active folding and blind thrust faulting induced by...

9Active Folding and Blind Thrust FaultingInduced by Basin Inversion Processes,Inner California Borderlands

Carlos Rivero1

Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A.

John H. ShawDepartment of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, U.S.A.

ABSTRACT

The present bathymetry, basin geometries, and spatial earthquake distribution in the innerCalifornia borderlands reflect complex basin inversion processes that reactivated two low-angleMiocene extensional detachments as blind thrust faults during the Pliocene to Holocene.The Oceanside and the Thirtymile Bank detachments comprise the inner California blindthrust system. These low-angle detachments originated during Neogene crustal extension thatopened the inner California borderlands, creating a rift system that controlled the deposition ofearly to late Miocene sedimentary units and the exhumation of the metamorphic Catalinaschist. During the Pliocene, a transpressional regime induced by oblique convergence betweenthe Pacific and the North American plates reactivated the Oceanside and the Thirtymile Bankdetachments as blind thrust faults. This reactivation generated regional structural wedgescored by faulted basement blocks that inverted the sedimentary basins in the hanging wall ofthe Miocene extensional detachments and induced contractional fold trends along the coastalplain ofOrange and SanDiego counties. Favorably oriented high-angle normal faultswere alsoreactivated, creating zones of oblique and strike-slip faulting and folding such as the offshoresegments of the Rose Canyon, San Diego, and the Newport-Inglewood fault zones. We eval-uate several different styles of geometric and kinematic interactions between these high-anglestrike-slip faults and the low-angle detachments, and favor interpretationswhere deep obliqueslip is partitioned at shallow crustal levels into thrusting and right-lateral strike-slip faulting.

Analyses of seismic reflection profiles, well data, earthquake information, and sea-floorgeology indicate that the Oceanside and the Thirtymile Bank blind thrust faults are active andrepresent important sources of earthquakes in this region. Restored balanced cross sectionsprovide aminimumsouthwest-directed slip of 2.2–2.7 km (1.4–1.8mi) on theOceanside thrustand illustrate the function of this detachment in controlling the processes of basin inversionand the development of the overlying fold and thrust belt.

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Rivero, Carlos, and John H. Shaw, 2011, Active folding and blind thrust faulting

induced by basin inversion processes, inner California borderlands, in K. McClay,

J. Shaw, and J. Suppe, eds., Thrust fault-related folding: AAPG Memoir 94,

p. 187 – 214.

1Present address: Structural Geology Team, Chevron Exploration Technology Company, Houston, Texas.

Copyright n2011 by The American Association of Petroleum Geologists.

DOI:10.1306/13251338M943432

INTRODUCTION

The southern margin of the inner California border-land province (sICB) comprises the San Pedro shelf andthe Oceanside-Capistrano Basin (Figure 1). This regionis limited to the north by the Los Angeles Basin, to theeast by the coastal plain of the Orange and San Diegocounties, to thewest by the SantaCatalina Island, and tothe south by theUnited States-Mexicomaritime bound-ary (Vedder, 1976; Drewry and Victor, 1997).

Geologic data from nearby islands, sea-floor sam-ples, and oil industry wells indicate that the sedimen-tary rocks deposited in the sICB aremainly Quaternaryto lower Miocene turbiditic and fluvial sand-shale de-posits overlying Cretaceous to Paleogene marine con-glomerate, sandstone to siltstone formations, and Me-sozoic Catalina schist (Legg, 1980; Vedder et al., 1986;Clarke et al., 1987, among others).

The main physiographic characteristic of the sICB isa series of elongated basins and ridges trending sub-parallel to the coast and to the relative slip vector betweenthe Pacific and the North American plates (Luyendyk,1991; Atwater and Stock, 1998). Based on the presenceof these physiographic features, geophysical data, andcoastal geology, several authors have attributed theearthquake activity in this part of the inner Californiaborderlands to a regional system of active strike-slipfaults similar to that in the onshore region around thePeninsular Ranges (e.g., Legg and Ortega, 1978; Clarkeet al., 1987; Legg, 1989). However, the inner Californiaborderlands do not display the apparent spatial corre-lation between earthquake activity and regional strike-slip fault zones that is observed around the onshoreregion of the Peninsular Ranges (Figure 1). In contrast,seismicity in this area is diffuse and scattered (Ziony andJones, 1989; Astiz and Shearer, 2000; Richards-Dingerand Shearer, 2000). Poorer offshore earthquake locationsbecause of limited station coverage presumably con-tribute to this pattern; however, they alone are insuf-ficient to explainwhy seismicity is not localized along afewmajor offshore strike-slip faults.Moreover, the focalmechanismof the 1986 (5.6 localmagnitude scale)Ocean-side earthquake, the largest recorded event in the re-gion, indicates faulting dominated by thrust motion(Hauksson and Jones, 1988; Pacheco andNabelek, 1988).Thus, we interpret these observations to reflect a com-plexmixtureof strike-slip andblind thrust faulting in theinner borderlands that is similar to the style of defor-mation in the onshore Los Angeles Basin (Hauksson,1990; Wright, 1991; Shaw and Suppe, 1996).

During the past decade, several authors have de-scribed the function of a Miocene system of low-anglenormal faults in the Neogene opening of the innerborderland province and in the clockwise rotation and

translation of the Transverse Ranges from the south toits present location (Yeats, 1976; Crouch and Suppe,1993; Nicholson et al., 1993; Bohannon and Geist, 1998;Ingersoll and Rumelhart, 1999). Rivero et al. (2000) sug-gested that the seismogenic inner borderland blindthrust systemoriginatedwhen twoof these detachmentswere reactivated by basin inversion processes initiatedin the late Pliocene, during the onset of the moderntranspressional regime (Figure 2). Herewe confirm andelaborate on these results, showing that large-scale thrustfaulting and folding are driven by structural wedging(Medwedeff, 1992) and argue that basin inversion pro-cesses are the primary tectonic mechanisms controllingthe physiography and geology of the southern marginof the inner borderlands.

To document these basin inversionmechanisms, weanalyzemore than10,000km(6214mi) of industry seismicreflection profiles, well data, seismicity, and sea-floorgeologic maps. We perform a structural analysis thatinvolves kinematic and forward modeling techniquesbased on quantitative structural relationships betweenfold and fault shapes (Suppe, 1983; Mount et al., 1990;Erslev, 1991; SuppeandMedwedeff, 1992;Allmendinger,1998). We also use advanced three-dimensional (3-D)modeling techniques to generate precise representa-tions of fault surfaces and key stratigraphic markers(Plesch et al., 2007). The lateral extent and geometry ofthe active blind thrust ramps and fold trends are de-termined by mapping of direct fault-plane reflectionsand folded reflections throughout the basin areas cov-ered by the seismic grid (Figures 2, 3). The 3-D mod-eling was also used to quantify the distribution of dipslip on the active fault system and to further constrainthe geometric analysis.

To illustrate our findings, we present new regionalstructural interpretations of the active submarine foldand thrust belt located offshore Dana Point, and newgeometric representations of the offshore Newport-Inglewood, Rose Canyon, and San Diego Trough faultzones. These new structural interpretations are consis-tent with basin inversion processes and the presenceof both active blind thrust and strike-slip faults in thesouthern inner California borderlands.

BASIN INVERSION

Formation of sedimentary basins commonly involvesextensional deformation of the crust, with concomi-tant development of rifting and normal faulting (Ballyand Snelson, 1980; Allen and Allen, 1990). Subsequentcompressional-transpressional tectonic phases gener-ally induce the contraction of the basins in a processcalled basin inversion or inversion tectonics (Gleinner

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and Boegner, 1981; Bally, 1984), characterized by up-lifting, flexure, and folding of the basin floor and thesedimentary infill (Ziegler, 1983; Williams et al., 1989;Letouzey, 1990) (Figure 4).

As the tectonic phase of compression or transpres-sion proceeds, favorably oriented normal faults reversetheir movement and become totally or partially inverted(Bally, 1983). The onset of reverse slip normally occursalong the deepest parts of the reactivated fault, typicallywhere the fault surface bounds a preexistent graben orhalf graben (Figures 4, 5). The upward movement of thehanging-wall block due to the reverse slip continuously

deforms shallower strata and older structures and mayinducedeep-seated structuralwedging andback thrust-ing (Roure et al., 1990). At shallow levels, the propaga-tion of the deformation eventually leads to the develop-mentofnew fault structures (blindor emergent shortcuts)in either the footwall or hanging-wall blocks (McClay,1992, 1995).

The reactivation of the faults produces broad anti-clines located directly on top of extensional rolloversand synextensional stratigraphic wedges (Bally, 1984;McClay, 1989; Letouzey, 1990), and tighter folds local-ized above the reactivated fault tips (Figure 5). As a

Figure 1. Map of the inner California borderlands and the study area. Major surface fault traces and earthquake locations(1977 to 2000) are shown (from Richards-Dinger and Shearer, 2000). The earthquake focal mechanism represents the 1986(5.3 local magnitude scale) Oceanside earthquake location from Astiz and Shearer (2000). Pacific NOAM slip vector (PACI) fromMcCaffrey (2005). L. A. = Los Angeles Basin; SCI = Santa Catalina Island; SCL = San Clemente Island. The bathymetric contourinterval is 200 m (656 ft). Digital southern California topography was generated from digital elevation data provided by the U.S.Geological Survey.

Active Folding and Blind Thrust Faulting Induced by Basin Inversion Processes 189

consequence, synextensional units in the hanging-wallblock are commonly preserved within the core of con-tractional folds at higher structural levels than their cor-relative footwall and basin regional elevations (Figure 5)(Williams et al., 1989). Commonly, the contractionalfolding is also recorded by syntectonic growth stratathat thicken away from the contractional folds, provid-ing a record of the kinematic evolution of the structure

(Shaw and Suppe, 1996; Suppe et al., 1997). As a result,we commonly observe that slip along an inverted nor-mal fault changes from net extension at deep strati-graphic levels to net contraction in the contractionalgrowth strata and shallowparts of the postrift sequence(Cooper et al., 1989; Hayward and Graham, 1989).

General observations suggest that inversion processesdo not always reactivate the entire suite of preexistent

Figure 2. Perspective view of the inner California blind thrust system: the Oceanside and the Thirtymile Bank thrusts. Theblue surface represents the top of the basement (Catalina schist). Digital shaded relief of the southern California topographywas generated from digital elevation data provided by the U.S. Geological Survey. PVP = Palos Verdes peninsula.

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Figure 3. (top) Migrated seismic reflection profile across the active submarine fold and thrust belt located to the south of Lasuen Knoll. The seismic profile imagesthe thrust front and a regional backthrust zone (see Figure 6 for the location). (bottom) Balanced geologic cross section XX0 derived from the seismic profile, lateralcorrelation of well information, direct fault-plane reflections, and fault-related folds theories. The cross section depicts the complex structural contractional trendsproduced by early Pliocene to Holocene reactivation of a series of northeast-dipping Miocene normal faults. The section also illustrates the role that basin inversionprocesses and associated structural wedging play to control the location of the thrust front, the SanMateo trend, and themonocline trend. Fm. = Formation; S. Onofre =San Onofre.

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normal faults; thus, we commonly observe isolatedextensional faults in close proximity with inverted andreverse faults (Lowell, 1995;McClay, 1995). Dependingon the coupling between the reactivated fault surfacesand the direction of the contractional slip, a varied suiteof blind or emergent structures such as back thrusts,structural wedges, and out-of-sequence thrusts may bepresent. All these elements give a complex picture tothe settingswherebasin inversionoperates,making themprone to be confusedwith flower structures (Roure andColletta, 1996; Colletta et al., 1997; Roure et al., 1997).

In this study,wedocument basin inversion processesin the inner California borderlands (Figures 1, 6) thatshare many of the characteristics documented in otherinverted basins (Bally, 1984; Badley et al., 1989; Lowell,1995), in analog (sand-box) experiments (McClay andBuchanan, 1992; Schreurs and Colletta, 1998), and innumerical models (Huyghe and Mugnier, 1991; Buiterand Pfiffner, 2003).Many of these previous studies, how-ever, consider reactivation of steeply dipping planar andlistric normal fault geometries, with little or no refer-ence to low-angle normal systems because activation of

upper crustal detachments is rare (Butler, 1989). Thus,the southern innerCalifornia borderlands offers a uniqueopportunity to investigate basin inversion processesoriginated by the reactivation of Miocene low-angle de-tachments. High-resolution seismic reflection profilesacross this region illuminate anddocument active growthfolding above blind thrust structures at Lasuen Knoll,CrespiKnoll,CoronadoBank, and theSanDiegoTrough–Thirtymile Bank region (Figures 3, 5–7). In these struc-tures, contractional Pliocene sediments and youngerunits overlayMiocene rocks deposited during the riftingphase that opened the inner California borderlands.TheseNeogene rocks commonly defined local asymmet-ric anticlines sitting on top of regional rollovers. Thenormal faults associated with these rollovers are com-monly synthetic to regional basal detachments. We in-terpret these offshore trends as the bathymetric expres-sion of folding and faulting processes generated by theonset of the present transpressional regime in the south-ern inner California borderlands in the Pliocene. Thistranspressional regime induced the inversion of theMio-cene depocenters (Figure 2) and reactivated a pair ofregional low-angle detachments as the inner Californiaborderlands blind thrust system (Rivero et al., 2000). Inthe next section,we integrate fault-bend fold theory andforward modeling techniques with seismic reflectionprofiles, well information, and seismicity to constructbalanced cross sections and kinematic models that de-scribe our interpretations of basin inversion and associ-ated structural wedging across the major active trendsof the southern inner California borderlands (Figure 6).

Our results are compatible with strike-slip displace-ment along the offshore segments of the Newport-Inglewood and Rose Canyon fault zones, and we pro-vide new constraints on the geometries and kinematicsof these fault systems. The results also provide insightinto the subsurface geometries of complex zoneswherecoeval strike slip and thrust fault interact.

ACTIVE SUBMARINE FOLD AND THRUST BELT

High-resolution seismic reflection profiles offshoreDanaPoint show thepresence of a shallowwest-vergentsubmarine fold and thrust belt defining a broad zoneof active thrusting and related folding sitting on top ofthe well-illuminated Oceanside thrust (Figures 3, 6).Themain characteristics of the submarine fold and thrustbelt are thedevelopment of a large-scale structuralwedge,a series of smaller forethrusts, and associated contrac-tional folds located in the hanging-wall block of theOceanside thrust. The thrust sheets are emplaced on,and structurally controlled by, the regionalMiocene rift

Figure 4. Conceptual model of basin inversion (modifiedfrom Bally, 1984). (A) Development of the extensional halfgraben and associated rollover structure. (B) Basin inversionphase characterized by the development of asymmetric con-tractional folds above the tip line of the old normal faultand structural wedging involving back thrusts within the oldhalf graben.

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system that is expressed in the seismic data as exten-sional rollovers and normal faults (Figures 2, 5). Al-though locally deformed in tight anticlines, the basicelements of the rift system are still clearly recognizablein the seismic data (Figure 2) (Yeats, 1976; Crouch andSuppe, 1993; Bohannon and Geist, 1998).

Several of these contractional and extensional struc-tures were previously interpreted as wrench-relatedthrust and folds and as flower structures produced byactive offshore segments of the Newport-Inglewoodstrike-slip fault (Legg and Kennedy, 1979; Crouch andBachman, 1989; Fisher and Mills, 1991). In contrast, weobserve that the contractional trends sole into theOcean-side thrust (Figures 3, 5, 7). This implies that the Ocean-side thrust is a regional detachment level for the con-tractional deformation observed in the southern innerCalifornia borderlands (Figures 3, 7). Folded strata andregional near-sea-floor unconformities delineate sev-eral active piggyback basins preserved on top of the

thrusts. Stratigraphic control provided by the nearbyMobilMSCH-1 San Clemente well and the Shell Ocean-side well (Figure 6) dates the onset of the basin inver-sion phase as Pliocene (Figures 3, 5).

Motion on theOceanside thrust generated four prom-inent contractional fold trends. Three of these trends areforeland-directed structures, namely the SanMateo, theSan Onofre, and the Carlsbad trends (Figures 3, 7, 8).These active structures are characterized by thrust sheetsthat extend laterally for several kilometers and produceprominent fold scarps in the sea floor. The fourth trendis characterized by hinterland thrusting,which ismani-fested in a laterally continuous monocline that controlsthe relief and bathymetric expression of the coastal shelf(Figure 6). Thismonocline is the result of the interactionbetween a shallow west-dipping backthrust system andthe deep-seated, east-dipping Oceanside thrust. In thefollowing section, we present detailed structural eval-uations of each of these trends.

Figure 5. Seismic examples ofbasin inversion structures asso-ciated with the activity of theOceanside thrust. (A) Half-grabenreactivation along a lateral rampof the Oceanside thrust. (B) Inver-sion structure developed bycontractional reactivation of theCarlsbad fault. In both cases, seis-mic reflections within the Mon-terrey Formation define the strati-graphic expansion of the synriftsequence. Similarly, the phase ofbasin inversion is recorded bythe contractional folds and thinningof the syncontractional Pico For-mation on the crest of the anti-clines. The location of seismic linesis shown in Figure 6.


SAN MATEO TREND

The SanMateo trend is located offshoreOrangeCountyat the southern margin of Lasuen Knoll (Figure 6). Thistrend extends for more than 12 km (7.4 mi), parallel to

the coastline betweenDanaPoint andOceanside.Crouchand Bachman (1989), Fisher andMills (1991), andMills(1991) originally mapped the trend as an active frontalthrust that is part of an outer complex zone associatedwith thedextralmotion of theNewport-Inglewood fault.

Figure 6. Map of major structural trends discussed in this study. The San Mateo (1), San Onofre (2), and Carlsbad trends (3) arethe three major forethrust structures; the shelf monocline trend (4) is a regional backthrust system that may extend intothe onshore San Joaquin Hills. Bathymetric contour lines illustrate the wide shelf offshore Orange County, which is producedby a regional southwest-propagating structural wedge. The locations of the mapped parts of the offshore Rose Canyon faultare shown (5). The breakaway zone of the Oceanside thrust is also indicated. Well locations A, B, and C correspond to theMobil MSCH-1 San Clemente well, the Shell Oceanside well, and the Point Loma well, respectively. Inset is the regional grid ofseismic reflection data used in this study. L. A. = Los Angeles.

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Figure 7. (top) Migrated seismic reflection profile across the San Mateo trend, offshore Dana Point (see Figure 6 for the location). Extensional Miocene structuresare observable next to the thrust front position. (bottom) Balanced geologic cross section YY0 developed from the seismic profile, well information, and directfault-plane reflections. The structural interpretation highlights the relationship of the contractional structures to a regional structural wedge and the reactivation ofthe preexistent Miocene normal faults during the phase Pliocene basin inversion. Restoration of the proposed structural scenario suggests 2.5 km (1.5 mi) as aminimum amount of Pliocene to Holocene shortening. San Clemente 1 = Mobil MSCH-1 San Clemente well.

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Fisher andMills (1991) inferred thatQuaternarywrenchdeformation produced by a broad basement anomalyat a left step of the Newport-Inglewood–Rose Canyonfault zone was responsible for this and other nearby con-tractional trends. In contrast, Rivero et al. (2000) sug-gested that this deformation results from thrusting onthe Oceanside detachment, whichmay be independentfrom strike-slip motion on the Newport-Inglewood–Rose Canyon fault zone.

Time anddepth-converted seismic reflection profilesacross the SanMateo trend used in our analysis indicatethat the structure consists of a shallow and relativelysymmetric faulted anticline that produces a series of pro-nouncedbathymetric scarps on the sea floor (Figures 3, 7).The backlimbof the structure is definedby a syncline thatcontains Quaternary and older sediments (Figures 3, 7).At the thrust front, two additional small anticlines de-form theuppermost sedimentary section (Figure 7). The

Figure 8. Detail of a seismicreflection profile across the SanOnofre trend (see Figure 6 forthe location). (A) Sea-floor scarps,shallow folding, and offset ofseismic reflections define the lo-cation of the thrust fault. (B) Struc-tural interpretation describingthe geometry of the San Onofreanticline and the origin of thistrend as a breakthrough thrust pro-duced during the reactivationof a Miocene normal fault. Fm. =Formation.

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observed folding of Pliocene and younger strata in thesyncline and the related sea-floor fold scarps above theanticlines suggest Quaternary activity of the SanMateotrend.

Direct fault-plane reflections and stratigraphic cutoffsconstrain the location of a 258 north-dipping fault planethat produces the anticline. We refer to this as the SanMateo thrust fault and apply fold-related fault theories(Suppe, 1983) to estimate the downdip projection of thefault to the northeast (Figure 7). This downward projec-tion corresponds to the location of a preexistent half-graben structure situated in the hangingwall of the basalOceanside thrust (Figures 3, 7). Thehalf graben is definedby awestward-thickening sedimentarywedge inwhichthe beds dip progressively steeper with depth, a typicalstyle of extensional half grabens. Stratigraphic controlprovidedby thenearbyMobilMSCH-1SanClementewell

indicates a Miocene age for the half graben, with the syn-rift sequence corresponding to the middle MioceneMon-terrey Formation. This rollover structure is laterally persis-tent along the entiremappedpart of the SanMateo thrustand shows evidence of structural inversion (Figure 7).

We combine these observationswith fold-related faulttheories and balanced forward modeling techniques(Mount et al., 1990; Shaw and Suppe, 1996) to interpretthe San Mateo anticline as a multibend fault-bend folddeveloped by the reactivation of the underlying Mio-cene normal fault associated with the described half gra-ben (Figure 9). The amount of contractional slip derivedfrom panel BB0 is 0.58 km (0.36 mi). This value is con-sistent with the estimated 0.55 km (0.34 mi) of contrac-tional slip recorded by kink-band DD0, which affectedthe Miocene rift basin, refolding the rollover and theyounger units deposited in shallower levels.

Figure 9. Kinematic modeling of the de-velopmentof theSanMateo trend. (A)Minordisplacement on the Oceanside thrust pro-duces the reactivation of a synthetic normalfault and the incipient development ofthe San Mateo thrust. Simultaneously, anupper structural wedge is also developedin the thrust front position, which dissipatescontractional slip as emergent back thrust-ing. (B) Generation of a subthrust (footwallshortcut), which imbricates and translatesthe San Mateo anticline. A coeval lowerstructural wedge related with the subthrustis formed at the thrust front position, in asimilar way as for the San Mateo thrust.(C) Final configuration of the San Mateotrend and the thrust front domain.


Weinterpret that the gently dippingOceanside thrustis not cut or deformed by the San Mateo fault, which islimited to its hanging-wall block (Figures 3, 7). This in-terpretation is based on the listric geometry of the SanMateo thrust with depth. This listric geometry is ob-served along the lateral extension of the trend (Figure 7)and is required to explain the rollover panel observed inreflections of the Monterrey Formation (Figure 7). Theinterpretation implies at least approximately 0.55 km(0.34 mi) of slip on the San Mateo thrust soles into theOceanside thrust.

Structural analysis of the backlimb reflections of theSanMateo anticline shows a kink-band panel (CC0) thatextends downward into the footwall block of the SanMateo thrust. Thus, the thrust fault appears to be foldedby this kink band based on the geometric criteria pro-posed by Shaw et al. (1999). This implies that the SanMateo trend is imbricated by an underlying youngerthrust (Figures 7, 9). This deeper thrust emerges to theeast,where its slip is transferred to shallowback thrustsanddissipated in contractional folding.This fault extendsto depth beneath and imbricates the San Mateo thrust.

Details of the structural geometry observed in theSanMateo trendappear tovary along strike. To thenorth,at location XX0 (Figures 3, 6), for example, the forelimbof the anticline expresses a stepper geometry that canbe explained by imbricated fault-related folding. To thesouth, the thrust geometry becomes amultibend geom-etry ending in a buried tipwedge.Despite this variation,shortening across the trend changes very little, rangingfrom 0.5 to 0.7 km (0.3 to 0.4 mi).

SAN ONOFRE TREND

The San Onofre trend is a large, northwest-strikingfaultedanticlinewithpronouncedbathymetric expressionlocated offshore San Onofre and Oceanside (Figure 6).The structure was previously interpreted as an active,near-vertical strand of the Newport-Inglewood faultzone that potentially represented the reactivated off-shore extension of theMiocene Cristianitos normal fault(Fisher and Mills, 1991; Mills, 1991). Based on severalobservations, Fisher andMills (1991) proposed a recentright-lateral strike-slip movement on the onshore Cris-tianitos normal fault. However, Shlemon (1992) andShlemon and Rockwell (1992) identified a regressive se-quence of undisturbed marine deposits of Quaternaryage (stage 5e �125 k.y.) concealing the surface trace ofthe Cristianitos fault at San Onofre beach. This indi-cates that the Cristianitos fault is inactive and thereforeunlikely to have produced the SanOnofre trend,whichis generally considered active. Moreover, our analysissuggests that the San Onofre trend is formedmostly by

slip on a northeast-dipping thrust fault above theOcean-side thrust, and thus, it is not a simple vertically dip-ping extension of theCristianitos orNewport-Inglewoodtrends.

High-resolution seismic reflection profiles across theSan Onofre trend image a complex ramp anticline withpronounced bathymetric relief (Figure 8). Folded andlaterally persistent unconformities and stratigraphic re-flectors of syncontractional Repettian–Venturian andyounger strata suggest the late Pliocene to Quaternaryactivity of the trend (Fisher andMills, 1991). A dippingzone of shallow reflection truncations observed in theseismic images also indicates that the San Onofre trendis produced by an emergent thrust fault (Figure 8). Seis-mic reflection terminations as well as offset axial sur-faces are used to interpret the shallow location and dip(32–348 northeast) of the thrust fault, whichwe definedas the SanOnofre thrust (Figures 6, 8). Using fault-bendfold theories (Suppe, 1983), we estimate that the thruststeepens to a northeast-dipping angle of 38 to 408,whichis consistent with the dipping value of the fault reflec-tions observed in depth-converted seismic images.

Similar to the San Mateo anticline, thickening of theMonterrey Formation toward the fault in the hanging-wall block of the San Onofre thrust suggests that thisyoung contractional structure reactivated aMiocene nor-mal fault. Using forward modeling techniques (Mountet al., 1990; Shaw and Suppe, 1996) and 3-D structuralanalysis (Rivero and Shaw, 2005), we interpret the SanOnofre trend as a faulted decollementwedge. Thiswedgedeveloped above a south-plunging segment of the under-lying SanMateo thrust (Figures 8, 10). The decollementwedge is defined by the presence of a southwest-dippingkink-band AA0 located below the San Onofre thrust-plane reflections. The wedge and the kink band extendupward into the hanging wall of the San Onofre faultbut are clearly offset by the thrust. Structural relief ofthe postrift top Repetto and Pico formations, which iscontrolled by lateral seismic correlation, confirms thefault offset.

On the basis of these geometric and stratigraphic rela-tionships, we interpret the San Onofre trend as a latePliocene to Quaternary fault-related fold originated byinversion of a half graben and thrusting that extendsto the sea floor (Figure 8). Our structural scenario forthe origin of the San Onofre anticline does not requirethe presence of amajor strike-slip fault in the SanOnofretrend nor its connection with other onshore trends. Incontrast, the proposed solution describes a series ofnortheast-dipping thrust ramps beneath the trend witha minimum foreland-directed slip of 1 km (0.6 mi) forthe San Mateo thrust and of 0.25 km (0.15 mi) for theSanOnofre thrust. These thrusts appear to sole at depthwith the Oceanside thrust.

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CARLSBAD TREND

Evidence for active thrusting associated with the innerCalifornia borderlands blind thrust system is also pre-sent at CrespiKnoll, adjacent to SanDiegoBay (Figure 6).At this location, thenorth-south–strikingCarlsbad trendextends for more than 25 km (15.5 mi) offshore betweenCarlsbad and Del Mar. The trend was originally rec-ognized by Crouch and Bachman (1989) and by Mills(1991) as a compressional thrust-related fold. Bohannonand Geist (1998) identified the trend as the offshore ex-tension of the Newport-Inglewood–Rose Canyon faultzone and noted the active character of the structure andits potential downward merging with the Oceansidedetachment.

In the industry seismic reflection profiles, theCarlsbadtrend is illuminated as awide, asymmetric, and laterallycontinuous contractional anticline overlying a Miocenehalf graben located in the hangingwall of theOceansidethrust (Figures 5B, 11). Reflection truncations also illu-minate a northeast-dipping thrust fault situated beneaththe trend, consistent with the observed southwest ver-gence of the anticline.We refer to this thrust fault as theCarlsbad thrust, following original terminology fromFisher and Mills (1991).

Downdip projection of theCarlsbad thrust coincideswith the location of a high-angle normal fault syntheticto the regional Oceanside detachment (Figure 11). Thenormal fault offsets the top of the regional acoustic base-ment defined by the Mesozoic Catalina schist. This faultalso controlled the formation of amiddle to lateMioceneasymmetric half graben in which the synrift MonterreyFormation thickens toward the fault (Figures 5B, 11).Highly reflective unitswithin theMonterrey Formationand the underlying San Onofre Breccia highlight thegeometry of the growth sequence within the broad roll-over. The high reflectivity levels may be related withMiocene volcanic rocks extruded and exposed in highlyextended regions of the inner borderlands during thepeak of the Neogene rifting phase (Vedder et al., 1986;Clarke et al., 1987; Crouch and Suppe, 1993). Similar seis-mic facies have been reported elsewhere within the To-panga Formation of theLosAngeles Basin and someotherareas of the inner California borderlands (Luyendyk,1991; Wright, 1991).

Regional stratigraphic correlation indicates that con-tractional growth sediments of the late Pliocene (Ven-turian) to Quaternary(?) age are present atop the Carls-bad trend (Figure 11). These growth strata are foldedand truncated at the sea floor in a way consistent withcontractional tip-line folding above the reactivated nor-mal fault. The structural relief of these andolderhorizonscan be explained in terms of basin inversion processesinduced by the reactivation of theOceanside thrust andby reverse motion on the described synthetic normalfault. At depth, the basal section of the Monterrey For-mation still preserves the original dip orientation pro-duced by the extensional phase. This suggests a limitedamount of reversemotion on the invertedCarlsbad faultbecause the null point is somewhere close to this strati-graphic level (Williams et al., 1989).

Our structural analysis indicates that the Carlsbadtrend is a contractional anticline formed above the tipof an inverted normal fault. The geometry of the struc-ture suggests that the Carlsbad anticline may have orig-inated by trishear fault-propagation folding (Figure 11)(Erslev, 1991; Hardy and Ford, 1997; Allmendinger,1998, 2000); although triple-junction kinematics (Narrand Suppe, 1994) are also possible (Rivero, 2004). Ideally,

Figure 10. Balanced sequential development of the SanOnofre and the San Mateo structures. (A) Activation of theSan Mateo thrust as a reactivated Miocene normal faultsynthetic to the Oceanside thrust. (B) Development of a struc-tural wedge and folding of the future San Onofre thrust sheet.(C) Slip on the young breakthrough structure cutting acrossthe northern limb of the structural wedge generates theSan Onofre anticline.


a geometric analysis of the contractional growth stratawould be sufficient to distinguish between the two al-ternatives (Hardy and Poblet, 1994; Ford et al., 1997;Suppe et al., 1997).However, contractional growth strataatop the Carlsbad trend define a very thin and shallowsedimentary sequence that is very difficult to analyze inthe seismic images.

In summary, our structural analysis indicates that theCarlsbad trend is a propagating tip fold produced by thestructural inversion of aMiocene normal fault soled intothe Oceanside thrust. Onlapping and modestly foldednear-sea-floor sediments of shallow Quaternary(?) ageare interpreted as evidence of Quaternary(?) activity onthe Carlsbad and Oceanside thrusts. Inversion of thefolded shape of the Carlsbad anticline using trishear es-timates between 0.4 and 0.6 km (0.2 and 0.4 mi) of totalthrust slip in the Carlsbad thrust.

SHELF MONOCLINE TREND

The offshore coastal shelf between Dana Point andCarlsbad exhibits a pronounced widening observed in

bathymetric contours of the sea floor (Figure 6). Thiswidening displaces the shelf break seaward, from anaverage distance of about 2 km (1.2 mi) in front of theSan JoaquinHills andCarlsbad, to amaximumof 10 km(3.2mi) offshore SanOnofre. Previous neotectonic stud-ies in the region attributed this variation to recent ac-tivity in the offshore trace of the Newport-Inglewoodstrike-slip fault zone (Legg, 1980; Fisher et al., 1988;Fisher andMills, 1991;Mills, 1991, amongothers). Theseauthors suggested that a complex andcontinuouswrenchfault systemassociatedwith the active trace of this strike-slip fault produced a flower structure with bathymet-ric expression that controls the sea-floor bathymetry ofthe area.

In contrast, industry seismic reflection profiles in thisregion illuminate a broad andwell-expressedmonoclinedipping at about 15 to 208 to the southwest, which un-derlies and forms thewide shelf (Figures 3, 7, 12). In theseismic images, the monocline is defined by northeast-dipping axial surfaces F and F0 (Figures 3, 7, 12). Southof San Onofre, the coastal shelf narrows again wherethe monocline is not present (Figure 6). We interpretthis monocline as a rift shoulder of the Oceanside de-tachment, which has been reactivated as a structural

Figure 11. Structural interpreta-tion of the Carlsbad trend in adepth-converted seismic reflectionprofile. A best-fitted model com-puted with the Trishear software(Allmendinger, 1998, 2000) forthe Carlsbad anticline assumingtrishear fault-tip propagation ki-nematics resolved the frontal limbof the Carlsbad anticline by re-verse slip of a steeply northeast-dipping thrust, consistent with thatobserved in the depth-convertedseismic image. The same solutionaccounts for rounded-hinge ge-ometries and an estimated 0.4 to0.6 km (0.2 to 0.4 mi) of displace-ment in the Carlsbad fault. Thevalue of the correlation coefficientfor this fittedmodel isR=0.98932.

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wedgeduring tectonic inversion. The rift shoulder devel-oped on the hanging-wall block of the Oceanside de-tachmentby east-directednormalmotion inducedduringthe Miocene extension that affected the inner Californiaborderlands.

Seismic correlation across the monocline providedby the Mobil MSCH-1 San Clemente and the Shell

Oceanside wells reveals shallow folding involving theRepetto and the Pico formations. These units appearcontinuous across the monocline trend but form a setof asymmetric, northeast-vergent folds consistent withhinterland-directed structural transport during tectonicinversion of the underlying rift (Figures 3, 7, 12). Theseismic images indicate that the style of deformation of

Figure 12. (A) High-resolutionseismic reflection profile illustratingthe geometry of the backthrustsystem in a region locatedoffshoreOceanside (see Figure 6 for thelocation). The seismic data illumi-nate a well-developed zone offault-related folding located in thehanging-wall block of the Ocean-side thrust. Compare with theshelf domain in Figure 7. (B) Retro-deformable structural solutionfor the backthrust system and themonocline structure located atthe coastal shelf of Orange Coun-ty. The solution also considersthe interaction of the monoclinetrend and the offshore part oftheNewport-Inglewood fault zone.In this solution, the strike-slipfault is offset by the structuralwedge, yielding offset fault seg-ments in different structural blocks.This solution allows the coevalactivity of the low-angle and thestrike-slip faults but reflects a highdegree of fault complexity. Thistype of complexity, induced by theOceanside thrust, is likely respon-sible for the along-strike geometricsegmentation of the Newport-Inglewood and the Rose Canyonstrike-slip faults south of DanaPoint.


this shallow folding changes laterally from single fault-bend and fault-propagation fold anticlines tomore com-plex imbricated systems (Figure 12).Normally, the crestsof these shallow anticlines appear truncated by themod-ern sea floor.

Direct fault-plane reflections and stratigraphic cut-offs of Miocene units at middle levels of the monoclinereveal the location of a backthrust fault dipping 21 to238 to the southwest (Figures 3, 7, 12). This and similarback thrusts underlie the shallow hinterland-directedfolds. In a stratigraphic sense, the position of the backthrust commonly coincides with the tilted contact be-tween the shaly Monterrey Formation and the under-lying San Onofre Breccia.

We apply analytical techniques provided by fault-related fold theories (Suppe, 1983; Medwedeff, 1992)and forwardmodeling strategies (Mount et al., 1990) tointerpret the observed fault-related fold and its relationwith the monocline geometry (Figure 12). Figures 3, 7,and 12 show three different structural cross sectionslocated across the continental platform between SanJoaquinHills andCarlsbad. Themonocline trend iswelldefined at the right side of the structural cross sections,where a southwest-dipping panel is limited by axial sur-faces F and F0. This panel extends continuously through-out thewide platform and is expressed in the low-slopegradient observed along this region (Figure 13). Thesouthwest-dipping panel is not observed anywhere be-low the structural level defined by the Oceanside de-tachment. Thus,we infer that themonocline structure islimited at depth by the Oceanside detachment, consis-tent with the proposed origin of the monocline as a Mio-cene rift shoulder and the presence of fore thrusts re-stricted to thehanging-wall blockof theOceanside thrust.

The structural and stratigraphic relations observedin the seismic data between the monocline trend, theshallow thrust-fold structures, and theOceanside thrustsuggest the presence of a wedge geometry (Medwedeff,1988) involving the metamorphic basement (Figure 13).In this scenario, a component of slip on the Oceansidethrust is directed offshore, leading to the developmentof the previously described submarine fold and thrustbelt composed of the San Mateo, the San Onofre, andthe Carlsbad trends. The development of a foreland-propagatingwedge tipwithin themonocline, however,also allowed some of the contractional slip to be trans-ferred to a shallow backthrust system ramping up fromthe Oceanside thrust (Figure 13). Thus, slip may havebeen partitioned between foreland- and hinterland-directed structures during themore recent evolution ofthis active submarine fold and thrust belt. Similarcrustal wedging processes have been reported in otherareas of California (Medwedeff, 1992; Shaw and Suppe,1996; Novoa, 1997) and represent a common mecha-

nism of deformation observed in many sedimentarybasins and fold and thrust belts (Ziegler, 1983; Cooperand Williams, 1989; Roure and Colletta, 1996; Collettaet al., 1997).

Rivero et al. (2000) andRivero (2004) noted the spatialcorrelation of this structural wedgewith Quaternary up-lift observed in adjacent coastal areas east of the anom-alously wide region of the coastal shelf (Figures 2, 6).Previous neotectonic studies based on Quaternary ma-rine terraces and strand lines located along the coastalplain of Orange and San Diego counties indicate con-tinuous uplift of this region during the last 120–80 k.y.(Lajoie et al., 1979, 1992; Barrie and Gath, 1992; KernandRockwell, 1992;Grant et al., 1999;Kier andMueller,1999). Moreover, some of the local high rates of tectonicuplift and foldingobserved in the San JoaquinHills havebeen recently associatedwith an active blind thrust faultdipping to the southwest (Mueller et al., 1998; Grantet al., 1999) and with activity in the Oceanside thrustitself (Rivero et al., 2000).

Figure 6 shows the bathymetric expression of themonocline structure at several locationsoffshore the SanJoaquin Hills.We use axial surface mapping techniques(Shaw et al., 1994; Rowan and Linares, 2000) and 3-Dstructuralmodeling to control the extent of the synclinalstructure and back thrusts related with the structuralwedge (axial surface F0). The results suggest that thesetrends extend toward the onshore region into the vicin-ities of the San Joaquin Hills. If the structural wedgeextends into the onshore domain, as suggested by themap patterns, then it is possible that an onshore seg-ment of the backthrust fault observed in the seismicimages is responsible for the Quaternary uplifting andfolding documented by coastal tectonics studies in theSan Joaquin Hills (Lajoie et al., 1979; Grant et al., 1999;Kier andMueller, 1999) (Figure 6). If so, this implies thatthe backthrust fault is active, which in turn suggeststhat the basal Oceanside thrust should be also active(Rivero et al., 2000).

STRIKE-SLIP AND BLIND THRUST FAULT INTERACTIONS

The inner California borderlands is cut by several right-lateral strike-slip fault zones that havebeendefinedbasedon sea-floor geomorphology, shallow-marine geophys-ical data, and seismicity (Wilcox et al., 1973; Yeats, 1973;Crowell, 1974; Sylvester, 1988, among others). The twomost conspicuous of these structures are the San DiegoTrough strike-slip fault, located on the western marginof the inner California borderlands (Vedder et al., 1986),and the easternNewport-Inglewood–RoseCanyon faultzone, which runs parallel to the coastline of the Orangeand San Diego counties (Harding, 1973; Vedder et al.,

202 Rivero and Shaw

1986). Here we consider the geometric and kinematicinteractions of these strike-slip fault zones with thelow-angle Oceanside and Thirtymile Bank detachmentsurfaces.

The San Diego Trough strike-slip fault is a continu-ouswrench fault zone that extends formore than 50 km(31mi) along a bathymetric lowknownas the SanDiegoTrough (Legg et al., 1991). This fault zone has been de-scribed as a characteristic dextral strike-slip system thatdissects low-angleMiocene detachment faults, includingthe Thirtymile Bank detachment (Legg and Nicholson,1993). These authors also suggest that the San Diego

Trough strike-slip fault is active on the basis of faultscarps, modern slump and flow deposits, and offset ofsubmarine channels observed at the sea floor.

The SanDiegoTrough strike-slip fault iswell imagedto depths of about 5 km (3.1 mi) in the industry seismicreflection data available for this study (Figures 6, 14A).The fault consists of one ormore steeply dipping splaysdefined by near vertically aligned truncations of reflec-tionswithin the sedimentary fill of thebasin (Figure 14A).Individual fault splays are somewhat discontinuous,but the fault system follows the central location of thetriangular San Diego Trough. The traces of these faults

Figure 13. Diagrammatic representation of the main structural elements and trends observed along three transects shownin Figures 3, 7, 8, and 12. The representation illustrates the lateral continuity of the offshore-dipping monocline and therole of the Oceanside thrust as a regional basal detachment level. The model also highlights the complex arrangements of themodern contractional trends within the active submarine fold and thrust belt, and the control induced by the Miocene normalfault system and the propagating structural wedge in their locations. Note that the most important fold structures in a profile donot necessarily correlate between sections (i.e., observe the transition of the San Mateo and San Onofre anticlines betweenlocations YY0 and ZZ0). Fm. = Formation; S. Onofre = San Onofre.


produce small scarps at the sea floor that coincide withthe location of the San Diego Trough strike-slip fault asmapped by Vedder et al. (1986).

We modeled the 3-D geometry of the San DiegoTrough strike-slip fault using the grid of high-resolutionseismic data, aswell as bathymetric data and seismicity

Figure 14. Seismic expression ofthe San Diego Trough strike-slipFault at Thirtymile Bank. (A) Me-dium segment of the San DiegoTrough strike-slip fault composedof two nearby fault branches.(B) Three-dimensional represen-tation of the San Diego Troughstrike-slip fault built from seismicreflectionprofiles similar to panel A,showing the relation betweenthe Thirtymile Bank thrust and theSan Diego Trough strike-slip fault.

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(Figure 14B). Our analysis indicates that the strike-slipfaulting and folding are confined to a zone along thecenter of the San Diego Trough. The zone is well ex-pressed next to Coronado Banks where the faults aremostly defined by discrete vertical segments, several ofwhich reach and offset the sea floor (Figure 14B). In con-trast, the fault zone is more diffuse toward the northernSanta Catalina Basin (Figure 6). The basin in this area isshallower, and the geometry of the San Diego Troughfault becomes more complex, with a southwesterly dipto the main fault splay and evidence of oblique slipmanifest in contractional folds adjacent to the fault.North of this region, the strike-slip fault seems to ter-minate into a zone of active thrusting and folding over-lying the Thirtymile Bank thrust (Figure 15). Near-vertical fault splays are no longer observed; instead,deformation is accommodated by north-northwest–trending contractional folds with bathymetric expres-sion (Figure 6).

Rivero et al. (2000) and Rivero (2004) described fourpossiblemodes of interaction betweenhigh-angle strike-slip faults and low-angle thrusts (Figure 16). Here weevaluate in detail each one of these modes on the basisof the results from our structural analysis.

The first two scenarios describe caseswhere the low-angle Thirtymile Bank detachment either dies out or isoffset by the active San Diego Trough strike-slip faultzone (Legg et al., 1991; Legg andNicholson, 1993). These

interactions imply that the strike-slip faults cut downthrough the entire seismogenic crust, offsetting and ren-dering inactive the low-angle thrust faults, as it is as-sumedbymost current hazard assessments (Figure 16A,

Figure 15. Structural cross section across the Thirtymile Bank thrust and the San Diego Trough strike-slip fault zone at theepicentral location of the 1986 (5.3 local magnitude scale) Oceanside sequence. The earthquake appears to have ruptured thedowndip extent of the Thirtymile Bank detachment and not the San Diego Trough strike-slip fault. The association of theearthquake and the Thirtymile Bank fault is consistent with the hypocentral locations and main-shock focal mechanism.

Figure 16. Schematic representation of different structuralscenarios considered in this study for strike-slip and blindthrust fault interactions (modified from Rivero et al., 2000).


B). In these configurations, some thrust motion mightstill be possible but only as part of a highly oblique slipderived from motion on the strike-slip faults. An alter-native set of solutions considers that the SanDiego strike-slip fault is either offset by ormergeswith the active low-angle detachment. For these solutions, both fault systemsare potentially active and possibly independent earth-quake sources (Rivero, 2004).

Seismic reflection data and hypocentral locations ofthe 1986 (5.3 local magnitude scale) Oceanside earth-quake sequence suggest that the Thirtymile Bank de-tachment is not truncated by the SanDiegoTrough faultbut extends to the northeast beyond the strike-slip zone(Figures 6, 15). Previous authors (Hauksson and Jones,1988; Pacheco and Nabelek, 1988) attributed the Ocean-side earthquake to dextral motion along the San DiegoTrough strike-slip fault rupturing at depths of about8 km (5 mi) in an unmapped restraining bend. In con-trast, Rivero et al. (2000) related the origin of the 1986(5.3 local magnitude scale) Oceanside earthquake se-quence to the activity on the gently dipping ThirtymileBank thrust fault based on spatial correlation betweenmapped parts of the detachment fault and the earth-quake cluster as relocated by Astiz and Shearer (2000)(Figure 15). Through our mapping, we note that theearthquake epicenter was located 6 km (3.7 mi) east ofthe closest mapped segments of the San Diego fault(Figure 6), further supporting the association of theearthquakewith theThirtymile Bank fault. This interpre-tation is consistent with scenarios where the San DiegoTrough strike-slip fault is either truncated by or mergesdown into the gently dipping Thirtymile Bank detach-ment (Figure 16C, D). As the Thirtymile Bank thrustmanifests little reverse slip at this location, we wouldexpect onlyminor offset of the SanDiegoTrough strike-slip fault if it extends into the footwall of the Thirtymiledetachment.

The seismic data do not have enough resolution torule out the presence of a deep segment of the SanDiegostrike-slip fault located in the footwall block of the Thirty-mile Bank fault. Therefore, themode of fault interactiondepicted in Figure 16C remains viable. However, sys-tematic mapping of the San Diego Trough fault and theThirtymile Bank detachment indicates that themergingpoint between these two faults generally coincideswiththe shallow hanging-wall cutoff of the crystalline base-ment on the Thirtymile Bank fault (Figures 14, 15). Thisrelation reflects that the strike-slip fault has brokenthrough the deepest part of the sedimentary basin andhence the weakest part of the upper crust. Moreover,the orientations of the two faults, with the strike-slipfault trending parallel to the strike of the detachment,are consistent with a system where oblique motion atdepth is fully partitioned into shallow zones of strike-

slip and dip-slip faulting (Fitch, 1970; McCaffrey, 1996;Colletta et al., 1997; Roure et al., 1997). Thus,we considerthat themode of fault interaction depicted in Figure 16Dwith shallow-slip partitioning is themost likely descrip-tion of the structural relationship between the Thirty-mile Bank and the San Diego Trough strike-slip faults.

The Newport-Inglewood–Rose Canyon fault zoneand the Oceanside thrust are other examples of strike-slip and thrust faults that interact at depth in the innerCalifornia borderlands. TheNewport-Inglewood–RoseCanyon fault zone is a near-shore zone of active right-lateral faulting along the coast line of Orange and SanDiego counties in southern California (Harding, 1973;Wilcox et al., 1973). Some of these authors defined acontinuous strike-slip fault zone that extends frommorethan 100 km (62 mi) between Newport Beach and theSanDiegoPeninsula (Harding, 1973; Legg andKennedy,1979; Ziony and Jones, 1989). However, these and otherauthors have proposedmany different locations for thesurface traces of theNewport-Inglewood–RoseCanyonfault zone (Fisher and Mills, 1991).

Although recent activity of theNewport-Inglewood–Rose Canyon fault zone is well expressed onshore inthe LosAngeles and SanDiego regions, seismicity alongthe offshore segments of this fault zone is scattered anddiffuse with no clear correlation between earthquakelocations and the proposed offshore traces of the faultzone (Figure 1). Nevertheless, these offshore trends areconsidered active strike-slip systems based on sets ofpositive flower structures with complex bathymetrictraces identified from offshore seismic data sets (Clarkeet al., 1987; Fisher et al., 1988; Ziony and Jones, 1989;Mills, 1991). Comparison of the proposed fault traceswith high-resolution bathymetric data and seismic re-flection profiles reveals the systematic alignment be-tween the locations of the shelf break and the mappedstrike-slip fault zones south of Dana Point (Figures 3, 7,12). In the Carlsbad region offshore from the San DiegoPeninsula, the shelf break is discrete and traditionallyattributed to the offshore segment of the onshore RoseCanyon strike-slip fault (Vedder et al., 1986; Ziony andJones, 1989; Rockwell et al., 1992). Crouch and Suppe(1993) reported sedimentary rocks of Eocene and Mio-cene ages exposed in this narrow bathymetric slopeand the coastal shelf. Stratigraphic control provided bythe nearby Oceanside well, combined with seismic re-flection data, indicates that these rocks are at higherstructural elevation than younger Pliocene (Venturian–Repettian) strata deposited farther offshore. We inter-pret much of this structural relief to reflect Neogene ex-tension or transtensional movement along the offshoreRose Canyon fault zone (Figure 17).

The offshore Rose Canyon fault zone is clearly recog-nized in seismic reflection profiles offshore Oceanside

206 Rivero and Shaw

Figure 17. Seismic reflection profiles illustrating the variation in the geometry of the offshore Rose Canyon fault along the strike (see Figure 6 for the locations).In all the profiles, the location of the shelf break is structurally controlled by the presence of a fault, with normal separation and southwest dip. Contractional faultingoccurs along faults that lie to the west of the shelf break. (A) Northern segment of the Rose Canyon fault. Pliocene contractional folding is associated with thenortheast-dipping Carlsbad thrust. (B) Asymmetric contraction developed on a fault splay in the hanging wall of the Rose Canyon fault. The contraction affectsMiocene and Pliocene–Holocene(?) sedimentary units. m = sea floor multiples; P = Top Pico Formation; R = Top Repetto Formation; C = Top CapistranoFormation; M = Top Monterrey Formation; S = Top San Onofre Formation.

Active

Foldingand

BlindThrust

FaultingInduced

byBasin

InversionProcesses

207

and Carlsbad (Figure 17A). The broad fault zone ex-tends continuously for more than 20 km (12.4 mi) be-tween Oceanside and Del Mar, dipping steeply to thesouthwest at about 55 to 658. Several splays relatedwiththe mapped fault zone show evidences of recent activ-ity. On some of these splays, east-vergent contractionalanticlines trending parallel to the coastline deform sedi-mentary rocks of Repettian and younger ages (Figure 6).Occasionally, the contractional trends define prominentfold scarps in the sea floor (Figure 17A). Other faultsplays, particularly along the shelf break, appear to ex-hibit normal offset. Presumably, these contractional andextensional structures represent local restraining andreleasing bends along the offshore extension of the RoseCanyon strike-slip fault.

Several authors havedefined the geometry of theRoseCanyon strike-slip fault in the San Diego Peninsula onthe basis of its surface expression between San DiegoandPoint Loma (Legg, 1980;Zionyand Jones, 1989; Fisherand Mills, 1991; Rockwell et al., 1992; Lindwall andRockwell, 1995). The projection of this onshore segmentnorth of Point Loma (and thus its connection with ourmapped Rose Canyon fault zone located offshore) isuncertain because of poor fault expression and lack ofseismic coverage at the La Jolla Canyon (Figure 6). AtLa Jolla Canyon, the mapped Rose Canyon fault zonechanges its orientation to a southwest-trending direc-tion, away from the SanDiego Peninsula (Figures 6, 18),whereas the projection of the onshore Rose Canyonstrike-slip fault continues to the north. This implies thatthe modern Rose Canyon fault zone has a pronouncedbend or other form of geometric segment boundary inthis area (Figures 6, 18). North of Oceanside, the off-shore Rose Canyon fault zone extends into an area ofactive thrusting and wedging associated with the sub-marine fold and thrust belt located above the Ocean-side thrust (Figure 6).

The offshore segment of the Newport-Inglewoodfault zone is present to the north of the submarine foldand thrust belt thatmarks the northern extent of theRoseCanyon fault. The structural character of the Newport-Inglewood fault iswell defined in the LosAngeles area,mostly because of its surface expression, and by theprolific amount of subsurface geologic and geophysicaldata obtained from oil fields located along its trace(Harding, 1973; Yeats, 1973; Wright, 1991). In this area,the fault geometry of the Newport-Inglewood faultzonehas been interpreted as a simple vertical fault zonethat changes to a more complex arrangement of seg-mented en echelon faults toward the south (Harding,1973; Petersen and Wesnousky, 1994). Recent tectonicactivity of theNewport-Inglewood fault zone in the LosAngeles region is documented by the 1920 (4.9 localmagnitude scale) Inglewood earthquake and the 1933

(6.3 local magnitude scale) Long Beach earthquake(Hauksson andGross, 1991;Wright, 1991). These eventsshoweda right-lateralmovement of anorthwest-trendingfault plane, dipping at around 708 to the east (Haukssonand Gross, 1991).

Offshore in the Dana Point and Carlsbad areas, theproposed traces of the offshore Newport-Inglewoodstrike-slip fault commonly lie above amonocline struc-ture thatwepreviously described in this study ashavingformed by structural wedging above the Oceansidethrust (Figure 12). Geologic interpretations of seismicreflectiondata generally showa continuous stratigraphiccorrelation of the Neogene units across the monoclinetrend. In many cases, the seismic data indicate that pre-viously interpreted strike-slip fault splays correspondto active hinges of contractional anticlines producedby Pliocene to Holocene backthrust motion on a deepstructuralwedge (Figures 3, 7, 12). Nevertheless, a set ofapparently truncated reflections within the monoclinedelineate a steep south-dipping zone that may corre-spond to the offshore trace of the Newport-Inglewoodfault (Figure 12A). These truncated reflections seem todefine an active strike-slip fault that may cut down intothe structural wedge and must intersect the Oceansidethrust at a depth of 4 km (2.5 mi).

The offshore Newport-Inglewood fault may extendnorthward to linkwith the onshoreNewport-Inglewoodfault segment (Harding, 1973;Wilcox et al., 1973; Crowell,1974; Legg and Kennedy, 1979; Sylvester, 1988; Cali-fornia Geological Survey, 2002). However, the pres-ence and activity of the Oceanside thrust in this regionpreclude such a simple geometric linkage, at least atshallow levels. As a consequence, we speculate that theoffshore San Joaquin Hills area most likely representsa geometric segment boundary between the onshoreand offshore traces of the Newport-Inglewood strike-slip fault.

To the south of Dana Point, the offshore segment ofthe Newport-Inglewood fault zone may extend southinto theOceanside area,where the offshoreRoseCanyonfault zone and the submarine fold and thrust belt con-verge (Figure 18). The direct linkage of the Newport-Inglewood fault zone and the offshore Rose Canyonstrike-slip fault in this area of active faulting and thrust-ing, however, is not clear (Figure 6). If the linkage exits,as it has beenproposed elsewhere (Harding, 1973;Wilcoxet al., 1973; Crowell, 1974; Legg and Kennedy, 1979;Sylvester, 1988; California Geological Survey, 2002,among others), then it is through a highly complicatedzone of fault splays and active folding observable inmapview and cross sections. This zone of complexity arisesbecause of the presence and activity of the Oceansidethrust. Thus, Rivero (2004) inferred that the offshoreNewport-Inglewood strike-slip fault and theRoseCanyon

208 Rivero and Shaw

fault zones are geometrically segmented from one an-other in this region and thus may represent indepen-dent sources of earthquakes (Figure 18).

At depth, the Newport-Inglewood and the Rose Can-yon strike-slip fault zones intersect with the Oceansidethrust. This intersection occurs at relatively shallow levelsof about 4 km (2.5 mi) to the north and deeper approx-imately 10 km (6.2mi) in the south.Data are insufficientto uniquely define the manner in which these two fault

systems interact. However, late Tertiary to Holoceneactivity on both systems and our documentation of sev-eral kilometers ofwest-directed shortening on theOcean-side thrust imply that the thrust fault is not truncatedby theNewport-Inglewood and the RoseCanyon strike-slip fault zones (cases A or B, Figures 16, 18). Instead,scenarios where the two fault systems interact at depthin amanner consistentwith their coeval activity (casesCand D) are favored.

Figure 18. Regional map view of theOceanside and the ThirtymileBank thrustsand the mapped and inferred offshoresegments of the strike-slip San DiegoTrough, Newport-Inglewood, and theRose Canyon faults. Suggested onshoreand offshore traces of the Newport-Inglewood and the Rose Canyon faults bythe California Geological Survey (2002)are also shown for comparison (blackand dashed lines). The footwall segmentof the San Diego Trough fault is notrepresented. HW = hanging wall; FW =footwall.


CONCLUSIONS

Structural interpretations of high-resolution seismic re-flection data, 3-D geologicmodeling,well data, and earth-quake information confirm the presence of a large blindthrust system underlying the coastal plain and offshoreregion of southern California. Our study indicates thatthe inner California blind thrust system consists of theOceanside and the Thirtymile Bank thrusts, two Mio-cene low-angle detachments that were reactivated asblind thrusts by Pliocene basin inversion processes. Thebasin inversion processes were initiated during the Plio-cene onset of the modern transpressional regime.

Our analysis suggests that active contractional fold-ingand faultingaredrivenby regional structuralwedgingand basin inversion mechanisms. These processes par-tially reactivated ramp segments of thedetachments andinduced the generation of northwest–southeast contrac-tional trends on the hangingwalls of theOceanside andThirtymile Bank thrusts. Many of these contractionaltrends have bathymetric and topographic expression inthe form of prominent fold scarps that control the mor-phology of the modern sea floor at the shelf and coastalplain of Orange County and the offshore Thirtymile Bankregion.

The characteristic structural style of the contractionaltrends changes in the study region from complex arraysof imbricated fold and thrust systems in the north todiscrete and individual broad fault-related folds in thesouth. These imbricated thrust systems are well ex-pressed offshoreDanaPoint, SanOnofre, andCarlsbad,which include foreland- and hinterland-directed thrustrampsof Plioceneandyounger age. In the south, obliquecontractional motion induced the reactivation of Mio-cene high-angle normal faults antithetic to theOceansidethrust as broad contractional anticlines. We attribute theorigin of the offshore segment of the RoseCanyon strike-slip fault to Pliocene to the Holocene structural reacti-vation of one of these high-angle normal faults.

Seismic reflection data and 3-D geologic modelingalso help define dextral strike-slip faulting in the off-shore SanDiego Trough region previously identified asthe San Diego strike-slip fault. We interpret this strike-slip fault as a young vertical structure developed byshallowstrainpartitioningof transpressional (southwest-directed) contractional slip transfer from the underlyingThirtymile Bank thrust.

Analysis of the potential fault interactions betweenthe strike-slip faults and the blind thrusts suggests thatthe vertical faults either merge at depth with or areoffset by the low-angle thrusts. Thus, both types of faultsystems are deemed likely to be active and should beconsidered in the context of regional earthquake haz-ards assessment.

ACKNOWLEDGMENTS

This research benefited from valuable contributions byM.Peter Suess andFrankBilotti. FreddyCorredor,ChrisGuzofski,AndreasPlesch, JamesDolan, andChris Sorlienprovided numerous discussions that improved manyof the ideas presented in this study.We also thank PeterShearer for assistance in integrating relocated earth-quake hypocenters into our analysis. This research waspartially funded by the National Science FoundationGrant EAR0087648,HarvardUniversity, and the South-ern California Earthquake Center (SCEC). Texaco, theMineralsManagement Service (MMS), andother indus-try sponsors provided thewell and seismic data used inthis research. Financial support to Carlos Rivero pro-vided by the Fulbright Grant ‘‘Energy for the XXI Cen-tury’’ is deeply appreciated.

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