JOURNALOF GEOPHYSICAL
RESEARCH,
VOL.103,NO. B10,PAGES23,813-23,825,
OCTOBER10,1998
Evolution of the Gorda Escarpment, San Andreas fault and
Mendocino triple junction from multichannel seismic data
collected across the northern Vizcaino block,
offshore
northern
California
NicolaJ. Godfrey
1,2,AnneS. Meltzer
3, SimonL. Klemperer
1,AnneM. Tr•hu4,
BeateLeitner
4,5,SamuelH. Clarke,Jr6 andAmyOndrus
3
Abstract. The GordaEscarpmentis a northfacing scarpimmediatelysouthof the Mendocino
transformfault (theGorda/Juan
de Fuca-Pacificplateboundary)between126øWandtheMendocino
triplejunction. It elevatesthe seafloorat the northernedgeof the Vizcainoblock,part of the
Pacificplate,-1.5 km abovethe seafloorof the Gorda/Juande Fucaplateto the north.
Stratigraphy
interpretedfrom multichannelseismicdataacrossandcloseto the GordaEscarpment
suggests
thatthe escarpmentis a relativelyrecentpop-upfeaturecausedby north-south
compression
acrossthe plateboundary.Closeto 126øW,the Vizcainoblock acousticbasement
shallowsandis overlainby sediments
thatthin northtowardthe GordaEscarpment.These
sediments
aretilted southandtruncatedat the seafloor.By contrast,in a localizedregionat the
easternendof the GordaEscarpment,closeto the Mendocinotriplejunction,the top of acoustic
basementdipsnorthandis overlainby a 2-km-thickwedgeof pre-11Ma sedimentaryrocksthat
thickensnorth,towardthe GordaEscarpment.This wedgeof sedimentsis restrictedto the
northeastcornerof the Vizcainoblock. Unlessthe wedgeof sediments
was a preexistingfeature
on the Vizcainoblockbeforeit wastransferredfrom the North Americanto the Pacific plate,the
strongspatialcorrelationbetweenthe sedimentarywedgeandthe triplejunctionsuggests
the entire
Vizcainoblock, with the SanAndreasat its easternboundary,hasbeenpart of the Pacific plate
sincesignificantlybefore 11 Ma.
1. Introduction
Three plate boundariesintersect at the Mendocino triple
junction(Figure la, inset). The Gorda/Juande Fuca plate is
subducting beneath the North American plate along the
Cascadiasubductionzone. The San Andreas fault is a rightlateral strike-slip fault separatingthe Pacific plate from the
North American plate. The Mendocino transformfault is a
right-lateral transform fault separating the subducting
Gorda/Juande Fuca plate and the Pacific plate. There is a
topographic high immediately south of the Mendocino
transform fault between 129øW and the Mendocino triple
junction [Krause et al., 1964; Silver, 1971]. Between 129øW
and 126øW this topographichigh is a ridge, the Mendocino
Ridge, which separatesbathymetricallylower (older) Pacific
plate to the south from bathymetrically elevated (young)
Gorda plate to the north. Between 126øW and the Mendocino
triple junction, in addition to the topographic high of the
Mendocino Ridge, there is the Gorda Escarpment, a north
facing scarp which forms the northern edge of the Vizcaino
block, part of the Pacific plate (Figure l a). The Vizcaino
block is boundedon the east by the San Andreasfault and on
the southwestby a break in continental slope (Figure l a)
interpreted as a paleosubductiontrench [McCulloch, 1987a].
The Vizcaino block is an anomalouslybroad,triangularpart of
the continental slope that is believed to consist of
accretionaryprism material overlying oceaniccrust stalled in
a fossil subductionzone [McLaughlin et al., 1994].
We expect a particularbathymetricrelationshipacrossthe
Mendocinofracturezone basedon the age of the lithosphere
on either side of the plate boundary;young, elevatedGorda
•Department
of Geophysics,
StanfordUniversity,Stanford,California. plate to the north versusold bathymetricallylow Pacific plate
2Now at Departmentof Earth Sciences,Universityof Southern to the south [Engebretsonet al., 1985]. West of 126øW, the
California,Los Angeles.
bathymetrynorth and southof the Mendocinotransformfault
and MendocinoRidge is as expected(see profiles 4 - 12 in
Bethlehem,Pennsylvania.
4College
of Oceanic
andAtmospheric
Science,
Oregon
StateUniversity, Figure 3 of Krause et al. [1964]). East of 126øW, the
Corvallis.
bathymetricrelationshipis also as expected,except within
3Department
of EarthandEnvironmental
Sciences,
LehighUniversity,
SNowatInstitute
of Geological
andNuclearSciences,
Dunedin11esearch 15 km south of the Mendocino fault, i.e., within the Vizcaino
Center,Dunedin,New Zealand.
Papernumber98JB02138.
block. Within the Vizcaino block, the bathymetryshallows
significantlyas the Mendocinotransformfault is approached
(see profiles 1-3 in Figure 3 of Krause et al. [1964])
culminatingin the bathymetricallyhigh Mendocino Ridge
adjacentto the steepnorthfacingscarp,the GordaEscarpment
0148-0227/98/98 JB-02138509.00
[Krause et al., 1964].
6U.S.GeologicalSurvey,MenloPark,California.
Copyright1998 by the AmericanGeophysicalUnion.
23,813
23,814
GODFREYET AL.:GORDAESCARPMENTAND VIZCAINOBLOCK
a) 126ø
125
ø
124ow
b) 126
ø
125
ø
41oN
40'
I
0
i
i
,
50 km
, MTJSE 94 seismic O
ODP Site 1022
USGSseismic O DSDPSite173
industry
seismic .... limits
ofOconostota
ridge
ß
I
industrywell
.•,•
onshore
region•
basin
trace of point P
lithological
sample
site
- - outline
ofVizcaino
block
Figure 1. Location map. (a) OffshorenorthernCalifornia showingthe Vizcaino block, bathymetry
(contoursin meters), and multichannelseismiclines. PAB, Point Arena basin; ERB, Eel River basin; and OR,
Oconostotaridge. Inset showsthe tectonicconfigurationof the Mendocinotriple junction. GOR, Gorda
plate;NAM, NorthAmericanplate;VB, Vizcainoblock;MTJ, Mendocinotriplejunction. (b) Enlargement
of
northernVizcaino block showingseismiclines referredto in text. Tick marks and numberson seismiclines
are for correlationbetweenmap and seismicsections(Figures2, 3, 4 and 6). MTJSE, Mendocinotriple
junctionseismicexperiment;
USGS,U.S. GeologicalSurvey;ODP, OceanDrillingProgram;DSDP, DeepSea
Drilling Project; MTF, Mendocinotransformfault.
This paper uses three multichannelseismicdata sets and
gravity data recorded across the Gorda Escarpment and
northern part of the Vizcaino block to investigate the
evolution of the Gorda Escarpment and to try and constrain
when the presentMendocinotriple junction-SanAndreasfaultMendocinofracturezone geometrywas established.
1.1.
Tectonic
Setting
The Mendocino (fault-fault-trench) triple junction and
Rivera (fault-ridge-trench)triple junction formed at -29-30
Ma, at about 31o N (in a modem fixed-North America reference
easternmostfaults (J. T. Freymueller et al., Kinematics for the
Pacific-North America plate boundary zone, northern
California, submitted to Journal of Geophysical Research,
1998).
The Mendocino triple junction is a broad zone of
deformation presently centered near the town of Petrolia,
onshoreCape Mendocino [Clarke, 1992] (Figure la, inset). It
is not a simple point in space but rather a region where the
North American, Gorda, and Pacific plates interact. It is
associatedwith an anomalouslyelevatedregion [Aalto et al.,
1995] with unusuallyhigh uplift rates [Merrits and Vincent,
frame), when the Pacific-Farallonspreadingridge intersected 1989].
From about 33 to 8 Ma (chrons 13 to 4), the direction of
the subduction zone along the North American margin
motion
of the Pacific plate was N60øW with respect to the
[Atwater, 1970; Stock and Molnar, 1988]. The Mendocino
triple junction has been associated with the Mendocino North American plate [Stock and Atwater, 1997; J. M. Stock
and T. Atwater, Pacific-North America plate tectonicsof the
fracture zone since at least 27 Ma [Stock and Molnar, 1988]
NeogenesouthwesternUnited States-An update,submittedto
and both have migrated northwest (relative to fixed North
International
Geological Review, 1998]. From about
America) to their present locations at about 40.5ø N (Figure
30
to
12
Ma,
the
Pacific plate velocity with respect to the
l a). The Rivera triple junction has, during the sameperiod of
North American plate was about 33 mm/yr and from
time, migratedsouth(with respectto fixed North America) by
12 to 8 Ma, it was about 52 mm/yr (J. M. Stock and T.
a seriesof jumps to about 23ø N, off the west coastof Mexico.
As the triple junctionsmigrateaway from one another,the San Atwater,Pacific-NorthAmericaplate tectonicsof the Neogene
Andreastransform systemlengthens,progressivelychanging southwesternUnited States- An update, submitted to
International Geological Review, 1998]. At about 8 Ma, the
the subductionregime to a transformsetting [Atwater, !970].
The transform margin in northern California is not a simple relativemotionbetweenthe Pacific and North Americanplates
plate boundary. Current motion on the northernmostSan changedto N37øW, althoughthe plate velocity remained
Andreasfault systemappearsto be distributedacrossat least 52 mm/yr [Stock and Atwater, 1997; J. M. Stock and T.
threemajor faultsover a 50-km-wideregion. The San Andreas Atwater,Pacific-NorthAmericaplatetectonicsof the Neogene
fault is the westernmost active fault [Castillo and Ellsworth,
southwestern United States- An update, submitted to
International Geological Review, 1998]. At about 3.5 Ma,
1993]. Late 20th century seismicity,however, is restrictedto
the Ma'acama and Bartlett Springsfaults which lie east of the the relative motion changedto north-northwest[Harbert and
San Andreas fault [Castillo and Ellsworth, 1993]. Geodetic
Cox, 1989].
modelsalso suggestthat most of the strike-slipmotion along
the plate boundary is currently accommodated on these
platesmovedeast with respectto the North Americanplate
The Farallon and later the Juan de Fuca/Gorda
(parallel to the Mendocino fracture zone) [Wilson et al.,
GODFREYET AL.: GORDA ESCARPMENTAND VIZCAINO BLOCK
1984], until about 6 Ma when the Juan de FucaJGordaplate
began moving E20øS, parallel to the Blanco fracture zone
[Wilson et al., 1984; Wilson, 1993]. These relative plate
motionsresultedin a componentof compression(the amount
varying over time) betweenthe Juande FucaJGordaand Pacific
platesalong their boundary,the east-westtrendingMendocino
transform fault [Wilson, 1986; Stoddard, 1987]. There has
been significant north-south compression across the
23,815
and nannofossilooze that contain little terrigenousmaterial
and that are overlain by upper Miocene through Pleistocene
terrigenousmudstonesand claystones [Shipboard Scientific
Party, 1973; McCulloch,
1987a]. There is a major
unconformitybetweenthe middle and upperMiocene units that
is imaged as a strongreflector in seismicreflection data (A. S.
Meltzer et al., manuscriptin preparation, 1998). The oldest
unit we can trace with confidenceto the northern edge of the
Vizcaino block correspondsto the Point Arena Formation (or
equivalent)which spans-- 10-16 Ma (see A. S. Meltzer et al.
(manuscript in preparation, 1998) for stratigraphic
Mendocino fracture zone since about 6 Ma (when the Blanco
fracture zone formed at an angle to the Mendocino fracture
zone). Basedon plate vectorsrecordedin magneticanomalies,
there was probably minimal north-southcompressionbetween
correlation).
10 and 6 Ma, probably very little or no compressionbetween
about 19 and 10 Ma but potentially significant compression
between 27 and 19 Ma (D. Wilson, personalcommunication,
Beneath the Point Arena basin sedimentary section, the
Vizcaino block is believed, on the basis of three lithological
samples [McCulloch, 1987a], seismic velocity [Henstock et
1997).
al., 1996; Henstock et al., 1997; Leitner et al., this issue], and
morphology of basement ridges imaged on migrated
multichannelseismicdata [Godfrey, 1997; A. S. Meltzer et al.,
manuscript in preparation, 1998] to consist of accretionary
There is only one lithologic sample site on the Gorda
prism material, comparable to the onshore Franciscan
Escarpmentitself (dredge haul FAN-BD-36 of Krause et al.
complex, overlying stalled, partially subductedoceanic crust
[1964] shown by the star on Figure l a). Cobbles and
or underplated mafic material [Bachman et al., 1984;
boulders of tholeiitic basalt, basalt conglomerate, andesire,
McLaughlin et al., 1994; Henstocket al., 1996, 1997].
calcareous ooze, and serpentinite were dredged from the
North of the Mendocino Ridge/Gorda Escarpmentis the
escarpment[Krause et al., 1964]. The andesireis Cretaceous
Juande FucaJGordaoceanicplate. For the remainderof this
in age [Fisk et al., 1993] and is probably part of the
paper we simply refer to the Juande FucaJGordaplate as the
accretionary prism believed to underlie the Point Arena
Gordaplate, althoughwe recognizethat the "Gordaplate"is a
sedimentarybasin. The roundednatureof the cobblesdredged broad deformation zone associated with the southernmost Juan
from the ridge indicate that the escarpmentmust once have
de Fuca plate [Wilson, 1986, 1989]. The Gorda plate is
been at sea level and has since subsided [Krause et al., 1964].
overlain by accretionary prism material associatedwith the
There are several sample sites west of 126øW on the
Cascadia subduction zone [Gulick et al., 1998] which in turn is
Mendocino Ridge [Krause et al., 1964; Duncan et al., 1994;
overlain by the sedimentary section of the Eel River forearc
Fisk et al., 1996]. The ages (11-22 Ma) of these samples
basin [Clarke, 1992] (Figure la). The Eel River basin
suggest the Mendocino Ridge consists of material that
contains, from its base upward, remnants of a Lower
originatedas part of the Gorda plate, but that is now part of the
Cretaceousclastic sequence,remnantsof a lower and middle
Pacific plate [Fisk et al., 1993; Duncan et al., 1994; Fisk et
Eocene marine sequence,a marine clastic Miocene section,and
al., 1996; Krause et al., 1997]. Rounded cobbles and gravels
Pliocene sandstonesand siltstones [Hoskins and Griffiths,
and a wave-cut platform on the northern flank of the ridge
1971; Clarke, 1992]. Young folds and thrust faults (mostly
suggestthat the Mendocino Ridge was also once above sea
post-Pliocene) trend north throughout most of the basin,
level [Duncan et al., 1994; Fisk et al., 1996; Krause et al.,
indicating recent (post-Pliocene) east-west compression
1997]. The material that makesup the MendocinoRidge was
associated with the Cascadia subduction zone [Clarke, 1987,
transferredto the Pacific plate some time after its formation
1992]. In the southernmost
part of the basin, the young folds
suchthat the plate boundary(the Mendocinotransformfault)
and
thrust
faults
trend
northwest
reflecting northeastnow lies at the base of the northern flank of the Mendocino
southwestcompression. The rotation of faults and folds to a
Ridge [Fisk et al., 1993]. It is not clear how the Mendocino
more east-westtrend in the southernmost
part of the Eel River
Ridge and Gorda Escarpmentare related, sincethere are very
basin and seismicity concentratedin the southernpart of the
few lithological or age data available from the Gorda
Gorda plate on some of these faults [Smith et al., 1993]
Escarpment.
suggests a present-day component of north-south
Much of the Vizcaino block is overlain by the Neogene
compressionclose to the Mendocino transformfault and triple
Point Arena basin [Hoskins and Griffiths, 1971; Ogle, 1981]
junction [Ogle, 1981; Stoddard, 1987; Clarke, 1992]. Recent
(Figure l a). The basin is boundedby the San Andreasfault to
strike-slip faults have also been mapped in the offshore Eel
the east, the Gorda Escarpmentto the north, and a structural
River basin [Gulick et al., 1997].
1.2.
Geologic
Setting
high encompassingseveral basement ridges collectively
known as the Oconostotaridge, to the southwest [Curray,
1966; McCulloch, 1987a; Godfrey, 1997; Leitner et al., this 2. Data
issue] (Figure l a). The Upper Cretaceousdeep to shallow
Our resultsare basedon three seismicdata setsand gravity
marine silty shale and fine sandstonesequencethat forms the
data.
lowermostpart of the sedimentarysectionin the southernmost
part of the basin [Hoskins and Griffiths, 1971; McCulloch,
2.1.
New
Seismic
Data
1987a, b] doesnot appearto extendas far north as the Gorda
In 1994, during the Mendocino Triple Junction Seismic
Escarpment(A. S. Meltzer et al., manuscriptin preparation,
1998). The Point Arena basin includes,in someplacesEocene Experiment (MTJSE), we collected marine multichannel
andlate Oligocene,and everywheremiddleMiocenediatomites seismic reflection (MCS) data [Tr•hu et al., 1995] in the
23,816
GODFREYET AL.: GORDAESCARPMENTAND VIZCAINO BLOCK
regionaroundCapeMendocino(Figure1). The MCS datawere
shotand recordedby the R/V Maurice Ewing usingan 8385
cubic inch tuned air gun array, firing shotsat 50-m intervals,
andrecordingdatato 14-stwo-waytraveltime (twtt) on a 4-km
streamer with 25-m hydrophone group spacing. This
geometryenabledus to producenominally40-fold common
midpoint (CMP) stackedsectionswith 12.5-m CMP spacing.
Four of the MCS lines, A, B, 5, and 7, (Figure 1) are discussed
in this paper. The 1994 MCS lines were all processedwith
similar and conventionalprocessingsequences
and processing
parameters,includingremovingnoisy traces,true-amplituderecovery, prestackand poststackband-passfilter, prestack
deconvolution,dip-move-out, normal-move-out,and stack.
For interpretationof the shallowsection,the top 6-s twtt were
time-migratedand depth-converted,using interval velocities
derived
from normal-moveout
2.2.
Other
Seismic
velocities.
Reflection
Data
MCS data were also recordedby the U.S GeologicalSurvey
(USGS) to 4-s twtt beneath the sea floor acrossthe Vizcaino
block aboardthe R/V Lee in 1977 (Figure la). Thesedata were
recorded on a 2.4-km streamer with 100-m group interval.
Shotswere fired every 50 m from a 1326 cubic inch air gun,
giving a nominalCMP-fold of 24 and CMP spacingof 50 m.
These previously unmigrated lines were time-migrated and
depth-convertedfor interpretationin this study. Two of these
lines (lines 25 and 32) are discussedin this paper (Figure lb).
The third seismic data set consistsof industry data made
available by Amoco Corporation. These data cover the eastern
part of the study area and extend south to the southernmost
part of the Vizcaino block, where well data provides age
constraints within the Point Arena basin (A. S. Meltzer et al.,
manuscriptin preparation,1998) (Figure la). This data set is
available as time-migrated sections.
2.3.
Age Constraints
Within
westernmargin of the Point Arena basin and bottomedin early
Pliocene-late Miocene(?) sediments at 387.7 m below sea
floor [Shipboard Scientific Party, 1997]. It doesnot intersect
any of the seismic lines interpretedin this paper. The site
surveyseismicdata for ODP Site 1022 intersectsthe southend
of line 5 on the southwesternside of the Oconostotaridge,
making correlation of horizons farther north along line 5,
northeastof the Oconostotaridge, very difficult.
Three other wells in the southernmostpart of the basin were
drilled for hydrocarbonexploration(small solid circles,Figure
l a). Industrydata with relatively densecoveragecloseto the
coast have been correlated with these wells, allowing the
mapping of dated unconformities,which are imaged as distinct
horizons in the MCS data, from the south of the basin to the
easternpart of the Gorda Escarpment [Ondrus, 1997; A. S.
Meltzer et al., manuscriptin preparation, 1998]. The Point
Arena Formation (or its time correlative equivalent) is the
oldestsequencethat can definitivelybe mappedfrom the south
to the northernpart of the basin. The unconformityassociated
with this formationcorrespondsapproximatelyto the base of
upper Miocene (about 11 Ma [Harland et al., 1982]). The
unconformity is a bright, distinctive reflector on lines 25
(Figures 1 and 2) and A (Figures 1 and 3). We infer the
positionof the base-of-upper-Miocenereflector on lines 7 and
B (Figures 1 and 4) by comparing the character of the
reflections on lines A, 7, and B.
2.4.
Gravity
Data
Gravitydatawerecollectedaboardthe R/V Maurice Ewingin
1994 along the same transectsas the MCS lines. The data
were collectedusing a BodenseewerkeKSS-30 marine gravity
meter, which logged data at 6-s intervals. The data were
corrected for drift, and an E6tvos correction and DC shift were
applied to obtain the free-air anomaly at 1-min time intervals.
Models derived from the shipboardgravity data along lines A,
the Point Arena
Basin
There
are few
wells
in the Point
Arena
basin
assignmentof ages to horizons imaged in the MCS data: two
deep drill hole sites in the northwesternpart of the Vizcaino
block (Deep Sea Drilling Project (DSDP) Site 173 and Ocean
Drilling Program (ODP) Site 1022) and three hydrocarbon
exploration wells in the southernmostpart of the Vizcaino
block (Figure la, circles).
The hole at DSDP
Site 173 was drilled
on the westernmost
basementridge making up the basementhigh known as the
Oconostota ridge (open circle, Figures la and lb) and is
thereforejust beyond the western margin of the Point Arena
basin.
The
hole
at DSDP
Site
173 bottomed
a)6
from the base of
the hole [Leitner et al., this issue, Figure 4]. This horizon is
not suitable for regional correlation since it pinches out
againstthe other basementridges that make up the Oconostota
ridge.
A morerecentdeephole drilled at ODP Site 1022 wasdrilled
16 km northeastof DSDP Site 173 (gray circle, Figuresl a and
lb) [Shipboard Scientific Party, 1997]. It was drilled at the
500
900
1300
o
b) m4
in undated
submarine andesitic volcanic breccia overlain by upper
Oligocene/lower Miocene(?) marine nannofossil ooze
[Shipboard Scientific Party, 1973]. MCS lines 4 and 5 cross
the DSDP drill site (open circle, Figures la and lb), and there
appearsto be good correlation between a bright reflection on
both seismic lines and the andesite recovered
100
to aid the
i,
Point Arena
b
c)
asi•
basement
6
VE - 4
west
east
20 km
Figure 2. Migrated, depth-convertedsectionsof line 25. (a)
True-scale section. Tick marks correspondto thoseon map
(Figure lb). (b) Sectionwith vertical exaggerationof 4, and
(c) line drawing.
GODFREYET AL.: GORDAESCARPMENTAND VIZCAINO BLOCK
900
500
I
a)
I
Gorda
100
I
Line A
23,817
I
1100
Escarpment
•
700
300
Escarpment
/
,--0
a)
C36
.
-UM
b)•
½•4-
,
o
b) =ø
0
900
500
I
I
100
I
10km
c)
I
I
I
I
• • ....
•E
•
C3 6
400
300
,ooloo
1-
UM
•2
VIE
--4.
_•_..................
•:•
0"
E
d)
ß ß....
-
ß ,:
•
..
.
3
B
VE=4
...........................................................
.......
d)
500
I
,-0
100
I
e) ••6
0
•--. 5ø •
•
6-
-UM
,, /,
•oø 10km .--:-P................
;;/B
•5ø
-.....
.,," /
400......•0
200
,'"'
e)
•
k•-••
-• -•
6
f) •E
2
•••••_••:
-
-B
•T -
] upper
Miocene
andyounger
sediments ........................
ß
....................................
:•...........
• UM
•4
ß
B
6/
10km
Figure 4. Migrated,depth-converted
sections
alongline 7
(Figures4a-4d) andline B (Figures4e and4f). (a) True-scale
sectionalongline 7 (tick markscorrespond
to thoseon map
(Figurelb) andarevalid for Figures4a and4b), and(b) same
sectionwith verticalexaggeration
of 4. (c) Enlargement
of
line 7. (d) Line drawingshowingthe basement
andtruncated
sediments
at the seafloor.(e) True-scale
sectionalonglineB
(tick markscorrespond
to thoseon map (Figure 1) and are
valid for Figures4e and 4f). (f) Same sectionwith vertical
exaggerationof 4. P is referred to in the text. B is basement.
Figure 3. (a) Unmigrated
deeptime sectionalongline A.
Arrowspoint to the top of accretionaryprismbasementand
the deep reflector. True-scalefor a velocity of 8 km/s,
corresponding
to a verticalexaggeration
of 2 for a velocityof
4 km/s. The tick marks along the top of the section
correspond
to themap(Figurelb) andarevalidfor Figures3a3d. (b) True-scale
sectionfor the upper6 km. (c) Migrated,
depth-converted
seismicsection. (d) Line drawingshowing
major sedimentary
featuresincludingbaseof upperMiocene
(UM) and top of accretionarywedge basement(B), H is a
horizonreferredto in text, andP is pointP referredto in the
text. For Figure 3c and 3d, verticalexaggeration
is 4. (e)
7, and B provide information about basement and basin
structurein regions where there are no coherentreflectionson
theMCS datadueto complexseafloortopography.
Figure5
shows
thegravitydataalonglinesA, 7, andB, alongwiththe
preferredmodels. Gravity modelsalonglines 3S, 4, and 5
(Figurela) arepresented
by Leitneret al. [thisissue].
3. Observations
Enlargement
of line A focusingon the sedimentary
wedge 3.1. Western Gorda Escarpment
closeto the Mendocino
Ridge.(f) Line drawing.Graycircles
Lines 5 (Figure 6), 32 (Figure 6), and 33 crossthe Gorda
showtruncated
layers;opensquares
showonlap. For Figures
3e and 3f, verticalexaggeration
is 3.5.
Escarpment
in thewesternpartof theVizcainoblock(Figure
23,818
GODFREY
ETAL.:GORDA
ESCARPMENT
ANDVIZCAINO
BLOCK
l a). The datedhorizonsfrom the hole at DSDP Site 173
(whichintersects
the southern
end of line5) that can be
maximum
thickness of about 1.5 km. The acoustic basement
shallowsto the north toward the Gorda Escarpment,with the
overlying sedimentsthinning as they approachthe top of the
correlatedwith reflectorsin the seismicdata [Leitner et al.,
thisissue,Figure4] cannotbe tracednorthalongline 5 with ridge (Figure6). Acousticbasementis exposedat the seafloor
any certaintybecause
the horizonsare disrupted
by the at the ridge crest (Figure 6). The most recent sedimentsare
Oconostota
ridge(Figure1). Figure6 showsthesedimentary tilted southand are erodedat the seafloor(Figure 6). The tilted
sectionoverlyingthe northwestern
Vizcainoblock has a sedimentsare restricted to within about 20 km of the ridge
crestand imply relativelyrecentuplift of the ridge. Rounded
cobblesdredgedfrom the ridge crest[Krauseet al., 1964; Fisk
Gorda
et al., 1993] imply the ridge was once at sea level and has
Escar3ment
since subsided,becausethe ridge is now 1.5 km below sea
900
500
level. West of 126øW, the crest of the Mendocino Ridge and
,
,
,
,
•r
20
GordaEscarpmentwere abovesealevel at about5-6 Ma [Fisk
et al., 1996]. If this is also true east of 126øW, it implies that
the pop-up of the escarpmentoccurredsometimeprior to 5-2O.• E 6 Ma, and then the ridge subsidedto its presentdepth.
a)
-40 &•
ILine A
-6o
b)
•////•/G/o
rda
o9earPl
'• 5//////.
/ / 3;';3')'•7'777•
.....................................
;•::•ean
crust
boneat
'"""'"••'•
c:l.
3.25 mantle
I- 20
900
500
100
-25 o•ro
c)
rOE
-50$•
d)
I
500
I
100
I
I
-
25
3.2.
Triple
Junction
LinesA (Figure3), 7 (Figure4) andB (Figure4), andseveral
industry lines cross the Gorda Escarpment and
northeasternmost
part of the Vizcainoblock (Figure 1). These
lines show the 1-4-km-thick Point Arena basin sedimentary
sectionand a prominentreflection at 2.5-5 km depth that is
the top of acoustic basement (Figures 3 and 4). A deeper
reflection (6.8-8 s twtt) is also imaged on lines A (Figure 3a),
B, 7, and 3S. This reflector can be traced intermittentlyfrom
about 5 km south of the Gorda Escarpmentto about 30 or
40 km southof the ridge and most likely representsthe Moho
[Henstock et al., 1997].
Lines A, 7, and B showa changein acousticcharacterof the
top of acousticbasementabout 13-16 km south of the Gorda
Escarpment(point P, Figures3 and 4). Southof P on lines A,
7, and B, the top of acoustic basement is a rough, highly
faulted surface(Figures 3 and 4). North of P, the interpreted
top of acousticbasementis a strongnorth dipping reflector
(Figures3 and 4). The top-of-basementreflector can be traced
on the MCS data to within 3-8 km of the Gorda Escarpment.
This reflector is indistinct beneath the ridge, but models
derived from gravity data along lines A, 7, and B suggestthat
the northern boundary of the Point Arena basin dips steeply
south beneath the crest of the Gorda Escarpment (Figure 5).
The top of basementin the gravity models is taken from the
MCS data wherever
Elsewhere,
-25
e)
._c5
-5oC_
Region
the acoustic basement reflection
the basement is modeled
to fit
is visible.
the short-
wavelength free-air gravity anomaly. The combination of
MCS and gravity data showsthat the basementdips north to
within about 13 km of the ridge (A, Figure 5b) before
-75
f)
south
•
20 km
..........
observed
data
preferred
model
shallow
basin
edge
(Line A)
:-:'":..'•.•
basin
•...... accretionary
prism
•
oceanic
crust
Figure 5. Gravity data and density models (vertical
exaggerationof 2) along multichannelseismic lines A, 7, and
B. (a) Model gravity (solid line) plotted against observed
gravity (dashedline) alongline A. Gray line is model gravity
for a model with a shallow basin edge. (b) Preferreddensity
model geometryalong line A. SegmentsA and B are referred
to in the text. (c) Model gravity (solid line) plotted against
observedgravity (dashed line) along line 7. (d) Preferred
densitymodelgeometryalongline 7. (e) Model gravity(solid
line) plottedagainstobservedgravity (dashedline) alongline
B. (f) Preferreddensitymodel geometryalong line B. Tick
marksaboveeachgravityplot are for correlationwith the map
in Figurelb. Numbers
aredensity
in g cm-3.
GODFREY ET AL.: GORDA ESCARPMENT AND VIZCAINO BLOCK
23,819
Gorda
800
south 400
I
1200
1
n rth
600
Escarpment
I
a)
o
E
•2
ß
0ø
L,ne32
l•::•m15
15oøø•
...................................
':•....:'
ø/
.-'"'J••
I
VE:
4
....
'" ::
-•; :...?•!!:,.•::.
i", '
,,•4-
...
j,_ 5ø•
0o
•
,
•
i VE-4
•
truncated
d) ] _
sedime,
•3
•
•i•n t
4
Gorda
100
500
Escar!•ment
e) •.
Line
5 - 10km - .........................
'-"
.__
2Itrue
scale
at4.8
km/s
0f
I
I
I
• 4 .... •
'
"•"'-'•
ß
• 6
600
700
i
f)
2,•
4o
800
i
i
true
4.8•
scale
at
"-'
,., .--.--••.-½-,
".•.,,.7•"'"./•
"
'•
5-
g)
•2-
truncated
sediments
ß•- 3,,•
4o
I
5-
10 km
Figure 6. (a) True-scale seismic section of USGS line 32. Tick marks along the top of the section
correspondto those on the map in Figure lb. (b) Migrated, depth-convertedsectionof line 32 with vertical
exaggerationof 4. (c) Enlargementof Figure 6b showingthe Gorda Escarpmentregion of line 32. (d) Line
drawing of Figure 6c. Dips correctedfor vertical exaggeration(Figures 6b-6d) are shown. (e) Migrated time
sectionalong MTJSE line 5. True scale at 4.8 km/s. Tick marks along the top of the sectioncorrespondto
thoseon the map in Figure lb. (f) Enlargementof Figure 6e showingthe Gorda Escarpmentregion of line 5
(true scaleat 4.8 km/s). (g) Line drawing of Figure 6f.
shallowing, first gently and then steeply, beneath the ridge
(B, Figure 5b). On line B, the farthestof the three lines (A, 7,
and B) from the triple junction, the northward dip of the
basementreflector is about 5ø (Figure 4b). On line A, the
closestof the three lines to the triple junction, this dip has
doubledto about 10ø (Figures3c and 3d).
The age of the sedimentsdirectly overlyingbasementin the
northernVizcaino block is unknown. We do, however, image
the baseof the upperMiocene section(horizonUM), the only
dated horizon we have to work with, which is 1 km or less
above the basement south of point P (Figures 3d and 3f).
North of P, where the basement starts to dip northward,
horizon UM rises, enclosinga wedge of sedimentsbetween it
and basementthat thickenstoward the ridge (Figures 3d and
3f). The sedimentswithin this wedge must be older than late
Miocene (about 11 Ma) by some unknown amount. The
sedimentsin this wedge onlap southwardonto the strong,topof-basementreflector and are truncatedtoward the ridge by
horizon H (no age control), which is parallel to and about
0.4 km deeperthan UM (Figure 3f). Sedimentsabove horizon
23,820
GODFREYET AL.:GORDAESCARPMENT
ANDVIZCAINOBLOCK
must have occurred since the late Miocene (about 11 Ma) as
the tilted and eroded sediments lie above the base-of-upper-
Gorda Escarpment
km
Miocenereflector(UM). This compression
causedthe ridge to
"pop-up",tilting the northernedge of the Vizcaino block and
exposing sediments at sea level, before subsiding to its
currentwater depth. Gravity models (Figure 5 and Figure 7 of
Leitner et al. [this issue]) show no crustal root beneath the
ridge suggestingthat present-daynorth-southcompressionis
dynamicallysupportingthe MendocinoRidge [Leitner et al.,
this issue]. Strong north-south compressionmost likely
began when the Blanco fracture zone initiated oblique to the
east-west trending Mendocino transform at about 6 Ma
[Wilson, 1986; Stoddard, 1987]. Prior to about 6 Ma, northsouth compressionacross the Mendocino fracture zone was
believed to be much less significant.
5 km
Figure 7. Cartoonshowingthe localizednature of the
sedimentary
wedgenearthe triplejunction. Light graysurface
is top of basement.Medium and dark gray showvertical
sectionsthroughthe sedimentarysection,medium gray is
above the base of upper Miocene and dark gray is the
sedimentary
wedgebelow upperMiocene. Bold solid lines
showbathymetry.White dottedline showshorizonUM, base
of upper Miocene.
4.2.
Triple
Origin of the Sedimentary Wedge Near the
Junction
We now discussthreepossiblewaysin whichthe pre-upper
Miocene sedimentarywedge may have formed: (1) north-south
compression
acrossthe Mendocinofracturezone (Figures8ac), (2) structure preexisting on the Vizcaino block when the
Vizcaino block was transferred from the North American to the
Pacific plate (Figure 8e), and 3) east-westextension(Figures
UM (late Miocene and younger)shallowtowardthe ridge and
are tilted south and eroded at the seafloor (Figure 4d), similar
to the sedimentarysequencenear the sea floor fartherwest on
the ridge (Figure 6).
8f-h).
4.2.1. North-south compression. The uplift of the
GordaEscarpmentthat causedthe tilting and truncationof the
youngest sediments (Figures 4 and 6) is evidence of northsouthcompressionwest of the triple junction sincesometime
Line 25 is an east-west USGS line located near the southern
after about 11 Ma (after the beginningof the late Miocene) in
endsof lines B, 7, and A (Figure lb). Line 25 showsboth the
the vicinity of line ? (Figure 1) wherewe have age control. It
top of basement shallowing eastward and the overlying
is therefore a separate,later north-southcompressiveevent
sedimentarysectionthinningeastwardtowardthe San Andreas
thanthat whichmay have formedthe sedimentary
wedge.
fault (Figure 2). The downwarpedbasementand northward
It is believed that there was no north-southcompression
thickening wedge of sedimentsbetween horizon UM and across the Mendocino fracture zone between 19 and 10 Ma.
basement are limited
to the north-easternmost
corner of the
Vizcaino block, close to the triple junction (trace of point P,
Figure 1, and cartoon,Figure7). The wedgeis seenon linesA,
7, and B, but is not seen on line 5 or 32 (line 33 does not
penetrate deep enough to see whether or not the wedge is
present)to the west and line 25 to the southeast(Figures 1, 2,
3, 4 ,and 6). In a north-southsense,the sedimentarywedge is
restrictedto within 13-16 km of the Gorda Escarpment,and in
an east-westsense,it is about 30 km wide at the escarpment.
It has a maximum
thickness
of about
3 km in the north-
easternmostpart of the Vizcaino block (Figure 7). The
lowermostage of sedimentsin the wedgeis unconstrained
and
the top of the sedimentarywedge is late Miocene in age. The
thin sedimentarysection above the thick wedge (i.e., those
sediments above horizon UM hence younger than late
Miocene) suggeststhat the processesresponsible for the
formationof the wedgedid not continueafter the late Miocene.
Between 27 and 19 Ma, however, north-south compression
acrossthe fracture zone may have been possible(D. Wilson,
personalcommunication,1997) (Figures 8a-c).
If north-southcompressionacross the Mendocino fracture
zone was responsiblefor the formation of the sedimentary
wedge, the onset of north-south compressionat any point
south of and close to the Mendocino fracture zone provides a
time at which the triple junction was locatedeastof that point
(in a fixed Pacific plate reference frame) (Figures 8a-c).
McCulloch [1987a, b] interpretedthe southwestmargin of the
Vizcaino block as a late Oligocene/early Miocene
paleosubduction-trench
(Figure 8d) and implied (though did
not explicitly state) that transformmotion betweenthe Pacific
plate and the North American plate was taken up by oblique
subductionat this paleotrenchuntil about 16 Ma when the
San Andreas initiated in its present-dayposition behind (east
of) the trench (Figure 8a). A model calling on north-south
compression, which is more likely before 19 Ma than
between 19 and 10 Ma, requires the Vizcaino block being
transferredto the Pacific plate before 19 Ma. Work on the
4.1.
Evidence
for Recent
North-South
Delgada fan, which overlies the Neogene Point Arena basin,
Compression Across the Gorda Escarpment
suggeststhat the present geometry between the Mendocino
triple
junction, San Andreas fault, and Point Arena basin has
The anomalouslyelevated Gorda Escarpmentand tilted
existed at least since the Delgada fan startedto form at 6-10
sediments eroded at the seafloor (e.g., lines 32, 5 and 7,
Figures 4d and 6) indicate recent north-southcompression Ma [Drake et al., 1989]. Prior to 10 Ma, Drake et al. [1989]
acrossthe Mendocino fracture zone. In the easternpart of the
also suggestthe plate boundarywas at the subduction
trench.
If the triple junction, Mendocino fracture zone, and Point
Vizcaino block (near line A where we have age control), this
4. Discussion
GODFREYET AL.:GORDAESCARPMENTAND VIZCAINOBLOCK
23,821
Arena basin have had their presentgeometry since the early 1990]. Griscom and Jachens' [1989, 1990] model is based on
Miocene or earlier, we should see evidence for compression the following: In the San FranciscoBay area, only about 22throughoutthis time period (prior to about 16-0 Ma) in the 23 km of a total of 300 km of strike-slip motion between the
north-eastcomer of the Vizcaino block (Figure 8a).
Pacific and North American plates has occurred on the San
A model by Griscom and Jachens [1989, 1990] proposes Andreasfault itself, the rest being presumablytaken up on the
that the triple junctionjumped eastover time (in one or more San Gregorio fault and now extinct Pilarcitosfault that lie west
stages)such that it has been at its current position, with of the San Andreas fault [Parsons and Zoback, 1997]. Farther
respectto the Pacific plate, only since about 5 Ma. This north, Griscom and Jachens [1989, 1990] correlate magnetic
estimatewas updatedto 3 Ma by Parsonsand Zoback[1997]. and gravity highs in the Vizcaino block with rare magnetic
In the Griscom and ./achens [1989, 1990] model, the triple and gravity highs to the southin the Franciscancomplex east
junctionfirst intersectedthe Mendocinofracturezone farther of the San Andreasfault. Realignmentof theseanomaliescan
west than its present position, at what is now the be done by restorationalong known faults only if there is also
a fault west of the presentSan Andreas fault in the Vizcaino
northwestern corner of the Vizcaino block (126øW in a
present-dayreferenceframe) [Griscomand ./achens,1989, block. Griscom and Jachens [1989, 1990] extrapolatethe
1990] (Figure 8b). Over time, the triple junction jumped Pilarcitos fault north through the Vizcaino block to the
eastward,accompaniedby eastwardjumps of the San Andreas Mendocinofracturezone, arguingthat it cannotrejoin the San
fault (or its past equivalents)and an eastwardlengtheningof Andreas fault south of the Mendocino fracture zone without
the Mendocino transform fault [Griscom and ./achens, 1989,
cutting across the trend of the magnetic anomalies in the
Thispaper- N-Scompression
Griscom& Jachens (1989, 1990b)
b)
SAF1
C)
sAF1
SAF
Ia) SAFPAC
I::•:;;•Z•:::
O;'
IO•j•
I •i/•I
••• • • ........
Cross-section
NAM
•'•i•.•a
Cross-section•"•.•
1.
PAC
Map
view
( •
Map
view
MFZ
'..• GOR
Map
view [. MTJ½.•
MFZ
•'
__
_mvmrL
.
•--,
Ear?•iocene
to }•.•
Prior
to19
Ma...................
- 3 Ma
until
present-iY
•:•"•"••
....
•'••••':•
•--I Farallon
Pacific/Gorda/
plate
basin
d)
Cross-section•
- 3Ma
to
td
Pacific/Farallon
SAF
m
plate
now
part
of
North American plate
accretionaryprism
Cross-section
.............
•
nOW
part
ofPacific
plate
Map
view
•,
MFZ GOR
i;!i}•iiii:i!•iii•ii
nowpartofNorth
accretionaryprism
[.•F..S.•.-'.F..-..;•-:..-:$
American
plate
,///• plate
North
American
transform fault
PAC
......
After 11 Ma
Prior to 27 Ma
I
f)
.........
relict transform fault
•
subduction trench
/x.
•
future transform fault
paleo-subduction
trench
spreadingridge
- -X line of cross-section
h)
SAF
SAF
.........
.•:•.•.•i:i:,•::•::•:.;.-.•=..
........•iii•:•:i•
.•ß
'
Cross-sect
ion•"'"'"""--,-,-•
•
Cross-section
•
Mapview !
MTJ.
Map
view
Map
view
•
-?•
X"
---::'
;•:•;.'•;.'•.•
•:,•-•j:.:•.::.-'.,,X:;.*.:•
•g,..i
untilpresent-day
This paper- E-W extension(I)
PAC
So'•":•'"•'
t' ft •"•"'
'"'••i'i•
"•:'
27M•:'"--'••• '-'"'"'
This paper- east-westextension(11)
Figure
8.
MTJ
MFZ
?GOR
PAC
Prior
to11
until
present-day
23,822
GODFREYET AL.: GORDAESCARPMENTAND VIZCAINO BLOCK
Vizcainoblock. Griscomand Jachens[1989, 1990] suggest prior to 11 Ma, it must have occurred prior to 19 Ma. If
that at about 29 Ma, transform motion initiated along the north-southcompressionis also related to the presenceof the
southwesternedge of the Vizcaino block, the paleosubduction
trench [McCulloch, 1987a, b], by oblique subduction. The
plate boundary then jumped east, probably in the early
Miocene, with a "proto-San Andreas" initiating within the
Vizcaino block along a continuation of the Pilarcitos fault
(SAF1 on Figure8b), and finally, at about3 Ma [Parsonsand
Zoback, 1997], a secondjump east formed the present-daySan
Andreasfault just west of the present-daycoastline(Figure 8c).
If
this model is correct,
we should see evidence
for
compression only from about 3 to 0 Ma in the northeast
corner of the Vizcaino block (Figure 8c) and compression
farther west prior to about 3 Ma (Figure 8b). We might also
expect to see evidencefor one or more major strike-slipfaults
cuttingthroughthe Vizcaino block (Figures8b and 8c).
North-south compressionacross the Mendocino transform
fault
concentrated
in the northeast
corner
of the Vizcaino
block near the present-dayposition of the triple junction may
have resulted in downwarping of the top of basement,
allowing the thick sedimentarywedge to be depositedagainst
the transform fault. Line A (Figures 3e and 3f) shows
sediments onlapping onto the basement, evidence for a
deepening basement close to the escarpment. If so,
compressionwas occurring sometime prior to about 11 Ma
(baseof upper Miocene), when the sedimentsbeneathhorizon
UM (the upper surface of the sedimentary wedge) were
deposited.The age of the oldestsedimentsin the northernpart
of thePointArenabasinis unknown,
sotimingof theonsetof
compressioncan be constrainedonly to a time before 11 Ma.
It is thought that there was no compressionbetween 19 Ma
and 10 Ma but that compressionprior to 19 Ma is possible
[D. Wilson, personal communication, 1997].
If the
sedimentary wedge formed as a result of north-south
compressionacrossthe Mendocino transform fault sometime
triple junction, the Mendocino triple junction, Mendocino
transform fault, and San Andreas fault must have had their
present-daygeometry from before 19 Ma to the present-day
(Figure 8a). If this geometry has existed since before 19 Ma,
it requiresmodificationof the eastwardjumping triple junction
model of Griscom and Jachens [1989, 1990] (Figures 8d, 8b
and 8c). The Mendocino triple junction and a proto-San
Andreasfault could only have been locatedfartherwest along
the Mendocino transformfault significantly before 19 Ma.
4.2.2 Preexisting structure.
An alternative model is
that the sedimentary wedge in the northeast comer of the
Vizcaino
block
is a remnant
of a forearc
basin related
to
Farallon subduction(white region marked "e", Figure 8d). In
this model, the forearc basin formed on the Vizcaino blocl,
when it was part of the North American plate and was
transferredwith the Vizcaino block to the Pacific plate when
the SanAndreasfault formedin its present-day
position(with
respectto the Pacific plate) (Figure 8e). It is a coincidence
that the wedgeendedup in the northeastcornerof the Vizcaino
block close to the triple junction. In this model, the
sedimentsat the base of the wedge overlying the basement
may be older than 27 Ma.
Offshore data from elsewhere in
California, however,show sedimentsof this age to have a
distinctly different reflective characterfrom the Neogene
sedimentarysectionand they are rarely concordantwith the
overlying sediments (A. S. Meltzer, manuscript in
preparation, 1998). This model suggeststhe San Andreas
formed in its presentpositionwith respectto the Mendocino
transformfault and Point Arena basin after the wedge was
formed, i.e., after about 11 Ma, implying that the triple
junction also arrived in its present-dayposition after 11 Ma.
This modelallowsa "Griscomand Jachens"[1989, 1990] -like
scenario,where the San Andreasfault jumps east over time,
Figure 8. (opposite)Tectonicmodelsto explainthe localizedsedimentary
wedgein the northeast
cornerof
the Vizcaino block near the Mendocinotriple junction. MFZ, Mendocinofracturezone; SAF, San Andreas
fault; SAF1, proto-SanAndreasfault; MTJ, Mendocinotriple junction;PAC, Pacific plate; FAR, Farallon
plate;NAM, NorthAmericanplate;GOR, Gordaplate. Schematic
crosssectionalongline x-x is shownabove
eachmodel. (a) Compressional
model(this paper). Hachuredarea showsthe regionof the Vizcainoblock
predictedto be affectedby compression
acrossthe Mendocinofracturezone. This regionextendsfrom the
paleotrench(southwestern
boundaryof the Vizcaino block) to the present-daySan Andreasfault, and no
intermediateSan Andreastransformis required. (b) and (c) Compressional
modelafter Griscomand Jachens
[1989, 1990]. This model assumes that the San Andreas fault initiated at the northwestern comer of the
Vizcainoblock west of its present-day
positionandjumpedto its presentpositionat the easternboundaryof
the Vizcainoblock accompanied
by an eastwardlengtheningof the Mendocinotransformfault at about3 Ma.
In Figure8b, hachuredareashowsthe regionof the Vizcainoblockwestof the proto-SanAndreasfault (SAF1)
predictedto be affectedby compressionacrossthe Mendocinofracturezone from 27 Ma to about 3 Ma. In
Figure 8c, hachuredarea showsthe region of the Vizcaino block west of the present-daytriple junction
predictedto be affectedby compression
acrossthe Mendocinofracturezonefrom about3 Ma to present-day.
(d) Startingpoint for all models:Farallon-Pacificspreadingprior to 27 Ma. The regionthat will becomethe
Vizcaino block after the San Andreasinitiatesis currentlypart of the North Americanaccretionaryprism.
White regionsmarkedby letter e representa preexistingforearcbasinon the North Americanaccretionary
prismthatwill be requiredby the modelshownin Figure8e. (e) Modelrequiringa preexisting
forearcbasinon
the North American plate accretionaryprism (white regionsin Figure 8d) which is transferredwith the
Vizcaino block to the Pacific plate when the San Andreasfault initiatesin its present-dayposition(with
respectto a fixed Pacificplate). (f) Extensionalmodelbasedon a pull-apartbasinformedat the northernendof
the SanAndreasfault dueto the SanAndreasfaultandCascadia
subduction
zonebeingnoncollinear.(g) and
(h) Extensionalmodelbasedon continuedspreading
at the subducted
Farallon-Pacific
ridge. In Figure8g,
sediments
are deposited
in a linearbasinformedby east-westextensionabovethe subducted
spreading
ridge.
In Figure 8h, the San Andreasfault preferentiallybreaksthroughthe crustat the locationof the now extinct
spreadingridge which lies beneaththe wedge(this may alsobe true for all othermodels).
GODFREY
ETAL.:GORDA
ESCARPMENT
ANDVIZCAINO
BLOCK
arriving in its presentposition (with respectto a fixed Pacific
plate) relativelyrecently. The seismicdata we have examined
showno compellingevidencefor a proto-SanAndreasfault, a
major plate boundary. The present-daySan Andreasfault is a
23,823
it was part of the North American plate that was transferredto
the Pacific plate with the Vizcaino block, these models all
suggestthat the Mendocino triple junction was east of the
sedimentarywedge during the time the wedge formed and
distinctive 2-4-km-wide shear zone [Ondrus, 1997], and we see
thereforeconstrainthe triple junction,San Andreasfault, and
Point Arena basin to their present geometry since at least
11 Ma. Although we cannot yet choosebetween the models
fault in the USGS data at the location of the Oconostotaridge. in Figures 8a, 8e, 8f and 8h, these hypothesescould be
Recent migrationof the USGS data set revealsthe Oconostota distinguishedby drilling for age control in the northeastern
ridge to consist of blind thrusts in the acoustic basement corner of the Vizcaino block.
nothinglike it fartherwest in the Vizcaino block. McCulloch
[1987a, b] cited evidence for a possible proto-San Andreas
rather than a crustal-scale near-vertical strike-slip fault
[Godfrey, 1997].
4.2.3. Extension. A third class of models explains the
formation of the wedge by east-west extension during the
"extensionalphase"(25-15 Ma) of Leitner et al. [this issue].
Two mechanisms
for extension in the northeast corner of the
Vizcaino block are (1) extension due to transtension across
the newly forming San Andreas fault, i.e., the sedimentary
wedge is evidenceof a pull-apart basin (Figure 8f) and (2)
extension related to continued spreading of the PacificFarallon ridge after it was subducted[cf. Murdie et al., 1993]
(Figures8g and 8h).
4.2.3.1. Pull-apart
basin formation.
East-west
extensionmay have occurredif the newly formed San Andreas
5. Conclusions
North-south compression sometime after about 11 Ma
(more likely after about 6 Ma) acrossthe plate boundary
betweenthe Pacificplate(whichincludesthe Vizcainoblock)
andGordaplateformedtheGordaEscarpment,
ananomalously
elevated region of the Mendocino fracture zone close to the
Mendocinotriple junction, eastof 126øW.
Alongthenorthern
boundary
of theVizcainoblock,except
closeto the triplejunction,the GordaEscarpment
is overlain
by a relativelythin sedimentary
sectionthat thinstowardsthe
crestof the escarpment.Closeto the triplejunction,thereis a
regionally-limited
sedimentary
wedgethat formedprior to
transform fault was not collinear with the Cascadia subduction
11Ma and that thickenstowardsthe escarpment
southof the
zone (Figure 8f). This geometry would result in east-west Mendocinofracture zone. The strong spatial correlation
transtension between the northern end of the San Andreas fault
betweenthe sedimentarywedgein the northeastcornerof the
and the southernend of the subductionzone, allowing a pull- Vizcainoblock and the Mendocinotriplejunctionsuggests
apart basin to form in the northeastcorner of the Vizcaino
block [Ingersoll, 1982b] (Figure 8f). This model requiresthe
triple junction to be east of the wedge (pull-apart basin)
throughout the period the wedge formed and therefore
constrains the triple junction to have been in its present
location (with respect to the Mendocino transform fault and
Point Arena basin) since before 11 Ma.
4.2.3.2. Extension over a subducted
spreading
ridge. If spreading continued for some time after the
Farallon-Pacific spreadingridge was subductedbeneaththe
North Americanmargin, extensionof the overlyingcrustmay
have occurred allowing a basin, now representedby the
sedimentarywedge, to form [cf. Murdie et ai•, 1993] (Figure
8g). Decompressionmelting at the subductedridge may also
have underplatedmafic material to the lower crust in the
northeastcorner of the Vizcaino block explaining the thick
mafic lower crustbeneaththe triple junction region [Henstock
the triple junction, San Andreas fault, and Point Arena basin
have had their presentgeometrysincebeforesedimentswere
depositedto form the wedge(sometimebefore11 Ma). Three
possiblemodels for the formationof the wedge are (1)
formation as a pull-apart basin due to transtensionat the
northernend of the newly forming San Andreasfault, (2)
formationdue to east-westextensioncausedby continued
spreadingat the subductedPacific-Farallonridge, or (3)
formation due to north-south compressionacross the
Mendocinotransformfault causingdownwarpingof the
accretionaryprism basement in the northeasternmostcorner
of the Vizcainoblocktowardthe fracturezone,allowinga
thickwedgeof pre-11Ma sediments
to be depositedaboveit,
againstthe Mendocinofault. Unlesswe explainthewedgeas
the remnantof a fortunatelyplacedpreexistingbasin, all
possiblemodelssuggestthat the triple junctionhas been in
its presentposition with respectto the Mendocino fracture
et al., 1996; Leitner et al., this issue]. One drawback of this
zoneandPointArenabasinsinceat least11 Ma, andpossibly
model is that we might expectan elongatebasinoverlyingthe
subductedridge ratherthan a wedge-shaped
basinin the comer
of the Vizcaino block (Figure 8g). The extinctridge beneath
the North Americancontinentmay have been a weak zone that
allowedthe San Andreasfault to preferentiallybreak through
the crustbeneaththe wedge(true for all models(Figures8a, 8e,
8f and 8h)). If the wedgeformedover the subducted
spreading
ridge andthe San Andreasfault brokethroughthe crustabove
the subductedridge, the triple junction is constrainedto have
been at its presentlocationsincethe wedgeformed,i.e., since
before 11 Ma, becausethe triple junction is always at the
northernend of the San Andreasfault (Figure 8h).
since before
4.3.
Summary
19 Ma.
Acknowledgments. We would like to thank Jon Childs and the
NationalGeophysical
Data Centerfor providingcopiesof the 1977
MCSdataandAmoco
Corporation
andSaraFoland
forproviding
copies
of theindustry
data.WethankCraigNicholson,
KateMiller,andRandy
Keller for criticalreviewsthat greatlyimprovedthis manuscript.
Fundingwasprovidedby NSF Continental
DynamicsgrantsEAR9218209, EAR-9219598, and EAR-9526116.
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GODFREY ET AL.: GORDA ESCARPMENTAND VIZCA1NO BLOCK
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(ReceivedDecember1, 1997;revisedMay 25, 1998;
acceptedJune19, 1998.)