Crustal structure of the northwestern Vizcaino block and Gorda ...

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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

.•:.,..:

.......

,,:.......... :..-..--

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

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