Oct 10, 1998 - College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis. Nicola J. Godfrey 2. Department of Geophysics, Stanford ...
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
..... '............... •-OR
123W
126W
OBSC4
125W
124W
123W
OBSA1
' ........... •..... :......... : • •'x'
(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
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.......
,,:.......... :..-..--
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 .•
... 2O
(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
