JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. B10, PAGES 22,513-22,527, OCTOBER 10, 1997
Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution RichardJ. Blakely,Robert L. Christiansen, Marianne Guffanti,Ray E. Wells,Julie M. Donnelly-Nolan,L. J. PatrickMuffler, MichaelA. Clynne,and JamesG. Smith U.S. GeologicalSurvey,Menlo Park, California
Abstract. Isostaticresidualgravityanomaliesin the southernCascadeRange of northern Californiaand southernOregonare spatiallycorrelatedwith broad zonesof Quaternary magmatismas reflectedby the total volume of Quaternaryvolcanicproducts,the distributionof Quaternaryvents,and the anomalouslylow teleseismicP wave velocitiesin the upper 30 km of crust.The orientationof Quaternaryfaults alsoappearsto be related to gravityanomaliesand volcanismin this area, trendinggenerallynorth-southwithin the magmaticregionsand northwest-southeast as they enter the neighboringamagmaticzones to the north and south.The relationshipbetweengravityanomalies,vent density,and fault orientationsmay indicatein a broad sensethe strengthof the middle and upper crust.The southernCascadeRange occupiesa transitionzone where horizontalstressis transferred from the northwest-southeast
dextral shear of the Walker
Lane belt to the east-west
extensioncharacteristicof the Cascadearc in central Oregon. Faulting alongnorth-south strikesin the volcanicallyactiveareasindicatesthe east-westextensionalstressesin thermallyweakenedcrust,whereasnorthwestfaultingbetweenthe volcanicallyactiveareas reflectsthe northwesttrending,right lateral shearstrain of the Walker Lane belt. The segmentation of the arc reflectedin Quaternarymagmatismmay be causedby differential extensionbehind crustalblocksof the forearcrotatingclockwisewith respectto North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and
LassenPeak in northern California are situatedalongthe southernparts of the trailing edgesof two distinctsegmentsof the forearcwhere additionalextensionis implied by their differential
clockwise rotation.
where a well-definedgravityanomalyover the volcaniccenters at Mount Shastaand Medicine Lake volcanois distinctlyisoThe southern Cascade Range in southern Oregon and lated from a similaranomalyover the Lassenvolcaniccenterto northernCaliforniaoccupiesa tectonicallycomplexpart of the the south and from gravity anomaliesto the north over the PacificNorthwest.Although clearlypart of the activevolcanic Oregon High Cascades.In this paper we examinethis part of arc associated with subduction of the Juan de Fuca and Gorda the Cascadearc, focusingon the spatialrelationshipbetween platesbeneathNorth America, the southernCascadeRange is gravity anomaliesand the distributionsof Quaternary vents, alsoinfluencedby interactionswith the Basinand Rangeprov- Quaternaryfaults, crustalearthquakes,seismicvelocitystrucince,the Mendocinotriple junction,and the SanAndreasfault ture, and other crustal manifestations of stress,tectonism, and system(Figure 1). Although the Juan de Fuca and Gorda geothermalconditions.The spatial correlationsamong these platesare convergingwith North America at an obliqueangle, various observationsreflect dynamic properties of the crust the maximumhorizontalstressbehindthe arc is orientedgen- and upper mantle and provide cluesconcerningthe Tertiary erally north-south[Zoback,1992].Zobackand Zoback [1985] and Quaternaryevolutionof the Cascadearc within the comsuggestedthat transform motion between the Pacific and plexlyevolvingwesternmarginof the North Americanplate. North American plates along the San Andreas fault system, rather than convergence betweenthe Juande Fuca and North Americanplates,is the dominantfactor in controllingregional 2. Data and Observations stressthroughouta broad region of the Pacific Northwest, a 2.1. Gravity Anomalies regionwhich includesthe southernCascadeRange. The southernpart of the Cascadevolcanicarc lies within a Residualgravityanomaliesare commonover volcanoesand broad, north-south depressionin isostatic residual gravity volcaniccenters,reflectingthe relativelylow densityof volcanic [Simpson et al., 1986] extendingmore than 600 km from north products,the geothermalconditionsof the crust,and the strucof the Columbia River to southof LassenPeak (Plate 1). The tural and magmaticunderpinningsof the volcanism.This reabrupt western margin of this north-south gravity trough is lationshipis particularlywell displayedin northern California causedprincipallyby lithologic contrastsbetween arc-related This paper is not subjectto U.S. copyright.Publishedin 1997by the rocksof the CascadeRange and forearc-accretedterranesof American GeophysicalUnion. the Oregon CoastRange and Klamath Mountains [Couchet Paper number 97JB01516. al., 1982;Blakely,1994]. The easternmargin of the trough is 1.
Introduction
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center is at the southwesternend of a broad, linear gravity depression,referred to here as the Lassengravitylineament, extendingnortheastward300 km into northwesternNevada and southeasternOregon (LGL on Plate 1 and subsequent figures).Similarly,the Mount Shasta-MedicineLake gravity low lies at the southwesternend of another linear gravitydepression,referred to as the Shastagravitylineament(Plate 1, SGL), lesswell definedthan the Lassengravitylineamentbut extendingnortheastwarda similar distance.A third gravity lineament,referred to here as the McLoughlingravitylineament, extends northeastwardfrom Mount McLoughlin in southernOregon (Plate 1, MGL). Trends other than those shownin Plate 1 can be found, to be
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Figure 1. Regional tectonicsettingof the southernCascade Range.Darkly shadedpatternrepresentsQuaternaryrocksof the CascadeRange, and the lined pattern is the Walker Lane belt. Line-circlesymbolsindicateorientationof maximumhorizontal stress[Zoback,1992]in the arc and backarcof Oregon and Washingtonand in the Walker Lane (only quality "A" determinationsare shown;seeZoback [1992] for an explanation). Arrow indicatesdirectionof convergence betweenthe Juan de Fuca and North Americanplates.Dashedbox shows the area of Figures3, 4, 6-8, 10, and 11.
lesswell definedgravitationally,reflectinga complexboundary between the arc and backarc regions.As discussedsubsequently,the complexityin gravityanomaliesresultsboth from the configurationof the base of Tertiary volcanicrocks and from structuresin the underlyingbasement. The north-southgravitytrough,althoughrelativelycontinuous from the ColumbiaRiver to LassenPeak, is pockmarked with severallocal gravitylowsspatiallyassociatedwith major volcaniccenters.The bestexamplesoccurin the southernmost part of the Cascadearc, where a broad gravitylow enclosing Mount Shastaand the Medicine Lake volcano is separated spatiallyfrom a gravitylow enclosingthe Lassenvolcaniccenter (Plate 1) [Pakiser,1964;LaFehr, 1965].LaFehr [1965]concludedthat the gravitylowsoverthe volcaniccentersat Mount Shasta and Lassen Peak are causedprimarily by lithologic variationsin the upper 10 km of crust. In the following discussionwe will make a distinctionbetween the roughly equidimensionalgravity lows discussed above and the gravity lineaments(labeled MGL, SGL, and LGL on the figures)that extendnortheastward from the Cascades.For example,the gravitylow over the Lassenvolcanic
sure.The roughlynorth-southpattern of normal fault blocks related to basin-rangeextensionproducesnumerousgravity anomalies.These and other local structuresstronglymodify the northeaststrikingpattern throughoutthe backarcregion, but the northeast pattern remains evident in the broader anomalies.Figure 2 showstwo mapsderivedfrom the gravity data of Plate 1. Figure 2a showsthe locationsof maximum horizontalgradientsof the gravityfield with trendsbetween0ø and 90øE,calculatedusingthe method of Blakelyand Simpson [1986]. Maxima in the horizontal gradient tend to indicate abruptdensitycontrastsin the middleand uppercrust.These calculationswere usedalongwith the isostaticresidualgravity anomaliesand other data discussedsubsequently to interpret the boundariessurroundingthe gravity lows and lineations shownin Plate 1 and discussedsubsequently. Figure 2b showsthe gravityanomaliesof Plate 1 analytically continuedupward10 km in order to emphasizedeeperpartsof the crust [Blakelyand Jachens,1990]. The Lassengravitylineamentand,to a lesserextent,the other gravitylineamentsare reflected in the upward continuedanomalies,implying that they originatefrom the upper and middle crust. The Shastaand Lassengravitylineaments(Plates 1 and 2) are separatedby a northeast-trending chain of gravityhighs, which Chapmanand Bishop[1968] and Griscom[1980b] suggestedcouldbe causedby structureswithin the basementunderlyingthe Modoc Plateau.The westernmostanomalyof this chain(Plate 1, T1) lies over a salientof the EasternKlamath belt of pre-Tertiaryrocks[Irwin,1966]exposedsouthof Mount Shastaand northwestof LassenPeak. Griscom[1980a] concludedthat this gravityhigh is causedby mafic and ultramafic rocksof the Trinity ophiolitecomplex,an early Paleozoicassemblagewithin the Eastern Klamath belt. The Trinity complex is also exposedwest and southwestof Mount Shasta, where it producesa pronouncedgravity anomaly [LaFehr, 1965; Griscom, 1977, 1980a] (Plate 1, T2). Aeromagnetic [Blakelyet al., 1985] and seismicrefraction [Fuis et al., 1987; Zucca et al., 1986] data suggestthat the Trinity complexextendseastwardwith shallowdip and may lie at a depth of km beneath
Mount
Shasta.
The Lassen,Shasta,and McLoughlingravitylineamentsare part of a northeasttrending "fabric" of gravity anomalies throughoutcentral and easternOregon, northeasternCalifornia, and northwestern Nevada, which we refer to here as the
OregonPlateausgravityprovince.This regionis boundedon the southby the northernlimit of pre-Tertiaryexposures in the Basinand RangeprovinceandnorthernSierraNevadaand on the north by pre-Tertiary exposuresof the Blue Mountains uplift. The most notable gravity anomalyof the region lies alonga northeasttrendingline from the KlamathMountainsin southwesternOregon to the Blue Mountainsin northeastern
BLAKELY ETAL.:SOUTHERN CASCADES GRAVITY,VENTS,ANDFAULTS
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10s years)Quaternary volcanism in vent datado not includethevolumeof rockeruptedfromeach northeastern Californiais distributedamongthreemajorareas with distinctive volcanicstylesandventpatterns.Mount Shasta is a largeandesiticto daciticstratovolcano builtwithinthe past 600,000 years [Christiansen,1985; Christiansenand Miller, 1989].It consistsof at leastfive overlappingconeswith satellitic flank vents.A north-southtrend of vent alignmentsat
vent, and thusvolcanicproductivityat largepolygenetic centers is underrepresented. The segmentation of the southernCascadearc discussed by Guffantiand Weaver[1988]is reflectedclearlyin Plate2. Sig-
nificantvolcanicactivityduringthe past7 m.y.is evidentin both the Mount Shasta-Medicine Lake segmentand the Las-
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Figure 4. Quaternaryfaultsof northern California and southernOregon. Faultsin California are from the statewidecompilationof Jennings[1994] supplementedby more recent geologicmapping(Muffler et al., unpublished mapping,1996);faultsin Oregonare from the Cascadegeologicmapof ShetrodandSmith[1989], supplemented by additionalinformationfrom D. R. Sherrod(writtencommunication, 1995)and C. R. Bacon (written communication,1995). Faults are not shownin Nevada and in easternOregon east of longitude 121øW.G is the Gillem fault;T is the fault alongthe eastmarginof Tule Lake graben;M is the Mayfieldfault; Mc is the McArthur fault zone;P is the Pittvillefault; H is the Hat Creek fault; and A is the Almanor fault. Outlined patternsindicate gravity lows at Mount McLoughlin, Mount Shasta-MedicineLake, and Lassen Peak. Light patternswithout outline representgravitylineamentsdiscussed in text: M GL, McLoughlingravity lineament;SGL, Shastagravitylineament;and LGL, Lassengravitylineament.Starsshowlocationsof major volcanoesas named(seecaptionsto Plate I and 2).
sensegment,but an interveninggap, about 35 km acrossin the north-southdirection,is nearly devoidof vents of theseages. The primary exceptionis an isolatedclusterof 1ow-Kbasaltic ventssituatedalong a Holocenefault systemthat extendsbetweenthe two volcanicregions,discussed subsequently. During
ping, 1995) (age data from Chesterman and Saucedo[1984]). All of these older volcanic centers fall within either the Mount
Shasta-MedicineLake or Lassengravitylows,exceptfor Yana that lies about7 km outsidethe Lassengravitylineament.It is of interest to note that like the youngervolcaniccentersat
the Quaternarythe MountShasta-Medicine Lakesegment has Mount Shasta, Medicine Lake, and Lassen Peak most of these oldercenters(Yana, Maidu, and SnowMountain)are situated
been isolatedfrom the Cascadearc to the north as well; very few ventsyoungerthan 2 Ma occurbetweenthe Mount ShastaMedicineLake segmentandMount McLoughlin(Plate2). The volcanicgapsnorth and south of the Mount Shasta-Medicine Lake segmentalsoare evidentin estimatesof the total volume of Quaternaryvolcanicrockseruptedin the area [Sherrodand Smith, 1990]. Plate 2 alsoshowsthe knownolder major volcaniccentersof
verynear the edgesof the roughlyequidimensional gravitylows. 2.4.
Quaternary Faults
A band of late Quaternary,extension-related normal faults stretchesfrom south of the Lassen Peak region to north of Medicine Lake volcano and structurallyconnectsthe major volcanicareasalongthe westernedgeof this part of the Basin the Mount Shasta-Medicine Lake and Lassen areas: Yana and Rangeprovince.Figure 4 showstheseQuaternaryfaultsin (3.1-2.7 Ma), Maidu (2.4-1.2 Ma) [Muffleret al., 1989],Ditt- and adjacentto the southernCascadeRange and their spatial mar (2.1-1.4 Ma) [Clynne,1984,1985],SnowMountain (2.7- relationshipto the gravity lows at Mount Shasta-Medicine 1.1 Ma) (J. G. Smith,unpublisheddata, 1995), and Rainbow Lake and LassenPeak. The faults in Californiawere digitized Mountain(about1 Ma) (R. L. Christiansen, unpublished map- from a statewidecompilationbyJennings[1994]supplemented
22,520
BLAKELY ET AL.: SOUTHERN CASCADES GRAVITY,
by more recentgeologicmappingin the Lassenarea (L. J.P. Muffler et al., unpublishedmapping,1996); faultsin Oregon were digitizedfrom the Cascadegeologiccompilationof Shetrod and Smith [1989], supplementedby informationprovided by D. R. Sherrod (written communication,1995) and C. R. Bacon(writtencommunication, 1995).(Faultsof similaragein the Basinand Rangeprovincefarther eastare not shown.) The orientationof Quaternaryfaultsin and adjacentto the southernCascadeRange appearsspatiallyrelated to gravity anomalies and to the density of Quaternary vents. Faults within the Mount Shasta-MedicineLake gravitylow are dominated by nearly north-southtrends (Figure 4). The Gillem fault (Figure 4, G), for example,projectsinto MedicineLake
VENTS, AND FAULTS
McArthur, Pittville, Mayfield, and possiblyother faults, extends a distance of over 200 km from south of Lassen Peak to
north of Mount McLoughlin. This regionallyextensivefault zone,whichwe informallyrefer to here as the Fall River fault zone [Guffantiet al., 1994], is part of a larger regionalstructural alignmenttermedthe Tahoe-MedicineLake trough[Page et al., 1993],a 400 km longseriesof right-stepping en echelon tectonicdepressions that form the easternedge of the Sierra Nevadaprovinceand southernCascadeRange. Because various faults discussed above have Holocene
and
youngerdisplacements[Wills, 1991;Muffler et al., 1994], we might expect to see a spatial relationshipbetween gravity anomaliesand the trends of earthquakealignmentsor trends volcano from the north with a north-south trend a distance of of earthquakefirst motions.A dramaticexampleoccurredin about 22 km [Donnelly-Nolanand Champion,1987]. From its 1993about25 km northwestof KlamathFalls,Oregon(Figure exposedsouthernend the Gillem fault is inferredon the basis 4), where three earthquakesof magnitude5.4-6.0 and afterof vent alignmentsto continuesouthwardanother35 km with shocksup to magnitude4.8 occurredat the southernboundary NNW-SSE trend [Donnelly-Nolan,1983]. At Mount Shastaa of the McLoughlin gravity lineament. This series of earthnorth-southalignmentof Quaternaryvents(Plate 2) probably quakeswill be discussed in more detail in the next section. representsnorth-southextensionalfractures.The north strikingfault alongthe eastmarginof the Tule Lakegraben(Figure 3. Discussion 4, T) lies east of the main gravitylow and appearsto be the eastern extent of extensivenorth-south faulting within the Isostaticresidual gravity anomaliesare causedby density Shastagravitylineament. variationsin the middleanduppercrust[Simpson etal., 1986], In contrastto the nearly north-southalignmentof faults and the spatial correlationsbetween isostaticanomalies,the within the Mount Shasta-MedicineLake gravitylow, Quater- distributionof Quaternaryvents, and the orientationof Quanary faultsto both the north and southhave,generally,more ternary faults in the southernCascadeRange likely reflect a northwesterlytrends. To the north of the gravity low, faults history of lateral variationsin physicalpropertiesand magtrend northwestward, the bend occurringapproximatelyat the matic conditions in the crust, now manifested as threenorthern edge of the gravity anomaly,and maintain this ori- dimensionalvariationsin density.The roughly equidimenentationto the southernedge of the McLoughlingravitylin- sional gravity anomalies of about 50-80 km diameter over Mount Shasta-Medicine Lake and over Lassen volcanic center eament,where they changeback to a more northerlystrike. The relationship between fault orientation and gravity may point, for example,to structuresin the pre-Tertiarybaseanomalies is less clear south of Mount Shasta. Here faults ment beneaththis part of the Cascadearc that relate directly typicallyhave a northwesterlyorientation,the changein ori- to the distributionof magmatism.Thisviewis supportedby the entation occurringapproximatelyalong the line of steepest spatiallink betweentheseroughlyequidimensionalanomalies gravity gradient at the south edge of the Mount Shasta- and the northeasterlytrend of the McLoughlin,Shasta,and MedicineLake gravitylow.Specificfaultsin thisregioninclude Lassen gravity lineaments and to the general fabric of the the Mayfield fault, the Pittville fault, and the McArthur fault entire OregonPlateausgravityprovince.Someof thesenorthzone (Figure 4), all of which have experienceddisplacement east trendinganomaliesmay be associatedwith pre-Tertiary duringthe Holocene [Wills,1991].Farther to the south,in and structures,particularlythe Klamath Mountains-BlueMounadjacentto the northern part of the LassenPeak gravitylow, tains gravitylineament[Riddihoughet al., 1986] and perhaps the fault orientation is again north-south,an orientationwell the southernedge of the Lassengravitylineament[McKeeet displayedby the northernand centralpart of the HoloceneHat al., 1990] (Plate 1), and it seemslikely that other northeast Creekfaultzone(Figure4, H) [Wills,1991;Muffleretal., 1994]. trendinganomaliesin the OregonPlateausgravityprovinceare This north-south trend continues southward to latitude related to concealedstructuresin pre-Tertiary rocks. 40ø39'N, where the Hat Creek fault zone bends back to a The northeasttrending chain of gravity highsbetweenthe dominantlynorthwesterlystrike.Southof latitude40ø35'N,the Shastaand Lassengravitylineamentsmay be a specificexamfault zone (albeit obscure) trends N10øW, linking up at ple of a causalrelationshipbetweenbasementstructureand 40ø24'Nwith a prominentsetof northwesttrendingfaultsthat volcanism. Plate 2 illustrates a clear correlation between the bound the northeast and southwest shores of Lake Almanor densityof Quaternaryvolcanicvents and the broad, equidi(Figure 4, A). mensional gravity lows associatedwith the Mount ShastaThus Quaternaryfaultsin thisregionappearto bend asthey Medicine Lake and Lassen Peak areas. Vents less than 2 Ma passin and out of regionsof relativelylow gravityand highvent are predominantlyrestrictedto the two low-gravityregions, density.Figure5 showsa stylizedviewof the "zed"patternthat and the interveningvolcanicgap corresponds to the northeast we observe.In general terms, Quaternaryfaults strike north- trendingchainof positivegravityanomaliesthat at its western south in the low-gravity,magmatic regions and northwest- end, overliesa salient of the pre-Tertiary Eastern Klamath southeastin the interveningamagmaticregions.This relation- belt. The high gravitybetweenMount Shasta-MedicineLake shipis bestdisplayedfrom the Mount McLoughlingravitylow and Lassenvolcaniccentersprobably indicatesa concealed, southwardto the northernpart of the LassenPeak gravitylow. structurallyelevatedblock of pre-Tertiary crustthat extends The relationshipis less convincingfarther north and south. northeastwardfrom the Eastern Klamath belt. Perhapsthe Togetherthis systemof Quaternarynormal faults,whichshows strengthand densityof this high standingblock, distinctrelaHolocene displacement on the Almanor, Hat Creek, tive to crustto the north and south,have inhibitedin someway
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Figure 8. The southern CascadeRange as a transitionalzone where horizontal stressis transferredfrom northwest-southeast dextralshearof the Walker Lane to the southto dominantlyeast-westextensionof the CascadeRangeto the north. Strike slip and extensionalstrainare partitionedamongdomainsin the southern Cascadesthat are distinguishedby their gravityanomalies.See Figure 4 for an explanationof patternsand symbols.
gravity highs.These relationshipsare best representedat the southedgeof the Mount McLoughlingravitylow and the north edge of the Mount Shasta-MedicineLake gravity low. Similarly, the distributionof Quaternary Cascadevolcanicvents correlateswith the gravitylows;ventsare muchlessabundant on the northeasttrendinggravityhighsat aboutlatitudes41øN and 42øN.Thus the crustof the high-gravityregions,which has experiencedlittle magmatismduring the Quaternary,may be coolerand possess greaterelasticstrengththan the contrasting crustof the neighboringlow-gravityregionsthat displayabundant youngvolcanism. This lateral variation in crustal strengthis reflected in the distributionof earthquakeepicenters.Figure 6 showsearthquakesgreaterthan magnitude2 occurringin thisregionsince 1973. A belt of magnitude2-4 epicentersextendsnorthwestward from the southernedgeof the studyarea (Figure6, A) to LassenPeak and perhapsbeyond.A secondbelt may extend southwardfrom the Shastagravitylow (Figure 6, B), where it includesthe swarm of earthquakesthat occurrednear StephensPass,California,in 1978 (Figure 6, S), and continues southwardto about latitude 41øN,longitude122øW.Between thesetwo belts,epicentersare broadlydistributedand showno clear alignments.
The Klamath Falls earthquakesof September1993, however, are of particularinterest.Theseearthquakesoccurredat an inflectionpoint in the trend of normal faults boundingthe Klamath Falls graben (Figure 7). South of the epicentralregion,the grabentrendsaboutN40øWand maintainsthis trend to near the California-Oregonborder;to the north the graben trendsabout N15øW towardCrater Lake. This changein fault trend is mirrored in both the focal mechanismsof the largest earthquakes[Dregeret al., 1995]and in the pronouncedcurvature of the aftershockdistribution[Qamarand Meagher,1993]. This change in fault orientation coincideswith the southern marginof the McLoughlingravitylineament(Figure 7, dashed line), a gravity boundary previouslynoted by Blakely and Jachens[1990] as one of a seriesof northeasttrendinglineations crossingthe Cascadearc and northwesternBasin and Rangeprovincebetweenthe Klamath Mountainsand the Blue Mountainsof northeasternOregon [Blakelyand Jachens,1990, Plate 2, D]. This lineation intersectsthe Quaternaryarc just south of Mount McLoughlin, correspondingto one of the segmentation boundariesof Guffantiand Weaver[1988]and to the northern edge of a zone of negligiblevolcanic activity duringthe past2 m.y. (Plate 2).
BLAKELY
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Figure 9. Interpretationalmodel for the nonhomogeneous focusingof volcanismalong the southernCascade arc. Stripedpattern indicatesregionsof positivegravityanomaliesin the forearc interpretedto reflect coherentcrustalblocks.Open circlesare volcanoeswith ageslessthan 2 Ma. Solid arrowsindicateCenozoic
clockwise rotationof forearcsegments alongzonesof accommodation (dashed lines);thesezonesarenotleftlateral faults but rather areas of enhanced east-west extension. Solid sectors indicate 95% confidence limits on
rotationsdeterminedfrom paleomagnetic studies:WC1 and WC2 from Magill and Cox [1980];WK from Bec. k et al. [1986];and KM from Mankinenand Irwin [1982].
4.
Regional Interpretation The southernCascadeRange is apparentlyactingas a tran-
sition zone between the Walker continuous
volcanic
Lane to the south and the
arc of the Cascades to the north.
Maxi-
mum horizontal stressin both regionsis oriented generally north-south(Figure 1) [Zoback, 1992]. This regional stress apparentlyis accommodateddifferently in the two regions. The westernmarginof the Basinand Rangeprovinceincluding the Walker Lane, is characterizedby spatialor temporalalternationsbetweenright lateral strike slip and normal faulting controlled by variations in the magnitude of the maximum horizontal stress relative to other components of stress [Zobackand Zoback,1989;Zoback,1989]or by partitioningof strain between domains [Jonesand Wesnousky,1992; Wesnouskyand Jones,1994]. The Cascadearc in central Oregon, north of our study area, is dominatedby east-westextension [Priest,1990;WeaverandMichaelson,1985].Strain,apparently, is partitioned in the southernCascadeRange as it transfers one styleof accommodationto the other. This transitionoccurs discontinuously througha seriesof stepovers(Figure 8), which are reflected by the Mount McLoughlin, Mount Shasta-
Medicine Lake, and LassenPeak gravitylows and their correspondingQuaternary volcanismand north trending Quaternary faults.We suggestthat the locationof the stepoversand
thus the distributionof volcanismare controlled,in part, b.y preexistingstructureswithin the underlyingpre-Tertiarybasement, structuresmanifestedby the northeasttrending gravity lineamentsof the Oregon Plateausgravityprovince. The correlation between fault orientation and gravity boundaries is excellent at the south edge of the Mount McLoughlin gravity low and the north edge of the Mount Sha. sta-MedicineLake gravitylow and lesssoat the southedge of the Mount Shasta-MedicineLake gravity low and north edgeof the LassenPeakgravitylow (Figure4). The correlation breaks down entirely farther to the south, where northwest trendingfaultsdominatethe southernhalf of the Lassengravity low. The Walker Lane probablyexertsa greater influence on the southernpart of the transitionzone, and the lack of correlationbetweenfault orientationand gravityboundariesat and southof LassenPeak may be related simplyto its relative proximityto the Walker Lane. It is interestingto note that the northwesttrendingfaults in
22,526
BLAKELY
ET AL.: SOUTHERN
CASCADES GRAVITY,
the amagmaticregionsare roughlynormal to the marginsof the northeasttrending gravity lineamentswhich lie over the magmaticregions(Figure4). Thisis especially apparentin the regionbetweenthe McLoughlinand Shastagravitylineaments (Figures4 and 7). The marginof the gravitylineamentscould correspondto discontinuities in averagecrustalstrength,crust in the magmaticregionsbeingthermallyweakenedrelativeto crust in the intervening amagmaticregions. The northwest trend of faults in the strongercrustmay be a "minimum energy"configurationin whichfaultstend to alignnormal to the discontinuity. The north trendingfaults,on the otherhand,are respondingto east-westextensionof the continuousextensional arc farther
north.
If our analysisis correct, the Klamath Falls earthquakes occurredon the northern edge of a zone of relatively strong crustand propagatednorthwardinto relativelyweak, volcanicallyactivecrust(Figure 7). It is possiblethat earthquakesin this regionmay preferentiallyoccurin the northeasttrending gravity highs,while extensionalstrain in the gravity lows is taken up partly by magmaticintrusion,followingthe rationale of Bursikand Sieh [1989],Parsonsand Thompson[1992],and Dzurisinet al. [1991].Large-magnitude earthquakesare rare in this region, and the seismicrecord is inadequateto test this hypothesison a regional scale.We note, however,that the earthquakeswarmthat occurredat Warner Valley near Adel, Oregon,in 1968, alsowaslocatedwithin a northeasttrending gravityhigh(Figure6, W) [Blakely andJachens, 1990,Plate2, B]. A complex interplay between oblique subductionof the Gorda and Juan de Fuca plates, distributedextensionin the backarc,transformmotionalongthe SanAndreasfault system, and strike slip on the Walker Lane belt [Zobackand Zoback, 1989; Dilles and Gans, 1995] is the engine that drives arc segmentationand ultimatelyproducesinhomogeneitiesin the distribution of Quaternary volcanismand the orientation of crustal strain. Gravity anomaliesand other geophysicaldata indicatethat the forearc in central and northern Oregon is a relativelycontinuousblock,whereasthe forearcto the north in Washingtonand to the southin southernOregonand northern Californiais brokeninto segments(Figure 9). Paleomagnetic data (summarizedby Grommdet al. [1986]and Wells[1990]) indicatethat with respectto North America duringthe Cenozoic the central Oregon forearc block and, to a lesserextent, the southernforearcsegmentsin southernOregonand northern California have rotated clockwisewhile simultaneously undergoingnorthwestward transport(R. E. Wells et al., 1997). If that rotation is continuingtoday, simultaneousclockwise rotation and translationof the varioussegmentsof the forearc would imply nonhomogeneousextensionalong the trailing edges of the forearc blocks. Specifically,maximum translational velocitymust occurat the southernendsof eachforearc segmentto accommodate the clockwiserotation,and thisextra velocitymust be accommodatedby increasedextension,magmatism, and volcanismin the wake of each rotating segment (Figure9). Accordingto thisview,the Mount Shasta-Medicine Lake, and Lassenvolcaniccentersare locatedalongthe southern part of the trailing edgesof two rotatingforearcsegments. This model implies sinistralslip betweenthe forearc blocks; i.e., alongthe dashedlines on Figure 9. We are not aware of any evidenceto support zones of left lateral faults in these regions,and deformationis more likelyaccommodated by enhanced east-west
extension.
The scenariosillustratedin Figures8 and 9 are not necessarilycompetinginterpretations.Both strainpartitioningand
differential
VENTS, AND FAULTS
rotation
of forearc
blocks could contribute
to the
segmentedpattern of gravityanomalies,volcanism,and faulting in the southernCascadearc. Acknowledgments. We are grateful to Dave Sherrodfor adviceon Quaternary faulting in the Klamath Falls graben and elsewherein Oregon,to Charlie Baconfor providinginformationon vent and fault locationin the Crater Lake area, and to Mary Lou Zoback for providing data from the World StressMap Project. Reviewsby Bob Jachens,Wes Hildreth, JohnDilles, Alan Herring, Paul Lowman,and PatrickTaylorgreatlyimprovedthismanuscript,asdid discussion with Bob Simpsonand Bill Stuart.
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