake, 1991). Therefore, therifted basin became active sometime after the beginning of the Relizian at 17.4 Ma, and it clearly wasactive prior to 13.9 Ma, the beginning of the Mohnian.Mohnian strata of the Modelo Formation also showevidence of thickening in the trough (Figure 8).The thick Miocene sequence exposed in the Cobblestone Mountain quadrangle is not obviously bounded by faults on the south and north, although thesoutheast-trending Agua Blanca fault zone (Figure 2)may be the northern boundary of the trough. TheAgua Blanca fault and associated folded beds as youngas early Miocene Rincon Shale to the north of this faultare covered unconformably by the Mohnian ModeloFormation (Yeats et al., 1985), indicating strong deformation along this zone in the middle Miocene. TheDevil Canyon fault cuts Mohnian strata and is itselfoverlain by Mohnian strata (Figure 2). It may form partof the southwestern boundary of the basement ridgeand the northeast boundary of the trough.Southeast of the Santa Clara River, deep-test wellsin the Newhall-Potrero and Castaic Junction oil fields(Figure 6D) and at the Pico anticline (Figure 6E) penetrated a much greater thickness of Modelo Formation than that exposed to the southwest and documented in the subsurface to the northeast. The crosssection in Figure 6E shows that the boundarybetween thin Modelo underlain by Sespe(?) Formation and thick Modelo beneath the Pico anticline isso abrupt that it must be a fault. Farther southeast,the breccia in the Modelo Formation in the Texaco

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East Ventura Basin

1-A Evans well (Figure 6F, well 15) is probably atalus deposit derived from a coeval fault scarp to thenortheast. Wells northeast of the 1-A Evans well penetrate thin Mohnian strata underlain by Sespe(?) Formation, whereas wells to the southwest penetratethick Mohnian underlain by Topanga Formation.The trough shallows farther southeast on thenortheast-trending lateral ramp of the Santa Susanafault at the edge of the San Fernando Valley (Figure8). The Mobil 1 Macson Mission well (Figure 3, well17) penetrated a relatively thin sequence of TopangaFormation underlain by basement; however, theMohnian in this well is much thicker than it is to thenortheast or southwest (Figure 8).On the footwall side of the Santa Susana fault,Modelo Formation of middle and late Miocene age(Luisian and Mohnian stages) is much thinner thancoeval strata cropping out on the hanging-wall side,and no gradation between these two sequences isseen in the direction of the fault (Figure 6CF). Thisrelation leads to the suggestion that the Santa Susanafault reactivated an old normal fault that marked thesouthwest margin of the Miocene trough.The Oak Ridge reverse fault in the west Venturabasin may have reactivated a Miocene normal fault(Yeats, 1987b). East of Torrey Canyon oil field (locatedon Figure 7), the south strand of the Oak Ridge faultchanges eastward from a south-dipping reverse fault toa north-dipping normal fault (Yeats, 1979, 1987a) (Figure 6B) that can be traced in the subsurface as far eastas Oakridge oil field (Figure 7). There, the Oak Ridgefault is overridden by the Santa Susana fault, and thechange in fault dip from south to north may be relatedto compression across the Santa Susana fault. The OakRidge fault probably continues southeast beneath theSanta Susana Mountains (Figure 6C), deeper than wellcontrol, where it may have served as the zone of weakness that guided the younger Santa Susana fault.North of the Santa Clara River, the Towsley Formation shows no evidence of the rifted trough, indicating that rifting ended there prior to 6.5 Ma. However,south of the river, the Towsley thickens to the southwest across the axis of the older rift, and is thickest inthe outcrop section in the Santa Susana Mountains(Figure 9). There is no Towsley in the footwall blockof the Santa Susana fault, indicating that the thickTowsley ended at a south-side-up fault near theyounger Santa Susana fault (Yeats, 1979), about thesame place as the Modelo rift boundary. Thus, the riftinfluenced Towsley thicknesses, but it was muchmore restricted areally, and its axis shifted southwest.San Gabriel FaultThe San Gabriel fault strikes southeast from nearFrazier Mountain (FM on Figure 1) and forms thesouthwestern edge of the Ridge basin (Figure 1). The

fault crosses the east Ventura basin, where it marks acontrast in stratigraphy of Miocene strata east andwest of the fault. The fault changes character in theeast Ventura basin. From Honor Rancho oil fieldnorthwestward, the fault is straight, strikes N40W,and dips steeply northeast with normal stratigraphicseparation (Figure 6C, D). Southeast of Honor Rancho, the fault is curved in plan and convex southward. At Saugus oil field, its strike is N55W; atPlacerita oil field, its strike is N70W. Dip is steeply tothe northeast, but with reverse separation rather thannormal (Figure 6E, F). Farther east, in the San GabrielMountains, the fault bifurcates into a northern strand,which strikes due east and is entirely within basementrocks of the San Gabriel Mountains, and a southernstrand (Vasquez Creek fault of Powell, 1993), whichmerges with the Sierra Madre reverse-fault zone marking the south-facing range front of the San GabrielMountains (Figure 1). The mapped trace of the fault,including the northern strand, extends 130 km fromthe Frazier Mountain area, where the fault is overlainunconformably by strata of the Pliocene Hungry Valley Formation, southeast to the east end of the SanGabriel Mountains, where both strands of the faultterminate at and are offset by northeast-trending faultsnear its junction with the San Andreas fault (Matti andMorton, 1993) (Figure 1).Crowell (1952) first suggested large-scale right-lateral displacement on the San Gabriel fault. Totalright-lateral displacement on the fault is controversial, ranging from 42 km (Powell, 1993) and 44 km(Matti and Morton, 1993) to 60 km (Bohannon,1975; Ehlig and Crowell, 1982). A review of the displacement history of the San Gabriel fault in the context of an overall reconstruction of southern California fault systems is provided by Powell (1993) andMatti and Morton (1993).We adopt a pre-late Miocene piercing-point offsetof 60 km across the fault (Figure 11) based on (1) offset of the Precambrian Mendenhall Gneiss andanorthositic rocks from near Frazier Mountain (FMon Figure 1) to the western San Gabriel Mountains(Crowell, 1962; Ehlig and Crowell, 1982), (2) offsetof Paleocene strata from the Caliente Range (CR onFigure 1) to the Paleocene San Francisquito Formation at the northern edge of the Soledad basin (Sage,1973) (located on Figure 1), (3) offset of theOligocene to lower Miocene Simmler Formation inthe Caliente Range and the Plush Ranch Formation inthe Lockwood Valley area (Carman, 1964) (Figure 1)to the Oligocene to lower Miocene Vasquez Formation in the Soledad basin (Bohannon, 1975, 1976)(Figure 1), (4) offset of the Blue Rock fault south ofthe Caliente Range, with evidence of Miocene movement, to the San Francisquito fault in the Soledadbasin (Bohannon, 1975; Ehlert, 1982) in order tomatch Charlie Canyon Megabreccia in the Soledadbasin (which overlies the Vasquez Formation with

Yeats et al.

1059

Figure 11Age vs. strikeslip displacement on theSan Gabriel fault. On theabscissa, R = Relizian, L =Luisian, M = Mohnian,D = local Delmontian(all four are benthicforaminiferal stages ofKleinpell, 1936), F = Fernando, and SA = SaugusFormation. Control fordisplacement are, fromright to left: VF, VasquezFormation; MC, conglomerate in MintCanyon Formation; DCF,Devil Canyon fault; DCC,Devil Canyon Conglomerate; CA, Castaic Formation; HA, Hasley Conglomerate; FM, Fernandodisplacements in SanGabriel Mountains; FV,Fernando displacementsin east Ventura basin; SA,Saugus Formation. Noage or displacement constraints in the directionof the arrows. Two estimates of slip rate areshown. The high sliprates were followednorthwest of HonorRancho oil field by lowslip rates of 1.3 mm/yror less than 1 mm/yr, assuggested by Kahle(1986). The low slip ratescontinue to the present.

angular unconformity) with a probable source in theLa Panza Range in the southern Coast Ranges (Smith,1977; Ehlig and Joseph, 1977; Joseph et al., 1982)(Figure 1, inset), and (5) offset of the MioceneCaliente Formation of the Caliente RangeLockwoodValley region from the Mint Canyon Formation of theSoledad basin (Ehlig et al., 1975; Ehlert, 1982).The lower displacement of Powell (1993) andMatti and Morton (1993) is based on (1) correlationof the San Francisquito fault with the San Andreasfault at Frazier Mountain, requiring a displacementof 4045 km, (2) basement offsets of 2223 kmacross the north strand of the San Gabriel fault in thecentral San Gabriel Mountains (Ehlig, 1981), combined with lack of evidence for large-scale displacement on the south strand (Vasquez Creek fault),(3) displacement of the left-lateral Malibu Coast faultand its eastern continuation north of the San GabrielValley to faults in the southeastern San Gabriel

Mountains, and (4) difficulty in integrating a 60-kmdisplacement into an overall scheme for southernCalifornia fault restorations.The offset of the depositional system of the MintCanyon Formation determined by Ehlig et al. (1975)and Ehlert (1982) is the same as that for all olderunits, indicating that strike slip on the San Gabrielfault largely postdates deposition of the Mint CanyonFormation. However, Ehlert (1982) proposed thatstrike slip began during deposition of the upper partof the Mint Canyon Formation, after the depositionof the lower Mint Canyon and Caliente conglomerates that show evidence of 60 km displacement. Stitt(1986) showed that the Mint Canyon Formationthickens southwestward toward the San Gabrielfault such that the basin configuration is that of ahalf graben. The Texaco A-1 Newhall and Exxon D-1NLF wells (located on Figure 3) contain sedimentarybreccia in the upper part of the Mint Canyon Forma-

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tion close to the San Gabriel fault. The brecciaincludes clasts of Mendenhall Gneiss(?), granite, andschist, which could not have come from the subModelo granite ridge on the southwest side of theSan Gabriel fault, but could have come from theAlamo MountainFrazier Mountain region to thenorthwest, which contains all rock types found inthe breccia. The breccia is an older analog to theViolin Breccia of the Ridge basin. In addition, thedepocenter for the coarse-clastic facies of the MintCanyon Formation shifted northward for the turbidite facies of the Castaic Formation, suggestingthat the entry point for these coarse clastic sediments was displaced northwestward along the SanGabriel fault (Stitt, 1982, 1986), as predicted by theconveyor-belt model of deposition along the coevalSan Gabriel fault proposed by Crowell (1982).Vertebrate fossils from the Mint Canyon Formation are referred to the Clarendonian and BarstovianVertebrate stages, the ages of which range fromabout 17 to 10 Ma (Berggren, 1969). Tuff beds in theupper part of the Mint Canyon Formation yielded zircon fission-track ages of 10.1 0.8 Ma and 11.6 1.2Ma (J. Obradovich and T. H. McCulloh in Terres andLuyendyk, 1985). Thus the age for initiation of strikeslip on the San Gabriel fault is in the range 1012Ma, as proposed by Crowell (1982).The lower Mohnian Devil Canyon Conglomeratein the Modelo Formation thickens, contains largerclasts, and grades to sedimentary breccia northeastward toward the San Gabriel fault (Crowell, 1952,1954). Castaic Formation of the same age, on theother side of the fault in the Ridge basin, could nothave been a source for the Devil Canyon Conglomerate. The conglomerate includes boulders as large as1.5 m in diameter of anorthosite, gabbro, norite, andgneiss similar to the rocks exposed in the westernSan Gabriel Mountains (Crowell, 1952, 1962, 1982).Right slip of at least 35 km but possibly as much as56 km is required to place the Devil Canyon Conglomerate next to its probable source (Figure 11).The Castaic Formation of Mohnian and Delmontian age grades southwest to the Violin Breccia,a narrow talus deposit adjacent to the San Gabrielfault. Right slip of about 35 km and possibly as muchas 60 km is required to match clasts of gneiss in theViolin Breccia with their most likely source in theAlamo MountainFrazier Mountain area west of thefault (Crowell, 1952, 1982). Powell (1993) suggested that the 35-km length of the Violin Breccia outcrop would approximate the offset accumulatedduring the time of deposition of the breccia. Thebreccia in the Mint Canyon Formation close to theSan Gabriel fault would add another 11 km to the offset, using this criterion.The Canton fault is an early formed strand of theSan Gabriel fault that juxtaposes gneiss on the eastagainst Whitaker Granodiorite on the west, a mini-

mum displacement of 23 km. The Canton fault cutsthe basal part of the lower Mohnian Devil CanyonConglomerate and is overlain by the upper part ofthe conglomerate, also Mohnian (Crowell, 1954;Yeats et al., 1985; Stitt, 1986) (Figures 2, 3, 6C).Therefore, at least 23 km displacement on the SanGabriel fault took place prior to the deposition ofthe upper part of the conglomerate. Because theCanton fault rejoins the San Gabriel fault north of theCastaic Hills oil field (Figure 3), the Canton faultcould not extend southeast into the San FernandoValley as proposed by Powell (1993, p. 40). However, examination of Figure 3 shows that the DevilCanyon fault, which in part forms the southwesternedge of a basement ridge, could extend into the SanFernando Valley and take up some right-lateral displacement measured on the San Gabriel fault farthernorth. If this fault occupied the position marked(A) on Figure 3, the Paleogene marine sequencewest of the Whitney Canyon fault would be offset2530 km from the Paleogene of Piru Creek ratherthan be correlated with the Paleogene of the SantaSusana footwall block (Figure 4). The disadvantageof this interpretation is that the questionable SespeFormation west of the Whitney Canyon fault is relatively fine grained, whereas the Sespe west of thefault is coarse grained, with evidence of a nearbyeastern source. In summary, the extension of astrand of the San Gabriel fault into the San FernandoValley is neither supported nor refuted by evidencefrom the east Ventura basin.The Hasley Conglomerate forms the base of theTowsley Formation, and it contains both Mohnianand Delmontian foraminifera, indicating an age ofabout 6.5 Ma. The conglomerate may be in partreworked from the underlying Devil Canyon Conglomerate, but most of it came from east of the SanGabriel fault, an area now underlain by coeval finegrained strata of the Castaic Formation. The nearestprimary source for the Hasley Conglomerate is thewestern San Gabriel Mountains, requiring that theapex of the Hasley submarine fan is offset at least 30km from its inferred source region (Figure 11).Displacement of the Pliocene Fernando Formation is clearly less than that of Miocene strata. Ehlig(1975) reported offsets of Pliocene strata in thewestern San Gabriel Mountains as follows (Figure11). (1) Conglomerate at the top of the lowerPliocene section is offset 1029 km from its provenance. (2) A sliver of marine sandstone in the SanGabriel fault zone is offset 1132 km from its provenance. (3) Middle to upper Pliocene conglomerateeast of Newhall is offset 619 km from its source.The uncertainties are based on the distribution ofpotential source rocks along the fault and on thepossibility of sediment transport parallel to, ratherthan at right angles to, the fault. An additional uncertainty is in biostratigraphic correlation of the largely

Yeats et al.

shallow-marine Fernando rocks of the San GabrielMountains with deep-water strata that have beenage-calibrated by Blake (1991).The Fernando Formation is the oldest formationwith electric-log correlation across the San Gabrielfault in the Castaic Hills and Honor Rancho oil fields(Schlaefer, 1978; Stitt, 1980), indicating that moststrike slip on the fault had been completed prior toFernando deposition. West of the modern trace ofthe San Gabriel fault, both the Castaic Hills andHonor Rancho oil fields contain abandoned strandsof the fault that juxtaposed Towsley and Modelo formations against the Castaic Formation prior to deposition of the Fernando Formation (Figure 6D, well29). The Fernando consists of shelf facies near thefault, so it is not possible to separate lower Pliocene(Repetto) from upper Pliocene faunal stages that arerecognized in deep-water Fernando rocks. The zeroisopach of Fernando Formation east of the SanGabriel fault, marking where the Fernando is overlapped by the Saugus, is offset across the fault at least2.4 km (Figure 10). The ease of correlation of the Fernando in electric logs across the fault makes it unlikely that the displacement is more than about 5 km.The present trace of the San Gabriel fault is east ofthe strands mentioned above in the Castaic Hills andHonor Rancho oil fields and east of the Piru fault ofShepherd (1960) that juxtaposes basement rocksagainst nonmarine strata of the Ridge basin 8 kmnorthwest of Castaic. The Quaternary behavior ofthe fault northwest of Honor Rancho oil field differsgreatly from its behavior to the southeast.Northwest of Honor Rancho oil field, verticalstratigraphic separation of the Quaternary SaugusFormation is 200250 m with the east side down.Surface dips in the Saugus average 2535 and donot increase appreciably at the fault. Near HonorRancho oil field, distinctive clast assemblages in theSaugus Formation have a right-lateral offset of about500 m (Weber, 1982) (Figure 11). Kahle (1986)reported linear ridges, trenches, hillside benches,and ponded alluvium along the fault trace, and excavations across the fault show evidence of Holocenedisplacement (Cotton and Seward, 1984; Cotton,1985, 1986). However, Kahle (1986) concluded thatQuaternary slip rates are low, less than 1 mm/yr. Atits northwestern end, the San Gabriel fault is overlapped by the Pliocene Hungry Valley Formation,and movement on the Frazier Mountain reverse faultalso postdates the San Gabriel fault (Crowell, 1982).The San Gabriel fault is overridden by the postSaugus Santa Felicia reverse fault northwest of Castaic (Stitt, 1986) (Figure 2), although Kahle (1986) andWeber (1986) disagreed with this interpretation.In contrast, the San Gabriel fault underwent relatively large Quaternary displacement southeast ofHonor Rancho oil field where it bends eastward. Surface dips in the Saugus are steep and locally over-

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turned at the fault. The fault itself decreases in dipfrom 80 to 85 north of Honor Rancho oil field to 60near Saugus oil field. Vertical separation of the baseof the Saugus Formation increases from 90 m atHonor Rancho to a maximum of 1600 m at Saugus oilfield, decreasing to about 450 m farther southeast.Accumulation of 1600 m separation since the end ofSaugus deposition 0.60.5 m.y. ago would be at a rateof 2.53 mm/yr., if all of it occurred in post-Saugustime. This interpretation is preferred due to theabsence of fault-related breccia in the subsurface onthe downthrown side of the fault. This displacementrate is faster than the slip rate farther northwest.As described above, the San Gabriel fault dipsnortheast throughout the east Ventura basin, withnormal separation from Honor Rancho oil fieldnorthwest and reverse separation southeast ofHonor Rancho oil field. However, in both segments,the fault dips approximately 90 to bedding in theSaugus Formation, which dips predominantly southwest at the fault (Figure 6D, E). When the SaugusFormation is rotated back to horizontal, the SanGabriel fault rotates to approximately vertical, a dipcharacteristic of other strike-slip faults in California.The normal vs. reverse separation changes to eastside down from Honor Rancho oil field northwestward and to west side down farther southeast.Pliocene Tectonics: Change from Extension toContractionThe Towsley Formation of latest MioceneearlyPliocene age (6.55 Ma) increases in thicknesssouthwestward toward the Santa Susana fault, yetthere is no Towsley farther southwest in the footwall block of the Santa Susana fault. The Towsleywas probably terminated southwestward by a normal fault that may have served as a zone of weaknessfor the younger Santa Susana fault as far east as theAliso Canyon oil field. Towsley isopachs closer tothe San Gabriel fault show a pattern of local thickerand thinner areas that reflect coeval displacement onthe San Gabriel fault. The Towsley is not thickerbeneath the syncline of Saugus near Newhall, indicating that that syncline is younger.In contrast, Fernando isopachs show relatively little control by the San Gabriel fault, suggesting thatthe fault was less active during Fernando deposition.The Fernando thickens southwestward from about1400 m northeast of the Newhall-Potrero anticline to2500 m southwest of this anticline, so that the anticline does not continue upward to the surfaceexcept locally (Winterer and Durham, 1962) (Figure6D). The limbs of this anticline formed at differenttimes. The northeast limb was formed after deposition of the Saugus Formation, but the southwestlimb formed much earlier, during deposition of the

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Fernando Formation (Figure 6D). Prior to postSaugus tilting, the Newhall-Potrero structure was asouthwest-facing monocline controlled by a reversefault that has not continued upward into stratayounger than Modelo Formation. The NewhallPotrero anticline cannot be a fault-propagation fold(cf. Suppe and Medwedeff, 1990) because it lacks abacklimb; the northeast limb formed after the monocline did. We have not been able to map the Repettoand Pico separately across this structure, so we havenot determined the time of growth of this monocline other than during Fernando deposition.Farther southeast, the Fernando is not sufficientlypreserved in the Oat Mountain syncline to demonstrate thickening across the Pico anticline analogousto that across the Newhall-Potrero anticline. Nevertheless, we propose that the Pico anticline (Figure6E) is another southwest-facing monocline formedduring Fernando deposition because (1) the Fernando isopach gradient north of the Newhall-Potreroanticline curves southward toward an analogousposition with respect to the Pico anticline (Figure10), (2) the northeast limb of the Pico anticline ispost-Saugus in age, similar to the age relations northeast of the Newhall-Potrero anticline, and (3) in thefootwall block of the Santa Susana fault in the AlisoCanyon oil field, the Fernando is more than 2000 mthick and increases in thickness northward towardthe fault in comparison to a thickness of 400600 mnortheast of the Pico anticline (Figure 6F, 10). Thisindicates that thickening must occur between theoutcrop north of the Pico anticline and the SantaSusana fault, which must have cut through the axisof the Fernando trough rather than bound it on thesouth, as it did the Towsley trough. In the footwallblock, the thick Fernando is terminated southwardat the Frew reverse fault (Figure 6E, F), which underwent near-vertical displacement during Fernandodeposition (Yeats, 1979, 1987a).North of the Santa Clara Valley, we speculate thatthe Temescal anticline (Figure 6B) and the HopperRanchModelo anticline (Figure 6A) in the Modelolobe of the San Cayetano fault are also folds formed asthe Fernando was deposited. The sharp isopach gradient at the Newhall-Potrero anticline continues westnorthwest to the vicinity of the Del Valle fault, wherethis gradient projects west to a position immediatelysouth of the Hopper RanchModelo anticline (Figure10). To the north, a gradient of 500800 m projectswest toward the Temescal anticline (Figure 10).There are two extreme possibilities (cf. Yeats,1983). (1) The Fernando and Saugus formations werenever deposited on the Modelo lobe, implying thatthe lobe was positive throughout the time of deposition. This extreme case is unlikely because, in contrast to the Red Mountain fault in the western Ventura basin (Yeats et al., 1987), the Fernando Formationin the footwall block shows no northward change in

grain size, and no sediments locally derived from thenorth, which could be considered as evidence for apositive Modelo lobe. (2) The Fernando and Saugusformations were as thick atop the Modelo lobe asthey are in the Santa Clara syncline in the footwall ofthe San Cayetano fault. This is also unlikely because itwould require that the Modelo still preserved wouldhave been overlain by up to 7 km of Towsley, Fernando, and Saugus formations. The porosity of Modelosandstone, which comprises the reservoir for severalsmall oil fields in the Modelo lobe, indicates muchless overburden than the full thickness of post-Modelo strata in the Santa Clara syncline. Furthermore, thethickness of Fernando in the Santa Clara syncline is2500 m, thicker than in any section measured northeast of the San Cayetano fault.The first extreme maximizes the displacement onthe San Cayetano fault prior to the end of Saugusdeposition and minimizes post-Saugus displacement.The second extreme requires all displacement onthe San Cayetano to be post Saugus. We favor anintermediate explanation that calls for displacementon a blind reverse fault generating the Temescal andHopper RanchModelo anticlines as monoclinesacross which Fernando thicknesses increase from900 to 2500 m. We project the thickness measuredin the east Ventura basin westward 1012 km to theModelo lobe, although it is likely that some westward thinning occurs, and strata younger thanTowsley may never have been deposited west of,say, Sespe Creek.In the footwall block of the Santa Susana fault, theFrew fault underwent displacement during depositionof the Fernando Formation (Yeats, 1979, 1987a). TheTorrey reverse fault cuts the Frew and is unconformablyoverlain by Saugus Formation, documenting reversefaulting prior to deposition of the Saugus (Yeats, 1987a).This would document Pliocene reverse faulting becausethe base of the Saugus south of the Santa Susana fault isas old as it is in the Newhall magnetostratigraphic section (Levi et al., 1986; Levi and Yeats, 1993).Contemporary TectonicsThe present-day structures began to form near theend of Saugus deposition. The appearance of locallyderived clasts in the Pacoima Formation near Newhalland the upper Saugus south of the Aliso Canyon oilfield document the beginning of uplift of the SantaSusana Mountains about 0.70.6 Ma (Levi and Yeats,1993). We view this uplift as related to upward ramping of the Santa Susana fault across the structural shelfto the southwest (tectonic inversion; Yeats, 1987a).The ramp dips 5055 northeast. Atop the ramp, theleading edge of the Santa Susana fault flattens in diplocally to near horizontal due to gravitational andtopographic effects. At the Aliso Canyon oil field, the

Yeats et al.

upper part of the fault flattened so much that the faultmoved into an unfavorable orientation with respect toplanes of high shear stress. This resulted in a youngerstrand of the Santa Susana fault breaking through theconvex-upward fault surface (older strand of the SantaSusana fault), which then became folded into an anticline (Yeats, 1987a) (Figure 6F).An alternate possibility is raised by the January 17,1994, Northridge earthquake (Mw = 6.7 on themoment-magnitude scale), which occurred on a previously unknown south-dipping blind thrust beneaththe footwall block of the Santa Susana fault. Uplift ofthe Santa Susana Mountains could be produced bydisplacement on the south-dipping fault in additionto displacement on the north-dipping Santa Susanafault. A corollary to this interpretation, suggested byT. L. Davis and J. Namson (oral communication,1994), is that the northeast-dipping limb of theNewhall-Potrero and Pico anticlines may representthe forelimb of this blind thrust.In the west Ventura basin, late Quaternary displacement on the San Cayetano fault was described by Yeats(1983) and emen (1989), and on the Oak Ridge faultby Yeats (1988, 1989). The post-Saugus Oak Ridge faultcontinues east-northeast along the southern margin ofthe Santa Clara Valley, apparently cutting stream terraces of the Santa Clara River, and dies out farther east.The South strand of the Oak Ridge fault turns eastsoutheast beneath the Santa Susana fault; until the 1994earthquake, it was believed that this strand was inactive(Yeats, 1987a). The 1994 earthquake indicates that asouth-dipping fault must continue beneath the SantaSusana Mountains, and Yeats (1994, unpublishedreport) suggested that this fault is the eastward continuation of the Oak Ridge fault.Near the town of Piru, the San Cayetano fault bifurcates into two strands (Figures 2, 6B). The northern(Main) strand cuts an alluvial fan west of Piru Creekand dies out. The southern (Piru) strand is close to,but south of, the northern margin of the Santa ClaraValley where it cuts overturned Saugus Formationrocks (Figure 6B) and dies out farther east.North of the Santa Susana fault and east of PiruCreek, the sequence, including the Saugus Formation,is deformed into a fold belt cut by several south-dipping reverse faults. The southernmost two faults (theDel Valle and Holser faults) have relatively large separations, whereas the more northerly faults (theHasley, Oak Canyon, and Santa Felicia faults) haveonly small displacements (Figure 6C). The northernmost fault (Santa Felicia) overrides the San Gabrielfault, bringing strata of the east Ventura basin overrocks of the Ridge basin. The Del Valle and Holserfaults occur close to a steep isopach gradient in theFernando Formation, which thickens southwardtoward the upthrown sides of these faults; thus thefaults are entirely post-Fernando. Subsurface relationsin the Ramona oil field (Figure 6C) show that the

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Holser fault cuts obliquely across the axis of theRamona anticline, suggesting that the fault postdatesthe anticline and is not the failed overturned limb ofit. Previous studies show the Holser fault continuingeastward to join the San Gabriel fault, but Stitt (1986,his Figure 21) showed that the Holser fault is notfound in a subsurface cross section southwest of andparallel to the San Gabriel fault. Folds appear to havemuch lower structural relief in the Saugus Formationthan in older beds (Yeats et al., 1985) (Figure 6C),although there is no major angular unconformity atthe base of the Saugus.The west Ventura basin contains an enormousthickness of Pliocene-Pleistocene strata in the footwall block of the San Cayetano fault (Figure 6A). Theeast Ventura basin contains a thinner Pliocene-Pleistocene sequence in a fold belt in the hanging wallblock of the Santa Susana fault (Figure 6BD). Thetransition between the west and the east Venturabasin is unclear. The San Cayetano and Santa Susanafaults appear to be part of a single south-vergingreverse-fault system, but they do not join at the surface. Both strands of the San Cayetano fault die outeastward, and the Santa Susana fault dies out westward. Between these fault tips, the thick PliocenePleistocene sequence of the west Ventura basin istilted upward in a narrow syncline that plungessteeply westward and is characterized by locallyoverturned limbs on both flanks (Figure 6B; steepsurface dips in Figure 6C). The base of the Saugusreaches the surface near the eastern tips of the OakRidge fault and Piru strand of the San Cayetano fault.Not only these faults but the Holser fault, Del Vallefault, and the active north strand of the Oak Ridgefault have their fault tips within 7 km of one another,all in the transition zone between the west and eastVentura basins. East of the fault tips, the synclinalplunge levels off, and the Saugus appears farther eastwhere the plunge is gently eastward (Figure 2).How is north-south shortening accommodated inthe zone where the Santa Susana fault and bothstrands of the San Cayetano fault die out? The mostlikely explanation is that this transition zone covers ablind thrust of the combined San CayetanoSantaSusana fault system. The Del Valle, Holser, and othersouth-dipping faults farther north would be backthrusts of this blind thrust system. The folds in thisarea may be related to blind backthrusts at depth.Finally, that part of the San Gabriel fault southeastof Honor Rancho oil field, discussed above, is part ofthe north-south shortening, as already documentedfor the southern branch of the San Gabriel fault farther east where it merges with the Sierra Madrefrontal fault marking the southern margin of the SanGabriel Mountains. Post-Saugus dip-slip displacement on the San Gabriel fault southeast of HonorRancho oil field is at a higher rate than strike-slip displacement farther northwest.

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Figure 12(A) EastVentura basin at 15Ma. Patterns represent the same formations as in Figure 1.Sixty kilometers ofright slip removedon the San Gabrielfault, 10 km shortening removed on westVentura basin, andclockwise rotation removed from areaswhere documented by magnetic northarrows (Hornafius, 1984; Terres andLuyendyk, 1985; Luyendyk and Hornafius,1987). South strand of future San Gabrielfault strikes east-west, an unfavorable orientation for strike slip. Note diverse orientations of normal-faulted basins, suggestingthat load stress greatly exceeded all horizontal stresses. (B) East Ventura basin at about6.5 Ma, the time of deposition of the Hasleyconglomerate, after accumulation of 36 km right slip on the San Gabriel fault. Alamo Mountain (AM) and FrazierMountain (FM) provide detritus for the Violin Breccia in the Ridge basin; western San Gabriel Mountains providedetritus for the Hasley submarine fan at the base of the Towsley Formation in the east Ventura basin. Soledad basinrotated clockwise to its present orientation, placing the San Gabriel fault in a favorable orientation to accumulatestrike slip.

Yeats et al.

DISCUSSIONRelation of Miocene Rifted Zone to YoungerBasin DepocentersCrowell (1973, 1976) and Yerkes and Campbell(1976) pointed out that the Ventura basin may havehad a Miocene normal-faulted precursor. Yeats(1987b, 1989) showed that Oak Ridge was a positivefeature controlled by normal faults, inferring that theOak Ridge fault bounding Oak Ridge on the northcould also have had a normal-fault precursor (Figure12A). But Miocene strata are too deep to documentthis structure, and industry multichannel seismic lineshave not revealed it either. The east Ventura basin hasstrong enough structural relief to shed light onMiocene geology, revealing the trough extending longitudinally northwest-southeast through the basin.This rift may contain an arm that extends westwardbeneath the west Ventura basin around the north sideof the Oak Ridge fault, as shown in Figure 12A. Thisrelationship does not apply to the Los Angeles basinon the south side of the Transverse Ranges in whichthe Pliocene-Pleistocene depocenter is unrelated toareas of thick middle and upper Miocene strata (Yeatsand Beall, 1991). However, the northwestern edge ofthe San Fernando Valley is controlled by theChatsworth normal fault (Figure 12A), which cutsmiddle Miocene rocks but predates deposition ofMohnian strata (Shields, 1977; Yeats, 1987b) becauseit controlled the position of the northwest edge of theMohnian Tarzana submarine fan (Sullwold, 1960).The diverse orientation of Miocene normal-faultedbasins suggests that the load stress was much greaterthan horizontal stresses, and the intermediate andminimum principal compressive stresses were closetogether in value. But what was the orientation ofthese basins with respect to north? Terres andLuyendyk (1985) showed that the Oligocene VasquezFormation was rotated 53 clockwise prior to deposition of at least part of the Mint Canyon Formation,and Hornafius (1984), Luyendyk and Hornafius(1987), and Luyendyk (1991) argued that clockwiserotation of this age and younger characterized mostof the western and central Transverse Ranges.Figure 12A is a speculative restoration of the Ventura basin and adjacent regions at 15 Ma, prior to thebeginning of strike slip on the San Gabriel fault. Clockwise rotations of the Soledad basin, the east Venturabasin north of the Santa Clara River, and the SantaMonica Mountains are restored. The Simi Hills, theSan Gabriel Mountains, and basement terranes northof the San Francisquito fault are assumed without evidence to have been rotated as much as the Soledadbasin and Santa Monica Mountains. However, theCuyama basin, north of the Big Pine fault, is not rotated, following Luyendyks model. (However, Ellis etal., 1993, found up to 20 clockwise rotation of

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Pliocene strata in the Cuyama basin.) Ten kilometersof post-Miocene contraction in the west Venturabasin is removed, and this contraction is removed inthe east Ventura basin as well. This restoration givesthe east Ventura basin rift and the granitic ridge northof it an east-west orientation in the late Miocene.Role of the San Gabriel FaultThe east-west orientation of the east Ventura basinrift and the granitic ridge to the north prior to clockwise tectonic rotation was unfavorable for the accumulation of strike-slip displacement. The San Gabriel faultis parallel to the southwest edge of the granitic ridge,suggesting that the fault may have had a middleMiocene normal-fault ancestor. Clockwise rotation inthe middle Miocene changed the orientation of thegranitic ridge and the rift south of it from east-west (Figure 12A) to northwest-southeast (Figure 12B). Becauseof the new favorable orientation, the San Gabriel faultbegan to accumulate right slip at 1012 Ma. This was atleast 23 m.y. after the initiation of the middle Miocenerift in the east Ventura basin. However, deposition inthe rift continued as major right slip on the San Gabrielfault accumulated (Figure 13), suggesting an extensional component of strike slip on the fault. An extensionalcomponent would have been present adjacent to theRidge basin for the same reason. This indicates that theSan Gabriel fault may have propagated along one of therift boundary faults at the time the boundary faultbecame favorably oriented for this to occur.Right slip began rather abruptly 1012 m.y. agoand continued through the Pliocene at rates of 69mm/yr (Figure 11). The arrows for DC and CA showthat the displacement could have occurred at anytime after the time shown; the arrow for DCF and HAshow that the displacement could have been largerthan that shown up to the maximum of 60 km. The69 mm/yr rate (4.55.5 mm/yr if the total displacement is 45 rather than 60 km) takes into account theinitiation of strike slip prior to the end of MintCanyon deposition, the relatively large displacementsof Fernando Formation in the western San GabrielMountains, and the small displacement of FernandoFormation in the east Ventura basin prior to deposition of the Saugus Formation, discussed below.Uncertainty in timing and amount of displacements shown on Figure 11 leads to uncertainty inthe age of the slowdown of strike slip on the SanGabriel fault. A slowdown by the Pliocene is suggested by the overlap of the San Gabriel fault at its northwestern end by the Pliocene Hungry Valley Formation (Crowell, 1982). However, the Fernandodisplacements in the western San Gabriel Mountains(Ehlig, 1975) (FM on Figure 11) suggest that theslowdown took place in the Pleistocene. The ease ofcorrelation of the Fernando Formation across the

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Figure 13Summary diagram of tectonic ev