JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B12, PAGES 30,101-30,115, DECEMBER 10, 1998 Location of the southern edge of the Gorda slab and evidence for an adjacent asthenosphericwindow: Results from seismic profiling and gravity Bruce C. Beaudoin,John A. Hole, • and Simon L. Klemperer Department of Geophysics,Stanford University, Stanford, California Anne M. Trdhu Collegeof Ocean and AtmosphericSciences, Oregon State University,Corvallis Abstract. As the Mendocino Triple Junctionmigrates northward along the California margin it is widely presumedto leave a "slab-free"or "asthenospheric"window in its wake. A 250-km-longsouth-north seismic refraction-reflection profile crossingthe transitionfrom transformto subductionregimes allows us to compareand contrastcrust and upper mantle of the North American margin before and after it is modified by passageof the Mendocino Triple Junction.From the seismicdata we have determined that (1) the crustis laterallyhomogeneous in velocityto a depth of 20 km (interpretedby us as Franciscancomplex), (2) below20 km depththe crustis characterizedby velocities of >-7.0 km/s for the southernhalf of the profile and by velocitiesof -<7.0 km/s for the northernhalf, (3) regionsof high reflectivityin the crustoccur below -20 km depth throughoutthe profile, and (4) the North Americancrust is thickest(-35 km) in the center of the profile and thins to -25 km at either end. From the gravity data we have determinedthat (1) asthenosphericdensities (3.2 g/cm 3) occursubjacent to theNorth Americancrust in the centerof the profile, and (2) a wedgeof lithosphericmantle density material(>-3.2 g/cm 3) is requiredon the southern end of theprofile. We interpretthese combinedresults to indicatethat our profile crossesthe southernedge of the Gorda plate and that directly adjacentto this edge is an asthenosphericwindow with overlyingmafic rocksin the crust.These mafic rocks and a reforming lithosphericmantle increasein thickness southward. 1. Introduction lithosphericscale, however, is only now being examinedwith seismicrefraction and reflectiontechniques. Northern California tectonicsare governedby the interac- In the currently most popular model for evolutionof the tions of the Pacific,Gorda, and North American plates. (We MTJ, first formulatedin a rigid-platemodel by Dickinsonand use the term Gorda plate while recognizingthat recent work Snyder[1979], an importantconsequence of MTJ migrationis suggeststhat Gorda deformationzone may be more appropri- that the North American plate slidesoff the Gorda plate, ate [Wilson, 1989].) The three plates come together at the leavingin its wake a void that is filled by upwellingastheno- MendocinoTriple Junction(MTJ) (Figure 1), a transform- sphere,referred to as the slablesswindow or slab gap [Dick- transform-trenchtriple junctionthat was initiated at 25-29 Ma insonand Snyder,1979; Severinghaus and Atwater, 1990]. Sup- by the collisionof a Pacific spreadingcenter with the North porting evidencefor a slablesswindow existsfrom heat flow American continent [Atwater, 1970]. As the Juan de Fuca- data [Lachenbruchand Sass,1980], gravity and magneticdata Pacific plate boundarymigrates northward relative to North [Griscomand Jachens,1989; Jachensand Griscom, 1983], America, subductionalong the Cascadiasubduction zone is teleseismicP wave delay studies[Benz et al., 1992; Verdonck replacedby the transformboundary of the San Andreas fault and Zandt, 1994;Zandt, 1981;Zandt and Furlong,1982], shear system(Figure 1). Today, the MTJ is definedby a broad on- wave velocities[Levander and Kovach, 1990], and changesin shore region near Cape Mendocino [Clarke, 1992] and pro- volcanism[Dickinson and Snyder,1979; Fox et al., 1985; Fur- videsa uniquelaboratory to studyplate interactionsat a triple long,1984; Johnson and O'Neil,1984; Zandt and Furlong, 1982]. junction. The transitionfrom a subductionregime to a trans- Thesestudies further suggestthat (1) the observedupper man- form regimehas long been recognizedfrom geologicalstudies, tle low-velocityzone is smaller and displacedto the east com- and its subsurfacemanifestations have been recognizedfrom pared to that predictedby rigid-platemodels; (2) the Gorda upper mantle velocitytomography and potential field studies. slabis fragmentingalong its southernedge; (3) arc volcanism How this transition is accommodated on a detailed crustal and is being replacedby bimodal volcanismthat exhibitsa north- ward decreasein age; and (4) the easternboundary of the •Now at Departmentof GeologicalSciences, Virginia Polytechnic Pacificplate has migratedeastward through a seriesof jumps Institute and State University,Blacksburg. of the MendocinoTriple Junction.Some of thesecomplexities Copyright1998 by the American GeophysicalUnion. maybe explainedby introducingmore complexrheologies and Paper number 98JB02231. thermal distributionsinto the simplerigid-plate model of Dick- 0148-0227/98/98 JB-02231 $09.00 inson and Snyder.For example,thermomechanical modeling 30,101 30,102 BEAUDOIN ET AL.: SOUTHERN EDGE OF THE GORDA SLAB 125 o 124 ø 123 ø 128 ø 125 ø 1.22ow 42 ø • EelMioceneRiver &sedimentsyounger __[•_ Klamath Juan de Fuca [---] MesozoicValley Great [• Tertiaryvolcanics ..............:.i_:-•BeltCentral Franciscan& Eastern _['7-7] Sierra Nevada [-•ß KingRange & CoastalBelt Franciscan 41 ø Line 6 . Redding•\# • //\\ Line 9 601 40 ø Line 1 Lk Willows 39øN -. ß ßß Clear Figure 1. Generalizedgeologic map of northern California. Squares,instrument locations, which commonly overlapto form solidlines; stars, shot point locations;BSF, Bartlett Springsfault; CSZ, Cascadiasubduction zone; MAF, Maacama fault; MFZ, Mendocino fracture zone; SAF, San Andreas fault; SEDGE, southern edge of the Gorda slab as determined from this study.Inset showstectonic setting of the Mendocino Triple JunctionSeismic Experiment. Wiggly lines, seismic profiles; large arrows,plate motion relativeto fixed North America; MTJ, Mendocino Triple Junction;GV, Great Valley. suggeststhat the boundaryat depth between the Pacific and data were part of the first phase of the Mendocino Triple North American plates shouldstep inboard with time as the Junction Seismic Experiment (MTJSE) collected in 1993 slablesswindow cools and new material is accretedto the edge [Beaudoinet al., 1996; Godfreyet al., 1995; Trdhuet al., 1995]. of the Pacificplate [Furlonget al., 1989];recent results suggest During this 1993 field seasonwe collected650 km of onshore that sucha processmay be occurringin the San FranciscoBay seismicrefraction data: line 1 samplingthe transformregime; region [Brocheret al., 1994].Alternatively, the slablesswindow line 6 samplingthe subductionregime; and line 9 samplingthe model may be fundamentallyincorrect for northern California. transitionfrom transformto subductionregimes (Figure 1). Bohannonand Parsons[1995] recently questionedthe long- The MTJSE is a multiyear, multiinstitutionaleffort to study standingparadigm of a slablesswindow for the MTJ region.On the lithosphericresponse to MTJ passageby comparingand the basisof analysisof plate motionsand geometries,Bohannon contrastingthe subductionregime to the transformregime. and Parsonssuggest that coastalCalifornia should be undedain by a stalledslab rather than a slablessgap; in their model a 2. Tectonic and Geologic Setting slablesswindow may existbut would be much farther inland. Our study focuseson the transition from subductionto 2.1. Plate Geometry and Cascadia Seismicity transformtectonics by analyzinga SE-NW trendingwide-angle As the Gorda-Pacific plate boundary migrates northward reflectionprofile herein referred to as line 9 (Figure 1). These relativeto North America, subductionalong the Cascadiasub- BEAUDOIN ET AL.: SOUTHERN EDGE OF THE GORDA SLAB 30,103 ductionzone is replacedby the transformboundary of the San wedgingbeneath the older Great Valley forearc basin and its Andreasfault system(Figure 1). North of the MTJ, the tec- ophioliticbasement [Godfrey et al., 1997]. tonics are governedby the subductionof young (---5 Ma) Gorda plate obliquelybeneath North Americanplate alongthe Cascadia subduction zone. South of the MTJ the tectonics are 3. Modeling: Technique, Results, and Errors governedby the transformboundary between the Pacific and The modelpresented herein is the resultof inversemodeling North American plates, the San Andreas fault system.Off- of travel times of both refracted and reflectedarrivals [Hole, shore, the boundary between the Pacific and Gorda plates 1992;Hole and Zelt, 1995] (Plate 1). The modelpresented has defines the Mendocino fracture zone which juxtaposesthe an overall absolute value rms fit of 82 ms to first arrivals. In young Gorda plate with the older (---26 Ma) Pacific plate. addition to travel time modeling,we have also incorporateda Kinematic modeling of northward convergenceof the Pacific depth-migratedsingle-fold image of the precriticalreflections plate with the Gorda plate indicatesthat the southeastern from offsets of _<30 km, and gravity modeling, to constrain portion of the Gorda plate, nearestthe MTJ, is undergoingthe possibletectonic models. greatestamount of compressionalstrain in a north-southdi- 3.1. Velocity and Interface Modeling rection[Wilson, 1986], further complicatingour understanding of the MTJ region. The velocity modelingwas carried out in two stages.First, The MTJ regionhas experienced over 60 earthquakesof Ms travel times for P•7, defined as the diving wave in the crust, -> 5.5 since1853 [Oppenheimeret al., 1993].Recent analysisof were picked, and thesetravel times were inverted