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
evidencefor an adjacent asthenosphericwindow:
Results from seismic profiling and gravity
Bruce C. Beaudoin,John A. Hole,• and Simon L. Klemperer
Department of Geophysics,StanfordUniversity,Stanford,California
Anne
M. Trdhu
Collegeof Ocean and AtmosphericSciences,Oregon State University,Corvallis
Abstract. As the MendocinoTriple Junctionmigratesnorthwardalong the California
margin it is widely presumedto leave a "slab-free"or "asthenospheric"
window in its
wake. A 250-km-longsouth-northseismicrefraction-reflectionprofile crossingthe
transitionfrom transformto subductionregimesallowsus to compareand contrastcrust
and upper mantle of the North American margin before and after it is modifiedby
passageof the MendocinoTriple Junction.From the seismicdata we have determined
that (1) the crustis laterallyhomogeneous
in velocityto a depthof 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 crustoccurbelow -20 km depth
throughoutthe profile,and (4) the North Americancrustis thickest(-35 km) in the
center of the profile and thins to -25 km at either end. From the gravitydata we have
determined
that(1) asthenospheric
densities
(3.2g/cm
3) occursubjacent
to theNorth
Americancrustin the centerof the profile, and (2) a wedgeof lithosphericmantle density
material(>-3.2g/cm
3) isrequired
onthesouthern
endof theprofile.We interpret
these
combinedresultsto indicatethat our profile crossesthe southernedge of the Gorda plate
and that directlyadjacentto this edge is an asthenospheric
windowwith overlyingmafic
rocksin the crust.These mafic rocksand a reforminglithosphericmantle increasein
thickness
southward.
lithosphericscale,however,is only now being examinedwith
seismicrefractionand reflectiontechniques.
Northern California tectonicsare governedby the interacIn the currentlymost popular model for evolutionof the
tions of the Pacific,Gorda, and North American plates.(We MTJ, first formulatedin a rigid-platemodelby Dickinsonand
use the term Gorda plate while recognizingthat recentwork Snyder[1979],an importantconsequence
of MTJ migrationis
suggests
that 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 upwellingasthenoMendocinoTriple Junction(MTJ) (Figure 1), a transform- sphere,referred to as the slablesswindow or slabgap [Dicktransform-trenchtriple junctionthat wasinitiated at 25-29 Ma insonand Snyder,1979;Severinghaus
andAtwater,1990].Supby 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 [Lachenbruch
and Sass,1980],gravityand magneticdata
Pacificplate boundarymigratesnorthward relative to North [Griscomand Jachens,1989; Jachensand Griscom, 1983],
America, subductionalong the Cascadiasubductionzone is teleseismicP wave delay studies[Benzet al., 1992; Verdonck
replacedby the transformboundaryof the San Andreasfault andZandt, 1994;Zandt, 1981;Zandt andFurlong,1982],shear
system(Figure 1). Today, the MTJ is definedby a broad on- wave velocities[Levanderand Kovach, 1990], and changesin
shoreregion near Cape Mendocino[Clarke, 1992] and pro- volcanism[Dickinsonand Snyder,1979;Fox et al., 1985;Furvidesa uniquelaboratoryto studyplate interactionsat a triple long,1984;Johnson
andO'Neil,1984;ZandtandFurlong,1982].
junction.The transitionfrom a subductionregime to a trans- Thesestudiesfurthersuggest
that (1) the observeduppermanform regimehaslongbeen recognizedfrom geologicalstudies, tle low-velocityzone is smaller and displacedto the east comand its subsurfacemanifestationshave been recognizedfrom pared to that predictedby rigid-platemodels;(2) the Gorda
upper mantlevelocitytomographyand potentialfield studies. slabis fragmentingalongits southernedge;(3) arcvolcanism
How this transition is accommodated on a detailed crustal and
is being replacedby bimodal volcanismthat exhibitsa northward decreasein age; and (4) the easternboundaryof the
•Nowat Departmentof Geological
Sciences,
VirginiaPolytechnic Pacificplate hasmigratedeastwardthrougha seriesof jumps
Institute and State University,Blacksburg.
of the MendocinoTriple Junction.Someof thesecomplexities
Copyright1998 by the American GeophysicalUnion.
maybe explainedby introducingmore complexrheologiesand
thermaldistributionsinto the simplerigid-platemodelof DickPaper number 98JB02231.
0148-0227/98/98 JB-02231 $09.00
inson and Snyder.For example,thermomechanicalmodeling
1.
Introduction
30,101
30,102
BEAUDOIN
125 o
42 ø
ET AL.: SOUTHERN
124 ø
EDGE OF THE GORDA SLAB
123 ø
128 ø
125 ø
1.22ow
Juan de Fuca
Miocene
&sediments
younger
Eel
River
__[•_
Klamath
[---] Mesozoic
Great [• Tertiary
volcanics
Valley
•
:.i_:-•
Central
&Eastern_['7-7]
Sierra
Nevada
..............Belt Franciscan
[-•ß
KingRange
& Coastal
BeltFranciscan
41 ø
Line 6
Redding
•\#• //\\
.
.
Line 9
601
40 ø
Line
1
Willows
39øN
-. ß
ßß
Lk
Clear
Figure 1. Generalizedgeologicmap of northern California. Squares,instrumentlocations,whichcommonly
overlapto form solidlines;stars,shotpoint 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 determinedfrom this study.Inset showstectonicsettingof the Mendocino Triple
JunctionSeismicExperiment.Wigglylines,seismicprofiles;large arrows,plate motion relativeto fixedNorth
America; MTJ, MendocinoTriple Junction;GV, Great Valley.
suggeststhat the boundaryat depth between the Pacificand
North American plates shouldstep inboardwith time as the
slablesswindowcoolsand new material is accretedto the edge
of the Pacificplate [Furlonget al., 1989];recentresultssuggest
that sucha processmay be occurringin the San FranciscoBay
region [Brocheret al., 1994].Alternatively,the slablesswindow
model maybe fundamentallyincorrectfor northernCalifornia.
Bohannonand Parsons[1995] recently questionedthe longstandingparadigmof a slabless
windowfor the MTJ region.On
the basisof analysisof plate motionsand geometries,Bohannon
and Parsonssuggestthat coastalCaliforniashouldbe undedain
by a stalledslab rather than a slablessgap; in their model a
slablesswindowmay existbut would be much farther inland.
Our study focuseson the transition from subductionto
transformtectonicsby analyzinga SE-NW trendingwide-angle
reflectionprofilehereinreferredto asline 9 (Figure 1). These
data were part of the first phase of the Mendocino Triple
Junction Seismic Experiment (MTJSE) collected in 1993
[Beaudoinet al., 1996;Godfreyet al., 1995;Trdhuet al., 1995].
During this 1993 field seasonwe collected650 km of onshore
seismicrefraction data: line 1 samplingthe transformregime;
line 6 samplingthe subductionregime;and line 9 samplingthe
transitionfrom transformto subductionregimes(Figure 1).
The MTJSE is a multiyear, multiinstitutionaleffort to study
the lithosphericresponseto MTJ passageby comparingand
contrastingthe subductionregime to the transformregime.
2.
2.1.
Tectonic and Geologic Setting
Plate Geometry and Cascadia Seismicity
As the Gorda-Pacific plate boundary migrates northward
relativeto North America, subductionalongthe Cascadiasub-
BEAUDOIN
ET AL.: SOUTHERN
ductionzone is replacedby the transformboundaryof the San
Andreasfault system(Figure 1). North of the MTJ, the tectonics are governedby the subductionof young (---5 Ma)
Gorda plate obliquelybeneathNorth Americanplate alongthe
Cascadia subduction
zone. South of the MTJ
the tectonics
are
governedby the transformboundarybetweenthe Pacificand
North American plates, the San Andreas fault system.Offshore, the boundary between the Pacific and Gorda plates
defines the Mendocino fracture zone which juxtaposesthe
young Gorda plate with the older (---26 Ma) Pacific plate.
Kinematic modeling of northward convergenceof the Pacific
plate with the Gorda plate indicatesthat the southeastern
portionof the Gorda plate, nearestthe MTJ, is undergoingthe
greatestamount of compressionalstrain in a north-southdirection[Wilson,1986],further complicatingour understanding
of the MTJ region.
The MTJ regionhasexperiencedover60 earthquakesof Ms
-> 5.5 since1853 [Oppenheimer
et al., 1993].Recent analysisof
the physicalcharacteristicsof the Cascadiasubductionzone
and seismicityof the Juan de Fuca-Gorda plate and surface
deformationindicatethe Cascadiasubductionzone may be a
well-coupledor fully locked systemwith potential for large
(Ms -> 8) subductionzone earthquakes[Carverand Burke,
1989; Clarkeand Carver,1992;Heaton and Hartzell, 1987]. In
the subductionregime,earthquakesare commonto depthsof
40 km and largely result from internal deformation of the
subductingGorda slab and slip alongthe Mendocinofracture
zone [Castilloand Ellsworth,1993]. In contrast,in the transform regime,earthquakesare normallyshallowerthan ---15km
and reflectthe right-lateralmotionbetweenPacificand North
American plates [Castilloand Ellsworth,1993].
2.2.
Geologic Overview
EDGE OF THE GORDA SLAB
30,103
wedgingbeneaththe older Great Valley forearc basin and its
ophioliticbasement[Godfreyet al., 1997].
3.
Modeling: Technique, Results, and Errors
The modelpresentedhereinis the resultof inversemodeling
of travel times of both refractedand reflectedarrivals[Hole,
1992;Hole and Zelt, 1995](Plate 1). The modelpresentedhas
an overall
absolute
value rms fit of 82 ms to first arrivals.
In
addition to travel time modeling,we have alsoincorporateda
depth-migratedsingle-foldimageof the precriticalreflections
from offsets of _<30km, and gravity modeling, to constrain
possibletectonicmodels.
3.1. Velocity and Interface Modeling
The velocity modelingwas carried out in two stages.First,
travel times for P•7, defined as the divingwave in the crust,
were picked, and thesetravel timeswere inverted for velocity.
The final inversion model has a 1 km square grid velocity
parameterization and a 20-km-horizontal x 4-km-vertical
smoothingoperator applied.The Franciscancomplexhas few
coherent
seismic reflectors.
Hence
we are unconcerned
with
midcrustaleventsin this data set sincethey are isolated features (observableon fewer than three shotgathers;for a detailed discussionof the effect the Franciscancomplexhas on
the wavefield,seeLendlet al. [1997]).Oncethe crustalvelocity
structure is established,we can infer first-order discontinuities
from the presenceof steep gradients,althoughthe method is
not designedto have explicitvelocitydiscontinuities.
Our inversionmethod doesallow the placementof a reflectorwithin
the model by "floating"the interfaceover the velocityfield to
minimizereflectiontravel time residualswithout updatingthe
model velocities.
Hence we introduced
interfaces
for the basal
crustaland Moho reflectionsbasedon the wide-anglereflecNorthern coastalCalifornia recordsa historyof subduction
tionsalongthe profile. The modeledlocationsof thesereflecand accretion that has been active since at least the Early
tors coincidewith many of the high-amplitudereflectionson
Cretaceous[Dickinson,1981].The overridingNorth American
the depth-migratedsingle-foldimage(seebelow).
plate in the MTJ regionconsistsof accretionarywedgesof the
3.1.1. The subduction regime (model coordinates --•110Mesozoic-CenozoicFranciscan complex [e.g., Blake et al.,
250 km). The Franciscancomplexbetween shot points 906
1985]overlainby the Eel River Basin,a Cenozoicforearcbasin and 911 exhibitsthe largestamount of lateral variation along
[Clarke,1992](Figure 1). The Franciscancomplexincreases
in the profile (Plate 1). Lateral variationsin velocityas large as
age and metamorphicgrade inboard and is largelycomposed 0.5 km/s in 25 km are modeled from refracted arrivals in the
of metasedimentaryrocks.It hasbeen dividedinto three belts: upper 10 km beneathshotpoints907, 908, and 909. There is no
the Coastalbelt, consistingpredominantlyof a Tertiary accre- evidenceof higher-velocityKlamath rocksbeneaththeseshot
tionarycomplex;the Central belt, a tectonicm61angeof Early points[Beaudoinet•al.,1996;Trdhuet al., 1995].Beneath10 km
Jurassicto Tertiary(?) age; and the Easternbelt, Jurassicto depththe velocityfield is laterallyuniformto 20 km depth(the
Cretaceousblueschist-facies
rocks (Figure 1) [Blake et al., 6.5 km/s contour).However,it is likely that variationsof the
1985]. Onshore,the Eel River Basinconsistsof up to 4 km of scaleobservedin the upper 10 km are presentbut not resolved
Miocene and youngersedimentaryrocksthat lie unconform- [Lendl et al., 1997]. Refracted arrivalssamplingdeeper in the
ably on Coastaland Central belt Franciscancomplex[Clarke, model definea gentlySE dippingvelocityboundaryto --•35km
1992]. Line 9 is roughlyparallel to the geologicstrike of the depth. Mantle diving rays, arrivals with apparent velocities
region and is therefore approximatelyin the sameFranciscan >-7.6 km/s, are observedfrom shot points 903, 905, 906, 909,
subterranethroughoutits length(Figure 1).
and 911 (Figure 2). Lower crustaland mantle ray coverage
North of the present-dayMTJ, the Franciscanbelts along from thesearrivalsis limited to a regionbetweencoordinates
the Cascadia subductionzone are overlain by the Klamath --•95 km and •215 km.
Reflections from the lower crust and Moho are observed in
terrane, composedof arc-relatedrocks of Paleozoicto Late
Jurassicage [Harper,1980].Accretionof the Klamath terrane both the wide-angle(Figure 2) and depth-migratedsingle-fold
to the North American margin occurredduring the Late Ju- data (Figure 3). The depth-migratedsingle-foldimagedefines
rassic[Blakeand Jayko,1986].The contactbetweenthe Klam- a segmentof an intracrustalinterfacewith an apparentdip of
ath terrane and the Easternbelt Franciscanis thoughtto be an --•10ø SE. Wide-anglereflectionsfrom shotpoints906 and 907
eastdippingthrustfault [cf.Beaudoinet al., 1996;Tr•hu et al., (Figure2) are modeledas an interfacethat is coincidentwith
1995].Southof the presentMTJ and Klamath belt, continent- the depth-migratedsingle-foldimage at a depth of 25-29 km
ward accretionof the Franciscanwas limited by crustal-scale between model coordinates 125 and 150. The inverse method
30,104
BEAUDOINET AL.:SOUTHERNEDGEOF THE GORDASLAB
L_
o
o
o
o
o
o
o
o
i i'•.....
! , •',
•
o
o
o
(tull) 41doCl
o
o
0
BEAUDOIN
ET AL.' SOUTHERN
EDGE OF THE GORDA SLAB
30,105
SE
NW
901
50
100
150
200
PmP
6
250 km
'
902
.
2
0
o
50
lOO
15o
200 km
6
2
0
-50
0
-50
50
0
100
50
150 km
100
150km
Figure 2. Record sectionsfrom line 9. Heavy black lines representcalculatedtravel times for all arrivals
usedin the velocityand interfaceinversion.Thin black lines representcalculatedtravel times for the gravity
Moho (seetext). Data are filteredfrom 2 to 16 Hz andtracenormalized.Time is reducedby 6 km/s.Pg, crustal
divingwave;PcP, reflectionsfrom the lower crustallayer;PmP, reflectionsfrom the Moho; Pn, mantle diving
wave.
models
this interface
with
an rms residual
of +_0.31 km in
depth (Figure 4a). Wide-angle reflectionsfrom the Gorda
plate Moho are observedon shotpoints904-911 (Figure 2).
Both forward and inversemodelingpositiona dipping inter-
face that is 35-36 km deep at model coordinate100 km and
shallows northwestward
km. The
inverse
method
to 26-27
models
km at model coordinate
this interface
residualof +0.96 km in depth (Figure 4b).
with
230
an rms
30,106
BEAUDOIN ET AL.: SOUTHERN EDGE OF THE GORDA SLAB
-50
0
50
100
150 km
2
0
-100
-50
0
50
100 km
1907
6
-150
-100
-50
50 km
0
8
6
4
_
2
0
-150
-100
-50
0
50 km
Figure 2. (continued)
these data are Pn arrivals that sample the mantle south of
model coordinate--•100km (shotpoints901 and 902 did not
propagateto distances->140 km).
Basalcrustalreflectionsfrom the depth-migratedsingle-fold
depth.Thisdepthcoincides
with the 6.5 km/scontourasin our
model for the subductionregime. Refracted arrivals from data are groupedin two sets,a shallowsetthat roughlyfollows
deeperin the model are confinedto an arrival observedon the 6.5 km/scontouranda gentlyNW dippingsetthat is --•5km
shotpoints901 and907 that definea highervelocity,--•7.0-7.5 deeper (Plate 1 and Figure 3). Wide-anglereflectionsfrom
km/s, between model coordinates--•30-90 km. Absent from shotpoints901, 904, and 905 definea shortreflectorbetween
3.1.2. The transform regime (model coordinates--•0-110
km). Crustal refracted arrivals from the transform regime
define a laterally uniform Franciscancomplexdown to 20 km
BEAUDOIN
ET AL.' SOUTHERN
EDGE OF THE GORDA
SLAB
30,107
o
-1•0
-2oo km
-100
-50
0
6
4
2
0
-200km
-150
-100
-50
0
6
40
911
4•5.0'
-
I!!!
t
6.0
0
-2
I
I
I
I
I
I
I
I
i
-200 km
I
I
I
I
-150
Source-Receiver
I
I
I
I
I
I
I
-100
Offset
I
I
I
I
-50
I
0
Figure 2. (continued)
901
902
SL
903
iii i
904
iiiiii
905
i
i ii
906
--
i
907
llllll
III
I
908
III i
i1111
909
ii i
910
II
911
-
I I I I
..........
10
20
30
50
SE
"'"
0
I'
20
' .......I .........'
40
I' '
60
'
I
80
'
I
1•
"
i ....... '
120
I
140
,
' t
160
,
!
180
'
I
2•
,
I
220
'
I
2•
Figure 3, Depth-migratedsingle-foldimage from precriticalreflectionsat offset of •30 km; a arrowsare
reflectionsinterpretedas top or near-topof Gorda crust,and b and c arrowsare basalcrustalreflections.
'
•
30,108
BEAUDOIN
ET AL.' SOUTHERN
lower-crustal
o _midpoints
* •Sz
5
VE5:I
10
refine the crustal structure used in these earlier models and
• 25
30
-2
35
40
SLAB
tomography[Verdonckand Zandt, 1994], and mantle tomography [Benzet al., 1992].This restrictedmodelsof the gravity
field to makingbroad assumptions
basedon thesesparsedata
and observations
of Benloft zone seismicity[e.g.,Jachensand
Griscom, 1983]. With our new seismicdata we are able to
reflector
wmodel
interface
0'
EDGE OF THE GORDA
rms:
_+0.93
km ;•
o
hence to utilize the gravity data to further constrainvarious
modelsof lower crustal/uppermantle geometry.
3.2.1. Jachensand Griscom model (Figure 7a). Jachens
and Griscom[1983]modeledthe isostaticresidualgravityfield
along six profiles oblique to line 9 (Figure 5). Five of their
profilesand our line 9 all samplea pronouncedgravitylow of
->50 mGal below regional average.Lacking any control on
crustal
rms:
_+0.31
km
-3
...... 1o'' "'264"
Model Coordinate (km)
thickness
Jachens
and Griscom
assumed
a uniform
North American(Franciscan)crustalthickness
of 20 km (Plate
2a) aboveGorda plate lithosphereto the north, and asthenosphere (slabless-window
model) to the south. The essential
componentsof their model are (1) a low-densitytabularbody
(Gorda crustand mantlejuxtaposedagainstasthenosphere
to
the south)creatinga gravitydecreaseto the north; and (2) a
wedgeof higherdensity(relativeto Franciscan)Klamathrocks
on the northern end of their profiles to create a gravity in-
....
m
ødei'interface
basal-crustal
reflector
0
o •midpoints
3
* bz
VE5:1
•i
•E
I
• •
a
crease at the northern
15
I +
2
1
0
35 ms:
••
0
•
_.
50
1•
•
-2
rms:
•.96km
150
Model Coordinate (km)
200
250
end of the model. Their introduction
of
the Klamath terraneinto the modelwaslargelyunconstrained
at the time of their writing.Althoughthe Jachensand Griscom
assumptionof constantcrustalthicknesshas the merit of simplicity, here [Beaudoinet al., 1996; Godfreyet al., 1997; T. J.
Henstockand A. Levander,Lithosphericevolutionin the wake
of the Mendocino Triple Junction:Structureof the San Andreasfault systemat 2 Ma, submittedto Journalof Geophysical
Research, 1998, hereinafter referred to as Henstock and
Levander,submittedmanuscript,1998] as elsewherein California [e.g.,Fuis and Mooney,1990]crustalthicknessincreases
by 5-10 km from the coastto 100 km inland, rendering their
conclusions invalid.
As seen in Plate 2a, Bouguer gravity along line 9 can be
Figure 4. Splinedinterfacesfor (top) lowercrustaland (bot- modeled incorrectlyusing a model similar to that of Jachens
tom) basal crustalreflectors(shown as hea• black lines in and Griscom(rms residualof 4.8 mGal). "Incorrect"because,
Plate 1), midpoints,and residualsfor the interfacesmodeled
unlike the profilesof Jachensand Griscom,line 9 does not
usingthe inversiontechnique.
crossany exposedKlamath rocks.Furthermore,the velocity
field presentsno indicationof significantvelocitydifferences,
and henceinferred densitydifferences,in the upper --•20km of
model coordinates
--•40-55 km that coincides with the shalcrust(Plate 1) [cf.Beaudoinet al., 1996].Without the Klamath
lower set of reflectionsfrom the depth-migratedsingle-fold body,this "Jachensand Griscom"-typemodel doesnot fit the
section. This interface has an rms residual of _+0.93 km in
observedBouguergravity (Plate 2a). We are therefore comdepth(Figure4a). Two deeperreflectorsare modeledfrom the pelled to evaluate a suite of models,based on our velocity
wide-anglereflectionsin this region of the transformregime. model, that vary only in lower crustalor upper mantle geomThe first,sampledby shotpoints901 and 903, is at a depthand etry in order to determinepermissibleplate geometrybeneath
a dip that agree with the single-folddata. This segmentis line 9.
modeledwith an rms residualof _+0.64km in depth (Figure
3.2.2. Gravity models derived from the velocity model.
4b). The secondsegment,definedby shotpoints903 and 906, Belowwe discussadditionalconstraintsthat gravitymodeling,
dipsgentlyto the NW, coincideswith the single-folddata, and basedon our velocitymodel, offers to understandinglithosextends from model coordinate
--•55-90 km. The inverse
phericstructurein the vicinityof the MendocinoTriple Juncmethodmodelsthis segmentwith an rms residualof _+0.46km tion and in particular along the southernedge of the Gorda
in depth (Figure 4b). Ray coveragefor this segmentdoesnot slab.A smoothedversionof our velocitymodel was converted
overlapwith the deepestreflectionnorth of modelcoordinate to MacRay format for the gravity modeling [Luetgert,1992].
100 km.
Featuresin thesedensitymodelsthat are held constantare the
densitystructureof the Franciscanrocksabove20 km depth
3.2. Constrained Gravity Modeling
and the apparentdip of the Gorda plate. In all of these2.5-D
Prior to collectingthe MTJSE data, little was known about models, the thicknessof Gorda lithosphereis based on a
crustalstructurein northernCalifornia.Existingdata included trenchageof 5.3 Ma and a half spreadingrate of 38 mm/yr for
unreversedseismicrefractiondata [Warren,1981], seismicre- the last5 Myr [Riddihough,1984]givingan approximateage of
fractiondata north of the MTJ [Beaudoinet al., 1994],crustal 8 Ma and thicknessof --•30-35 km for the lithospherebeneath
BEAUDOIN
125 ø
ET AL.: SOUTHERN
EDGE OF THE GORDA SLAB
124ø
123 ø
30,109
122 ø
41 ø
909
40 ø
39øN
Figure 5. Bougucrgravityin the regionof line 9 [after Oliveret al., 1980]. Dashedlinesrepresentprofiles
modeledbyJachens
and Griscom[1983].Shadedare regionsof -< -100 mGal anomaly.Stippleis regionof ->
-50 mGal anomaly.
line 9. A densitycontrastof 0.04 g/cm
3 is usedacrossthe
3.2.2.3. Preferred model (Plate 2d): Finally,we introduce
a lateral changein upper mantle densityas predictedby the
3.2.2.1. Direct conversion (Plate 2b): Our first model is a slabwindowmodel. The lack of Pn arrivalsfrom shotpoints
direct conversionof the velocitymodel to densityusinga sim- 901 and 902 (Figure2) and the extentof modeledray coverage
ple velocity-densityconversion appropriate for Franciscan (see Plate 1) suggesta zone of increasedscattering,intrinsic
rocks[seeGodfreyet al., 1997,Figure 7) (Plate 2b). From this attenuation,or both that affectsenergy propagationthrough
model it is evident that the apparent dip of the Gorda slab the centerof our model[cf.Beaudoinet al., 1996].We attribute
(corresponding
to yellow and orangecolorsin Plate 2b) will thiszone of attenuationto the southernedgeof the Gorda slab
provide much of the needed northward increasingBougucr (SEDGE). With this in mind, our preferredmodel introduces
anomalyon the northernend of line 9. The large mismatchon a slabedge(SEDGE) that coincideswith the envelopeof ray
the southernend indicatesthat further adjustmentto the lower coverage(Plate 1; dashedwhite line, Plate 2). By introducing
3 tabularbodyonthenorthernend,we are
crustor uppermantleis requiredin thisregion.This modelhas this3.20-3.23g/cm
lithosphcr½-asthcnosphcr½
boundary.
an rms residual
of 26.4 regal.
3.2.2.2. Adjustedgravity Moho (Plate 2c): The first logical modificationto our direct conversionmodel is to adjustthe
gravity Moho to coincidewith the modeled reflector depths
from the wide-anglereflections.This is justified becausethe
tomographicinversion smoothesthis contrast, as discussed
above. A density discontinuitywas therefore introduced to
coincidewith the basalcrustalreflectors.With this change,the
observedBougucranomalyis remarkablywell matched(rms
residualof 9.7 mGal), implyingthat mostof the gravityanomaly can be accountedfor by structurein the lower crust.The
predictedanomalyis only weakly sensitiveto the variationsin
mantle geometryif the mantle densityremainsconstantalong
the profile.
requiredalsoto includea sub-Moho3.20-3.23g/cm
3 highdensitywedge on the southernend of the profile to fit the
observedgravity.The boundarybetween Gorda crust and the
southern basal crustal layer was also adjusted from model
coordinate110 km to 95 km to better fit the gravitylow in the
centerof the profile.The final modelhasan rmsresidualof 5.8
regal with the largestmisfitsbetweenmodel coordinates90130 kin. Removing either the high-densitywedge at Moho
depthson the southernend or the Gorda mantle slab on the
northernend degradesthe fit (rms residualsof 11.7 regal rms
and 20.0 regal rms, respectively,Plate 2d).
Our preferredmodelhasfour main features:(1) The North
American crust is thickestin the center of the profile. This
thickeningof low-densityrocksin the middle of the profile is
30,110
BEAUDOIN
ET AL.: SOUTHERN EDGE OF THE GORDA SLAB
-20-
_40'•
•
-60:
Gorda Plate w/Klamath
ß
ß
C9 -802
•"•-•oo2
-120-
rms:
4.8mGal
•4o5.A--]
20
o
40
lO
60
80
2.67
100 i":•0 l•t'0' 'l(•0 1'•'0 "•"60" 'Kl"im,4th'"
North American crust
2O
3. e5
E 30
.c
40
•
50
2.67(Gorda
upper
crust)
ewer crust
or,
a
60
70
'
0
20
40
60
80
100
120
140
160
180
200
220
240 km
-20-
•E
-8o
•,o - 100-
-] 2o-'
.140.,.,
•..,,B•.
Direct
Conversion
(rms:
26.4
mGal)
0
20
40
60
0
80
2.60
100
120 140
160 180 200
220 240'km
-
North
America•'•rust
20_.
30
ß
,.
3.•)
40
........
50
ß...........
Gordamantle
60
70
SE
0
NW
30'
20
40
60
80
100
120
140
160
180
200
220
240
km
Plate 2. Gravitymodelsalongline 9. Dottedwhite line, the ray coverageof our velocitymodel(see
Figure2). Solidwhitelines,densitycontours.Red circles,the observedBouguergravityalongline 9. (a)
Jachensand Griscom[1983]model (profileE-E' in Figure5) rotatedand modifiedto fit the Bouguer
gravityalongline 9. (b) A directconversion
of the velocitymodelto density.(c) Sameas Plate2b with
the Moho adjustedto match basalcrustalreflectors.(d) The best fit model with changesmade to
densitiesonly beneath20 km depth. Also shownare the effectsof removingthe Gorda lithospheric
mantleon the north and postulatedlithosphericmantlewedgeon the south.(e) Variationsin location
of southernedge of Gorda slab.
-20
-40
•
-60
o -80q
Og
-
e•.
ß©
-
140 Mo,
hoCorrected
!rms:
9..7
mOa!) .....
0
20
40
60
80
loo 120 140 160 l•o 260 "2'•.0240'km
!
"
North American
".
•- 20
•
30
•,
40
•
crust
2 80
ø'ø
-
' • .'7
.,,
50
60
'-,i
70
%%
0
20
40
60
80
-20L•_
100
120
140
160
180
200
220 240 km
pretErred
(black)
-40_[..__•
nomantle
wedge
SE(red;
rms:
11.7
mGal)
•o -60q
•x.,.•
*•••••_•
-80-1
'•Y•-'•-••
no
Gorda
mantle
NW
(green;
rms:
20.0
regal))
©ß
ß©
ß
•,'• -! 00
i14020.... Preferred
odcl
(
0
20
40
60
' .............
,
80
........, .........
100 120 140 160 180 2(50 2•.0 240km
0
ioJ
North American crust
,..
•.
•
.
•,.r,,
,,o••---•-•,
30
•"
..
2.80
"''•"•' ':'*/'•//'"-• Gd(({a
crust""
ßß
E• 40
____
ß ..
........
.
320
Gorda mantle
.
c2 50
60
\ \\
70
0
20
40
60
80
100
120
140
160
180
200
220
240 km
-20-
preferred(black;rms:5.8 mGal))
slabedge+25 km NW (green;rms:6.3 mGal)
slabedge-25 km SE (red;rms:6.4 Gal)
-40-•
.
•
-6o
co -80
regal)
.
..
•o -100-
-I 2o-'
.
140
,E---•
Extent
ofSouthern
edge
0
20
40
60
80
160
1•0
l•i0
l•J0 1•0
2130 2"•.
0 240'km
i
North American
crust
1o
•' 20
-.-- 30
•
"•
-.-
-- z/.• crust
"•orca
'.......
.....,"'
,/•. / • 7
E. 40
t2
.......
Gorda mantle
50
60
70
20
40
60
80
100
120
140
160
180
200
220
240
VE
2.50
2.75
3.00
Density(g/cm3)
Plate
2.
continued
3.25
km
I' 1.25
30,112
BEAUDOIN
ET AL.: SOUTHERN EDGE OF THE GORDA SLAB
A) Line9 Interpretation
901
902
903
V
904
905
906
V
! _ v
907
908
909
V .
910
v
911
v . .V-
1o
t,,:..Oo
ø"-'7:.'?
.:•:..•....-.,.?..øo•o
ø:q}"•.,",a•.
•'•:0,
• • o• •/,•,.•,,.,,•,.,,•
o.Y,.,,,J,
-oO:
ø•' ø North
Ooo
American
crust
' oo'..'.,•-
.:c.--;'-'-'.",,..'.:,,:,: •,
20J. •,:' :,.,,•:.
- - -
',,',
m
••..,•,
-7" ...-'• ........
40
ß
'"
tectonicall
.-.•,,.
.
"o
.,'
g, ,.,,.,•.•, ,,,,•o:
•, ,;,3,3,z
.:
•, :_-?.=-,
.,.•,,.
o o
.'zz.•
o
_ oo
•7
..
.... ,.,.•.•.:."..•.....,.z•.o
-
o o•5o,•x"'•__9oes 8'••oo
z.,'zZ///
rdaOcrust
•,•
:'//' •da½cfust
-
".
.
o o ..
-o
'
o
_
....
'3oceanm
.crust
or ""•.•..?
2'.'"".;:.•.
i............................................
Oorda
mantle
-
50_]
rnagmatm
newly
accreted
•' ','
6o-]underplate &thickened nartial
melt/
m SE
-•a'}
'""1 " ' ' I
0
'
20
! ' ' '-I
40
'
I"
60
•'
erosion
80
'
I
'
100
oo
I
120
'
!
140
•oø•
asthenosphere
2øø'/
NW
Iso'
]-
'
i
'
160
I
180
''
I
"
200
I
220
Magnitudes'o2+
03+04+ 05+ C)6+
'
I
240 km
VE1.1
B)decompression
melting
& quenching:
requires
extreme
rates D)slow
decompression
melting
+ tectonic
scraping
& assembly
NorthAmerican
Crust(fixed)
NorthAmerican
Crust(fixed)
......
//
.•ew.Pacific'
%•
t • • •'///•//GordaPlate•/
C)preexisting
feature:
requires
extreme
coincidence
//Gorda
Plate/•/
preferred
NorthAmerican
Crust(fixed)
_•pre•North
America'
• Gorda
Plate
•/
Plate 3. (a) Interpretation of our velocity model. Overlain are interfacesdetermined from wide-angle
reflections(heavyblack lines), regionsof high reflectivityfrom the depth-migratedsingle-foldimage (hachured),velocitymodelray coverage(dottedwhite line), and hypocenters
for 1984-1994from the Northern
CaliforniaSeismicNetwork catalogwithin 7.5 km of the profile (octagons,hypocentershaveerrorsin depth
of <-2.5 km and lateral positionof <-1.5 km). Below are three possibleexplanationsfor the body of mafic
materialat the baseof North Americancrustand immediatelyadjacentto the Gorda slab.(b) Decompression
meltingand rapid quenchingaddingmaterial to the North Americanplate. (c) A preexistingfeature of the
North Americanplate.(d) Slowdecompression
meltingcoupledwithtectonicoff-scraping
andassembly
of the
basal crustalmaterial as part of the Pacificplate.
to the location of the southern edge of the Gorda plate
(SEDGE). Usingour preferredmodel (Plate 2d) as a starting
velocities
at 23 km deptharemodeled
with0.1g/cm
3 density point, we have calculatedgravity along our profile for three
increase relative to the Gorda crust densities.This layer's models and compareit to both the observedBouguergravity
thicknessis basedon reflectionsfrom both the wide-angleand along line 9 and the calculatedgravity from our preferred
depth-migratedsingle-foldimage.(3) Gorda lithosphereis ex- model. In thesemodelsthe southernedgeof the Gorda slabis
tendedonly as far southas model coordinate---110km where movedfrom modelcoordinate105 km by -50 km (rms residmantle lithospheredensitiesgradeinto asthenospheric
density ual of 7.9 mGal), -25 km (rmsresidualof 6.4 mGal), and + 25
over 10 km (e.g., centeredabout model coordinate105 km). km (rms residualof 6.3 mGal). We concludethat our preferred
(4) A high-densitywedgeis presentat Moho depthsbeneath location of SEDGE could be in error by _+25km but that a
the southernend of our profile. However,the thicknessof this shift of 50 km is precludedby our modelling.
wedge is poorly determined.
In summary,first-order features from the velocity model
3.2.2.4. Southern edge of the Gorda slab (Plate 2e). In
that we demonstrateare needed to fit gravity data are the
this final modelwe test the sensitivityof our preferredmodel thickeningof the overlyingFranciscanterrane near the center
basedon the well-sampled,low velocitiesobservedin the inversemodel. (2) On the southernend of the profile the high
BEAUDOIN
ET AL.: SOUTHERN
EDGE OF THE GORDA
SLAB
30,113
of our profile, the existenceof a higher-density(relative to
overlyingFranciscan)lower crustallayer on the southernend
of our profile, and the truncationof the Gorda plate near the
center of our profile. Furthermore, given these featuresconstrainedby our seismicstudy,the gravity model requiresthe
presenceof high-densitymaterial below the Moho on the
migrated single-fold section, we can trace the Gorda crust
reflectionsfrom shotpoint 911 to just southof shotpoint 907,
coincidentwith the terminationof Gorda slabseismicity(Plate
3a). At this point we are unable to determinewhether the
reflectivitybeneathshotpoint 906 is geneticallyassociatedwith
southernend of our profile.
locally strong, reflectivity to the south. However, based on
first-arrivalray coverageand the wide-anglereflections,Gorda
Moho extendsbeyond these Gorda crust reflections to between shot points 905 and 906 (Plate 3a). Finite difference
modeling of amplitudesof Pn and PmP on shot point 911
supportsthis interpretation[Lendl, 1996].
We interpret the deep structurebeneath line 9 determined
from seismicand gravity data to indicate that the southern
edge of the Gorda slab is between shot points 905 and 906
(Plate 3a). Changesin Gordacrustreflectivityandcessationof
slab-associated
seismicity--•30 km north of this edge may be
due to either changesin slab continuityor a changein lithologies and therefore impedancecontrastand coupling,across
the North American-Gorda plate boundary. On the basis of
our velocity and gravitymodelingour model is generallycompatible with that of Jachensand Griscom[1983], in which the
Gorda slab extendsas far south as approximatelyshot point
904, althoughour preferred model placesthe southernedge
Discussion
Travel time tomographyusingfirst arrivalsalongline 9 producesa modelof relativelyuniformvelocity(<6.5 km/s) crust
to depthsof--•20 km (Plate 1). We interpretthesevelocitiesto
indicate
that Franciscan
rocks extend to at least 20 km beneath
our profile [Thompsonand Talwani, 1964]. At depthsgreater
than 20 km, south-to-northvariationsin both the velocityfield
and model ray penetration are observed.In our "transform
regime"betweenshotpoints903 and905, highvelocities(7.07.5 km/s) occur at 23 km depth. A similar onset of higher
velocities(an increasefrom ---6.5to 7.0 km/s)is modeledalong
crossingprofile line 1 [Godfreyet al., 1997; Henstock and
Levander, submittedmanuscript,1998]. Dip on this "layer" is
poorly constrained.In our "subductionregime" from shot
points 906 to 910, velocities>7.0 km/s define a southward
dippingzone from 22 km depth beneathshotpoint 910 to 28
km depth beneath shot point 906.
Within the North American crust, line 9 indicatesa deepening(-10 km) of the 7.0 km/svelocitycontourin the center
of our profile.A similarresult,interpretedasthickenedNorth
American crust along the eastwardprojection of the Mendocinofracturezone, hasrecentlybeen reportedby Verdonckand
Zandt [1994]. Verdonck and Zandt interpret their model to
indicate that the Gorda slabis flexing downward6-12 ø to the
south and that the local thickeningof the North American
crust is either due to compressionalforces associatedwith
Pacificplate convergenceor underthrustingof the accretionary
complex along the CascadiaSubductionZone. Our line 9
model is only 10ø oblique to the local trend of the Cascadia
SubductionZone, and therefore we expectno more than half
of the crustalthickeningto be attributed to moving downdip,
southwardalong our profile (-4-5 km depth variation between shotpoints911 and 906 for a 10øobliquityand a 12ødip
on the Gorda slab). If, however,the Gorda slab is flexing
southward as suggestedby Verdonck and Zandt, our data
suggesta maximum of 5ø southwardflexing. We prefer to
interpret the overthickenedNorth American crustas tectonic
thickeningin responseto the northward convergencebetween
Pacificand Gorda plates rather than underthrustaccretionary
complex[Beaudoinet al., 1996;Tr•hu et al., 1995]becausewe
are forced (Plate 3) to adopt a similar tectonic thickening
model to explainour uppermostmantle observations.
the reflective
band to the north
about 25 km farther
or with the more
diffuse
but
north.
The lower crust/uppermantle reflectivityand velocitystructure observed near the southern end of line 9 is consistent with
that modeled for line 1, which intersectsour profile north of
shotpoint 902 (Plate 1) [Godfreyet al., 1997;Levanderet al.,
1998; Henstock and Levander, submittedmanuscript,1998].
The -5 km of maficmaterial(6.5 km/s<_Vp <- 7.5 km/s)
overlyingmantle can be interpretedas either tectonicallyunderplatedoceaniccrust [cf. Benzet al., 1992] or magmatically
accretedmafic material [cf. Furlonget al., 1989]. We cannot
distinguishbetweenthesetwo modelson the basisof velocity
alone.
The modeledshallow(---23km) highvelocitiesand densities
in the center of our profile suggestup to 10 km of mafic
material
exists at the base of the North American
crust imme-
diately adjacentto the southernedge of the Gorda slab.Here
we explorethree possibleexplanationsfor this body of mafic
material. The first explanation is a localized region of basal
underplating(Plate 3b). Thermomechanicalmodeling indicatesthat in -5 m.y. 5-10 km of basalticunderplatecould be
generatedafter passageof the MTJ by decompression
melting
of the rising asthenosphere[Furlongand Fountain, 1986;Liu
and Furlong,1992].However,for decompression
meltingto be
the mechanismcreating this layer requires that we invoke
maximumdecompressionmelting, completemelt segregation,
and nearly instantaneousquenchingto producethe structure
observed,i.e., conditionsthat seemunlikely.A secondexplaReflections from the lower crust and Moho are observed in
nation is that this body is a preexistingstructureof unknown
both the wide-angleand depth-migratedsingle-folddata (Plate origin (Plate 3c), which also seemsunlikely sincewe see no
1 and Figure3). North of shotpoint 905, a southdippingzone evidencefor similarlower crustalstructurenorth of thisregion.
of high-velocitymaterial is coincidentwith south dipping re- It seemsan unlikelycoincidencethat sucha preexistingfeature
flectorsinterpretedas Gorda plate Moho. The high-amplitude would coincidewith the southernedgeof the Gorda slabtoday,
reflectionsaboveGorda plate Moho (arrowson Figure 3) are rather than arbitrarily in the past or future. Our preferred
interpretedto originateat or near the North American-Gorda explanationis a modelwherebybasalunderplatingis occurring
plate boundary.This interpretationis consistentwith models at expectedratesbut at a lower rate than requiredto createthe
for MTJSE line 6 [Beaudoinet al., 1996;A.M. Tr•hu et al., entirethicknessof maficmaterialmodeledhere (Plate 3d). As
manuscriptin preparation,1998],earthquakehypocenters,
and this new material cools,it is accretedto the easternedgeof the
projectedGorda slabdepth[cf.Beaudoinet al., 1994].Utilizing Pacificplate [cf.Furlonget al., 1989]and movesnorthwardwith
both the wide-angle reflection modeling and the depth- respectto North America. With this northwardmovementthe
BEAUDOIN
30,114
ET AL.: SOUTHERN
EDGE
OF THE
GORDA
SLAB
Important in our observationsand interpretation are the
variationsof lowercrustalreflectorsalongline 9 and the thickner, the mafic body is constructedby both magmaticunder- ening of mantle lithosphereto the south. The variationsin
plating and tectonic thickening.Although this model can lowercrustalreflectorsand the apparentabsorptionof seismic
explain observationsalong our profile, inconsistencies
exist energyfrom the southernshotssupporta zone of increased
whencomparingto observations
from otherwide-angleseismic scattering,intrinsic attenuation,or both, resultingfrom meprofiles [Hole et al., 1998]. Most notable is evidencefrom chanicalmixingof lithologiesand/orpartial melt in the central
onshore/offshore
line 1 that suggests
the southernbasalcrustal region of line 9 beneaththe onshoreprojectionof the Menlayer, like the overlyingNorth American crust,is faulted and docino fracture zone. This zone of attenuation combined with
thereforemostlikely part of the North Americancrust[Hen- the southwardthickeningmantlelithosphereindicatesthat our
stocket al., 1997].Thisrequiresthat the basallayerbe coupled profile crossesthe southernedgeof the Gorda plate and that
to the North Americanplate and henceunableto movenorth- directlyadjacentto this edgeis an asthenospheric
window.
ward to act as a bulldozeroff-scrapingmaterial alongits leading edge.
Acknowledgments. Supported by National Science Foundation
In our model (Plate 3a) the lithosphericmantle is thinnest grants EAR-9218209, EAR-9219870, EAR-9218968, and EARjust south of the southernedge of the Gorda plate and in- 9219598and by the U.S. GeologicalSurveyDeep ContinentalStudies
creasesin thicknessto produce a wedge to the south. This program.Volunteersfrom Stanford,Rice, OregonState,Lehigh,and
geometryis consistentwith a model in which the astheno- Humboldt State Universitiesand the U.S. GeologicalSurveywere
to the success
of thisproject.We particularlythank Ed Criley
sphererises to the base of the crust in a slablesswindow crucial
and the Crustal StudiesGroup at the USGS. Seismographs
from the
createdby the northwardmovementof the Gorda plate and Program for Array SeismicStudiesof the Continental Lithosphere
then cools,creatingmagmaticunderplatingand new mantle (PASSCAL), StanfordUniversity,and the GeologicalSurveyof Canlithosphere,as a functionof time sincepassageof the triple ada were used in this research.Land use, graciouslypermitted by
junction.Our proposedmodel for the thickeningof the lower individuals,corporations,and government,was essentialto our success.Specifically,we would like to thank Bob Bremner,Craig Brown,
crustalmafic layer overlyingthe region of thin mantle litho- Lee Bucknell, David Dodd, Bill Feeney, Gilbert Martin, Michael
sphereimpliesthat this mantle lithosphereand the overlying Ward, AndersonSolid Waste (Rick Morton), Bureauof Land Manmaficcrustareweldedto the Pacificplate andmovenorthwith agement(Michael Truden), CircleX Ranch(Marilyn Brooks),Georit relative to North America. This result supportsthe offset- giaPacificCo. (Tom Ray), Hoff Trust(Marie Hoff), LouisianaPacific
(Dave Shantzand Roger Krueger),MendocinoNational Forest
plate-boundary
model,as developedby Furlonget al. [1989], Co.
(DavidRoak), PacificLumberCo. (Tom Herman),RedwoodNational
rather than the modelof Bohannonand Parsons[1995]which ParkService(EdwardHaberlin),SierraPacificIndustries(JackFrost),
predictssignificantly
older, and thereforethicker,lithosphere SimpsonTimber Co. (JuergenMomber), Six RiversNational Forest
(Meredith Smith), Soper-WheelerCo. (James Soper), and Stover
adjacentto the subductedGorda slab.
South of our interpretedSEDGE, the region of thickened Ranch (Libby George). For use of their facilitiesduring our field
operationwe extendthanksto HumboldtStateUniversityand Blosser
Franciscancomplexand basalunderplatingoverliesa zone of Lane ElementarySchool(CharlesDavison).
increasedscattering,intrinsicattenuation,or both, resulting
from mechanicalmixingof lithologiesand/or partial melt left
in the wake of the Gorda plate. Our modelsuggests
that there References
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Conclusions
The MTJSE providesnew seismicdata that imagechanges
in crustaland uppermostmantle structureassociated
with the
passageof the MendocinoTriple Junction.The crustalvelocity
modelshows,on average,a laterallyhomogeneous
velocityfor
the Franciscanterraneand the greatestthicknessof -<7.0km/s
lithologiesin the centerof our profile.The high densitiesand
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(ReceivedAugust11, 1997;revisedJune 18, 1998;
acceptedJune 19, 1998.)