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Architecture of transpressional thrust faulting in the San Title Bernardino Mountains, southern California, from deformation of a deeply weathered surface.

Author(s) Spotila, James A.; Sieh, Kerry.

Spotila, J. A., & Sieh, K. (2000). Architecture of transpressional thrust faulting in the San Bernardino Citation Mountains, southern California, from deformation of a deeply weathered surface. Tectonics, 19(4), 589–615.

Date 2000

URL http://hdl.handle.net/10220/8429

© 2000 AGU. This paper was published in Tectonics and is made available as an electronic reprint (preprint) with permission of American Geophysical Union. The paper can be found at DOI: [http://dx.doi.org/10.1029/1999TC001150]. One print or electronic copy may be made for personal use only. Rights Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper is prohibited and is subject to penalties under law. TECTONICS, VOL. 19, NO. 4, PAGES 589-615 AUGUST 2000

Architecture of transpressional thrust faulting in the San Bernardino Mountains, southern California, from deformation of a deeply weathered surface

JamesA. Spotila Departmentof GeologicalSciences, Virginia Polytechnic Institute and State University, Blacksburg

Kerry Sieh Divisionof Geologicaland Planetary Sciences, California Institute of Technology,Pasadena

Abstract. To investigatethe architectureof transpressional boundariesexists, strike-slipdisplacement alone cannot deformationand its long-termrelationship to platemotion adequately accommodate relative movements of the in southern California, we have studied the deformation lithosphere [Dewey et al., 1998]. Instead, far-field patternand structuralgeometry of orogenywithin the San horizontal plate motions are manifest as complex Andreas fault system. The San Bernardino Mountains deformationalong the plateboundary, which can involve haveformed recently at the hub of severalactive structures significantvertical strain [Teyssieret al., 1995]. The that intersectthe SanAndreas fault eastof Los Angeles. accommodationof horizontal plate motion by convergent This mountainrange consistsof a groupof crystalline or divergentstructures is not a simpleprocess, and its blocksthat have risen in associationwith transpressive characteristicsarenot predicted by simpletectonic theory. plate motionalong both high- and low-anglefaults of a As a result, there are many modelsof transpressive complex structural array. We have used a deeply deformationthat are debatedin termsof geometry[e.g., weathered erosion surface as a structural datum to constrain Sylvester,1988; Lemiski and Brown, 1988] and mechanics the patternof vertical deformationacross fault blocksin and [e.g., Wilcoxet al., 1973; Mount and Suppe,1987] of adjacentto this mountainrange. By subtractingthe structures. hangingwall and footwall positionsof this preuplift In southernCalifornia the obliquitybetween plate horizonwe havedetermined vertical displacement along motiondirection and the San Andreasfault system,the two major thrust faults. We concludethat one fault, the primarytransform, is 27ø [DeMets,1995] (i.e., the "Big North Frontalthrust, has played a moresignificant role in Bend";Figure 1). The resultingoblique convergence is raisingthe largefault blocksand can explain the uplift of accommodatedin the far field by a wide fold andthrust belt all but a few crustalslivers. On the basisof thepattern of (the western TransverseRanges [Namson and Davis, displacementassociated with this thrust fault we have also 1988]). In the near field the San Andreas fault exhibits inferredfault zone geometry beneath the range. Rather than nearlypure strike-slipdisplacement, although localized simplysteepening into a high-anglefault zone or flattening rockuplift has occurred within the faultzone itself (e.g., into a decollement,the thrustfault may have a complex, Yucaipa Ridge [Spotila et al., 1998]) and acrossnarrow curviplanargeometry. The patternof rock uplift also beltsass6ciated with local geometricperturbations of the enablesusto calculate the total motion accommodated by fault zone (e.g., the restraining bend at SanGorgonio Pass thisorogeny. We estimatethat >6 km of convergence.(5% [Allen,1957; Jones et al., 1986]). Obliqueplate motion of the total plate motionin the last 2 Myr) has occurred. is thusaccommodated by two architecturesof deformation: This horizontalshortening is associatedspatially with the wide, internally deforming thrust sheets (i.e., 15-km-widerestraining bend in the SanAndreas fault zone decollements)and narrow, high-angle zones (i.e., flower near SanGorgonio Pass. The entirerange may thushave structures). These general cases mimic the two broad risenbecause of a smallgeometric complexity in the San architectures that have been observed elsewhere in Andreasfault ratherthan the obliquityof far-field plate transpressiveenvironments [Sylvester, 1988; Lemiskiand motion. Brown, 1988; Lettis and Hanson, 1991; Vauchez and Nicolas, 1991; BeauJoin, 1994]. However, the 1. Introducti6n relationshipbetween these deformation styles is not clear. By definition,continental transform plate boundaries are First, it is not obviouswhy two stylesexist. Does far- dominatedby horizontal plate motions. However, along fielddeformation represent distributed slip partitioning due transpressiveor transtensionalmargins, where significant to obliqueconvergence, while near-fielddeformation results obliquity between plate motion vectors and plate from localgeometric complexities along the strike-slip fault itself?.Second, it is not clearhow low-angleand Copyright2000 by the AmericanGeophysical Union. high-angle deformation zones interact at intermediate distancesfrom strike-slip fault zones. Do thrust faults Paper number 1999TC001150. merge with the strike-slip zone itself, or are they 0278-7407/00/1999TC001150512.00 completelydecoupled in the subsurface?

589 59O SPOTILAAND SIEH: ARCHITECTUREOF TRANSPRESSIONALTHRUST FAULTING

19 ø

Sierra Nevada

broad zone of convergence,sinistral shear, and clockwise rotation

Mojave Desert diffuse zone of dextral shear

Pacific Ocean California

[ 100km .:?,• .•...... sinistralslip faultsand clockwise rotation

Arizona

Mexico

Figure 1. Activetectonic context of the SanBernardino Mountains (SBMs). The "Big Bend"of the SanAndreas fault occurs south of the Garlockfault (GF) andnorth of the smallrestraining bend at San GorgonioPass (circle). Relativeplate motion vector is from DeMets [ 1995]. ECsz,eastern California shearzone (gray); eTR, easternTransverse Ranges (ruled); N-IF, Newport-Inglewoodfault; Pen. Ranges,Peninsular Ranges; PMF, PintoMountain fault; SAF, SanAndreas fault (m, Mojave;sb, San Bernardino;cv, CoachellaValley segments);SGMs, San Gabriel Mountains;SJF, San Jacintofault; SJMs,San Jacinto Mountains; SM-CF, SierraMadre-Cucamonga fault; W-EF, Whittier-Elsinorefault; wTR, westernTransverse Ranges.

To better understandhow horizontal plate motion is San Andreasfault zone or the low-angledeformation that manifestas crustaldeformation along transpressive strike- occursat greaterdistance from its trace. Although both slip fault systems,the architectureand kinematichistory of have been proposed as models for the SBMs [Sadler, structuralarrays at intermediatedistances to strike-slipfault 1982; Li et al., 1992], the subsurfacestructural geometry zonesmust be quantified. The San BernardinoMountains and displacements of the faults have not been well (SBMs) extend from near-field to intermediate distance constrained. Previous efforts have been stymied because from the San Andreasfault zone (Figure 1) and have been the SBMs consistmainly of crystallinebedrock that cannot elevatedover the pastfew million yearsin associationwith be palinspasticallyrestored in balancedcross sections. transpressivedeformation [Dibblee, 1975;Meisling and We have used a geomorphicfeature as a structural Weldon, 1989; Spotila et al., 1998]. This young datum to quantify the patternof rock uplift in the SBMs mountainsystem thus representsan ideal opportunityto and thereby place constraintson the displacementand study long-term transpressiveorogenesis associated with geometryof the major structures. Atop the SBMs and the San Andreas fault' system. The bulk of the range acrossthe surroundinglowlands is a low-relief, deeply consistof the broad Big Bear plateau, which has been weathered,granitic erosion surface (Figure 3). Geologic raised along two east-west trending thrust faults with and geomorphicrelationships argue that this surfaceis opposingdips (North Frontal thrust system(NFTS) and relict, in that it formed prior to recentmountain building Santa Ana thrust (SAT)) (Figure 2). Critical to and was subsequentlyseparated by the major thrustfaults understandingthe structuralkinematics of the SBMs is of the range [Oberlander, 1972; Meisling, 1984; Spotila, knowingwhether the subsurfacegeometry of thesethrusts 1999]. We have used the distribution of this easily mimics the high-angledeformation observed within the identified geologic marker to reconstructthe vertical SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 591 592 SPOTILAAND SIEH:ARCHITECTURE OF TRANSPRESSIONALTHRUST FAULTING

displacement field with some certainty. Our results Mio-Pliocene Santa Ana Sandstone [Sadler, 1993]. demonstrate that the NFTS has been the dominant Basalts in this unit indicate a post-6.2 Ma initiation of structureresponsible for uplift of the SBMs and likely thrustingalong the southernplateau margin [Woodburne, undercutsthe SAT at depth. On the basisof the patternof 1975]. Much of the SAT is buried by Quaternary rock uplift we proposea complex subsurfacestructural deposits,however, and late Pleistocenescarps occur only geometrythat bearssimilarity to bothhigh-angle and low- locally [Jacobs, 1982; Dibblee, 1964a, 1974; Spotila, anglemodels of transpressivedeformation. The patternof 1999]. Some young scarpsoccur near intersectionswith vertical displacement also suggests a link between northwesttrending strike-slip faults and may represent horizontal shortening in the SBMs and the restraining local activity [Jacobs, 1982]. The SAT's trace has been bend in San Gorgonio Pass. These results provide disarticulatedlocally by strike-slip structures. The Deer importantnew insighton the architectureof deformationin Creek fault, for example, offsets the SAT by •-200 m the SBMs and improve our understandingof transpressive dextrally. Complexitiesnear the Mentone fault zone may orogenesis. Our constraints on fault geometry and also indicate disruption of the SAT (Figure 2). This displacement also help characterize regional seismic relationshipsuggests that dextral faults postdatethe SAT, hazards. but the historyof activity along the thrustfault is not well known. On the basis of similaritiesto the Squaw Peak thrust systemto the west, Meisling and Weldon [1989] suggestedthat the SAT was active mainly in the Miocene. 2. Geological Background The WatermanCanyon fault is a strandof the SquawPeak The North Frontal thrust system bounds the 80-km- thrust system and has similar trend, escarpment, and long northernrange front, an impressiveescarpment with structural position as the SAT (Figure 2). These two ample geomorphicindicators of recentuplift [Bull, 1977; thrust faults may indeed be the same structure. A lack of Miller, 1987] (Figure 3). It displayspure thrust motion directconstraints on the timing of the SAT and Waterman and is considereda fault systembecause it consistsof a Canyon fault, however, yields uncertaintyas to whether complex, kilometer-wide band of discontinuous fault they were active throughout the Pliocene or terminated strandsand folds [Meisling and Weldon, 1989; Sadler, duringthe Miocene. 1982]. The fault hasjuxtaposed crystalline basement over The subsurfacegeometry of the NFTS and SAT are not Plio-Pleistocenesediments, including the Old Woman well known. Both thrust faults display a range in dip at Sandstone[Woodford and Harriss, 1928; Dibblee, 1975]. thesurface (100-60 ø) but,on average,dip •-30ø-35 øtoward Clast provenance,palcocurrents, and facies in this unit each other [Miller, 1987; Meisling, 1984; Woodford and recordthe initiation of the thrustingas post 2-3 Ma [May Harriss, 1928; Rzonca and Clark 1982; Jacobs, 1982]. and Repenning, 1982; Sadler, 1982]. Late Pleistocene The NFTS and SAT probably meet at some depth below alluvium is also extensively faulted along the northern the plateau,but whether they merge or truncateeach other range front, with scarpslocally reachingheights of 60 m is not known. A seismic reflection profile across the [Meisling, 1984; Miller, 1987]. Although a segmentof NFTS shows that it retains its low dip in the upper the NFTS has been seismicallyactive as recently as 1992 kilometer of crust [Li et al., 1992], but little else is well (M5.4 [Feigl et al., 1995]), Holocene scarpsoccur only constrained.One hypothesisfor the architectureof these locally [Bryant, 1986]. Thus there is debateas to whether faults is that the NFTS is the main structureresponsible the fault is as active today as it has been throughoutthe for uplift of the plateau and becomes a low-angle Pleistocene[Meisling and Weldon, 1989; Miller, 1987]. detachmentat depth that undercutsthe SAT. This is The northern margin of the plateau is penetrated by basedon a southwarddeepening base of microseismicity northwesttrending dextral faultsof the easternCalifornia (•10øS dip [Corbett,1984]), a similarlyoriented zone of shearzone (Figure 2), which couldindicate that thrusting high seismic reflectivity [Li et al., 1992], and several hasbeen replaced by strike-slipdeformation. Although the small, deep earthquakeswith shallow nodal planes [Webb occurrenceof thesefaults in both the upperand lower thrust and Kanamori, 1985]. An alternative hypothesisis that plateshas beenused as an argumentthat the NFTS is the two thrusts steepen and merge into a mushroom- largely inactive [Sadler, 1982], no clear crosscutting shaped, high-angle zone of distributed transpression relationshipor directconnection between faults north and [Sadler, 1982]. This idea is basedon the symmetryof the south of the NFTS have been mapped [Miller, 1987]. opposingthrust faults, which appearas antitheticstructures Thus these two fault systems might both be active of roughly similar length and displacement. The [Spot#a, 1999]. displacementpatterns along the thrustfaults have not been The Santa Ana thrust bounds only a portion of the defined, however, and thus their relative roles in southernmargin of the plateau(Figure 3). Its escarpment accommodatingcontraction are uncertain. The importance is alsohigh andrugged, indicating the importantrole of of high-angletranspressive zones is evidentin the southern the SAT in upliftingthe plateaurelative to the low Santa SBMs, where crustal slivers have been uplifted within Ana Valleyto the south(Figure 2). The disappearanceof strandsof the San Andreasfault zone [Spotilaet al., 1998]. the escarpment eastward into high mountainous The kinematicrelationship between th•ese high-angle, topography,however, shows that the SAT couldhave been oblique slip faults and the thrust fault• to the north, responsiblefor uplifting only a fraction of the SBMs however, cannot be understood until geometry of the (Figure3). The fault hasjuxtaposed basement over the thrusts are constrained. SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 593 594 SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

Thus the structural pattern within the SBMs is [Neville, 1983] and Mio-Pliocene sedimentsoverlie it on complex. Plate motion has been accommodatedby the the west (Crowder Formation [Meisling and Weldon, opposedthrust faults, the high-anglefaults in the south, 1989]) and in Santa Ana Valley (Santa Ana Sandstone and diffuse dextral faults that penetratethe rangeon the [Sadler, 1993]) (Figure 3). Although the lack of overlying north,all of which have interactedin the pastfew million units elsewhere prevents the conclusion that the entire years. Becausethe subsurfacegeometry and displacements surfaceis relict (i.e., preuplift),other evidence argues that it of these structures are not well known, the functional representsa horizon of minimal synuplifterosion. Where relationship between plate motion and transpressive developed on granitic bedrock, the surface consistsof orogenyis not well understood.The limiting factorthus corestonesand regolith (grus) that represent a typical far in constrainingthe architectureof this systemis the profile of deep weathering [Ollier, 1975]. Argillaceous unconstrainedpattern of vertical deformation acrossthe soils locally cap this saproliteand indicateextreme decay range. of the original parent rock. Becauseof the slow rates of chemical weathering on granites in nontropicalclimates 3. Methods this thickness of weathered debris may have required several million years of isolation from erosion to To definethe verticaldisplacement pattern across the accumulate [cf. Spotila, 1999]. The magnitude of SBMs, a widespread,easily identifiedstructural datum is exhumation indicated by thermochronometry is also needed. In section3.1 we describea deeplyweathered consistentwith minimal erosionatop the Big Bear plateau erosionsurface on graniteand how it servesthis purpose in the Cenozoic [Spotila et al., 1998]. Wherever [Spotila, 1999]. We have constructeddetailed structure preserved,the weatheredsurface thus represents a horizon contourdiagrams of this weatheredhorizon throughout that has experiencedminimal erosivelowering since uplift boththe SBMs(hanging wall of NFTS) andsurrounding initiated. Its shape is therefore primarily a function of Mojave Desert(footwall of NFTS). The methodsused to paleotopographyand subsequenttectonic deformation. definethese contours are explainedbelow. Oncedefined, If the weatheredsurface indicates where surface uplift has these contours can be subtracted to determine the vertical equaled rock uplift (i.e., no exhumation), the displacementson the NFTS andSAT. The displacement displacements along faults can be determined by fieldalso constrains the subsurface pattern of thrustfaulting subtractingits position along footwall and hanging wall and the patternof horizontalplate motionaccommodated blocks. If the paleotopographyof the surface can be by uplift. We describethe techniquesand results of these constrained,its present distribution can also be used to structuralanalyses in the subsequentsection. constrainthe vertical displacementfield of blocksand the subsurfacegeometry of faults. The low-relief surfacelikely 3.1. Weathered Surface had minor topographicundulations before uplift, owing to The Big Bear plateauis characterizedby an extensive, an irregular weathering front associated with lateral variationsin bedrock weathering rate (i.e., etchplanation low-reliefupper surface perched at highaltitude (from 1.5 [Twirlale, 1990]). Chemically resistant metamorphic to 2.5 km elevation),which is isolatedfrom surrounding rocks,for example,outcrop an averageof-0.4 km higher lowlandsby steepfault escarpmentsand canyonwalls thanweatherable granitic bedrock atop the plateau[Spotila, (Figure 3). While fresh bedrockis exposedalong the 1999]. Significant long-wavelength topographic localizedsteep slopes, up to 30 m of deeplyweathered rock variations, however, were probably absent. The deep occursalong the gently rolling uplandsurface [Spotila, etchplanationalong the surface would have required a 1999;Meisling, 1984;Brown, 1976]. Low-reliefpatches protractedperiod of weathering during which a nearly of deep-weatheringalso occur atop the San Gorgonio horizontal, regional pediment would have developed Block,in the structurally lowSanta Ana Valley, and in the [Spotila, 1999]. The Neogene stratigraphicrecord also surroundingMojave Desert [Spotila, 1999;Jacobs, 1982; indicatesthat all but the westernmostSBMs had very low Oberlander,1972] (Figure3). Rock alongthese surfaces relief prior to the last few million years[cf. Spot#a, 1999; share the same structural level with respect to Meisling and Weldon, 1989; May and Repenning,1982; thermochronometricisochrons that formedat-75øC (-3 Sadler, 1982, 1993; Allen, 1957]. Clast provenancein km depth),suggesting the surfaceswere continuousprior preuplift sedimentssurrounding the plateaudoes indicate to displacementalong block-bounding faults [Spotilaet that minor positiverelief existedat what is now the central al., 1998]. This continuoussurface likely developedin a plateau [Sadler and Reeder, 1983]. However, this relief low-energyerosional environment that existedprior to can be explained by the difference in elevationbetween a uplift but is now in disequilibriumwith its positionabove granitepediment and localized resistantquartzite that the base level [Spotila, 1999]. The surface'swidespread sedimentswere derived from. Granitic bedrockmay thus distributionmakes it a potential datum to constrainthe have been approximatelyflat prior to uplift, so that the regionalpattern of verticaldisplacement. weatheredgranitic surfacereflects the patternof vertical Becausethe low relief surfaceexhibits deep weathering, displacement. it had to have been accumulatingthe residuumof rock decay for a long period. This requires that minimal erosionhas occurred where the surfaceis preserved.The 3.2. Structure Contour Diagram Atop the SBMs surfacehas experienced zero post-Miocene erosion locally, We constructeda structure contour diagram of the where 6-10 Ma basaltsoverlie it on the easternplateau weathered surface, on the basis of a detailed map of its SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 595 distribution and a screening process that attempted to the weathered surface. There are liable to be artifacts from minimize differences in its original shape (Figure 4). the weatheringfront's paleotopography and minor erosion Spotila [1999] mappedthe entire weatheredsurface using in these structurecontours. However, we can estimatethe airphotos and field observationsat a scale of 1:62,500 resulting uncertainties. Uncertainty due to erosion is (contour interval is 80 ft (24 m)). From the mapped small. Locally, erosionof well-developedsoils or thick distribution of the weathered horizon, we selected -3000 regolith (i.e., grus and saprolite) of the horizon has spot elevationsfollowing severalcriteria. Spot elevations exposeda hard rind of corestones[Spotila, 1999]. The were taken from all patches of the surface >0.5 km in preservationof deepweathering wherever spot elevations width. Each elevation representsan approximate local havebeen taken, however, indicates that the magnitudeof median in that the lowest pocketsand troughsand highest recent erosion from the surface has been less than the total inselbergs and tors were avoided. This minimizes thicknessof the original,characteristic weathered profile elevation differences associated with variations in the (<50 m). The uncertaintyassociated with the original original weatheringfront geometry. Spot elevationswere geometryof the weatheringfront is somewhatlarger. Even also selectedfrom the same bedrock type to ensure that within uniform bedrocktypes, local variationsin bedrock elevationdifferences were not the result of etchplanation. weathering rate may have produced small undulationsin Locations on Figure 4 shown as solid circles are from the weathering front [Twidale, 1990]. Our selection bedrock of quartz- or biotite-monzonite, whereas solid processshould have avoided most of these undulations. squaresindicate granitic gneiss, granite, or granodiorite However, hummockytopography is ubiquitousacross the bedrock. The resultingspot elevationswere contouredby weathered surface, and we cannot be sure that small hand at 200 ft (61 m) intervals. Contours were drawn as variations(<100 m) in contourelevation are not due to the continuouslines, except where fault escarpmentsclearly short-wavelength (-1 km) paleotopography of the disruptthe weatheredsurface. weatheringfront [Spotila, 1999]. The structure contour diagram exhibits a smoother Thesetwo sourcesof uncertaintyshould generally work shape than the present topography. Contours of the in oppositedirections' erosion resulting in contoursthat weatheredsurface are higherthan present topography where shouldbe higher and undulationsdue to resistantbedrock erosion has lowered or removed the weathered horizon, resulting in contoursthat should be lower. Conservative whereascontours are lower than presentelevation where confidencelimits of the degreeto which structurecontours resistant, nongranitic bedrock has produced resistant locally representa smooth,near-horizontal, preuplift geomorphic forms. Contours increase smoothly in structurallevel are thus about +100 m. We are less certain elevation from 3800 ft (1.2 km) on the western rim and aboutthe degree to whichthe surface was level across long 4400 ft (1.3 kin) on the easternrim to 8000 ft (-2.4 km) wavelengths.As discussedabove, however, available data on the north central rim. The weathered surface also crests are most consistentwith a regional, near-flat pediment nearthe centerof the southernrim of the plateau(-8200 ft prior to uplift. We thus make the assumptionthat the (-2.5 km)). Betweenthe northernand southerncrests is a surfacewas essentiallyhorizontal prior to analyzingits slighttrough (-7000 ft (-2.1 km)) that givesthe weathered form. horizonatop the centralplateau a saddle-likeshape. The Additionaluncertainty in contoursresults from sparsity highest contoursoccur south of the plateau, however, of data. Acrossparts of the range,weathered granite has wheretraces of the weatheredsurface rise steeplyalong the been removed by erosion or buried by Quaternary northernflank of the San Gorgonioblock (> 10,000 ft (>3.1 sediments or is absent simply because nongranitic km)). Thesesouthern contours terminate where remnants lithologiesare present. The contoursalmost certainly of weatheredgranite disappear along the crestof thisblock. missdetails of the weatheredhorizon's shape across these The structurecontours also reveal a complex geometry, areas. Across the Big Bear Valley, for example, the which reflects several phenomena. Most noticeableare contoursindicate a gentle saddleshape, although it is discontinuitiesin the surfacealong major and minor faults uncertainif weatheredgranite has been removed by erosion (Figure4). The contoursare perpendicular to the northern from the valley or is presentlyburied by the alluviumand and southernthrust escarpmentsof the NFTS and SAT. lacustrine sediment therein [Brown, 1976]. Another The southernbreak in contoursseparates weathered surface specialcase is the SantaAna Valley, the footwall block of atopthe plateaufrom the weatheredgranite and Tertiary the SAT. Although we determined the elevation of sedimentsin the low, tilted SantaAna Valley (rises from weatheredgranite in the westernvalley beneathTertiary -3600 (-1.1 km) to -5600 ft (•-1.7 km), from westto east). '-'sediments[dacobs, 1982], thick Quaternaryfanglomerates Many shorterfaults also vertically displace the weathered obscureits connectionto the weatheredsurface near high surface,including segments of the inactive SquawPeak Onyx Mountain on the east(Figure 4). Subsurfacedata in thrustsystem on the west [Meislingand Weldon,1989] and active strands of the eastern California shear zone on each of these locationswould be required to refine the geometryof our structurecontours. the northeast[Dokka and Travis, 1990]. The degreeto which the weatheredhorizon is deformedis not surprising, giventhat it may be as old as lateMiocene and record 10 3.3. Structure Contour Diagram of the Mojave Desert Myr of faultingwithin the plate boundary. North of the NFTS, thick alluvial depositscover a large Minor variations in structure contours also betray portionof the footwallblock. Weatheredgranite occurs on limitationsin our ability to reconstructthe fine detailsof local basementexposures, but sedimentsobscure much of 596 SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 597 598 SPOTILAAND SIEH: ARCHITECTUREOF TRANSPRESSIONALTHRUST FAULTING the weatheredsurface's distribution (Figure 5). To define is possibly as high as 20-30% (+100-200 m in most structurecontours in the footwall block, we thus had to use cases). There is additional uncertainty as to whether the the elevationof buried basementas a proxy. Becausemost basementis truly weathered granite and to what degree bedrock in the adjacent Mojave Desert is granitic and such a weathered surface is smooth and noneroded. cappedby a deeplyweathered profile [Oberlander,1970], The structure contours of basement on the footwall we assume that bedrock beneath alluvium is weathered block indicate that the surface topography of Lucerne granite as well. Given the widespreaddistribution of Valley does not provide a good approximation of the weatheringin the area and the slim chancethat basement elevation of the weathered surface. The contours show a buried under thick Tertiary and Quaternary deposits nontrivialdeparture from topographicelevations and clearly experiencedgreater erosion than the weathered surface atop define deepbasins that have depressedbedrock. Alluvium the exposedBig Bear plateau, we feel that this is a in the trough-likebasin north of the NFTS is as thick as reasonableassumption. We used several data sets to 1.2 km and consistsof debris derived primarily from the constrain the elevation of buried basement and combined SBMs [Sadlet, 1982]. The alluvium thins to negligible the elevationsof exposedweathered granite to constructa thicknesseseast of the Helendale fault and eventually structurecontour diagram of the weatheredsurface atop the exposesweathered granite, where the footwall andhanging footwall block. wall blocks meet at the eastern terminus of the NFTS. Existing borehole data provide direct control on the Severalspurs and troughsare also apparentin the contours, depthto basementnorth of the range[Aksoy et al., 1986; where northwesttrending faults of the easternCalifornia Riley, 1956; John S. Murk Engineers, 1985]. Although shearzone have created small pressurehorsts or grabens. mostwells are very shallowand confinedto alluvium, a Depression of basement by sediments, downwarp of numberpenetrate basement or a considerabledepth of basementalong the NFTS, and the local displacementdue alluvium and are thus very useful. These deep wells are to strike-slip faulting have considerably altered the labeledon Figure 5. A secondconstraint on the depthto geometryof the weatheredsurface of the footwall block basementis providedby a seismicreflection profile across from what was presumablyonce a smoothershape. the westernNFTS [Li et al., 1992]. The two-dimensional geometryof the LucerneValley basinappears clearly as a reflection from the sediment-basement interface. This 4. Structural Analysis contactdips southwardand is offset acrossthe NFTS, Figure 7 depictsa simplified structurecontour map of creatinga bedrockwedge in the hangingwall that has the weathered surface both north and south of the NFTS. overthrusta trappedwedge of alluviumin the footwall . According to the above methods and assumptions, We interpretedbasement depths from the two-waytravel contoursof the weatheredsurface approximate the present times on this profile by assuminga 2.5 km/s P-wave structurallevel of a smooth,preuplift horizon. This surface velocityfor the alluvium(Figure 5). has been only insignificantlyeroded during uplift of the Another useful constraintfor determiningthe depth to SBMs. The widespreaddistribution and smoothnessof basementis Bouguergravity data. Basementdepth can be surfaceremnants also suggestthat the deeply weathered calculatedfrom gravity anomaliesby assuminga density horizonwas roughlyuniform in elevation(i.e., low-relief) contrastbetween unconsolidated sediment and crystalline prior to deformation. The lack of sedimentologicevidence bedrock. Aksoy et al. [1986] determinedthe depth to of pre-Pliocene relief argues that long-wavelength bedrockalong two profilesin the centralLucerne Valley topographicfeatures were absentprior to the recentuplift (dd' andee' in Figure5). We calculatedthree additional [Sadletand Reeder, 1983], while .the contour construction depthprofiles (aa', bb', andcc') using a two-dimensionalmethod avoids minor undulationsin the weatheringfront modelingprogram (S. Biehler,personal communication, due to etching. Figure 7 shouldthus approximaterock 1998)on completeBouguer gravity anomaly data [Biehler uplift and subsidencerelative to the low-relief etal., 1988]and assuming a 0.4 g/cm 3 densitycontrast. paleotopography of the widespreadweathered granite Theseprofiles are probablyaccurate to within several surface.Although we cannotbe absolutelycertain of the hundredmeters. We also interpret a lack of gravity predeformationalgeometry of the weatheredsurface, our anomalies on the east to indicate that alluvium is <100 m comparisonof the relationshipbetween its hangingwall thick(S. Biehler,personal communication, 1998). Small and footwall positionand the distributionof structures squareson Figure5 indicatewhere we considerbasement normalizesany effectof paleotopographicrelief. We can to be within 100 m of the surface based on this. Note that thususe the weatheredsurface's constraint on rock uplift to becauseof complexgeometry below the thrustfault, we studythe tectonics of thethrust faults. refrainedfrom usinggravity data to constrainbasement depthnear its trace. 4.1. DisplacementAlong the Near-SurfaceNFTS We contouredelevations of exposedweathered granite and SAT and buried basement of the footwall block at a scale of 1:62,500and an intervalof 200 ft (61 m). The resulting The geometryof structurecontours along the Big Bear structurecontour map showsa low (•03400ft (•01.0km)), plateaubears a remarkablecorrespondence tothe location of smoothform (Figure6). The smoothnessresults partly theNFTS (Figure7). Elevationsare low at eitherend but from the sparsedata set used. The uncertaintyin the crestin the centralsegment of the thrustfault. On the elevationof basementrepresented by contoursacross basins eastern terminus of the thrust the weathered surface of the 599 600 SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

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oo 'q:D • t'"q 0 tull '(•IP) •u.tu•.•oqs SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 601

me Be A A ! 2500 I I I f I I I I I I I I I I I I I I I I I I I I I 2000, _ (8202') ' ' ' ' 2000_ hangingwall (6562)INFTS • mean=O74m-_ (BB Plateau) 1500 ß' HF•""-' "•' 1500 . ß ß HF2 ' .I""'"' dr BH2"," i mlm,,mll 1000 ,ooo i 500 r •footwall•• (!640')• (MojaveDesert) 0 •,, • I,•,l,,,•l,•,,I,,,,I,,,,!,,1,1,•,, L 0 10 20 30 40 50 60 70 00 1020 30 40 50 60 70 !0 distance, km distance. km

re De B B' B 3000c .... • .... • .... • .... • .... i,, ,', 200 (9843')• SAT hanging-wall t937') SAT mean = 647 m ooo

2500• (BBPlateau• combined 2000•'•anging wall J 800 .... • .8r'" •fSAT (overlar)) / footwall 600 13UU•,•••,,•..=,• ofSAT SAT 1000•-,o•otwal'• -• ' 400' 200 (1564000,)• OfWCF WCF v 0 10 20 30 40 50 60 0o 10 20 30 40 50 60 distance, km distance, km Figure 8. Vertical displacementalong the NFTS and SAT, whichequals the differencein weathered surface elevationsatop hanging wall and footwall blocks. (a) Comparisonof structurecontour elevationsatop the Big Bearplateau and Mojave Desert adjacent to the surfacetrace of theNFTS (along line AA'; see Figures4 and 6). HF1 and HF2 are strandsof the Helendale fault that bound the Helendalegraben; BH2 is a strandof the Bighornfault zone. (b) The differencebetween the hanging wall and footwall elevationsalong AA', which equalsthe verticaldisplacement along the NFTS. (c) Comparisonof structurecontour elevations atop the plateau and Santa Ana Valley adjacentto the surface traceof the SAT (alongline BB'; seeFigure 4). Note that the footwallelevations are differentfor the WatermanCanyon fault andthe main SAT strand.Where the two strandsoverlap, the elevationsof the hangingwall for the main SAT strandwere takenas the elevationof the footwallto the Waterman Canyonfault. (d) The differencebetween the hangingwall andfootwall elevations along BB', which equalsthe verticaldisplacement along the SAT. Solidcircles represent displacement on the Waterman Canyonfault, whereas solid triangles represent displacement on the mainstrand of the SAT. The line aboverepresents the sumof thesetwo.

footwall and hanging wall blocks actually merge. This toward each side to resemblea smooth,quasi-parabolic pattern strongly suggeststhat the NFTS has vertically distribution. Displacementdrops to zero on the eastbut displaceda surfacethat was previouslynearly level. By lingers with moderatemagnitude (-•610 m) west of the subtractingthe elevationof contoursatop the hangingwall NFTS. This is because the NFTS terminates within the and footwall blocks we have directly measured the MioceneSquaw Peak thrustsystem, which locally offsets magnitude of this vertical displacement. This theweathered surface [Meisling and Weldon,1989] (Figure measurementis not dependenton assumptionsabout the 2). Other complexitiesin the displacementprofile also originalgeometry of the surface. occur where different fault segmentsare encountered. Because the structure contours of the weathered surface Grabensbetween strands of the Helendalefault and Bighorn in the footwall block are lower in elevation than present fault zones,for example,down-drop the weatheredsurface topography,the resulting plot of vertical displacement of the hangingwall block (Figures2 and4). Lack of data along the NFTS is greater than the height of the fault preventsseeing a similar down-dropin the footwall block escarpment(Figures 8a and 8b). Vertical displacement (Figures5 and 6), but if it exists,the displacementprofile peaksnear the centerof the fault trace(1650 m) but drops shouldextend smoothly across these grabens (shaded line 602 SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

4Akrn•N x=0.95 km 34•15' '..... 34•5' S----•A' S--1.02km OnyxMtn. x _-m <5=21d) alluv weat•recWeat/-e •ace •o=65(3) 10km- ', • g I

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ß ß• ...... • • •ventC I • • XkA II

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Figure 9. SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 603 in Figure 8b). Owing to the general limits in data that south)may insteadrepresent downdip variationsin thrust define contour elevations the entire displacementplot fault dip. The contour pattern is similar to the probably has an inherent uncertainty of a few hundred geodeticallyobserved coseismic vertical displacements for meters(•10-15%). individual ruptures of thrust faults [e.g., Savage and We measuredvertical displacementalong the SAT in a Hastie, 1966], which can be used to define fault dip at similar manner, but several complicationsresulted in a depth. We usethe structurecontours similarly to explore larger uncertaintythan along the NFTS. The weathered subsurfacegeometry of the NFTS and SAT in three north- surfaceis locally not exposedalong the southernfootwall, southcross sections (Figure 9). owingto burial by thick fanglomeratesof SantaAna Valley The antithetic symmetry of the NFTS and SAT has on the east, where no data constrained the depth to previously inspired the hypothesisthat the subsurface basement. The weathered surface has also been locally structureof the rangeis shapedlike a flower or mushroom, removedby erosionon the west, where the SAT outcrops in which low-angle faults near the surface steepento a midway up the southwesternescarpment of the plateau vertical zone at depth [Sadlet, 1982]. However, we have (Figure 4). Another complication is the Waterman several reasonsto proposethat the NFTS undercutsthe Canyon fault on the west, which sharesthe SAT's trend, SAT. The SAT is the lesser of the two faults, having positionon the escarpment,and senseof separation. This significantlyless vertical offset and shorterlength (Figures fault may representa westward continuationof the SAT, 2 and 8). Although the SAT has uplifted the plateau implying that its displacementbe summedwith the SAT. abovethe SantaAna Valley, the valley disappearswithin a We thus measuredthe displacementalong the southern high-elevation bulge in structure contoursto the east thrust faults by differencing the elevation of structure (Onyx Mountain; Figure 7). Because the topographic contours of the hanging-wall block with those of the expressionof the SAT is lost there as well, it cannothave footwall blocks of both the SAT and Waterman Canyon contributedto the uplift of this bulge. Similarly, the trace fault separately (Figure 8c). Where not exposed, the of the SAT itself appearsto have been uplifted, rising elevation of the weathered surface of the footwall was eastwardto over 2.5 km elevation. This implies that the approximatedas the elevation of the surfacetrace of the NFTS undercutsthe SAT, uplifting the secondarythrust thrust fault itself. The resulting combineddisplacement fault alongwith the plateauitself. The high contoursatop profile for the two faultsreveals a bell-shapedcurve that is the San Gorgonio block further imply that the NFTS similar to the NFTS profile (Figure 8d). The maximum penetratesthe crust south of the SAT. Rather than vertical offset along the SAT is 1100 m, but displacement merginginto a high-anglefault systemmidway beneath the averages•650 m. The uncertaintyof the displacement plateau,the NFTS may thusextend southward to the San valuesare probablyas greatas 25% (-200 m). Andreas fault. We focus our analysis to constrainthe geometryof the NFTS, the more importantstructure. However,the area of theweathered surface underlain by 4.2. Subsurface Geometry of the NFTS and SAT the SAT hasexperienced deformation due to boththrust The geometry of structurecontours of the weathered faults. To learnabout the geometryof the NFTS, we must surfaceare clearly associatedwith the boundingthrust first removethe vertical displacementproduced by the faults (Figure 7). The along-strikeincrease in contour SAT. We attribute a fixed component of vertical elevationstoward the centerof the plateaurepresents an displacementto the SAT, using two end-member increasein displacementalong the faultsfrom their eastern scenarios. In the first case, we assumethat the entire and westernterminations. Cross-strikechanges (north to hangingwall blockhas a uniformvertical displacement,

Figure 9. Shapeof the NFTS in north-southcross sections across the SBMs (AA' is at 116ø40'W, BB' is at 116ø55'W,and CC' is at 117ø05'W;locations are shownin Figure2), on the basisof the deformationof the weatheredsurface. Topographyis shownas an irregularlight line withoutvertical exaggeration.The dark line near the surfacetopography represents the elevationsof the weathered surface(w.s.) from structurecontours (dashed where inferred). Orientationof the NFTS is basedon analysisdescribed in the text. Displacementsat the trace of eachthrust are listed (S, net slip;X, horizontalcomponent). Fault orientationsat the surfaceare labeled(•5o) and referenced with subscripts (1, Miller [1987]; 2, Dibblee [1967b]; 3, Dibblee [1967a];4, doneset al. [1986]; 5, Rzoncaand Clark [1982]; 6, dacobs[1982]; 7, Allen [1957]; 8, Dibblee [1974]; 9, Li et al. [1992]). Other data are from Sadlet[1993] andDibblee [1964a, b]. Faultsare dashed where orientation is inferred,and fault type is indicatedby arrows(vertical motion) or circles(circle with crossindicates lateral motion away; circle with dot indicateslateral motion toward). Focal mechanismsfrom Figure 11 are given as upper hemisphereplots projected onto the perspectiveplane of sectionBB' (e.g., eventsA-D representthrust motion). MB, Morongo block; MCF, Mill Creek fault; MFZ, Mentone fault zone; MsCF, Mission Creek fault; OWSF, Old Woman Springs fault; OWss, Old Woman Sandstone;SAF-SB, San Bernardinostrand of the SanAndreas fault; SAss,Santa Ana sandstone;SAV, SantaAna Valley; YRB, YucaipaRidge block (otherabbreviations are as in Figures2, 4, 6, and 7). 604 SPOTILAAND SIEH:ARCHITECTURE OF TRANSPRESSIONALTHRUST FAULTING

ae Pre-deformation: MojaveDesert Bi• Bearplateau SAV

Components of SAT motion: Case #1 Be MojaveDesert Big Bearplateau I v_ .•..•.._.•__.•;-••-...... •,•^v • measured vertical on 0.8measuredCC' =1.0 kmvertical on CC// . 'V"i'(]C')=1.0 km

Case#2 BigBear plateau 0.4 B' •?a:Sv•.....:..•I•e•ert . • 02•) BB'----"'"• 0.7•B' 10 0 distancefrom SAT scarp,km

Postdeformation:Big Bear plateau M0Jave..D.esert.•,r' "'...... ,,•,:'•AV :" •-.:"':•• .•.• Ce AA' d!p...... 1.5 .,•...... -...½.'" ,- 40 •

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0 i i .... I .... I .... I .... 0 o 0 5 10 15 20 25 30 o 5 1o 15 20 25 30 distancefrom NFTS scarp,km distancefrom NFTS scarp,km CC' Case #1 /'", ,', , 1', , , , I .... i .... i .... iq32 di ...... •dipt24 \x 6 C•

ß

0 • • • i i • • i • i .... i , , , • i • • • • i 1• 0 5 10 15 20 25 • % 5 10 15 20 25 0 distancekom N•S sca•, • distancefrom NFTS scarp,km

Figure 10. SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 605 equalto the value measuredat the surfacetrace of the SAT To use elevations of structure contours to learn about (Figure 10a). Both the plateau and Mojave Desert would the shapeof the NFTS, several more assumptionsare have thus been uplifted the same amount relative to the required. On the basis of available geologic data we SantaAna Valley. Given that the NFTS has uplifted the assume that the weathered granite surface was plateau and Santa Ana Valley relative to the Mojave approximately horizontal prior to uplift. The Desert, no vertical displacementattributable to the SAT paleogeographyof the regionis not uniquelyconstrained, must be removed prior to analyzing the orientationof the however,and this assumptionrepresents the weakestaspect NFTS. of our analysis. We also assume that horizontal In the secondcase, we assumethat vertical deformation componentsof thrust motion at depth do not vary due to SAT motion decayswith distancenorthward away significantlyfrom surfacemeasurements. This assumption from the fault surfacetrace (Figure 10a). Becausevertical is well foundedif convergenceis associatedwith far-field, elastic displacements associated with individual purely horizontal translation of plates along the San earthquakesare confinedto narrow zonesnear faults [e.g., Andreasfault. Along-strikevariations in slip existalong Savage and Hastie, 1966], it is plausible that the long- the NFTS as a function of eastern and western fault term deformation associated with the SAT has been terminations(e.g., Figure 8), but we do not attributethese similarly limited to a fixed distancefrom its surfacetrace. to cross-strike changes in fault orientation. Last, we This requires that a component of plateau uplift be assume that deformation of the weathered surface was subtractedprior to analyzingstructure contour elevations to produced primarily by the NFTS and SAT. The San learn aboutNFTS geometry. To maximize the difference Andreas fault and secondaryreverse faults may have between our two cases, we assume that deformation deformed the weathered surface in the westernmost SBMs associated with the SAT is confined to a narrow distance (e.g., westernSan Bernardinoarch [Meisling and Weldon, from the fault. This results in significant reduction of 1989]), but elsewhereit is clear that the thrust faults have vertical displacementassociated with the NFTS in the b4enthe main structuresresponsible for uplift. southernmostBig Bear plateau, while not affecting that Given these assumptions,variation in the elevationof associatedwith the NFTS in the northernplateau (Figure structure contours should reflect the orientation of the 10a). King et al. [1988] have shown that the width (i.e., underlyingthrust faults. Higher regionsshould indicate normalto strike) of long-termdeformation associated with greater vertical components of fault motion, in turn dippingfaults, although greater than the width of coseismic implying steeperunderlying fault dips. Lower elevations elastic deformation, is highly dependenton the flexural insteadimply more shallowdips. This representsa space rigidity of the crust and the degree of erosion and balance approachto constrainingdowndip changesin sedimentation.It is most narrow when postseismicstress thrustfault geometry. The specificconstraints used for the relaxation of the crust is minimized by a thin apparent subsurface orientation of faults are described below. elasticthickness and minimal isostaticadjustment (i.e., no 4.2.1. Constraints on fault geometry. W e erosion). Figure 10b shows how vertical displacement constructed cross sections using a 1:62,500 scale shoulddecrease away from the SAT surfacetrace, based on topographicbase and elevations of the weathered surface the resultsof King et al. [1988] for an uneroded,45 ø from the contourmaps (Figures 4 and 6). The elevationof dippingreverse fault that penetratesa 15-km-thick brittle basementin the San BernardinoValley on sectionCC' is crust with an apparentelastic thicknessof-4 km. We from Willingham[ 1981]. The magnitudeof rock uplift for subtractthese componentsof vertical displacementfrom southern fault blocks is based on thermochronometry elevationsof structurecontours on both crosssections prior [Spotila et al., 1998]. Fault location and orientation to analysisof NFTS geometry. observedat the surfacewere boundaryconditions for cross-

Figure 10. The dip of the NFTS in crosssections based on changesin elevationof structurecontours away from the surface trace of the thrust. To make this calculation, the componentof vertical displacementattributable to the SAT mustbe removed. (a) Schematicdiagram showing two plausible casesof vertical deformationon the SAT. In case 1, slip doesnot decreasewith distancenorthward from the SAT's trace, so that the plateauand Mojave Desert are displacedequally relative to the Santa Ana Valley (SAV). This casedoes not require any displacementof the weatheredsurface between plateauand Mojave Desert to be removedprior to the NFTS dip analysis. In case2, slip decreases away from the SAT, so that a correctionmust be madeprior to calculationof NFTS dip. (b) Plots of the decay in vertical displacementattributed to the SAT on crosssections BB' and CC' for case 2, basedon a modelof long-termelastic deformation and stress relaxation for a 45ø dippingreverse fault in a 15-km-thick brittle crust with a 4-km-thick apparentelastic thickness[King et al., 1988]. This displacementwas subtractedfrom plateauelevations prior to analysisof NFTS dip. (c) Plots of the elevation of structurecontours (V) as a function of distancefrom the NFTS for each crosssection. Results for both cases of assumed SAT deformation are shown. Differences between these elevations andthe elevationdirectly above the NFTS scarpare taken as indicationsof dip changesat depth,as V/X - tan;5(;5 is dip andX is constanthorizontal shortening). Resulting values of dip are plottedas well. 606 SPOTILAAND SIEH' ARCHITECTUREOF TRANSPRESSIONALTHRUST FAULTING sectionconstruction (as referenced;Figure 9). Faults for dipsto intersectthis 45øSplane (BB' andCC'). A fourth which no subsurfaceinformation is available were simply thrust rupture occurred at 2.6 km depth near the NFTS projectedto the baseof the brittle crustat their near-surface trace (event D; BB'). This was associated with attitude. This is the casefor most of the strike-slipfaults displacements visible on synthetic aperture radar in the sections.For many high-anglefaults, there were no interferograms[Feig! et al., 1995]. This rupture dipped dataavailable on surfaceorientation other than topographic 28+4øS based on an elastic dislocation model of its expression[cf. Spotila, 1999]. These are drawn vertically, displacementbut may have occurredon a secondaryblind so that faults with dip-slip motion are not inferred to be thrust responsible for folding north of the main trace normal or reverse (Barton Flats and Green Valley fault [Spotila, 1999; Miller, 1987; Dibblee, 1964b]. The depth zones). distributionof recent seismicityalso delineatesthe brittle- We determinedthe orientation of the NFTS using the plastic transition beneaththe SBMs (Figures 9 and 11). elevation of structure contours and the displacements This rheologicalboundary marks the lower limit to which measuredalong its surfacetrace. Apparentnet slip S along brittle faultsare projected. Our geometryfor this is similar the traceof the NFTS was measuredby projectingthe fault to that in previousreports [Corbett, 1984; Magistrale and upwardto intersectthe projectionof the weatheredsurface Sanders, 1996]. northof the escarpment(Figure 9). The weatheredsurface 4.2.2. Resulting cross sections. Because cross is unlikely to bend downwardas a fold at the escarpments, sectionAA' cuts the plateau east of the SAT's apparent given that thermochronometricisochrons remain nearly termination, none of the weathered surface's vertical horizontalat boththe northernand southernmargins of the displacement had to be restored prior to performing plateau [Spotilaet al., 1998]. The horizontalcomponent analysisof NFTS geometry. The weatheredsurface rises X of apparentnet slip S was used as a fixed boundary steadily to the south, away from the surfacetrace of the conditionfor the determinationof downdipfault orientation NFTS. This requires the NFTS to steependowndip in in each cross section. The elevations of the weathered our analysis (Figure 9a). The distinctly convex NFTS surfaceV were plottedaway from the NFTS on eachcross steepensto a dip of 52øSbefore it intersectsthe brittle- sectionat •1.5 km intervals(Figure 10c). We determined plastic transition at-18 km depth. The vertical fault dip below (b)as V/X = tan(b). Resultingdips were displacementof the weatheredsurface, however, continues plotted sequentiallydownward for each vertical offset to rise south of the NFTS's intersection with the brittle- measured but were offset from where calculated by a plastictransition, beyond where elastic deformation would distanceequal to the horizontalcomponent of motion(to be expectedto occur. To extendthe NFTS farther south accountfor vergence).Small high-angle faults with minor beneaththe rise of Onyx Mountain, we envision that it vertical separationof the weatheredsurface (Barton Flats couldmerge with the mechanicaldiscontinuity at the base and Green Valley fault zones) were backslippedprior to of brittle crust to form a detachment. In this case, the calculatingthrust fault dip. This was completedfor the bulge in the weathered surfacecould result from internal two casesof prescribedSAT displacements(lines A andB; deformation(i.e., kink bandmigration) associated with the Figures 9 and 10a). Note that this space balance bend betweenthe detachment(i.e., flat) and the steeply techniquewas used rather than an elasticmodel because of dippingNFTS (i.e., ramp). Alternatively,the bulgecould the insensitivity of such models to fault dip and their have beenproduced by the steeplynorth dippingMission dependenceon mechanicalassumptions [e.g., King et al., Creek fault, althoughthis strandof the San Andreasfault 1988]. zone doesnot have significantvertical motion along its Seismicity is often used to constrainthe subsurface trace. orientationof faultsbut providesonly minor constrainton The weathered surface in cross section BB' is deformed our crosssections. Recent seismicity beneath the SBMs is by both thrust faults, and thus we backslippedthe SAT dominated by aftershock clusters along the strike-slip prior to analyzinggeometry of the NFTS (Figure 9b). In rupturesof the 1992Landers-Big Bear sequence[Hauksson one scenario we assumed motion on the SAT does not et al., 1993; Jonesand Hough, 1995] and the oblique slip decreasenorthward from its trace, as shownin Figure 10 1986 Palm Springsrupture [Joneset al., 1986] (Figure (Case 1). Given this assumption, the NFTS flattens 11). The 1986 event definesa 45øN dip for the San slightly(to 30ø) until-14-km depth,where it steepens(to GorgonioPass fault zone (AA'; Figures9 and 11). Most 49ø) to accountfor uplift of the San Gorgonioblock. faults, includingthe NFTS and San Andreas,cannot be However,the fault meetsthe brittle-plastictransition north definedby hypocenters.A few diffuse clustersof small of the high part of this southernblock. It is thus not clear 1992 aftershocksoccur near the NFTS (AA' and BB'), but that the NFTS can explainthe San Gorgonioblock uplift theseprobably represent secondary deformation within the in this case. In the second case, motion on the SAT is footwall. Severalmoderate events with thrust-typefocal assumedto decreaseaway from its trace (Case 2; Figure mechanismsoccurred along the NFTS and SAT in 1992 10). This meansthat a significantfraction of the weathered [Haukssonet al., 1993;Jones and Hough, 1995]. Two of surface'sdeformation is removedand results in pronounced, these occurreddeep along the projection of the NFTS listric-like flattening (to 12ø) of the NFTS. The fault (eventsA and B; BB'), but definingthe orientationof the remains above the brittle-plastic transition beneath the NFTS based on these is tenuous. Event C is a 45øS entireSan Gorgonio block, where it steepensto accountfor thrustrupture that occurrednear the SAT (sectionBB'). the localized uplift. This case suggeststhe NFTS can We havesteepened the SAT fromits 26 ø and36øS surface explainthe uplift of this southernblock (with the help of 607

-10

-25

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...... • , o . ß • oø•I ...... ----•.• , 'Eve• ' ....i99• C ...... / ' . .... • ...... L......

-25 B = 8/18/92 C = 7•9•92 M5.3 -30 D = 12/4/92 20 40 o C NETS oo SATSAF N _ : oo, 0,o'/ ,o,, • • o • • o• -•o• - •• •-•e• o'o0. •-o . . .

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-3½ -4 (•-3 0-2o-1 20 40 60 distance, km Figure 11. Seismicityconstraining the extentof brittle crustand the geometryof somefaults beneath the SBMs. North-southcross sections AA', BB', and CC' depictearthquakes from 1981-1998 from within the boxes shownon Figure 2 (data are from E. Hauksson,manuscript in preparation,1999). Event locationsare basedon the velocity structureshown by the labeledthin dashedlines (km/s). The thick dashedline representsthe brittle-plastictransition, inferred to equalthe baseof seismicity. Focal mechanismsof eventsA-D appearon Figure9. EventsA andC actuallyoccur outside of the window of BB' shownon Figure 2 but are projectedonto the plot from severalkilometers east [Jones and Hough, 1995]. Clustersof eventsare labeled(PS, PalmSprings; BB, Big Bear). Locationsand near-surface orientationsof major faultsare shown. MCF, Mill Creek fault; NFTS, North Frontalthrust system; SAF, SanAndreas fault; SAT, SantaAna thrust;SGPFZ, San Gorgonio Pass fault zone. 608 SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING the Barton Flats fault zone). We considerthis second thrustfault escarpmentsis restoredby projectingthe scenarioto bemore realistic, given that displacement along weatheredsurface to intersectthe upward projection of each the SAT probablydecreased away from its trace. In neither thrustplane. For the thermochronometry,the preerosion casecan the NFTS accountfor the majorrock uplift of elevation of each block is restored and extended crustalslivers south of the SanGorgonio block [Spotila et horizontallyin either direction to the projectionof al., 1998]. bounding faults. The resulting estimate of the A lesscomplicated NFTS geometryresults from the paleotopographicsurface is thusa discontinuousline across analysisalong cross section CC' (Figure9c). In the first each profile. By assumingthis was approximately case of SAT motion (Figure 10) the NFTS steepens horizontalprior to deformationand uplift of the SBMs, slightlywith depth(25 ø to 32ø) to explaina slightrise in this line can be straightenedto estimatethe horizontal theweathered surface near Butler Peak. Thissteepening is shortening(change in line lengthdL) for eachprofile. less apparent in the second case, in which the NFTS Theseestimates are lessprecise than the measurementof flattens (to 15øS) because of the share of vertical horizontalmotion along individualfaults in crosssection displacementattributed to the SAT (Figure 10). This but provide constraint on how horizontal motion is representsa minor difference,however, and the NFTS and distributedacross the entirerange. Uncertaintyin these SAT can adequatelyexplain the uplift of the plateauin valuesis difficultto assessand most dependent on whether both cases. faultprojections are valid, the originalhorizontality of the The geometryof the NFTS implied in each of these topography,and whethersignificant internal deformation threecross sections is very different. SectionBB' revealsa hasoccurred in the heavilyeroded southern blocks. contorted shape, because of the complexity in the The largestestimated value of horizontalshortening weatheredsurface across the plateau, low SantaAna Valley, occursin the center of the range,just northwestof the and high San Gorgonio block. SectionsAA' and CC' restrainingbend in the SanAndreas fault at SanGorgonio imply oppositegeometries (convex on the east, listric on Pass (profile C, dL = 6.3 km). This representsa the west)because the SAT accountsfor someuplift on the minimumestimate of the total shorteningin the SBMs, west but none on the east. Accordingto this analysis, given that the thermochronometricconstraints provide however,displacement along the NFTS andminor motion minimum values of rock uplift and do not accountfor on the SAT (and BartonFlats fault zone)can adequately internal deformation. This convergenceresolves to a account for deformation of the weathered surface across the N37øW componentof-5 km, which is 5% of the total entire Big Bear plateau and San Gorgonio block. Pacific-NorthAmerican motion since 2 Ma [DeMets, Althoughour structuralanalysis is nonuniqueand limited 1995]. To the east and west, convergencedecreases by the assumptionswe have made, it thus providesa smoothly,as shownin the upperpart of Figure7. The plausibleexplanation for the shapeof structuresbeneath the resultingdistribution is similar to howslip varies along range. the NFTS and SAT (Figure 8). On the basisof these profiles,we estimatethe totalvolume of rockuplifted 4.3. Horizontal Shortening in the SBMs duringconstruction ofthe SBMs to be -4400 km 3 (Figure Although the weatheredsurface places valuable new 12). The magnitudeof erosionis also evidenton these constraintson the uplift of the Big Bear plateau,the thrust profilesand is greateston the thrust fault escarpments and faultsbounding the plateauare not the only transpressive southernmostfault bfdcks. The total volume eroded from deformation in the SBMs. Fault blocks in the southern therange is -1200km 3, whichshould fit intothe deep part of the rangehave experiencedmajor rock uplift and alluvialbasins in thesurrounding region (Figure 12). The exhumationdue to oblique slip on high-anglestructures averageincision depth along each profile, calculated as the [Spotilaet al., 1998] (Figure 2). Horizontalplate motion quotientof the erodedarea and the profilewidth, varies in the SBMs is thus accommodatedby a variety of from0.16 to 0.71km (Figure12). Theseestimates imply mechanisms.To learn about the causeof transpressive a whole-mountainaverage incision of 0.39 km, uplift in the SBMs, we mustfirst summarizethe patternof correspondingto an averageerosion rate of-0.16 mm/yr convergencerepresented by the structuralrelief of the entire (assumingca. 2.5 Ma initiationof uplift [May and range. Repenning,1982]). This is slightlylower thanfor other The horizontal plate motion that has been activemountains in California(e.g.,-0.2-0.3 mm/yr, accommodatedby uplift of the SBMs canbe estimatedby SantaCruz Mountains[Anderson, 1990], -0.5 mm/yr, simplevolume balance. The weatheredsurface provides a SantaMonica Mountains [Meigs et al., 1999]). constrainton the magnitudeof rock uplift acrossmuch of the range,whereas the minimumrock uplift anderosion for 5. Discussions blocksin the southcan be inferredfrom radiogenichelium thermochronometry[Spotila et al., 1998]. By restoring the volume eroded from each block a paleotopographic 5.1. DisplacementsAlong Thrust Faults surfaceis approximatedthat canbe restoredto constrainthe Both the NFTS and SAT exhibit large vertical magnitudeof horizontalshortening. We havedone this for displacementsin the SBMs (Figure 8). Giventheir gentle five north-southprofiles in Figure 12. Similar to the dips,the net displacementsalong these faults are even cross-sectionanalysis presentedabove, the erosion near larger (Figure 9). Using maximum vertical offset and SPOTILA AND SIEH' ARCHITECTURE OF TRANSPRESSIONALTHRUST FAULTING 609

ou• c• -- ß tU'UOtl"AOI•r• tU'UO!le^•IO • I.U'UOtl,AOIO [.• i.U'UOt•,XOIO 610 SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

Figure13. Schematicblock diagram of theprimary structural architecture of deformationin the SBMs inferredfrom our analysis ofthe weathered surface. The NFTS is shown with a contortedgeometry that includesboth convex and concave portions. The SAT appears as a simple,planar thrust. The dextral faultsof the eastern California shear zone are shown as penetrating theupper plate of the thrust faults to indicatethe possibility that both have been active during uplift of the range. Although this diagram is schematicand only represents ourbest guess for the structural geometry ofthe range, it givesa senseof thecomplexity and variability of transpressionaldeformation in theSBMs.

averagedip orientationfor eachfault (35øS,NFTS; 35øN, independentlyor in concertto uplift the plateauand to SAT), respectivemaximum throws are 2.9 (NFTS) and determine whether the NFTS has slowed in the late 1.9 km (SAT). The averagethrow along each fault is Quaternary. lower (-1.7 km, NFTS; -1.0 km, SAT). The horizontal components of these thrust motions make up a 5.2. Architectureof Transpression-RelatedStructures considerableportion of the total shorteningin the SBMs, in the SBMs which cumulativelyrepresents 5% of the motion between To synthesizeour interpretationsfor the structural the Pacific and North AmericanPlates over the pastfew geometryof theSBMs, we havesketched a simple block million years [DeMets, 1995]. diagram in Figure 13. The NFTS is shown as The largerof the two thrustfaults (NFTS) thusdisplays undercuttingthe SAT andnearly meets the SanAndreas a significantlygreater magnitude of slip. Interestingly,the differencein slip along each thrust fault seemsto scaleto faulton the south. On the east the NFTS has a convexup fault length (70 km, NFTS; 50 km, SAT), in that both shape,as it steepensto accommodate a greater component of verticalmotion beneath Onyx Mountain. In themiddle displaymaximum slip-to-length ratios of-4xl 0-2. Thisis the NFTS is shownwith a complexshape, flattening a typical long-term value for large faults [Cowie and beneaththe low SantaAna Valley andsteepening to Scholz, 1992]. The greatermagnitude of slip along the explainthe high San Gorgonioblock. On the westthe NFTS is consistentwith the idea that it hasplayed a more NFTSdisplays a moresimple, listric shape. Although significantrole in raisingthe Big Bear plateau. Given that littledata constrain the orientationof the SAT, its small the exacthistory of eitherfault remainsuncertain, however, size indicatesthat it is a minor backthrustto the NFTS. it is difficult to fully assesstheir relationship. The SAT Togetherthese thrust faults adequately account for the may have been a secondaryback thrust to the NFTS if upliftof theBig Bear plateau, although whether they were coeval or simply a smaller, preexisting structure. The coactiveis notknown. Imprinted across the entire plateau SAT has likely been inactive throughout much of the is a diffusezone of dextralshear, whose mechanical role is Quaternary [Sadlet, 1993], but the NFTS also appearsto notwell understood.This system may postdate uplift, have deceleratedin the last few hundred thousandyears. althoughexisting fault history data and a lack of The maximumvertical displacementon the NFTS implies crosscuttingrelationships suggest the deformational an averageuplift rate of 0.83-0.55 mm/yr (net slip rate is systemsmay be coactive[Spotila, 1999]. 1.45-0.97 mm/yr) over the past 2-3 Myr [May and It issignificant that the NFTS can explain rock uplift in Repenning,1982], whereaslate Pleistocenescarps indicate theSan Gorgonio block. Minor high-angle faults add -0.5 slower rates of 0.05-0.30 mm/yr [Meisling and Weldon, km of upliftto thisblock relative to SantaAna Valley 1989; Bryant, 1986]. Tighter constrainton fault timing is (Figure9b), but no otherstructures are required. On the needed to test whether the opposing thrust faults acted basisof thermochronometricisochron shapes [Spotila et SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 611 al., 1998], this block is a-30-km-wide antiform that been coactive illustrates an interesting dilemma of how plungesgently northward. If the NFTS has uplifted this intersectingfault systemsof different kinematic behavior block, it must be confined to this narrow east-west extent. can work togetherto accommodatecrustal deformation. We have thus drawn a localizedthrust patch that extends The structural array required to accommodate farther souththan the easternor westernsegments (Figure convergencein the SBMs is thus an intricate pattem of 13). This allows the NFTS to connect directly to the deformation that resembles several general notions of small convergentstepover at San GorgonioPass. It is in transpressive systems. Both high- and low-angle this vicinity that the greatestdeformation has occurred, structuresappear to have worked in concertto transform includingthe uplift of crustalslices along the high-angle plate motion into orogeny. This three-dimensionalpattern strandsof the SanAndreas fault zone (Figures7 and 9). of faulting shouldimprove modelsof seismogenicsources This synthesis bears similarities to two general in the southernCalifornia region. Given that we now have conceptionsof transpressivedeformation. Given that the a better idea of the geometry and kinematics of these NFTS appearsto undercutthe SAT and extendsouthward structures,we can also addressthe mechanismresponsible to the San Andreas fault, a high-angletranspressive zone for transpressionin the SBMs. (i.e., flower structure[Sylvester, 1988; Sadler, 1982]) probablydoes not exist beneaththe plateau. The core of 5.3. Source of Convergence in the SBMs the SBMs may thus be likened to a fold and thrust belt that internallydeforms the crustadjoining a strike-slipfault A distinct aspectof deformationin the SBMs is the zone (e.g., the westernTransverse Ranges [Namson and narrownessof the region that has experiencedsignificant Davis, 1988]). The more gently dipping sectionsof the horizontal shortening. The east to west distribution of NFTS may alsobe likenedto a subhorizontaldetachment convergencehas a roughparabolic shape only a few tensof [Corbett, 1984; Liet al., 1992], which are predictedto kilometerswide (Figure 7). This convergenceis the only decoupleupper and lower crust along transpressive margins significantzone of transpressionaldeformation anywhere [Beaudoin,1994; Lemiskiand Brown, 1988]. However, east of the San Andreas fault in southernCalifornia, and its our interpretationof NFTS shapeis morecomplex. The peak intensitylies just northwest of a localizedrestraining easternportion of the fault appearsto steepen,rather than bend in the transform fault. This spatial coincidence fiatten, and the fault's complex shape is much more suggestsa possiblecausal link between transpressional orogenyand strike-slipfault geometry. irregularthan a uniformly shallow dipping decollement. Furthermore,the extremerock uplift within the high-angle Othershave suggestedthat the San GorgonioPass left strandsof the San Andreasfault zone on the southimplies stephas been responsible for convergentuplift of the SBMs that a portionof the transpressivesystem does resemble the [Dibblee, 1975;Matti and Morton, 1993;Humphreys and classic "flower structure" shape [Spotila et al., 1998] Weldon,1994]. Our analysisnow permitsa quantitative (Figure 9b). Like the Spitsbergentranspressive orogen comparisonbetween convergent uplift and horizontal [Lowell, 1972] or othermountain systems with significant motion in the stepover. If the 15-km-wide left step is a true gap in right-lateral faulting, it shouldaccumulate an strike-slipmotion [ Vauchezand Nicolas, 1991;Schwartz et al., 1990; Lettis and Hanson, 1991], the San Andreas excessvolume of rock due to the 25 mm/yr fault-parallel motion on the northwestand southeast[ Weldonand Sieh, fault zoneappears to be a high-anglezone of partitioned lateraland vertical strainand is itself responsiblefor large 1985;Sieh, 1986]. If this geometryhas been stable for the last 2 Myr, the excessvolume of crust should have been magnitudesof rock uplift. An added complication to the architecture of -11,250km 3 (assuminga 15-km-thick brittle crust; Figure deformationis the presenceof high-anglestrike-slip faults 14a). This value representsthe maximum volume of rock within the upperplate of the NFTS (Figure9). Faultsof that convergence in San Gorgonio Pass could have the easternCalifornia shearzone that display significant providedfor crustalthickening in the lifetime of the SBMs. dextral motion and have been active in the late Pleistocene Our estimateof the volume of rock uplift in the SBMs is arepresent both north of andwithin the plateau(Figure 2) -4400km 3. If theanomalously deep seismicity below the [Spotila,1999]. Becausecrosscutting relationships along SBMs representsdepression of the brittle plastictransition fault traces have not been demonstrated, however, it is due to forcedrock subsidence,-1300km 3 of crustal unknown how these relate to the NFTS [Sadler, 1982; thickening must be added to the rock uplift (based on Miller, 1987]. The trace of the NFTS bendssouthward seismicityof Magistraleand Sanders[1996] andassuming where it meetsthe Helendale-Blackhawkfaults, implying a a 15-km-deep presubsidencebrittle-plastic transition). possibleright-lateral offset alongthe strike-slipsystem This simple volume comparison implies that crustal (Figure 2). The Helendalefault appearsoffset by the thickeningin theSBMs required addition of•5700 km 3 of NFTS itself,however, as it stepsright acrossa grabenand rock, or roughly half of what the left step in the San connectsto the Pipes Canyon fault within the plateau. Andreasfault couldhave provided. The forcefuladdition of Northward motion of the hanging wall block along the a large volume of rock in the stepovermay thus have thus NFTS could have offset the trace of a once-continuous been responsiblefor the convergentrise and northward dextral fault between upper and lower plates. The movement of the SBMs. mutuallydisrupted traces of the strike-slipand thrust faults For the entire SBMs to have grown due to crustal could indicate that they have interactedsynchronously thickeningspawned by the San GorgonioPass stepover, duringuplift. The possibilitythat these fault systems have however, their present-dayspatial coincidencemust have 612 SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

Ae SanGorgonio Pass Stepover After2 Myr: -,o%%,•,• • 11250km3

Be Before the ECSZ, After severalMyr of activity,

Carter et al., 1987: Humphreysand Weldon, 1994:

localizedconvergence localized convergence

Figure 14. Plots illustratingthat the SBMs may have been built in responseto convergenceat the restrainingbend at SanGorgonio Pass. (a) Schematicdiagram of the 15-km-wideleft stepin the San Andreasfault zoneat SanGorgonio Pass. If thereis zero lateralmotion across this stepover,the 25 mm/yrrate along the San Andreas fault would create an excess crustal volume of 11,250km 3 in2 Myr. This is abouttwice as largeas the volumeof rock upliftedin the SBMs. (b) Two kinematicmodels of the SanAndreas fault, eastern California shear zone, and easternmost Transverse Ranges showing that a left step shoulddevelop in the San Andreasfault at San GorgonioPass due to the transrotationof blocksto the eastand left slip alongthe PintoMountain fault [afterCarter et al., 1987;Humphreys and Weldon,1994]. Althoughmagnitudes of slip and rotationto the eastdiffer betweenthe models,both imply that the left step in the San Andreasfault would be stationaryrelative to the deformational domainsto the east,including the SBMs. existedfor the pastseveral million years. This wouldhave The SanGorgonio Pass restraining bend presently lies beenpossible only if the restrainingbend has been fixed at the junction of two distinct structuraldomains on the with respectto Mojave plate eastof the SanAndreas fault. NorthAmerican side of the SanAndreas fault (Figure 1). If the stepoverhad insteadbeen fixed with respectto the To the northeast,the diffuse,northwest-trending eastern PeninsularRanges, the thickenedcrust of the SBMs would Californiashear zone has accumulated-65 km of right-slip havebeen steadily shuffled off towardthe southeast.In this sincethe middleMiocene [Dokka and Travis,1990]. This case,we would expectthe SBMs to appearmore like the zoneis a structuralconduit for transferringslip from the steady-statetopography of the Santa Cruz Mountains. SanAndreas fault to the Basinand Rangeand presently This northernCalifornia range is highestand youngest at accommodates-12 mm/yr dextral shear [Sauberet al., the smallrestraining bend in the SanAndreas fault whereit 1994]. To the southeast,the west-trendingfaults of the is forming but becomesolder and decreasesin elevation easternmostTransverse Ranges have accrued-50 km of (due to erosion)in the directionof strike-slipmotion sinistraldisplacement in the past10 Myr [Powell,1981]. [Anderson, 1990]. We must thus ask whether there are By rotating-41 o clockwisethis system of left-lateralfaults any geologic featuresto the east of the San Andreas fault has accommodated52-100 km of dextral shear,a value that may have controlledthe locationof its left bend at San similarto the platemotion accommodated by the eastern GorgonioPass. Californiashear zone to the north [Carter et al., 1987; SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING 613

Dickinson, 1996]. At the boundarybetween the eastern and rotation within structural domains to the east. These Californiashear zone and easternmostTransverse Ranges domainsaccommodate plate motion and help the North lies the Pinto Mountain fault. This structure has a America plate negotiatethe Big Bend in the San Andreas maximumof 17 km of left slip and covnectsdirectly to the fault (Figure 1), and thus the SBMs may ultimately be the San Andreas fault in the vicinity of San Gorgonio Pass product of obliquity between plate motion and the [Dibblee, 1992]. The separationof the San Andreasfault transform plate boundary. Alternative explanationsfor in the stepovermatches the magnitudeand senseof motion convergence in the SBMs, such as upper crustal along the Pinto Mountain fault, which implies that the convergencedue to a negativelybuoyant slab in the upper restrainingbend grew as crust to the east was displaced. mantle [Humphreysand Hager, 1990], cannotbe ruled out Matti and Morton [1993] similarly proposedthat the left but seem unlikely given the symmetry of deformation bend grew while the Mission Creek fault was the active about San GorgonioPass. strandof the San Andreasfault zone, from roughly 3.5 to 0.5 Myr ago. The San GorgonioPass stepover may thus 5.4. Implications for Seismic Hazards have remained fixed with respectto the North American The NFTS is a large structure with significant side of the transform, consistentwith the idea that the left displacement,which hasplayed a primaryrole in uplifting bend was causally linked to the structural boundary the SBMs over the past few million years. Becauseof its between the transrotating domains to the northeast and size and distance from the San Andreas fault it should be southeast. consideredan independentstructural component of the Two kinematicmodels of block rotationand strike-slip transpressive deformation system [Lettis and Hanson, faulting in the eastern California shear zone and 1991]. It thus representsa significant potential seismic easternmostTransverse Ranges predict that a deflectionin hazard to the growing populationof southernCalifornia's the San Andreas fault should have developed near San Inland Empire. Given that the 80-km-long thrust fault Gorgonio Pass over the last few million years. In one appears to have uplifted the entire Big Bear and San model [Carter et al., 1987] clockwise rotation and left- Gorgonioblocks, its down dip width is likely -30 km lateral faultingto the southeastand slightcounterclockwise (Figure 9). Its dimensionstherefore imply a maximum- rotation and dextral faulting in the Mojave createa slight sized event of momentmagnitude Mw = 7.5 (assuming-3 left deflection in the San Andreas fault that is fixed to the m throw per event [Wells and Coppersmith, 1994] and boundarybetween the southernand northerntransrotating domains(Figure 14b). A secondmodel [Humphreysand rigidityequal to 3x10ll dyn/cm2).This is as large as the Weldon,1994], which usesa differentinterpretation of the major thrust events feared in metropolitanLos Angeles rotation pattern to the northeast [cf. Dickinson, 1996], [Dolan et al., 1995]. Such an event would also be quite similarlyresults in a left deflectionof the SanAndreas fault damaging,based on the significantdamage associated with at San Gorgonio Pass. Both of these kinematic models the smaller(M6.5) Big Bear earthquake(1992). However, illustrate how the small left-bend in the San Andreas fault it is also possible that the NFTS fails only in small could have evolved at the western end of the boundary segmentsor is eveninactive because of the degreeto which between two distinct structural domains to the northeast it is intersectedby northwesttrending strands of the eastern and southeast. In detail, the interaction of the San Andreas California shear zone [Sadler, 1982]. These possibilities fault and these structural systemshas likely been more indicatethe need for paleoseismicstudies along the NFTS complex and may have resultedin slight migrationof the to quantifyits futurerupture capability. restraining bend with respect to North America [Humphreys and Weldon, 1994; Johnson et al., 1994; 6. Conclusions Dokka and Travis, 1990]. To the first order, however, a mechanismof fixing the bend with respectto the SBMs The weatheredsurface atop the SBMs is a deformed seems to exist. structuraldatum that is useful for understandingthe Becauseof their apparentlong-term spatial coincidence, architectureand kinematics of transpressiveuplift. On the we proposethat growth of the SBMs has been directly basisof thevertical displacements measured along traces of associatedwith the left-stepin the SanAndreas fault at San the opposingthrusts of the Big Bear plateauthe NFTS is GorgonioPass. Our volume comparisonimplies that the the principlestructure and the SAT is a minor backthrust crustalconvergence in the stepovercould have beenmore withinits hangingwall block. Togetherthese thrusts have than twice that neededfor crustalthickening in the SBMs. accommodated-4 km of north-south horizontal The excessconvergence was likely avoidedby lateral convergence,a minor componentof total Pacific-North transferof rock throughthe restrainingbend (i.e., at half of Americamotion in the past few million years. On the the San Andreasfault's rate). If the SBMs have beenbuilt basisof analysisof the shapeof the overlyingweathered owing to convergenceat San GorgonioPass, then their surfacethe NFTS hasa complexgeometry at depth. The transpressiveorogenesis has been the result of a local fault partly flattens into a decollement on the west, geometriccomplexity along a strike-slipfault. They may steepensto a convexup shapeon the east,and is contorted thus be distinct from the crustal thickening within the beneaththe centralpart of the range. Boththe Big Bear >200-km-wide westernTransverse Ranges, which may be plateauand San Gorgonio block appear to haverisen along a more direct functionof obliqueplate motion [Dickinson, the NFTS, while obliquereverse motion along strands of 1996; Namsonand Davis, 1988]. However, the left stepat the San Andreasfault zone is associatedwith uplift for San GorgonioPass may itself be the result of strike-slip crustalslivers to the south. The total horizontalplate 614 SPOTILA AND SIEH: ARCHITECTURE OF TRANSPRESSIONAL THRUST FAULTING

motion accommodated by uplift of these blocks is Acknowledgments. We thank Egill Haukssonfor help constrainedby balancingthe volume of rock uplift that is with seismicity data, Shawn Biehler for help modeling evident in the weatheredsurface and thermochronometry. Bouguer gravity anomalies, Miles McKenney for sharing Maximum convergence(6.3 km) occursin the centralpart boreholedata, and Doug Yule, Brian Wernicke,Ken Farley, of the range just northwest of San Gorgonio Pass. Rob Clayton, Leon Silver, and Slawek Tulaczyk for ideasand commentsthroughout the courseof this study. We alsothank Horizontal motion decreasesrapidly to eastand west, thus Peter Sadler and John Crowell for valuable reviews. This defining a very localized deformational pattern that is researchwas supportedby the SouthernCalifornia Earthquake closely associatedwith the restraining bend in the San Center (contribution 482), which is funded by NSF Andreasfault zone. This suggeststhat transpressiveuplift Cooperative Agreement EAR-8920136 and USGS of the SBMs has been producedby a localized geometric Cooperative Agreements 14-08-0001-A0899 and 1434-HQ- perturbationin the strike-slip system, which itself may 97AG01718. Contribution 8593 of the Seismological have resulted from the transrotation of blocks within shear Laboratoryat the CaliforniaInstitute of Technology. zones to the east.

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