JOURNALOF GEOPHYSICALRESEARCH,VOL. 103,NO. B10,PAGES23,795-23,812,
OCTOBER10,1998
Crustal
structure
of the northwestern
Vizcaino
block and Gorda
Escarpment, offshorenorthern California, and implicationsfor
postsubductiondeformationof a paleoaccretionarymargin
BeateLeitner• andAnneM. Tr6hu
Collegeof OceanicandAtmosphericSciences,OregonStateUniversity,Corvallis
NicolaJ.Godfrey
2
Departmentof Geophysics,StanfordUniversity,Stanford,California
Abstract. The Vizcainoblockis an anomalously
shallowregionof the westernU.S. continental
marginlocatedsouthwest
of theMendocinotriplejunction.It originatedaspartof the
accretionary
prismof theNorthAmericaplateandwastransferred
to thePacificplatein the
MioceneasthePacific-NorthAmericaplateboundarymigrated-130km eastward,formingthe
GordaEscarpmentat its northernboundary.We presenthybridcrustalmodelsfor the
northwestern
part of the Vizcainoblockderivedfrommarineseismicandgravitydata.The
velocityanddensitystructureof thenorthwestern
Vizcainoblockarecompatiblewith
paleoaccretionary
complexmaterialsimilarto SanSimeon/Patton
terraneoverlyingoceaniccrust
or a maficlayer.The mostsignificant
resultof ourmodelingis an abruptincreasein Moho dip
from-5 øto -20-30 øbeneaththewesternedgeof the Oconostota
ridgealongthenorthwestern
marginof the Vizcainoblock.This Moho dip is steeperthanobservedanywherealongthe
Cascadiasubduction
zone,indicatingpostsubduction
deformation.
We suggest
thatthe
paleotrench
wasdeformedby compression,
whichreactivated
preexisting
thrustfaultsin theupper
crustandthickenedthecrustwithinthisapparentweakzone.At leastpartof thedeformation
predateslatePliocenePacific-NorthAmericaplateconvergence
andmayresultmainlyfrom
north-south
compression
betweenthePacific-Juan
de Fucaplatesacrossthe Mendocinotransform
fault.North-southcompression
continues
todayandmay dynamicallysupporttheuplifted
northernmarginof the Vizcainoblock,althoughtheprimarylocusof deformationshiftedto the
relativelyweak Gordaplatesometimepriorto 3 Ma.
1. Introduction
strikeand addressthe questionof why deformation
is focused
alongthe seaward
partof the accretionary
wedge.Resolution
of
The active Cascadiamargin north of the Mendocinotriple
the
Moho
dip
and
its
trend
is
better
in
this
study
than
in
many
of
junction has a shallow-dippingMoho and base to the
the
earlier
studies
because
of
the
range
of
strikes
along
which
accretionaryprism. In contrast,results from severaltransects
thisfeaturehasbeensampled.
acrossthe paleoaccretionary
prism making up the California
margin show steeplydippinglower crustand Moho segments
[e.g., Trdhu, 1991; Meltzer and Levander, 1991; Howie et al.,
1993; Holbrook et al., 1996; Miller et al., 1992, 1996],
indicating postsubductiondeformation.Marine seismic and
gravity data from the Mendocino Triple Junction Seismic
Experiment[Trdhuet al., 1995] allow furtherinvestigation
of the
tectonicprocesscausingthis deformationin the northwestern
Vizcainoblock. The hybrid velocityand densitycrustalmodels
derivedin this studyimagethe lower crustaldeformationalong
•Nowat Instituteof Geological
andNuclearSciences,
Dunedin,New
Zealand.
1.2. Vizcaino
Block
The Vizcainoblock formsan unusuallywide, triangularshaped,shallowregionon the continental
margin(Plate la)
offshorenorthernCalifornia whose tectonichistory and
basementstructurehas been the subject of considerable
speculation.
It is boundedby the Mendocinotransformfault to
thenorth,the SanAndreasfaultzoneto the east,andtheNavarro
discontinuity
to thesouthwest,
andit approximately
followsthe
continental
slopeandchangeof trendin magneticanomalies
at
its westernmargin(for a summary,
seeMcCulloch[1987a,b,
1989]).Its westernmarginis characterized
by a buriedbasement
high,knownas the Oconostota
ridge,whichis approximately
'-Now at Departmentof Earth Sciences,Universityof Southern parallelto topographic
contours.
California,Los Angeles.
Vizcainoblock basement
is juxtaposedagainstthe onshore
Papernumber98JB02050.
KingRangeterrane
(middleMiocene[McLaughlin
et al., 1982])
andcoastal
Franciscan
belt(Cretaceous
to Eocene
accretionary
complex[BlakeandJones,1981]) alongthe SanAndreasfault,
and abutsagainstthe Salinianterrane(Cretaceous
plutonic
0148-0227/98/98JB-02050509.00
terrane [Mattinson, 1978]) and Franciscanor San Simeon
Copyright1998 by the AmericanGeophysicalUnion.
23,795
23,796
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
(a)
BATHYMETRY
(b)
GRAVITYFIELD
60
30
20
10
-
0
- -10
- -20
- -30
- -40
-50
-60
-70
-5
-100
mGal
EL__
126W
125W
(c) LINE 3S
0 ;-•:
.
124W
LINE 4
.....
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•-OR
123W
126W
OBSC4
125W
124W
123W
OBSA1
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25
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--- DEEP
REFLECTIONS
.:E•
.RAY
COVERAGE
OBSCA NE
5.5
20
4.5
30
40
(d) LINE4
LINE 5
i
, 1•, I
50
60
70
5.0
80
4.0
DISTANCE
(km)
I
•1
I
LINE 3
1', ,,
,
3.5
,!,
3.0
2.5
2.0
•1o
1.8
m 15
0o0
• 20
25
.
10
0
20
30
40
(e) LINE5
60
....
MOHO FROM GRAVITY MODELING
DISTANCE
(km)
OBH24
E 5
50
OBSC3
'"' -"'-'':-.........
OBH'I 8
OBH 17
OBSCl
OBH22
--
:[::10 •,,,•
uJ15
20
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0
N
10
20
30
40
50
60
DISTANCE(kin)
70
80
90
100
110
120
LEITNERETAL.:CRUSTALSTRUCTURE
OFTHENORTHWESTERN
VIZCAINOBLOCK
terranes (Cretaceous-Oligocene accretionary complex
[McLaughlin et al., 1994]) at the Navarro discontinuity.The
Vizcaino block is presentlypart of the Pacific plate and is
migratingnorthwardrelativeto the North Americaplate along
the present-daySan Andreastransformfault system[Atwater,
1970, 1989]. At present,much of the transformmotion in the
San Andreasfault systemappearsto be taken up in the Coast
Range east of the San Andreasfault along the Maacamaand
BartlettSpringsfaults[Castilloand Ellsworth,1993]. Seismicity
in the offshore region, an indicationof present-daytectonic
deformation,is concentratedalong the Mendocinotransform
fault andwithin a triangularregionin the southeast
comerof the
Gorda plate adjacentto the Vizcaino block (Plate l a). The
Vizcainoblock thereforeappearsto representa rigid part of the
Pacificplate.
The Vizcaino block basement is believed to have formed in an
accretionaryprism. This conclusionis supportedby lithologic
basementsamplesfrom four localitiesin the westempartof the
Vizcainoblock [McCulloch,1987a;McLaughlinet al., 1994], a
deepreflectioninterpretedto be the top of the subducted
plate
beneaththe westernmargin[McCulloch,1987a;Henstocket al.,
1996, 1997; Godfrey, 1997], and multiple basementridges
throughoutthe westem part of the Vizcaino block (Plate l a;
yellow lines)thathavebeeninterpreted
asblindthrustssimilarto
thoseobservedin accretionary
complexes
[Godfrey,1997].The
satellite-derivedfree-air gravity anomalydata over the westem
margin show a gravity low (Plate lb), typical for a subduction
zone trench.This low, however,is less prominentthan the
observedgravity low along the Cascadiatrench north of the
Mendocino transform fault or the paleotrenchsouth of the
Vizcaino block in centralCalifomia (Plate 1b). One objectiveof
this project was to determinewhether this is entirely due to
differencesin the accretionaryprism geometryor if structural
deviationsfrom a typical paleotrenchboundaryare requiredin
the underlyingcrust.
1.2. Gorda Escarpment
The morphologyof theMendocinotransformfault changes
at
about 126øW. East of 126øW, the Mendocinotransformfault is
at the base of the Gorda Escarpment,a steep,north facing
escarpment
wherethe Vizcainoblock is elevatedup to 1.5km
23,797
abovethe Gordaplateseafloor(Plate1a). This topographic
step
is oppositeof what is predictedfrom the agedifferencebetween
the Pacific(old) and Gorda(young)plates.West of 126øW,the
seafloorof the Pacificplateis, on average,1 km deeperthanthe
adjacentGorda seafloor,consistentwith the age and density
contrastbetweenthe two plates[Engebretson
et al., 1985], and
the transformis immediatelynorth of a 5-15km-wideridge
knownastheMendocinoRidge.Basaltcobbles
dredgedfromthe
ridge and submersibleobservations
indicatethat the Mendocino
Ridgewasonceabovesealevel [Krauseet al., 1964;Duncanet
al., 1994]andhassincesubsided.
Agesof rocksamples
fromthe
MendocinoRidgewestof 126øWrangebetween7 and 22 Ma,
indicatingthat Gorda plate material was obductedonto the
Pacificplate [Duncanet al., 1994]. In contrast,the only dated
samplefrom the GordaEscarpment
is Cretaceous
[Fisk et al.,
1993]. These observationssuggestthat the evolution of the
Gorda Escarpmentis connectedwith the developmentof the
Vizcainoblock and may havean uplift and subsidence
history
that is different from that of the MendocinoRidge west of
126øW. A secondobjective of this study was to determine
whetherthe basementridge along the Gorda Escarpmentis
gravitationallycompensated
by a crustalroot.
1.3. Plate Tectonic History
The present-daystructureof the Vizcaino block and Gorda
Escarpmentis the integratedresultof plate boundaryprocesses
sincethe Oligocene.Numerousworkershave reconstructed
the
plate boundaryevolutionin time and spacebasedon offshore
magnetic anomalies, fault offsets in California, and other
geologic constraints [e.g., Atwater, 1970; Graham and
Dickinson, 1978; Engebretson et al., 1985; Atwater and
Severinghaus,1989; Drake et al., 1989; Sedlockand Hamilton,
1991;Fernandezand Hey, 1991;Lonsdale,1991; Wilson,1993;
McLaughlinet al., 1994;Nicholsonet al., 1994;Bohannonand
Parsons,1995;McCroryet al., 1995].The four importantstages
of plateboundaryevolutionin the historyof the Vizcainoblock
are subduction,extension,transferto the Pacific plate, and
Pacific plate motion (Figure 1). The Farallonplate subducted
beneath the North
American
continent until about 27-25 Ma
(Figure l a), whenthe Pacific-Farallon
ridgereachedthe trench.
Spreadingand subduction
of the ridgebeneaththe accretionary
Plate 1. (top)Map of theMendocinotriplejunctionregion.Red(datapresented
in thispaper)andwhitelinesmark
theR/V Ewingshiptracksalongwhichseismic
andgravitydatawerecollected
duringthe 1994Mendocino
triple
junctionseismicexperiment.
Grayline is a seismic
reflectionline of 1977vintageshownin Figure9. SAFZ, San
Andreasfaultzone;MTF, Mendocinotransform
fault;ND, Navarrodiscontinuity;
PFZ, Pioneerfracturezone;VB,
Vizcainoblock,OR, Oconostota
ridge,PA, PointArena;BSF,BartlettSpringfault;MF, Maacamafault.Faultsto
theeastof the SAFZ andtheCascadia
trencharefromCastilloandEllsworth[1993].The westernmarginof the
Vizcainoblockis shownasa dashedline [afterMcCulloch,1987a].Opentrianglesmarktheapproximate
location
of the paleotrench
southof the ND; solidtrianglesmarkthe positionof the Cascadia
trench.(a)Bathymetry
(NationalGeophysical
Data CenterCD-Romrelease1.0, Geophysics
of North America).Basementsample
locations
shownasyellowstars[McCulloch,1987a]andbasement
ridgesafterGodfrey[1997] shownasyellow
lines.DSDP Site 173 is markedby a yellowsquare,ODP Site 1022is markedby a yellowcircle.Soliddotsare
earthquakes
with M>3 (northern
Califomiaearthquake
datacenter,1970-1996).The insetlocatesthemapregion
andVizcainoblockin Califomia.(b) Globalmarinegravitydata[Sandwell
andSmith,1997].The solidboxmarks
theregionof theMohokinkdiscussed
in thisstudy.(bottom)Velocitymodelsof lines3S,4 and5 incorporating
basement
depthandsediment
velocities
fromMCS data.Theportionof themodelconstrained
by seismicdatais
colored.OR, Oconostota
ridge;PAB, Point Arenabasin.(c) Velocitymodelof line 3S. Note the absenceof a
continuous
deepreflectorbetweenkm 15 and45. In thisregionbasement
topography
is roughandenergygets
scattered
in thedeepersection.(d) Velocitymodelof Line4. Onlyoneinstrument
recorded
thedata,andthemodel
is constrained
by theintersecting
lines3 and5 velocitymodels.(e) Velocitymodelof line5. The crustsouthof the
GordaEscarpment
is twiceasthickasto thenorthandincreases
abruptlybeneath
theGordaEscarpment.
LEITNER ET AL.- CRUSTAL STRUCTURE OF THE NORTHWESTERN
23,798
VIZCAINO BLOCK
Vizcaino Block History
• (a)Subduction
Phase
/
",,(b)
Extensional
Phase
/
1:•15
priørtøabøut27'25Ma/112
to20-15Ma
1.23.5xx27-25
/ / 114
33
I ¾, •%_. North
'// 13335
I •
A•erica]
_1 ' Farallon•
/plate
I
x (c)Transfer
Phase/
26-18
to?-11
Ma /11
\
124.5\
35
4•I ,• ,•
IJDF
P,,•
• I ,plate
%_
/ I
25• ...........
ß
ß
•t• !2527
112
121.5
Active Trench
Paleotrench
Fault
r/
...........
R
123.5
2733
114
124.5
x (d)Pacific
Plate
Phase
/
?-11
Ma
topresent
/119
/
\
128.•
• .40
43'
NAP
/
//
,
II.GordaP'\
43
//NAP
i !/\'¾•',•
I '.,'
'
nterey
FZ
• ß //
•
. 33
115
37
128.5
119
Trench
Reactivated
byExtension
--------Subducted
Spreading
Center•
Oceanic
Crust/Mafic
Layer
Present
Coastline
(hypothesized)
...• Region
ofTransfer
Magnetic
Anomaly
(Ma)
Spreading
Center
Figure1. Cartoonof theplateboundary
evolution
(adapted
fromBohannon
andParsons[ 1995]at 30, 20, 10,and
0 Ma) highlighting
the importantphasesin Vizcainoblockhistorysincethe Oligocene.
Locations
of the cross
sections
areindicated
by thesolidlineonthemapview.Agesin Ma areupperandlowerbounds
for thegiven
processes.
MFZ, Mendocinofracturezone;PFZ, Pioneerfracturezone;JDFP,Juande Fucaplate;NAP, North
Americaplate;PP, Pacificplate.(a) Subduction
phase;subduction
of the Farallonplatebeneaththe NorthAmericancontinent
untiltheridgecollidedwith thetrenchabout27-25 Ma. The Vizcainoblockis partof the
accretionary
wedgeat thewestern
marginof theNorthAmericaplatenorthof theMendocino
fracturezone.(b)
Extensional
phase;
followingridgecollision
about27-25Ma, thePacificplateandthePacific-Farallon
ridgelikely
continued
subduction
andridgespreading
[McLaughlin
et al., 1996]untilabout20-16Ma. The ridgepossibly
continuedto moveeastwardand couldhavecausedrelativehigh heat flow, magmaticunderplating
and the
formation
of thePointArenabasinin itswake.(c) Transfer
phase;offshore
transform
initiated20-18Ma possibly
on a detachment
surfacebeneaththe accretionary
wedge(dashedline in crosssection)and connectedthe
Mendocinofracturezonewith the northwardextension
of the Pacific-Farallon
ridge systemto the southby
initiatingtheSanAndreas-Pilarcitos
faultsystem[afterMcLaughlinet al., 1996].The detachment
regionbeneath
theaccretionary
wedgemarkedin graygrowseastward
with time.At about12.-10Ma, the SanAndreasfault is the
masterfault andthe Vizcainoblockis now partof the Pacificplate.(d) Pacificplatephase;the Vizcainoblock
moveswith thePacificplatealongthe SanAndreasfaultsystem.About3.5 Ma, nearPointArena,the SanAndreas
faultchanged
its trendto a morenortherlydirectionresponding
to a clockwisechangeof Pacific-North
America
relativeplatemotion.
prismmay have continueduntil 20-16Ma [Severinghaus
and
Atwater,1990;Bohannon
andParsons,1995;McLaughlinet al.,
1996](Figure1b).
Between20 and 12Ma, therewas a changefrom oblique
subduction
to transform
motionontheSanAndreas
faultsystem.
Theevolutionof transform
motionin timeandspaceduringthis
periodis poorlyconstrained,
McCroryet al. [1995] speculated
thatbetween20 and 16Ma, transform
motioninitiatedalonga
subsurface
detachment
at theslab-accretionary
complexinterface
andthatthislow-angletransform
boundarypropagated
eastward
and upward(Figurel c) formingthe GordaEscarpment
in its
wake.It remainsuncertain
if theeastward
migrationof theplate
boundary
wasa continuous
process
throughtimeor occurred
in
distinctsteps[e.g., Dickinsonand Snyder,1979; McCulloch,
1987b;Drake et al., 1989; Griscomand Jachens,1989; Sedlock
andHamilton,1991;Smithet al., 1993;McLaughlinet al., 1994,
1996].
Al•out12-10Ma the primarylocusof transform
motion
apparentlyshiftedto the Pilarcitos/San
Andreasfault system
[McLaughlin
et al., 1996].Griscom
andJachens
[1989]propose
that an offshoretransformfault west of the present-day
San
Andreasfault zone was activeuntil about5 Ma. The apparent
absenceof strike-sliptypedeformationin seismicreflectiondata
from the 6-10 Ma Delgada fan [Drake et al., 1989] and
underlyinglate Miocene sedimentsthat overlie the Vizcaino
block[Hoskinsand Griffiths,1971;McCulloch,1987a;Drakeet
al., 1989; Godfrey, 1997; Ondrus, 1997], however,has been
usedto argueagainstthis interpretation.
Godfreyet al. [this
issue]presentevidencethat the present-day
Mendocinotriple
junction may have been at its presentlocationrelativeto the
Mendocinotransformfaultsignificantlyearlierthan11 Ma.
The uplift of both the Mendocino Ridge and Gorda
Escarpmentmay have been a result of compressional
stresses
acrossthe transformcausedby changesin relativeplatemotion,
[e.g., Riddihough, 1984; Wilson et al., 1984; Duncan and
Clague, 1985; Cox and Engebretson,1985; Wilson,1988, 1993]
and by differencesin thermaland viscoelastic
propertiesof the
adjacentoceanic lithospheres[Bonatti, 1978]. The latter are
partlycontrolledby the agedifferencebetweentheplates,which
changedpolarity at about 12 Ma [Engebretsonet al., 1985].
North-south compressionincreased considerablyafter the
inceptionof the Blancotransform(about6 Ma) andcontinues
to
LEITNERET AL.: CRUSTALSTRUCTUREOF THE NORTHWESTERNVIZCAINO BLOCK
23,799
the present-day
[Wilson,1993]. Part or all of the present-day were digitized from the record sections.A strongmultiple
north-southcompression
is absorbedby the Gordaplate,where (markedm, Figure2) arrivingwith a timedelayequalto thetwothe curvature of magnetic patterns and shorter magnetic way travel time (twtt) throughthe water columnabovethe
anomalies on the east side versus the west side of the Gorda
instrument
is apparent
on all instruments.
The amplitude
of this
ridge indicate internal deformation [e.g. Silver, 1971; arrivalis oftenstrongerthanthe first arrival,andit wasusedto
Riddihough, 1980; Wilson, 1986, 1989; Stoddard, 1987; verify andextendtherangeof first-arrivalpicks.
Denlinger,1992].Thatthemorphology
of theGordaEscarpment A startingmodelfor 2-D inversionof traveltimes[Zelt and
is different from that of the MendocinoRidge west of 126ø, Smith,1992] for line3S utilizedbasement
depthstakendirectly
whereasthe sametectonicforcesare active, suggeststhat it is from a depth-migrated
reflectionprofile[Henstock
et al., 1996].
The startingmodelsfor lines4 and 5 were constructed
from
stronglyaffectedby thecrustalstructure
of theVizcainoblock.
bathymetricand time-migrated
MCS data.For lines4 and 5,
seafloor to basementtravel times were convertedto depth
2. Data Acquisition,Processing,and Modeling
assuminga constantvelocitygradientof 0.7 kms-•km
'• and a
We presentlarge-aperture
seismicandgravitydatacollected constantvelocityof 1.8kms'• at the top of the sedimentlayer.
over the northwestern
part of the Vizcaino block, the Gorda This velocitygradientis an averageof the velocitygradients
Escarpment,
andthe Gordaplateduringthe 1994phaseof the obtainedfrom 1-D travel time modelingof near-offsetdata for
MendocinoTriple JunctionSeismicExperiment[Trdhuet al., the line 5 instruments.In addition,the velocity model at the
1995]. The R/V Ewing recordedmultichannel
seismic(MCS), northernend of line 5 was constrainedin the inversionby
potentialfield and bathymetric
dataalong14 profiles.Results velocityinformationfromline 6 [Tr•hu et al., 1995].
fromthreeof theseprofilesarepresented
in thisstudy(Plate1a).
Velocity models(Plate l c-le) were determinedthroughan
Four oceanbottomseismographs
(OBS) and four oceanbottom iterativeforward/inverse
modelingapproachin whichthe initial
hydrophones
(OBH), deployedfromtheR/V Wecoma,
recorded inversion was performedfor a three layer model (water,
shotsfromtheR/V Ewingtunedair gunarrayalonglines3S, 4, sediment,and crust)with velocitiesin the water and sediment
and 5 (Plate l a, markedin red). We constructhybrid crustal layersfixed and velocitynodesto be determined
by inversion
modelsusing the MCS data to constrainsedimentthickness placedat 20-kmintervalsalongthetopandbottomof thecrustal
alongtheprofiles,thelarge-aperture
datato constrain
theP wave layer. Becausevelocitiesbetweennodes increaselinearly,
velocityandthicknessof the crustwherepermittedby the ray velocitydiscontinuities
and gradientchangesin the crustwere
coverage,andgravitydatato extendconstraints
on thegeometry modeledby introducingadditionallayers,and the data were
of the lower crust and Moho. Details of MCS processingand
reinvertedafterthe additionof eachlayer.
The introduction
of an uppercrustallayer,allowingrelatively
et al., this issue;T. Henstock,personalcommunication,
1997;A.
low velocitiesanda high-velocitygradientin theupperbasement
S. Meltzer,personalcommunication,
1997].Magneticfield data of the Vizcaino block, reducedthe travel time misfit for lines3
arenot discussed
in thispapersincethe magneticfield variation and 5. Constraintson the velocity and thicknessof the lower
overthe Vizcainoblockis smallalonglines3S, 4, and5 southof crustof the Gordaplateon line5 wereincludedasindicated
by
the Mendocinotransformfault.The magneticfield of the Gorda resultsfrom line 6, wherereversedarrivalsareobservedfrom the
interpretation
are discussed
elsewhere
[Godfrey,1997;Godfrey
Escarpment
wherecrossed
by line5 iswellmodeledassuming
an
averagesusceptibility
of oceaniccrust.Directly north of the
MendocinoRidge, magneticlineationsfrom the Gorda crust
trendobliquelyto line5, andtwo-dimensional
(2-D) modelingis
not valid.
lowercrustanduppermantle[Tr•huet al., 1995].The modeled
Pn and Prop arrivals were not included in the inversion. The
Moho depthof lines3 and 4 were constrained
by MCS and
gravitydata,wherepossible.The final modelsfor lines3S, 4,
and 5 containtwo, one, and three crustallayers,respectively.
Final travel time misfits are summarized in Table 1.
2.1. Modeling the Large-Aperture SeismicData
2.1.1. Line3S. Line 3S lies entirely within the Vizcaino
block, extendingdiagonally from its western boundaryat
125.2øW,39.7øN to the coastnear CapeMendocino(Plate la).
The profile is orthogonal
to the Oconostota
ridge and mapped
basement
ridges,providinga crosssectionof theVizcainoblock.
shotson lines3S, 4, andthe middle sectionof line 5 were shotat Water depth decreasesgradually from west (3 km) to east
20 s intervals(about 50 m) to optimizethe MCS data. This (0.5 km) along the profile, with a relatively rapid decrease
resulted
in bandsof previous
shotnoise(PSN)thatinterfere
with betweenkm ! 3 and 18 (the seawardedgeof the Vizcainoblock)
is shallowest
betweenkm 18 and38,
crustalarrivalsin the OBH/OBS recordsections(Figure2). Shot (Figure3a). The basement
to the Oconostotaridge. A deep sedimentary
spacingat bothendsof line 5 was 50 s (about130 m), whichis corresponding
preferable for large-aperturedata when seafloor depths are basin (the Point Arena basin) overliesthe easternend of the
thickenwestof theOconostota
ridge.
greater than 1000m [Christesonet al., 1996]. Although the profile,andsediments
For this profile, we focuson usinglimited seismicdata in
signal-to-noise
ratio on the OBHs was generallyhigherthan on
the OBSs, the OBHs were more stronglyaffectedby PSN conjunctionwith gravity data to explorea range of possible
models for the Moho configurationbeneaththe Oconostota
(Figure 2). PSN could be reduced,but not eliminated,by
effortsby Henstocket al. [1996]
applyingan F-k filter to the data after shiftingthe datato align ridge.This work complements
to determinestructurebeneaththe triple junction region using
tracesalongthe previousshotinstant[Christeson
et al., 1996].
data along two long profiles
After applyinga time domainband-pass
filter (4-30 Hz), gain MCS, OBS, and onshore-offshore
amplificationas a functionof rangeand a 6.5 kms-• reduction that crossapproximatelyat the triple junction. We presenta
velocity to the OBHs/OBSsdata, and after adding2 adjacent detailedanalysisof a subsetof the seismicdata usedfor their
traceswhen data were shotat a 20 s interval,travel-timepicks model.
Large-aperture
datacoverageis beston line5, alongwhich6
instrumentsrecordeddata (Plate l a). At the intersectionof
lines4 and5, OBH 24 recordedair gunshotsfrombothlines.On
line 3S only2 of 3 instruments
deployedrecordeddata.Air gun
23,800
LEITNER ET AL.: CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
SW
LINE 3S OBS C4
NE
6
(a)
5
•
4
I:: 2
-40
-30
-20
- 10
SW
0
10
20
30
LINE 3S OBS A1
40
NE
(b)
-60
-40
-50
-30
LINE 40BH
-20
-10
24
(c)
NW
[,
4O
(d)
s
<
> <
3O
20
LINE5 OBH24
shotspacing20 S
10
>
•7
N
FK Filtered
7
10
20
30
40
50
60
70
30
40
50
60
70
RANGE (km)
Figure2. Processed
recordsections
(seetextfordetails)of oceanbottominstruments
recording
(a) and(b) line3S,
(c) line4 and (d)-(i) line 5 MCS air gun shots.Shotspacingfor line5 instruments
is indicatedat top. PSN,
previousshotnoise;m, watermultiple;EQ, M w5.0 earthquake
locatedoffshorePetrolia,California[Braunmiller
et al., 1997].Calculatedarrivaltimesfor thevelocitymodelsin Plate1 (c)-(e) areshownby the solidlines.White
arrowsindicatepossiblelowercrustalor Mohoreflections
notconstrained
by ourline 3S velocitymodel.Possible
mantlearrivalsonOBSC3 andOBH 18 (marked'?') beneath
theGordaplateareobscured
by PSN.
The large-aperture
seismicdataonly sparselysamplethe crust high-velocitylower crust at a depth of 9 km beneath the
(Plate l c). OBS C4 (Figure 2a) recordeddiving waves (Pg), northeastern
end of line 3S [Henstocket al., 1996]. OBSA1
samplingthe upper4-6 km of basement.A possiblewide-angle (Figure2b) recordedPg arrivalsto an offsetof 75 km, sampling
reflection from the mid crust beneath the northeastern end of the
the lowercrustto a depthof 18 km. Ray coveragein the lower
line is markedby white arrows(Figure2a) andis consistent
with crustis limitedto km 30 to 60 in themodel.An arrivalthatmay
onshore-offshore
observations,
which require a thick layer of representa reflectionfrom eitherthe top of the oceaniccrustor
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
LINE 50BS
(e) S
<;.
•
50 s
><
C3
N
shot spacing20 s
>
:;:-:":•?.?'".•
•.•.-'..-•-!t-,:.;•-.,:•.••l••
•l•J••"•'
'•••.• "•'"•"'
'•'
ß'" •-'• '
•'•
......
• 4
'"•.
:•
ß ...............
[:22
.:" EQ:'•I [ :•
I
-20
-10
s
<
(f) 7
0
10
20
30
40
LINE
50BH 18
50 s --><
N
shot spacing20 s
>
6
•'5
c64
½3
2
1
-40
-30
S
<
(g)
-20
50 s
- 10
0
10
20
LINE5 OBH17
> <
shot s
20 s
30
> <-• 50 s-•>
N
7 ,
%- 6
-50
S
<
(h) 6,
-40
50 s
•
-30
-20
- 10
0
LINE50BS C1
><
20
30
N
--><--
shot spacing20 s
50 s--->
.-
•
--.• -• •
I
I
-60
-70
I
--•
I
-50
. -.
- •
I
-40
I
-30
-20
I
-10
S<
20s
><
.........
I
I
0
10
LINE 50BH
(i) 6
10
20
30
40
22
shot
spacing
50s
>N
5
-50
-40
-30
-20
RANGE (km)
Figure 2. (continued)
50
- 10
0
23,801
23,802
LEITNER ET AL.: CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
Table 1. RMS Valuesof VelocityModelsbasedonPg Arrivals
(Plate1c-1e) andGravityModels(Figure7)
Models
Line 3S
Line 4
Line5
Velocity model
0.13
0.077
0.097
2.83
1.78
1.94
3.5
Gravity models
Model I
Model II
Model III
3.1
2.87
Misfits in s (velocity)andmGal (gravity).
fromthe Moho is markedby whitearrows(Figure2b), but due
to our limited ray coveragein the lower crust, we cannot
the 3-km-deepGordaplateoceanfloor.MCS dataconstrain
the
sedimentthicknessalong the profile (Figure 3d). Sediment
thickness
increases
towardtheridgefromthe southbetweenkm
20 to 40 and pinchesout againstthe ridge crest.North of the
ridge,thebasement
is offsetby largefaultsthatcutthrough
the
sediment
and,in at leastonecase,offsettheseafloor.
No deep
reflections are observed.
Rapidbathymetric
andsediment
thickness
changes
dominate
the record sections(Figures2d-2i). Modeled arrivalsin the
Vizcaino block are diving waves (Pg) throughthe crust
(OBH24, OBSC3, OBH 18, OBH 17 south, and OBSC1
south).Shotsfrom the southernendwererecordedasfar northas
OBS C1 (offsetof 76 km). F-k filteringenabledus to verify
uniquelymodelit.
A lateral velocity changein the basementrocks at about
traveltimepicksbetweenoffsetsof 28-45 km on OBH 24, where
previousshotnoise(PSN) obscured
the arrivalson theunfiltered
km 40 to 50 is requiredto modelthe near-rangearrivalsof
OBSC4, which are only slightly asynm•etricdespite a
data(Figure2d). The final velocitymodelsouthof the ridge
(Plate l e) hasa 1.5-km-thickintermediate
velocitylayer(2.54.5 kms'•) thatis underlainby rocksin whichvelocityincreases
pronounced decrease in sediment thickness west of the
instrument(Figure3a). In orderto matchthesearrivals,a 2-km-
approximatelylinearly from 5 to 7.4 kms'•. The thick crust
thick layer with velocitiesof 3.5-4.5kms'• was addedjust
extends
beneath
theMendocinoRidgeto theGordaEscarpment,
beneaththe basementwestof OBS C4 (Plate lc). The thickness northof whichcrustalthickness
decreases
abruptly.
The northerninstruments
werein an areaof roughbasement
of andvelocityin thislayeragreewith an intermediate
velocity
layer modeledat the southernend of line 5 and on the western topography,
leadingto largediffractions,
considerable
scattering
part of line4 (seediscussion
of theselinesbelow).Beneaththis of seismicenergy, and maximum observedoffsetsof about
30 km (OBH 22, OBS C1 north, OBH 17 north). Instruments
depth,velocitiesincreaseapproximately
linearlywith depth.A
dip in the velocitycontours
beneathOBSC4 maybe an artefact northof the ridgeare modeledby uppercrustalvelocitiesof 5of uncertainties
in the detailedconfiguration
of the baseof the 6 kms'• overlyinga lower crustwith velocitiesof 6.5-7.6kms'•.
overlyinglayer.
The mid-crustal
boundaryis at 6.7 km depth,andtheMohois at
2.1.2. Line4.
Line4
extends for 70km
across the
10km depth(Plate1e). Mohodepthis constrained
primarilyby
northwestern
part of the Vizcainoblock,crossing
line 5 at km 7 resultsfrom line 6 [Tr•hu et al., 1995] and is consistent
with a
andline3 at km 50. Almosttheentireline lieswithintheregion possibleProp arrival on OBH 18 and Pn arrival on OBH 17
of the Oconostotaridge. MCS data define the top of the (Figures2f and2g).
basement(Figures3b and 3c), and no deeperreflectionsare
We can compareour velocitymodel at the southernend of
observed.Basementtopographyis rough, and the general line5 to resultsfromtheDeepSeaDrillingProject(DSDP)Site
appearanceof the datais similarto that of line 3S west of km 40
173 at the intersection
of lines4 and5 (Figure4). The 0.4-kmandline 5 southof theGordaEscarpment.
thicksediment
layerin our velocitymodelis in agreement
with
OBH 24, located at the intersectionof line4 with line 5,
the drilling results[Kulrn et al., 1973]. The top of our
recordedPg and possibleProp and Pn arrivalsto an offset of
intermediatevelocity layer has velocitiesof 2.5-4.5kms'• and
60 km (Figure 2c). With only one receiver,we cannotresolve
coincideswith the recoveryof andesiticbasaltat the bottomof
dipping structureand velocity gradientsuniquely,although
comparison
with modelsfor lines3S and 5 help constrainthe
model.The resultingvelocitymodel(Plate1d) fits the datawell
(Table1). Beneaththe sediments,
an intermediate
velocitylayer
(3.5-4.5kms-•) of 2 km thickness
overliesa layerwith velocities
thedrill holeat 330m. The velocities
in this 1.5-km-thick
layer
are compatiblewith velocitiesof highly porousbasalt or
SimeonterraneoffshorecentralCalifornia[Howieet al., 1993;
Holbrooket al., 1996].The lateralextendof the andesitic
basalt
of 5.0 to 7.4 kms '•.
found in DSDP Site 173 remains uncertain.
2.1.3. Line 5. Four OBHs and two OBSsrecordedair gun
shotsalong the 120-km-longline5 (Plate l a). Southof the
GordaEscarpment,
the profile lies in the Vizcainoblock andthe
seafloorrisesfrom 3 km at the southern
end of the profileto
1.5km depthat the MendocinoRidge.It thendropssteeplyto
We usedelasticfinite differencewavefield modeling(code
developedby Rodriguesand Mora [1992], adaptedby Lendl
[1996], summaryby Lendl et al. [1997]), to verify the model
alongline5. Forthiscalculation,
thevelocitymodelwasgridded
at a 50-m spacing,and an explosivepoint sourcewith a
consolidated sediments and are observed in Franciscan/San
Figure3. (a)Line3Sbathymetry
fromshipdataandtopofbasement
fromdepth
migrated
MCSdata.(b)Migrated
multichannel
seismic
dataalong
MCSline4 show
several
basement
ridges.
Sediment
horizons
above
theridges
are
deformed
andinterrupted.
(c) Line4 bathymetry
fromshipmeasurements
andtopof basement
fromMCS data
alongline 4 constrain
the velocityandgravitymodels.(d) Migratednear-trace
datafor line 5 shownwith
amplitudes
corrected
forspherical
spreading.
Thetopofthebasement
iswelldefined
along
theentire
profile.
The
pattern
of upliftandtiltingof sediments
ontheMendocino
Ridgeissimilar
tothatreported
bySilver[1971
]. A
moredetailed
discussion
of stratigraphy
based
onMCSdataispresented
byGodfrey
etal. [thisissue].
(e)Line5
bathymetry
fromshipdataandtopof basement
fromtimemigrated
MCSdata(A. S.Meltzer,
unpublished
data,
1997).Seetextfordetails
of thetwtttodepth
conversion.
Bathymetry
andsediment
thickness
changes
between
0
and2 km alongtheprofile.
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
(a) _INE
3S
LINE
4
OBSC4
' 0 [ • [ • I [ , , ] I • • , , v ....---... I ....
• 5 ....
c•10
, OBSA1
I .............
--" P•inl•
Ar'en•.
'
Oconostota
ridge
SOUTHWEST
[ ....
....
0
,
23,803
]
10
....
[
20
....
i
30
'
'
'
40
[
i
50
60
NORTHEAST
] ....
....
70
80
DISTANCE (km)
(b)
LINE4
NORTHWEST
½
Oconostota
ridge
SOUTHEAST
_
.........
LINE5
(c)
.-..5 .-•,•..-..•.••-
DISTANCE
(km)
...•
LINE3
0
•'
OBH24
2
•
-1-
LU 4
VE:2.5:1
•
0
]
10
20
,
,
!
,
30
40
50
60
LINE 5
(d)
SOUTH
i
Oconostota i
2
•DP 173 ridge
.......
r",,•
'•'•'•••
'•
"
Mendocino
Ridge
••'•
Gorda
__•-•-•--•?.•••••v-scarpmem
_
.
•-'-•"•'
'••••::!:
"••
'•--'-•"••'
•''
'- '
OBSC3
Basement
ridge
•;'":.•
'•/ :'½-f
' ''
%''"' .... :-";:
[:,:-?'?g",
:•:•,:,,";",,
,'!::7:,':,:"'i?,
......................
OBH24
NORTH
I
[
onGordaplate
' ....
: • -½?.
'"::""
....
ß?: '
•" ':,;"'"
:':..:'
.' ":•'.'"•.-.".-::½'"•..•,•
•
..... :"..........
:....: .....
...... .,.:.
..........
.•....7'.?
......•.,,
OBH18
OBH17
OBSC1
OBH22
(e) o
_
E
•v20_34-
0
I
I
I
I
10
20
30
40
VE 5:1
I
I
I
I
I
50
60
70
80
90
DISTANCE (km)
Figure 3.
I
100
11o
12o
23,804
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
DSDP
NW
173
Line 4
sea-floor
U/L Miocene
basement
•
•..
.
;;..,•,..'.,.,-.,,
,:•,••.•-...,.
:',,::r:.,
:,•.::.?•,•,•;;;.:•½;.,
.,....:
";,:-.:
.';,,:.•.,'...;_.,':i•.;:..'.'.•:',.'..
•'"':
:5
............
Figure4. Blowupof MCS dataforlines4 and5 attheirintersection
andDSDPSite173.Depthof themiddle/late
Mioceneboundaryconverted
fromtwtt is approximate,
m•dno distinctsediment
horizonis apparent.
We
maximumfrequencyof 10 Hz was put at each instrument waves,multiples,shearandconvertedwavesarereproduced.
location. The wave field was recorded at 50-m intervals on the
surface(i.e., the oppositeof the actualexperiment
geometry).
The syntheticrecordsectionfor instrument
OBH 17 showsa
closematchto the data (Figures5 and 2g). The travel time
advanceresultingfrom propagation
throughthe ridgematches
the data satisfactorily,and the relative amplitudesof direct
concludethat our modelgenerallyexplainsthe amplitudeof the
observedwave field evenin the complicatedridgearea.
2.1.4. Velocities. We comparevelocitiesin our modelsto
velocities derived for
the
onshore Franciscan, offshore
Franciscan/San
Simeon(paleoaccretionary
complexassemblages
of pre-Mioceneage) and onshoreSalinian(graniticbatholith)
"::':'
.......
.:"
•:'•,'•,:,,:,,,J."%:??::,•,:.
S'"':'::'!,.:
ii:!:.
'"•:•?:•':"•:.!•i
'•'•::•:.:"'::
..,::•i½?•:?:iii•,ii%?:i
.....
:•::''............................
p-s......
...'%,.:':.•:.i?½':::i!:i•.•%::')i::.
'::'.:.'.:•'r
•:'.".,.:..i•i:i::..?"•..."•:'.:?'"•"!
................
f 5-
S
-1
o
i
i
,
50 s
1
i
N
i
i
i
i
-
i
•
ß
:....,•,•
:.,;t•'.t ' :•a':•;;•'•
"' 't i'
•
I
-50
-40
-30
-20
- 10
0
10
20
30
RANGE (km)
Figure5. Amplitudeplotsof (top)synthetic
and(bottom)obse•edrecordsections
of OBH 17;m, multiplecaused
by reflectionat the water surhcedirectlyabovethe instrument;
S, shearwave throughthe ridge;p-s, P-to-S
conversion
at basement.
For comparison
seealsotherecordsectionin Figure2g.
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
VELOCITY (krn/s)
3
4
5
6
7
0
2
4
6
E 8
•
10
r• 12
14
16
18
20
Figure 6. Velocity-depth
profilesof lines3S, 4, and 5 of the
velocitymodelsshownin Plate 1 (c)-(e).The resolution
of our
velocitymodelsis bestin thetop6 km anddecreases
withdepth.
The indicatedregions(seelegend)markthe rangeof velocities
23,805
layerby replacingthemwith materialof averagecrustaldensity
(2.75 gcm'3in this study).The Bougueranomalyreflectsdensity
variationsbeneaththe sedimentlayer and can be comparedto
other gravity profiles with different topographyand basin
structure.
The main goals of the gravity modelingwere to test and
extendthe velocitymodels,puttinglimitson the Moho whereit
is not imagedby the seismicdata.We did not attemptto match
the observeddata exactly and focusedon explaininglong
wavelengthvariations,which reflect densityvariationsbeneath
the sediment layer. Starting gravity models incorporated
bathymetry,changesin sedimentthickness,
andlayerboundaries
from the velocitymodel.The averagevelocitywithin layersof
variablevelocity was usedto determinedensity.Velocity was
converted
to densityusinglaboratory
andfieldmeasurements
summarizedin Table 2. The crustwas split into middle and
lower crustallayerswith densitiesof 2.75 gem'3and 2.9 gcm'L
Lithospheric
and asthenospheric
densities
Werederivedby
addingdensitycontrasts
of 0.4 and0.36 gem-3respectively
to the
densityof the lowercrust(Table2). Two-dimensional
gravity
for Salinianterraneonshore[Walterand Mooney,1982;Howie modelingandinversionto obtairithe crust-mantle
boundarywas
et al.• 1993], Franciscan
terraneonshore[Walterand Mooney,
done with GM_SYS softWare (Northwest Geophysical
1982;Howieet al., 1993;Holbrooket al., 1996],andPatton/San
Simeon
terrane
offshore
[Howie
et al., 1993;Holbrook
et al., Associates,Corvallis,Oregon),followingthe theoryof Talwani
et al. [1959].RIMSvaluesfor the modelsoutlinedin thisstudy
1996].All velocity
profiles
arenormalized
to thetopof the
basement as 0 km. Velocities between 6.5 to 7.5 kms-' are too
are summarized in Table 1.
2.2.1.Line3S. We constrained
thedepthto theMohoat the
highfor Salinian,Franciscan,
or SanSimeon/Patton
terraneand
northeasternend of line 3S to be 20 km based on a deep
represent
maflcmaterialor underlying
oceanic
crtist.
reflectionobservedon the MCS dataat about8 s twtt [Henstock
et al., 1996]. This eventis nearlycontinuous
on MCS line3S
fromkm 45-76 [Henstock
et al., 1996].It is alsoseenon other
Vizcainoblock [GodJ•ey,
terranes
(Figure
6).Basement
along
lines
3S,4,and5 (south
of profilescrossingthe northeastern
the Mendocinotransformfault) is likely composedof material 1997;GodJ•eyet al., thisissue].
The Bougueranomaly(Figure 7b) decreasesby 150 mGal
similarto the Franciscanand SanSimeonterranes.The velocityfrom
westtoeast.Thischange
is3 timesgreater
thanthechange
depthprofilefor line 3S at km 30, representative
of thewestern
thissignal
part of the Vizcainoblock, has velocitiesin the uppercrust in flee-airgravity(50 mGal,Figure7a). We interpret
.
typicalof thosefor SanSimeon/Patton
terrane.In contrast, to primarilyreflectthechangein Mohodepthalongtheprofile.
velocitiesat a givendepth at km 55 are slightlyhigherandare
consisten{
with Franciscan
or Salinianterrane,whichare
Three differentdensitymodelsthat all incorporate
bathymetry
and sedimentthicknessand which are compatiblewith the
typically
6-6.3kms
-•.Velocities
inthelower
crust
along
line3S, gravitydataare shownin Figures7c-7e.All 3 gravitymodels
howeleer,
aretoohighto beaccretionary
complex
material,
anda
tectonically
or magmatically
underplated
maficlayeris likely.A
reflectionat 6-7 s twtt is apparenton a 1977 U.S. Geological
SUrveyMCS reflectionline located10-20km west of line5
[McCulloch,1987a;Godjbey,1997]. Using our velocity-depth
show(1) aneastdipping,
lowercrustal
layerbetween
km0 and
15(dip<= 5ø),(2) a transition
zonebetween
km i5 and30 with
a steeplydippingMoho (modelI = 21ø, modelII = 30ø, model
III = 54ø), (3) an abruptlateralchangein densityat somedepth
in the crustor uppermostmantlebetweenkm 40 and 50. This
profile,thisreflector
coincides
withthe6 kms-'contour
at about densitychangeis neededto accountfor a 20-mGalincreasein
a low9km depth, supportingthe interpretationof tectonically thefree-airgravityanomaly.ModelI (Figure7c) assumes
density
body
(2.55
gcm
'3
versus
2.75
gcm
'•)
at
a
shallow
level
underplated
oceaniccrust.
2.2. Modeling
theGravityData
We modeledflee-air gravityanomaliesrecordedby the R/V
EwingalongtheMCS profilesandmergedwiththenavigation.at
1-mintime intervals.The E6tvoscorrectionwasapplied,andthe
Table2. Velocity-Density
Conversion
References
Material type
Density
Sediment
2.0-2.3
2.75
1980theoretical
gravityfield wasremoved.
Datacorrection Franciscan
Upper
oceani•
crust
procedures
andinstrumentation
ontheR/V Ewingaredescribed Lower oceanic crust 2.75
2.9
by Diebold[1995]. Gravitydataderivedfrom satellitealtimetry
[Sandwell and Smith, 1997] supplementthe ship-board
measurements
at thetrackends.A 5-10 mGalshiftof theglobal
marinegravitydatawasnecessary
to overlapthedatasets;thisis
within observedshiftsbetweenglobalmarinegravity data and
Uppermantle
3.3
Nafe andDrake [1957]
Stewartand Peselnick[1977]
Christensenand Salisbury[1975]
Christensenand Mootrey[1995]
basedOn0.4 gcm-3densitycontrast
to lower crust
Asthenosphere
3.26
[Jachens
andGriscom,1983]
basedon -0.04 gcm'3density
contrastto uppermantle
Idachens
andGriscorn,
1983]
Densities
wereobtainedby usingthe averagevelocitiesfromthe
velocitymodelsandconverting
to an averagedensitybsingdatafrom
..
ship board measurements
[Sandwelland Smith, 1997]. In
References
(gem-')
addition to the flee-air anomaly, we calculateda Bouguer
finomalyby removingtheeffectof thetopography
andsediment the appropriatereference.
23,806
LEITNER ET AL.: CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
(a)
•.
LINE 3S
TRACK
START
zll• I ....
I i , , , I , , - - ! ....
! .....
LINE 5
TRACK START
TRACK END
OBH22
LINE
4 OBSC4 OBSA1
(i)60....................
¾V
.........
•......
¾.......
•......
*.................
'•....
½
....
i ....
! ....
! ....
! .....
•BH:•4OBSC30BH18
OBH17
OBSCl
! ....
E 2
• -20
ø
• -40
..SOUTH
-------• _--/ •1O
STEP
IN
LITHOSPHERE
• • •
NORTH
..........
(b) ,<AL•ME•Y
DATA
>,
300j
.... ' .... ' ..........................................
•
(C) 0
•
DEEP SEISMIC REFLECTIONS
TRACK
START
• '- .... •'-' ' ' ' ..........
' ....
LINE
4 OBSC4
• .........
* ......
OBSA1
•
180t
5 •$•:•::•::;::•.::•:::•:::•:::•:•:•+•:•:•://:s•:•::{•::f•::•::*•:•.;•:•::.:::::•::•::•f::•.:::;:•c:::::•:::.:::•*•*:*•*::•:•*•:•::•
..............................
...•....•..•..•:.:::>
(k)
,
TRACK
START
oj:..."' "'
:-:".-::
.........
----"-"-----"
'
TRACK
END
2.2
....""-:"......
--'.:-:'::-"'::--:'-'
:'::-'
..'........:._. ' '
"
25............... , .... , .... , .... ,
p-- 10
d)
' 0'
TRACK
START
.....
! ....
w•.,.,,
LINE
4 OBSC4
i., .......
. ............
t,1.ø3
::•:?-:-.:-•:.:•
E•
OBSA1
.•:.,..:
.......
,,:..........
:..-..--
2.3•1-
•!i--..-!•:,•
::•i•i•:•,•:,•:•::•i:::,:•:•,'•'"'"'"'"'"""••i!:•
(i) o
STEP IN LITHOSPHERE
,,,,,,,,,i,,
.......
t .........
, .........
, .........
THICKNESS
, .........
, .........
, .........
•.....
======================y&•.`:.;.L.2..•&..•.:.;.`•.:•:`•``•.•.•;•.:•....•.:.•.•.•:•:.:•,•:•
-:-:-..--.----•-..--••:•:::::::-.?.•a•a:•-:•:•:•-•.--.--•...-.•
.--
:.....................
::.:::•:•.•::.:::.:*:::•:*!:.:..`.i.•:?:.•.:.:.:`.:•i:•::.1.:ii:•:•:•:•%•.•:;;...:i•:i::•:::•.;`:i:•:::•:i•!:i:.i•:iii:i.•..::&:•!:::ii:!•i•::•>:..::
.....,• ..... •,,,:,,: ..,?.,.,,..•..............
. ß .•
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(e)0
' ' ''1
''
TRACK
START
LINE4OBSC4
' "• ....
' ....
' ....
OBSA1
t 103--'
.....
2....
-20
-10
0
10
20
30
40
50
2.3•1E
60
70
80
DISTANCE(km)
(f)
• -10
-40
-20
0
20
4O
6O
8O
100
120
DISTANCE (km)
TRACK
STARTLINE
4
TRACK
END line 3S of threepossibledensitymodelsshownin Figures7(c),
7(d) and 7(e). Note the-20mGal changein free-air gravity
betweenkm 40-50. (b) Bougueranomalyof line 3S is dominated
by the densityanomalyof the dippingoceaniccrust.(c) Density
modelI alongline3S has a 2-km-thicklow-density
body of
2.55 gcm'• in theuppercrustof theOconostota
ridge.(d) Density
SOUTHEAST
modelII alongline 3S has a higher-density
lower crusteastof
the Oconostota
ridge.(e) Crust
in modelIII along
line3S hasan
-3
ß
n''60
_•.
250
L'..............................................
(• 20
ø
n' 15
10
õ
TRACK START
(h)r:
0
TRACK END
LINE
5
LINE
3
averagedensityof 2.8 gcm andthereforeexplmnsthe changein
free air anomalyas a changein depthto the Moho. (f)Free-air
anomalyof the line 4 densitymodel.It changesonly 30 mGal
alongthe profile.(g) Bougueranomalyalongline 4. (h) Density
modelof line4. Moho dips 7ø eastwardto modelkm 50. (i)
Free-airanomalyfor the line 5 densitymodelwith (solid line)
andwithoutstepin lithosphere
thickness(dashedline) acrossthe
MTF. (j) Bougueranomalyalongline 5 is dominatedby a broad
low in the southindicatinga thick crustand abruptthinningat
km 50. (k) Densitymodel along line 5 has a shallowdipping
Moho to km 0, Moho depth increasesto 15 km and the thick
crustextends
under-the
ridgecrest.The Gordaplateedgeis at
km 55, and the Moho is flat to the north. Densitiesbetweenkm
60 and 90 km are lower than in the adjacentbasementblocks.
The dataareequallywell modeledwith a blockof 2.65 gcm'• in
the uppercrust(modelI in Table 1) or alternatelythickeningand
Figure 7. Observed(circles)and calculated
(solidline) free air thinninglowercrust(dashedline, modelII in Table 1). (1) Same
andBougueranomalies
anddensitymodelsfor lines3S, 4, and as density model in Figure (k), but incorporatesa step in
5. Altimetrydataextendthe profilesat the trackends.Average lithosphereaccordingto the age contrastof Pacificand Gorda
block densitiesare given in gcm'3. (a) Free-airanomalyalong plates.-20
-10
0
10
20
30
40
DISTANCE (km)
50
80
70
80
LEITNER ET AL.: CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
beneaththe Oconostotaridge, as suggestedby our velocity
model. The observedMoho dip of 20ø beneaththe Oconostota
ridge is a minimum value; a shallowerdip would require
unreasonablelow-density values. Alternatively, the density
contrastcan be modeledby a relativedensityhigh in the lower
crust east of model km 45 (model II, Figure 7d), which is
23,807
3. Discussion
The density and velocity models along all three profiles
indicatean abruptincreasein Moho dip at the westernmarginof
the Oconostotaridge and a thickenedupper crust within the
Oconostotaridge. Along line 3S, the dip of the Moho changes
compatible
with the velocitymodelderivedfrom the onshore- abruptlyfrom <5ø at the westernend of the profile to 20-30ø
offshoreseismicdata [Henstocket al., 1996]. Model III was (densitymodelsI andII, Figures7c and7d) overa distanceof at
determinedfrom inversionof the gravitydatafor Moho position most 16 km beneaththe seawardboundaryof the Vizcainoblock
assumingno lateral density variationswithin the crust and and Oconostotaridge. Line 4 is entirely within the Vizcaino
ridgebut trendsobliquely
illustratesthe changein crustalthicknessneededto fit this end- block in the regionof the Oconostota
membercase(Figure7e). Sincewe do not expecta westward to it. The part of line 5 that is southof the Mendocinotransform
increase in crustal thickness and both models I and II are
fault also crossesthe Oconostotaridge obliquely. The Moho
ridgeon lines4 and 5 has
supported
by seismicdata,ourpreferred
modelis a combination kink modeledbeneaththe Oconostota
of models I and II with relative weights presentlypoorly true dips of 17ø and 20ø, respectively,when correctedfor
constrained.
obliquity(Table3). This is consistent
with theminimumvalueof
Moho dip modeledalong line 3S (model I). If the Moho dip
2.2.2. Line 4. The f•ee-air anomaly(Figure 7f) changesby
only40 mGal alongtheprofile.The Bougueranomalydecreases along Line 3S is in fact larger than 20ø, it would imply an
from westto east,reflectingthe increasein Moho depth.Moho increasein Moho dip along strike,but this cannotbe resolved
dip increases
from4øto 7ø at km 8. Betweenkm 8 and50, Moho with our data set. This Moho dip is much steeperthan that
depthincreases
linearlyfrom12 to 17km, andeastof km 50, the observedbeneaththe accretionarycomplex anywherein the
Moho is flat. We did not force the models to agree at the active Cascadiasubductionmargin [Tr•hu et al., 1994, 1995;
intersectionswith lines 3S and 5, and thereforethe mismatch Flueh et al., 1997; Parsonset at., 1998], suggestingthat the
betweenthe modelsgives a qualitativemeasureof model relict subductionboundarymakingup the westernboundaryof
uncertainty.
A mismatchin Moho depthof about3 km between the Vizcainoblockhasbeensignificantlydeformed.
Northeastof the Moho kink on line 3S, the Moho flattensat a
the gravitymodelsfor lines3S and4 canbe partlyremovedby
introducing
an additionallow-densitylayerin the line 4 model depthof about20 km, and we observea lateraldensitychange
uppercrust,which supports
the existence
of an intermediate- within the crust 10-15 km east of the Oconostotaridge. This
velocitylayer beneaththe basementalongline 4. We did not density changecan be modeled either as a changein crustal
includethislayerin.our gravitymodelsinceit is only basedon material,as thinningof the uppercrust,or as thickeningof the
modelingthelarge-aperture
datanearOBH 24. The observed
dip lower crust(or somecombinationthereof)andcoincideswith an
of 7ø alongthe line 4 profilecontrasts
with the steepdip on line apparentchangein crustalreflectivity and the appearanceof a
3S in thisregion,suggesting
that line 4 trendsobliquelyto the deepreflectorto the northeast[Henstocket al., 1996; Godfrey,
1997; Godfrey et al., this issue]. In contrast,no regionally
sharplydippingstructure
modeledon line 3S.
2.2.3. Line 5. The free-airgravityanomalyalongline 5 is coherentdeep reflectionsare observedon any of the profiles
dominatedby large anomaliesover the MendocinoRidge (50- crossingthe northwesternregionof the Vizcainoblock.
Beneaththe Gorda Escarpment,the densitymodel of line 5
70mGals), over a bathymetrichigh on the Vizcaino block
(10mGal) and over a basementhigh on the Gorda plate (Figure 71) indicates abrupt crustal thinning to the north.
datarequiresthat the Mendocino
(20 mGal) (Figure7i). The Bougueranomalyis dominated
by Matchingthe long-wavelength
thechangein Mohodepthsouthof theGordaEscarpment,
where transform fault continues vertically through the plate
a broad low of 50 mGal outlines the thick crust of the Vizcaino
lithospheres,
similarto }gilson's
[1989]gravitymodelsacross
the
block(Figure7j). In addition,theBouguergravityhasa 10mGal MendocinoRidgewestof 126ø. The high velocitiesandlack of a
low between km 60 and 100.
crustalroot beneaththe MendocinoRidge suggestthat it was
The densitymodel (Figures7k and 71) derived from the formedby uplift and tilting of the Vizcaino block in responseto
velocitymodelneededthreechanges
to fit thedatasatisfactorily: Pacific-Gordacompressiveforces.
The low-densityblock and/or Moho variation beneaththe
(1) A northward
dippingMoho (maximumdip 10ø) beneathkm
0 and 15, (2) on the Gordaplate,eithera low-density
block basementhigh on the Gordaplatemodeledon line 5 (Figure7k)
(Figure7k) or, alternatively,
thinningand thickening
of the are not imagedin the large-apertureseismicdata.Neither of our
oceaniccrustbetweenkm 70 and 100 (dashedline, Figure7k), density models are compatiblewith the high being a flexural
(3) a contrast
in uppermantledensities
corresponding
to a bulge, as this would result in a relative density high. The
lithosphere-asthenosphere
boundarycalculated
from the age basementhigh may have formed either throughthrustingand
contrast
[Fowler,1990,p. 238]between
theGorda(5-6Ma) and thickening of the Gorda crust in responseto north-south
compression,similar to the crustal thickening beneath the
Pacific(27 Ma) plates.
Table3. Geometry
of LinesandApparent
andTrueDipsof MohoKink
Line
ApparentMoho Dip Along Trend
Angle of Line to N45øE,
of Line,del•
del•
True Dip of Moho Kink, deg
3
4
20-30 (Figures7c and 7d)
7
(Figure7f)
0
58
20-30
5
10
45
20
(Fil•ure7k)
17
23,808
LEITNER ET AL.: CRUSTAL STRUCTUREOF THE NORTHWESTERNVIZCAINO BLOCK
1992, 1996]. It is not observed, however, across the
southwestern
boundaryof the Vizcaino block [Henstocket al.,
1997]. Offshore central California, the Moho kink has been
attributed
to
crustal
imbrication
on
thrust
faults
and/or
deformationalongburiedtransformfaults.
In order to reconstructthe tectonicprocesses
that lead to the
20 km
Moho kink and thickened crust, we look at several reflection
LINE 3S
profilescrossingthe Oconostota
ridge (red and gray lines,Plate
t..x
<.....
OR ..... >
L•
0
, I • i , i
i , i , I , i , i , , l a). Theseprofilesshowa seriesof basement
ridgesoverlainby
folded sediments(Figures 3b, 4, and 9). Becauseof their
• •::•'"'::••••
T••h•.•••.••••:••i:!.•Backsto
.................
e-act
i'vat
ed
p•
•' 10
•:••"•••'!-'.i'•"•i'""•'
...................
•;'•"'"'":'"";:•. Thi•.k,
nw•r similarityto ridgesseenin the activeaccretionaryprismof the
Cascadiasubductionzone [Goldfingeret al., 1992;Pratsonand
:••.-'-'3:-':•
:½::..'3•:•:•
•:•:•.%
•; g-':-:.-'•
•,
Haxby, 1996], Godfrey [1997] suggestedthat these ridges
-20 -10
0
10
20
30
40
50
60
70
80 formedin an accretionary
complexduringthe subduction
regime
DISTANCE(kin)
and were later reactivated.This outer zone of the accretionary
OROconostota
ridge MTFMendocino
transform
fault
wedgewasrelativelyyoung,thin,weakenedby preexisting
faults
PABPointArena basin SAFZ San Andreasfaultzone
andthermallyweakenedby the subducted
Farallonridgebeneath
Figure 8. (a) Three-dimensional
cartoonof the crustalstructure it (Plate lb). It is thereforenot surprisingthat deformationwas
'in the northwestern
part of the Vizcainoblock.The Moho dip focusedin this region.Reactivationof thesethrustfoldsclearly
abruptlyincreases
to about20ø in the northand20-30ø to the continueduntil relatively recently(Figures4 and 9), but the
south. The section ends at the southwest-northeast
trending absenceof seismicityor of noahwesttrendingridges on the
line 3S, where our data set ends.The lower crustis shownin seafloorsuggeststhat they are not active today. We speculate
darkgrayandanythingabovein lightergray.The approximate that the Moho kink representsshorteningof the lower crustin
regionof the Oconostota
ridgeis markedin light gray.Mapped
responseto the same compressionthat reactivatedthe thrust
basementridgesafter Godfrey[1997] are solid lines, dotted
complexmaterial
where uncertain.Arrows show directionof compression.(b) faultsandthickenedthe overlyingaccretionary
Sketchof line 3S, whichis a crosssectionof the Vizcainoblock. (Figure 8b). Placing constraintson the time of initiation and
201
......
-'•-:--,"Sh?rten,no,
_......
.
We suggests
thatcompression
shortened
the lowercrust,kinked
it, andthickenedthe uppercrustalongpreexisting
thrustfaults;
faultingand foldingthe uppercrustand sediments.
At model
km 45, we seea crustalchangeand possiblythickeningof the
oceaniccrustor marie layer.
Oconostotaridge, or it may be a relict structurecreatedby an
excessof magmatismat the spreadingcenter.We preferthe first
explanationbecausethe basementhigh corresponds
to a
southwest-northeast
trendingseafloorridge visible in Gloria
sidescandata [EEZ-SCAN84ScientificStaff 1986], which is
approximatelycoincidentwith a persistantlineation in the
patternof Gordaplateseismicityandwith the aftershockzoneof
a magnitude7 intraplateeventthat occurredin November1980
[Smithet al., 1993].
cessationof reactivation, however, is difficult because of the
small number of drill holes in the region, the discontinuous
nature of the seismicreflections,the presenceof intervening
basementridges,and activeseafloorerosion(Figures4 and 9).
High resolutionseismicdata near the ODP Site 1022 (yellow
circle,Plate 1a) suggestthat deformationceasedat about3.4 Ma
(Lyle et al., 1997).
Another significantresult of our modelingis the apparent
thinningof the uppercrustand/orthickeningof the lower-crust
northeastof km 45 on line 3S (Figure 8b). The trend of this
changein distributionof upperand lower-crustal
materialis
presentlypoorlyconstrained
because
of sparseseismicdataand
because
the wavelength
of the gravityanomalyon whichthis
featureis basedon is too shortto be resolvedin globalmarine
gravitydata.Basedon a generalnoahwest-trending
patternin
Neogenestructures
and gravity data (Plate l a and lb), we
assume
a noahwesttrend.Possible
explanations
for the change
in crustalstructure
include(1) the samecompressive
eventthat
4. Tectonic Implications
resultedin the Moho kink, whichmighthave imbricatedand
Our primaryresultis thatthe paleosubduction
zonepreserved thickenedthe lower crust, (2) extensionand magmatic
along the seaward edge of the Vizcaino block has been
significantlydeformed.This is suggestedby the increasein
Moho dip from about5ø westof the Oconostota
ridgeto 20-30ø
beneath the western margin of the Vizcaino block and the
Oconostotaridge. This dip is consistenton all three profiles
when the apparentdip is correctedfor the obliquity of the
profilesrelativeto the northwesttrendof the structureindicated
by globalmarinegravity(Platelb andTable3) andmultichannel
seismic data [McCulloch, 1987a; GodJ?ey,1997]. Crustal
thicknessfarthereastis approximately
constant,resultingin a
kink in the Moho that strikes about N45øW and underlies the
underplatingin responseto subductionof the Pacific-Farallon
ridge, and (3) lateral variationsin crustalstructurewithin the
forearcformedduringFarallon-North
Americaplatesubduction.
We speculate
thathigher-velocity
anddensitymaterial10-15km
east of the Oconostotaridge may representa mechanical
backstopthat contributed
to localizingdeformation
beneaththe
Oconostota
ridge(Figure8b).
A third result of our modelingis a new constrainton the
composition
of materialformingthe MendocinoRidgeandthe
geometryof changesin crustalthicknessassociatedwith the
GordaEscarpment.
The modelfor line5 showsthatthevelocity
and densitywithin the MendocinoRidge are not resolvable
structure is not observed farther north in Cascadia but is
differentfromthatwithinthebasement
of the adjacent
Vizcaino
observedon many crossingsfarther south on the California block.Combined
withstratigraphic
evidence
forupliftandtilting
continentalmargin [e.g. Trdhu, 1991; Meltzer and Levander, of theMendocino
Ridge[Silver,1971;Godfrey
et al., thisissue],
1991;Howieet al., 1993;Holbrooket al., 1996;Miller et al., this suggeststhat the MendocinoRidge east of 126ø is
westernmarginof the Vizcainoblock (Figure8a). This type of
LEITNER
ET AL.' CRUSTAL
STRUCTURE
OF THE NORTHWESTERN
VIZCAINO
BLOCK
23,809
,
2
R•
I Deformatio
Figure9. Migratedseismic
reflection
profilecollected
by theUSGSin 1977[McCulloch,
1987a;Godfrey,1997,
Godfreyet al., thisissue].Arrowsmarkthelocation
of thecrosscuttingbasement
ridgesseenthroughout
the
northwestern
Vizcainoblock (Plate l a). Bottomfigureshowsrecentsedimentdeformation
aboveone of the
N45øW trendingthrustfaults.
constructedfrom tectonically uplifted accretionarycomplex block as inferredfrom our crustalmodels.Figure 10 showsthat
acrossthe Mendocinotransformwassmallduringa
material. We cannot, however, rule out tectonic mixing of compression
accretionarycomplexrocks and uplifted oceaniccrust. The relativestablephaseof Pacificplatemotionfrom about19 to 10
configurationof the Moho beneaththe Gorda Escarpment Ma, increasedslightlyin the periodbetween10 and 6 Ma, and
indicatesthat the MendocinoRidge is not underlainby a crustal has been relatively strong since then. It also showsthat the
root and hasthereforebeenfon•nedand maintaineddynamically lithosphericagecontrastacrossthe Mendocinotransformnearits
by compressive
forcesacross[hePacific-Gorda
plateboundary. intersectionwith the Oconostotaridgeflipped 12.5 to 10 Ma and
that Pacific-North America relative plate motion was
transtensional
prior to -3.5 Ma. In spiteof strongnormalforces
acrossboth plate boundariesat the presenttime, the lack of
seismicity (Plate l a) and sedimentdeformationsince about
3.4 Ma [Lyle et al., 1997] indicatesthatthe Vizcainoblockis not
deforminginternallyunderthe currentstressregime.Sinceat
least 3 Ma, north-south compressionhas been absorbed
deformation.
internally by the Gorda plate [Wilson, 1989] and east-west
compressionhas been accommodatedfarther east, within the
The formationof the Oconostotaridge and underlyingMoho
kink and the formationof the Gorda Escarpmentand adjacent PointArenabasin[Ondrus,1997].
We suggestthat most of the internal deformationof the
basementridge both suggestcompressive
stressin the Vizcaino
in responseto
block duringand/orafter its accretionto the Pacificplate,but Vizcaino block resulted from transpression
de Fuca/Gordaplatecompression
(darkgrayshaded
stratigraphic
constraints
on the timingof compression
are poor. Pacific-Juan
acrossthebounding
Becausethesefeatureslikely occurredalongpreexistingfaults, regions,Figure10) whenthenormalstresses
the directionof the maximumcompressive
stressis alsopoorly transformswere relativelylarge(about6-3 Ma) and/orwhen the
constrained.
Vizcaino block lower crust was very young, weak, and
In order to obtain additional insights into when the juxtaposedagainstthe older Juande Fucaplateto the north(18deformationor uplift may haveoccurred,we returnto the plate 12.5 Ma). Internaldeformationis lesslikely in the late Miocene
tectonichistorysummarized
in Figure 1. In Figure 10, we show (6-12Ma), when both north-south compressionand the
agecontrastwererelativelysmall(lightgrayshaded
the lithosphericage contrast across the Pacific-Juande lithospheric
Fuca/Gordaplateboundaryandthe component
of normalstress region, Figure 10). The observationthat internaldeformationof
across the Mendocino transform and San Andreas fault zones
the Vizcaino block seemsto have stoppedand beentransferred
stresswithinthe
andrelatetheseparameters
to thetectonichistoryof theVizcaino to the Gordaplate,eventhoughthe compressive
Reportedsubsidence
of the MendocinoRidgesince5 Ma [Fisk
et al., 1993] doesnot contradictour observations
but indicates
that compression
is decreasing
over time. Timing of the uplift
and subsequent
partial subsidence
of the MendocinoRidge in
this region is likely related to changesin the amount of
compression
acrossthe Mendocinotransformfault and to the
degreeto which compression
is accommodated
by intraplate
23,810
LEITNER ET AL.' CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK
Extensional
Phas•
[Oligocene[
Eady
Miocen,
northeastof the Oconostotaridge, but it is unclearif this is a
productof the samedeformationor wasa preexistingfeatureand
therefore acted as a backstop during the deformation.
Deformationwithin the Vizcaino block appearsto have ceased
and shifted to the Gorda plate sometimeprior to 3 Ma. We
suggestthat the spatialandtemporalshiftsof the primarylocus
of deformationresultedfrom changesin relativemotionsacross
the Pacific, Juan de Fuca/Gordaand North America plate
Late
MioCe
n:ilil]?•!iii:..•11•11:•:!
,eI e I
boundariescombinedwith changesin lithosphericage and age
contrast across these boundaries.
5•
Acknowledgments. We thank the participants of the Mendocino
Triple JunctionSeismic Experiment:Special thanks go to the crews of
the R/V Ewing and R/V Wecomaand to Ken Peals,BeechefWooding
and Greg Miller for running the OBS/OBHs. Thanks to T. Henstock,
who provided the depth-migratedMCS profile for line 3S, and to A. S.
Meltzer, who providedthe time-migratedprofile for line 5. Reviewsby
10o< T. Brocher,R. Keller, and K. Miller greatlyimprovedthis manuscript.
0
B. Leitner gratefully acknowledgesthe supportof J. L. N/tbelekand D.
EberhartPhillips.Gerald Connardat NorthwestGeophysicalAssociates
AGE (Ma)
(Corvallis,Oregon)provideda test copy of GM_SYS for Unix. Plots
were createdusingGMT [Wesseland Smith, 1995]. This researchwas
Figure 10. Factorsinfluencingthe likelihoodof compressional supportedby NSF grants9219870-EAR and 9527011-EAR to Oregon
.......................
deformationin the Vizcaino block. At the top, solid linesmark
lower age and dashedlines mark the upper age limit of the
Vizcaino block phasessketchedin Figure 1. Normal forces
acrossthe transformsare calculatedfrom stagepolesnear the
Oconostotaridge-MTF intersection[Wilson, 1993] and just
southof the Mendocinotriplejunctionregion[Hatbert and Cox,
1989] and are projectedto a fault at 90ø (MTF) and 319.5ø
(SAFZ). The strikeof the SanAndreasfault northof PointArena
(since 3.5 Ma) is about 330% and thereforethe presentplate
motion normal to the San Andreasfault is negligible.Velocity
normalto the MTF of the Pacific-Farallonplatesbetween19 and
10 Ma was relatively small and uncertain.Age of the Farallon
and later Juan de Fuca plate [Engebretsonet al., 1985] are
estimatesassuminga constantspreadingrate. Pacificplateageis
indicatedby the dark gray regionwhichmarksthe possibletime
span of ridge cessationat about 25 to 20 Ma. The horizontal
dotted line marks the onset of Gorda plate deformation
(minimumage). When consideringthe possibleVizcainoblock
deformationin the earlyMiocene,we assumethatthethenyoung
Pacific plate deformedat about the sameage. Light gray area
indicates small likelihood for Vizcaino block deformation; dark
gray areasindicatetimesof likely deformation.
Pacificplate shouldbe largernow thanat any time in the past
historyof triplejunctioninteraction,suggests
that lithospheric
agehasa majoreffecton intraplatedeformation.
5. Conclusions
Our data show evidencefor significantdeformationof the
crustin the northwestern
part of the Vizcainoblock.The internal
deformationresultedprimarilyfrom transpression
in response
to
Pacific-Juande Fuca plate compression
when normal stresses
acrossthe boundingtransforms
wererelativelylarge(6-3.4 Ma)
and/orwhen the Vizcaino block was considerablyyoungerand
weakerthanthe adjoiningJuande Fucaplate(18-12.5Ma). This
compressionappearsto have reactivatedpreexistingcrustal
faults,thickenedthe crustof the Oconostota
ridge,and formeda
kink in the Moho beneath the northwesternmargin of the
Vizcainoblock. It alsotilted and upliftedthe Vizcainoblock at
its northernmargin,formingthe GordaEscarpment
andadjacent
basementridge.A lateralchangein structureis observedto the
StateUniversity
andEAR-92182'09
toStanfor•l
University.
References
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Atwater, T., and J. Severinghaus,
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Nicola J. Godfrey, Departmentof Earth Sciences,Universityof
SouthernCalifornia,Los Angeles,Ca 90089.
Beate Leitner, Instituteof GeologicalandNuclear Sciences,Dunedin
ResearchCenter,Private Bag 1930, Dunedin,New Zealand.
Anne M. Trthu, College of Oceanic and AtmosphericSciences,
OregonStateUniversity,Corvallis,OR, 97331.(email:nicola•usc.edu;
beate•eos.otago.ac.nz;
trehu•oce.orst.edu)
(ReceivedNovember5, 1997;revisedMay 1, 1998;
acceptedJune9, 1998.)