Nov 10, 1990 - Lassen region of NE California and surround five longer-lived, large-volume, intermediate to silicic volcanic centers. .... Washington as well as in northern California, although not in ...... pp., Univ. of Calif., Berkeley, 1961.
JOURNAL OFGEOPHYSICAL RESEARCH, VOL.95,NO.BI2, PAGES 19,453-19,464, NOVEMBER 10,1990
LateCenozoic Volcanism, Subduction, andExtension in theLassen Region of California,Southern Cascade Range MARIANNE GUFFANTI
U.S. Geological Survey,Reston,Virginia
MICHAEL A. CLYNNE,JAMES G. SMITH,L. J.P. MUFFLER, ANDTHOMAS D. BULLEN U.S. Geological Survey,MenloPark,California
Hundreds ofshort-lived, smalltomoderate-volume, mostly mafic volcanoes occur throughout the
Lassen regionof NE California andsurround fivelonger-lived, large-volume, intermediate to silicic
volcanic centers. younger than3 Ma.Volcanic rocks older than7 Maarescarce intheLassen region. Weidentify 537volcanic vents younger than7Ma,andweclassify these intofiveageintervals andfive compositional categories based onSiO2content. Mapsof ventsbyageandcomposition illustrate regionallyrepresentative volcanictrends.By 2 Ma, the easternlimit of volcanism had contracted westwardtowardthe late Quaternary arc. Late Quaternary volcanism is concentrated aroundand
northofthesilicicLassen volcanic center. Thebeltofmostrecentvolcanism (25--0 ka)hasbeenactive sinceat least2 Ma.Mostmaficvolcanism iscalcalkaline basaltandbasaltic andesitc. However,lesser volume oflow-potassium olivine tholeiRe (LKOT),ageochemically distinctive basalt typefound inthe
nortlZtern Basin andRange province, also haserupted throughout theLassen segment oftheCascade
arcsincethePliocene. Thusmodels of themantle source andtectonic control of LKOTmagmatism shouldbe applicableboth withinand behindthe subduction-related arc. Normalfaultsand linear
groups ofventsareevidence ofwidespread crustal extension throughout mostoftheLassen region.
NNW alignments of thesefeatures indicateNNW orientation of maximum horizontal stress(ENE extension), whichis similarto thestress regime in theadjacent northwestern BasinandRangeand northern SierraNevadaprovinces. Thelarge,long-lived volcanic centers developed justwestof a zone ofclosely spaced NNW trending normal faults.Withinthatzoneoffaulting, pervasive ENE extension hasprecluded growthof large,long-lived crustalmagma systems. Weinterpret thewestern limitof the zoneof NNW trending normalfaultsasthewestern boundary of theBasinandRaageprovince where it overlapsthe Lassensegment of the Cascade arc. In ourview,theLassenvolcanic regionoccurs abovethe subductingGordaNorth platebut alsolieswithina broadzoneof distributedextensionthat occursin the NorthAmericanlithosphere eastandsoutheast of thepresentCascadiasubructionzone.
An episode of ENE extension thatbeganin thelateMiocenein thenorthwestern BasinandRange provinceappearsto havetriggered widespread lateMioceneto Quaternary maficvolcanism in the Lassenregion.The scarcityof volcanicrocksolderthan7 Ma suggests that a morecompressive lithospheric stressregimeprior to the late Mioceneextensional episodemay havesuppressed volcanism,eventhoughsubduction probablywasoccurringbeneaththe Lassenregion. INTRODUCTION
Medicine Lake volcaniccentersto the north by an area in which there are no recognizedQuaternaryvolcanicvents, beneath theNorthAmericanplatehascontracted northward forms a distinctsegmentof the Cascadearc [Guffantiand since theMiocene,in conjunction with northward migration Weaver, 1988]. Within the Lassen region (120o30' to of theMendocino triplejunction[Dickinson and Snyder, 122ø10'Wand 40ø15 ' to 41øN, an area of approximately 1979].At 20 Ma, the arc was active as far south as Las !0,000km2),thevolcanic record isoverwhelmingly younger Vegas.Sincethe late Pliocene,the southernterminusof the than 7 Ma. Hundreds of short-lived, small- to moderatearchasbeenlocatedin the vicinityof the Lassenregionin volume, mostlymarievolcanoessurrounda few long-lived, The volcanic arc associated with Cenozoic subduction
northeastern California(Figure 1). In its positionat the large-volume, intermediate to silicic volcanic centers southern endof the Cascadearc, the Lassenvolcanicregion younger than 3 Ma (Figure 2).
'des nearthejunctionof threeactivetectonicregimes: (1) a In order to characterizevolcanicactivity on a regional subduction regimeto the northresulting fromthrusting of scale, we located as many volcanic vents as we could in the theJuande Fucaplatesystem undertheNorthAmerican Lassenregion(537 vents)and assignedeachone to an age Plate; (2)a transform regime to thesouthresulting fromthe category and a compositionalcategory using a variety of relative motionof the NorthAmericanandPacificplates; geochemical andgeochronological data. We presentmapsof and(3) an extensional regimeto the eastin the Basinand vents by age and compositionto illustrateregionallyrepre-
Range province (Figure1).In thispaper,weconsider the
sentative volcanic trends.
effects of theadjacent tectonicenvironments on volcanism We propose that some of the volcanic and tectonic fea-
inthesubduction environment oftheLassen region.
turesof the Lassenregionare best understoodas resulting
TheLassen region,separated fromtheMountShasta and from the spatialoverlapof the extensionalBasinand Range T'tfis paper isnotsubject toU.S.copyright. Published in 1990 by province on the subduction.relatedarc. Basin and Range theAmerican Geophysical Union, influenceon the CascadeRange previously has been sug. Pa•raumber 90JB00246.
gestedby others,for example,Magill eta!., [!982], Priest et 19,453
19,454
GUFFANTIET AL' VOLCANISM, SUBDUCTION, AND EXTENSIONiN LASSEN !14 ø 51 ø
upper surface isestimated to bebetween 80kmandatleast
130kmdeep.Subcrustal earthquake hypocenters along a
Wadati-Benioff zonewestof theLassen region define stab Canada
dipof 25ø between depths of 50-90km [Walter, !'986;
Cockerham, 1984].Subcrustal earthquakes, however, have notbeendetected at depthsgreaterthan90kmbeneath t:he Lassen segment oftheCascade arc.Weestimate contours of plate depthgreaterthan 90 km by continuedeastward projection of the25øplatedip,recognizing thatactual plate
Washington
Fuca Plate
Oregon
dip may be steeper than that.
The southernedgeof the subducting slabis a function of
theposition of the northward migrating Mendocino tri•e
Basin and
junction [Dickinsonand Snyder, 1979]. At present,the subductingslab extends as far south as about 39ø45'N [Jachensand Griscom,1983;Wilson,1986].Migration Mendocinotriple junction has been correlatedwith extensionin the northernCaliforniaCoastRanges[McLaug•in, 1981]and the SacramentoValley [Harwood, 1984]but_not correlated with extensionfarthereastin the Lassenre,on. Geophysicalstudiesin the Lassenregion suggest thata
Range
Nevada
Pacific
Plate
Pacific Ocean 0
few kilometersof late Cenozoic volcanic rocks are undefiain by granitic crust. The thickness of the cr,•st beneath
20O km
131"
126•
• 120"
114"
Lassenregionis 38 ---4 km [Mooneyand Weaver,!•.
Berge and Stauber [1987] concludedfrom a two-dimensional Fig. 1. Plate tectonic setting of the Lassen volcanic region. seismic refraction study that volcanic rocks in the Lasset• Trianglesindicate major Quaternary volcanic centersof the Cascade region range from about 1 to 4 km in thickness and that Range. Small rectangle enclosesLassen region. S, Mount Shasta; Sierra Nevada basement probably underlies the volcaff.•c M, Medicine Lake volcano. (Modified from Mooney and Weaver [1989].)
al. [1983], Smith and Luedke [1984], and Guffanti and Weaver [1988]; in this paper, we attempt to enlarge upon their insights in the southern Cascades. GEOLOGIC AND TECTONIC
SETTING
rocks at least as far northwest as Lassen Peak. This conclu.
sion is compatible with the fact that fault blocks of Sie• graniticrock are exposedsoutheastof Eagle Lake (Figure2•. Blakely et al. [1985, Figure 7, feature F] suggestedthatan aeromagnetic anomaly in the northern part of the Lassea region may represent an isolated fragment of mafic or ultramaficrocks at shallow depth in the crust. This aero• netic feature could be part of either the Trinity ophio!ite complex of the Klamath Mountains or an ultramaficbody in the Sierra Nevada basement complex. Thus a transition from Sierran to Klamath basement may occur beneaththe
The Lassen region is distinctive because the record of volcanism prior to the late Miocene is very sparse. Thick sectionsof early Oligoceneto middle Miocene volcanogenic northern part of the Lassen region. rocks of the Western CascadeRange are widely exposed Evidence from gravity modeling indicates that upl•r from west of Mount Shasta north to Mount Rainier. In crustal structure in the Basin and Range continuesintothe contrast, Oligocene and early Miocene rocks are not evident Lassen region. Blakely and Jachens [this issue] describea south of Mount Shasta, and only a few small areas of middle NE trending gravity depressionthat extends from the LasMiocene (--•15 Ma) rocks depositedon Mesozoic and Paleo- sen region hundredsof kilometers into Nevada. SimilarNE zoic basementare exposedin the westernpart of the Lassen trending gravity features occur throughout Oregon region. The time span from 17 to 7 Ma appearsto be an Washington and largely correlate with the Cascade.arc interval of relative volcanic quiescencein the Cascadesin segmentation of Guffanti and Weaver [1988].Blakelyand Washingtonas well as in northernCalifornia, althoughnot in Jachens[this issue]proposedthat the NE trendingfeatures central Oregon [Smith, 1989; Sherrod and Smith, 1989]. reflectuppercrustalstructures thathaveactivelyinfluenced Whereas nearly all volcanic rocks within the Lassen the locus of more recent volcanism. region are younger than 7 Ma, volcanic rocks east of the Voluminous, intermediate to silicic volcanism has oc. Lassen region are mostly older than 7 Ma. Adjacent to the curred in the Lassen region at four, large, late P!iocene east of the study area and north of Honey Lake, widely Quaternary centers,Dittmar, SnowMountain,Maidu,aM scatteredstratovolcanoesare Miocene in age, commonly Lassen(Figure2). A fifth largecenter,Yana, of latePliocer•e about 10 Ma [Grose and McKee, 1986].Decreasein age of age [Lydon,1968]liesjust outsidethe studyareato volcanic rocks from east to west occurred during the late south.Thesecentersarelocatedin the westernpartof Miocenenear Susanville,on the easternedgeof the Lassen volcanic regionwheretheupperpartof thesubducting sl•b region [Grose and McKee, 1982; Grose, 1985]. is projected to lie between depthsof approximately 85 The Lassen volcanic region is associatedwith subduction 110km.Thecenters arelong-lived, oftheorderofhund.red.s of the Gorda North plate, part of the Juan de Fuca plate ofthousands of years,compared to smaller volcanoes in system.Contoursof estimatedplate depthand locationsof surrounding region.The youngestlarge centeris L.as• recognizedvolcanicventsare shownin Figure2. Most vents volcaniccenter(63%SiO2)rocks(roughly halfof basalt, 48-53%; basaltic andesite, 53--57%; andesire, 57-
itstotal eruptive volume) is the largest Quaternarysilicic 63%;dacite,63-70%;rhyolite,>70%. The basaltcategoryis magmasystem in the Cascades.
Faultsare more abundant in the Lassen region than elsewhere in the CascadeRangeexcept, perhaps,around Medicine Lakevolcano.Manyfaultshaveventsalongthem, although not all magmarose alongfaults observableon the
surface. In thenorthernpartof theregion,normalfaultingis concentratedin zones of closely spaced individual faults
having separations of about0.8 km; otherareaslackclosely s•ced faults.On the westernmost sideof the Lassenregion istheBattleCreekfaultzoneof ENE trending,lateQuatermary,dominantlynormal faults [Helley et al., 1981]. The
subdividedinto low-potassium(80 km deep and also west of the area in which NNW trending normal faults indicate ENE extension. In contrast, within the area dominated by NNW trending normal faults only volcanoes that are small and not highly evolved occur. Thus pervasive ENE crustal
extension
that is characteristic
of the northwestern
marin of the Basin and Range province may preclude growth of the long-lived crustal magma systemsrequiredfor large volcanic centers. The
influence
of extension
on late Miocene
volcanism
adjacentto the northwesternBasin and Rangeprovince also is evident in the central Oregon Cascades.Extension-related
allowadiabaticdecompresslye meltingof mantleto generate both primitive magmas,as well as provide accessto the surface.
We concludethat the Lassensegmentof the Cascades and adjacentmargin of the Basin and Range provincesharea common stress regime, and we suggest that the tectonic origin of extension is similar in both provinces. Various modelsfor the origin of extensionhave been proposed for the two provinces.Slow, oblique subductionactingonthe centralOregonCascadearc has been suggestedby Smithet al. [1987] and Hughes and Taylor [1986]. Gravitatic•nal
instabilityof thickenedlithosphere[Wernickeet al., 1987] and distributedsheardue to North American-Pacificplate transformmotion [Atwater, !970] have been proposedfor the Basinand Rangeprovince.Althoughwe do not specify an extensional mechanism for the Lassen region, we do suggestthat any model formulated for the southernCascades also shouldbe applicable to the adjoiningmargin of the Basin and Range province, and vice versa. SUMMARY
volcanismdbminatedby basaltand basalticandesitealso
A spectrumof widespread volcanic activity in the Lassea
began about 7 Ma in the Three Sisters to Mount Jefferson
regionis representedby 537 mostly mafic volcanicvents
area, near the intersection
formed during the past 7 million years. ENE-WSW extem sion of the upper crust produced NNW trending vent al'•,nments and normal faults throughout much of the Lassen region. Four long-lived, large, late Pliocene to Quaternary volcanic centers formed immediately west of the areacutby closely spaced NNW trending normal faults. The eastern
of the Cascade arc and the
northern margin of the Basin and Range province [Smith et al., 1987; Hughes and Taylor, !986, Taylor, this issue]. There, asin the Lassen region, north to NNW trendingfaults and vent alignmentsindicate intra-arc extension, and LKOT and calcalkalinebasalt younger than 7 Ma are intermingled. Smith et al. [!987] and Hughes and Taylor [1986] consider the mechanismof intra-arc extension in central Oregon to result from the plate margin processof slow, oblique subduction.
At about the same time that extension along the northwestern margin of the Basin and Range province may have triggered Lassen volcanism, the Sierra Nevada province south of Lassen also underwent extension-relatedchanges. Using a gravity model for isostatic adjustmentof the batholith, Chase and Wallace [1986] argue that between 5 and 10
Ma Basin-and-Rangeextensionbroke the lithospherealong
limit of volcanicactivity in the Lassenregionhadcontracted westward by 2 Ma. Late Quaternary volcanism is conce• trated around and north of the silicic Lassen volcanic center.
The Lassenvolcanicregionoccursabove the subducti• Gorda North plate but also lies on the western edgeof a broad zone of distributed extension that exists within the
North American lithosphere east and southeastof the present Cascadia subduction zone. Tectonism and magrna-
tism of the Basin and Rangeprovinceoverlapthe subduction-relatedarc in theLassenregion.The onsetof anepisode of late Miocene lithospheric extension in the northwestern
BasinandRangeprovince appears to havetriggered onset of widespread volcanism in the Lassenregion.Largecrustal The distributions of LKOT and calcalkaline basalt in the magmasystems thatproduced long-lived differentiated volLassenregionoverlap in spaceand time. Calc-alkalinebasalt canoesdeveloped westof the zoneof NNW normalfaults
the eastern side of the Sierra Nevada, which allowed sudden uplift of the batholith to occur.
which are characteristicof the ENE extension of the n •orthvolcanicmaterial. Calc-alkalinebasaltis similarto orogenic western Basin and Range province. lavas from continentaland mature island arcs [Gill, 1981] Low-potassium olivinetholeiiteisa geochemically •stincis dominant in both number of vents and total volume of
and thus appears to be closely related to subduction.LKOT
tivebasalt typeof theBasin andRange province thata•so
is not an importanteruptivecomponentof the largevolcanic occurs intheLassensegment of theCascade arc.Them.anfle centersof the Lassenregionbut contributesto the regional sourceandprocessof magmageneration of thisbasalttype mafic volcanism and is found throughoutthe Basin and apparently are the same across the two environments.
GUFFANT! ET AL.: VOLCANISM,SUBDUCTION,AND EXTENSIONIN LASSEN
Acknowledgments. We appreciate reviewsby CathyHickson,
•9,463
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Robert Jachens, JamesRytuba,andGarySmith,Thisworkwas
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(Received August 30, 1989; revised January 19, 1990;
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