Prism' Structural Diversity in a Small Volume of the Lower Slope

JOURNAL OF GEOPHYSICAL

RESEARCH,

VOL. 97, NO. B4, PAGES 4439-4459, APRIL 10, 1992

Three-DimensionalSeismicImagingof the CostaRica AccretionaryPrism'
StructuralDiversity in a Small Volumeof the Lower Slope

Tt4OMAS H. $HIPLEY

Institutefor Geophysics,
Universityof Texasat Austin
Kuu{ D. McI•rrosi-I ANt>ELI A. Snv•

Board of Earth Sciences,Universityof California,SantaCruz
PAUL L. STOFFA

Departmentof GeologicalSciences
andInstitutefor Geophysics,
Universityof Texasat Austin

Conventional
two-dimensional
seismicreflectioninvestigations
havebeengenerallyrelieduponto provide
imagesof largeto mediumscalestmcturalfeaturesin accretionaryprisms.We undertooka three-dimensional
seismicreflectionsurveyof a smallpartof a prismarcwardof the Middle AmericaTrenchoff CostaRica to
more correctlyimage structureandto usethe improvedstructuralinformationto examinethe processes
of
accretion. This surveyreveals small features,with dimensionsof hundredsof meters, while also defining
featuresthousandsof metersin lateral extent,both of which were underappreciated
in conventionaltwodimensional
datafromthe samearea. We haveimagedactiveoffscrapingat thetrenchandbothduplexingand
out-of-sequence
faulting a few kilometersarcwardof the trench. Fault spacingandreflectorgeometryvary
dramaticallyovera spaceof severalhundredmeters. Someof thesevariationsare relatedto visiblechanges
in morphologyof the underlyingoceanicbasement,but othersare not so easilydocumented.Fault surface
reflectionsdefinean architecturewhichmay controlgrossfluid motionthroughthe prism. This architecture
is apparentlyformedby duplexingandout-of-sequence
faultingandhasbeenmaintainedby periodicmotion

faults. The slopesedimentapronrecordsmultiplephasesof deformation.
on someof the out-of-sequence
Abundantsmalloffsetreversefaultsbreakthe seafloorandindicaterecentshorteningof a broadregionof the
underlyingprism.A primaryresultof thissurveyis appreciation
of thestructuraldiversityacrossa smallwidth
of an accretionaryprism.

INTRODUC•ON

gent margins have improved steadily. We will present here
resultsfrom the first three-dimensional
(3-D) seismicsurveyof
Investigations of the internal structure of modern accrean active accretionary margin that overcomes some of the
tionary margins i'ely on the combinationof drilling and seisimaging problems.This data set coversa 8.5 km wide x 21.6
mic reflection profiles. The relationshipbetweenthe observed
km long x 4 km thick volume of the accretionaryprism just
structureswithin cores and seismic images remains poorly arcward of the Middle America Trench off Costa Rica.
understood
but is thoughtto be a functionof scale.Brown et al.
Many models of convergentmargin tectonic processeshave

[1990] have demonstrated
this problemoff Barbados,wherethe
been developedfrom studiesof rocks now exposedin ancient
results of five drill sites and a seismic cross section over a 7deformedbelts [Boyer and Elliott, 1982; Suppe, 1983]. These
kin-wide transectof the marginproducestwo differentpossible modelshave been appliedto modernmargin settingsand used
line-balanced cross sections. This difference is thought to
to guide interpretationof seismiccrosssections.For example,
result largely from lack of significantconstraintsby the seisCowan [ 1985] related North American Cordillera melangetypes
mic data. Another possibilityis that the diversity of structures
to specificpositionswithin a model convergentmargin, while
possiblewithin this tectonicenvironmenthas not been fully
Byrne [1986] andSampleand Moore [1987] useda modelof
appreciated.This uncertaintymight reflect our inability to drill
convergentmargin structureto explain deformationstyles on
enoughholes or to seismicallyimage the structuresproperly.
Kodiak, Alaska. These comparisonsare possiblebecauseof
Structural studies of modem convergent margins present a
geochemicaland structural features recorded in the rocks.
challengingproblem. Great water depths and hole instability
However, the internal anatomyof a modem accretionaryprism

make deep sampling a difficult technical problem, while the
is poorly known. The generalknowledgeof the internal strucrapidly varying velocity fields and possiblethree dimensionture comesfrom a variety of seismicreflectionstudiesacross
ality of structures degrade conventional seismic reflection
numerous active margins. Some exceptionally spectacular
imaging [Coltrin et al., 1989]. Even so, investigationsof the
seismicreflection images include Nankai [Aoki et al., 1982;
structuralframework and tectonicprocessesat modem convetMoore et al., 1990], Aleutians [McCarthy and Scholl, 1985;

Ryan and Scholl, 1989], Peru [yonHuene et al., 1985a], and
Copyright1992 by the AmericanGeophysicalUnion.
Papernumber91JB02999.

Barbados [Westbrook and Smith, 1983; Westbrook et al.,
1988]. Even these,however,show few detailsmore than 10 km

0148-0227/92/91

arcward of the trench axis.

JB-02999505.00

4439

4440

SHIPLE•ET AL.: THREE-DIMENSIONAL
STRUC'FURE
OFCOSTARICAPRISM

Thepurposes
of ourprogram
were(1) to determine
thedegree Normalfaultingis evidentseawardof the trenchaxis(at -2, -4,
to which3-D acquisition
andprocessing
techniques
improve -6, -9 km in Figure2). Becausethe trenchaxisdeepensto the

imagingwithinthe accretionary
margins,
and(2) to examine

the internalanatomyof one suchmarginto investigate
the
basictectonicprocesses
associated
with its growth.The first
objective
hasbeenexamined
by Stoffaet al. [1991].We chose
to studythismarginbecause
of conflicting
structural
interpretations derived from conventionalseismic data which poorly

northwestand the absenceof significantwestwarddrainage

systemsin NicoyaPeninsula,
the trenchaxisis nearlydevoid
of turbidites.Reflectionprofilesshowthat all of the oceanic
pelagicsediments
are subducted

beneaththe lowerslope,while
someof the hemipelagicsectionis scrapedoff abovea welldefined decollement (between 0 and 5 kin). Most of the

imagedthe internalstructureof the prism.For examplethis hemipelagicsectioncontinuesto dewateras it is subducted
partof theMiddleAmericaTrenchis about500 km southwest beneaththe slopeseveralkilometersto aboutwhere the 3-D
of the area off Guatemalawhere Deep Sea Drilling Project surveystarts[Shipleyet al., 1990]. The shallowzone of
in part, for the nearly
(DSDP) drilling has shownthe slope to be composedof offscrapingappearsto be responsible,
Cretaceousophioliteand litfie net accretionof sedimentary reflection-freesedimentwedgeat the baseof the slope(0 km to
material since the Cretaceous [Aubouin et al., 1982]. This
combined with evidence of a similar ophiolite complex on-

4 km in Figure2).

bodyandthatthereis at leastsomemodemoffscraping
[Crowe
and Buffier,1983;ShipIcyand Moore, 1986].Crosssections
of previously
acquired
datahavebeenusedto suggest

thatthe
deformed
wedgeof sediments
maypredatetheunusually
thick,
relativelyundeformed
sedimentary
apronsuggesting
thepossibility of a nonaccreting
margin[CroweandBuffier,1983].On

decollementand an increasein prism thickness(line 1, Figure
2). From25 km to beneaththe shelfedgeat 55 km, a zoneof

The small amountof offscrapingand the frontal accretion
shorethe Nicoya Peninsulahas led to speculationthat the suggestthat much of the growthof the prismmust be by
An importantzone of this underplatingoccurs
offshoreslopeof CostaRicais similar[Bourgois
et al., 1984]. underplating.
Even so, mostdata suggestthat the marginis a sedimentary between 10 and 25 km associated with a drop in the

reflections 500 to 1000 m thick with variable amplitude and

continuityis well developedjust abovethe downgoingslab.
Someof thesereflectionslie subparallelto the basement,then

rampupwardat 20 to 30 km fromthetrench.
Theupperextent
of theserampsareoftenassociated
with smalloffsetsat the top
a distinctive
highas growingthrustduplexeswith the slopesectionpartially of .theprism.The topof theprismproduces
isolatedby roofthrusts.
Datacollected
previously
off theCosta amplitudereflectionsurfacewhichhasbeenhighly disrupted
Ricanmarginincludedsparsemultichannel
seismicwork of and offset by numerousfaulting episodes(labeled rough
1976-1978vintage[CroweandBuffier,1983],high-resolution surface).
watergunreflection,andSeaBeamcoverage

in 1982[ShipIcy The overlyingslopesedimentaproncontainslow-amplitude

the otherhand,Silver et al. [1985] interpretedthe structurehere

and Moore, 1986], limited deeptow seismicsin 1986 [Moore

reflections

that become less coherent and less continuous

and ShipIcy,1988],anda singleDSDPsite(565) thatreached toward the trench. Several unconformities and a general
328 m subbottom[vonHueneet al., 1985b].In addition,there
has been continuingwork on the geologyof exposedupper

trenchwarddownlappingreflectionconfigurationare obvious.
Normalfaultsare distinctlyimagednearthe arcwardendof the

Jurassic to lower Cretaceous marine sediments and oceanic

profilebetween30 and45 km, with fault offsetgreatestand

crustal rocks onshorethe Nicoya Peninsulaand elsewhere mostprevalentin the middleandlowerportionsof the slope

The DSDP site 565 penetrated
328 m of
alongthe margin[Bourgoiset al., 1984;Lundberg, 1982; apronstratigraphy.
Baumgartneret al., 1984;Corrigan et al., 1990]. Another terrigenousmuds bottomingin lowest Pliocenesediments
foraminiferal
zoneN18 [StoneandKeller, 1985])
primaryreasonfor choosing
CostaRicais thatit is oneof the (planktonic
few locationswhere the entire margin is within line-of-sight over300 m abovethetopof theprism(Figure2). Baltucket al.
from the structuralfabricof the coresthat
radio rangeof the shoreline,crucialfor the high precision [1985] demonstrated
the slopeis unstableandsubjectto significantcreepandslope
navigation
requiredfor thefieldexperiment
(Figure1).
in theuppermost
40-50 m.
In this paperwe briefly reviewthe CostaRica continental failure,particularly
margingeology,includingresultsfrom regionaltwo-dimen- Thus the 2-D data define the downgoingslab, a thick wedge
sional(2-D) seismicreflectiondata.Followingthe review,the with few coherentinternal reflections,a rough contactbetween
resultsof the3-D surveyaredescribed
with sections
devotedto an unusuallythick seawardprogradingsedimentaryslope
of frontal
theexternalprismstructure,
surfacereflectionamplitudes,
and sectionandtheunderlyingwedge,andsmallamounts
internal prism structure.In the discussionwe presenttwo accretion. One of the most unusual features revealed in the new
variantsof a model showinglower slopegrowthprocesses.
COSTA RICA CONTINENTAL MARGIN GEOLOGY:

Two-DIMENSIONAL SEISMICDATA

The topof the downgoing
slabwasobserved
fromthetrench
axis to the shelf edge on 24-fold multichannelseismiclines
acrosstheCostaRica margin[Croweand Buffer, 1983]andin

recenthigher-resolution
48-fold data (Figure2). The age of
is late Oligocene[Klitgordand
oceaniccrustbeingsubducted
Mammerickx, 1982] and the convergencerate is about 8.7

2-D data set is a well-defined series of reflections in the arcward
third of the sections which def'me a ~1000 m-zone above the

downgoing
slab.We haveno definitiveexplanation
for these
reflections,but the possibilitiesrange from basementduplex
to a subductionchannel[i.e., Cloos and Shreve,1988].
RESULTS OF THE 3-D SEISMIC STUDY

We chosethe 3-D coverageto includethe zone of initial
andextended
underplating
it 22.6 km arcward.We locatedthe
grid so thatwe wouldimagea pair of bathymetric
reentrants

cm/yr [DeMetset al., 1990]. The Cocosplate stratigraphy between3200 m and 3000 m which might indicatestructureat
consistsof about250 m of oceanicpelagiccarbonates
overlain depth.We alsopositioned
thegridto takeadvantage
of DSDP
with about 175 m of more terrigenous.hemipelagic
material site565 whichsampledpartof the slopesedimentapron.Some
in line numberingwill be usefulfor later reference
deposited
as the platedeepened
with ageandapproached
the conventions
continental sediment sources [Shipley and Moore, 1986]. to the figures. First, the basic 3-D portion of the survey

SHIPLEY ET AL: THREE-DIMENSIONAL STRUCTURE OF COSTA RICA PRISM

NICOYA

4441

PENINSULA
•Nlcoya

COSTA

RIGA

80

9.5

50

Caribbean
Plate

1050

-86.5

-86.0

100 o

95 ø

90 o

85 ø

80 o

-85.5

Fig.1.Mapshows
bathymetry
offshore
oftheNicoya
Peninsula
ofCosta
Rica.Thebathymetry
isacompilation
oftheUniversity
6f
Texas
andScripps
Institution
ofOceanography
data.
Inset
shows
plate
tectonic
setting.
Tenlong
diplines
are48-fold
seismic
profiles.
Stippled
area
is8.5x 21.6km3-Dgridconsisting
of88lines100mapart
forming
the3-Dvolume.
DSDP
site565shown
bydot.

consistedof 88 parallel lines shot 100 m apart normal to the
trenchtrend and 23 km long. Processingresultedin 170 lines,
50 m apart,for a width of 8.450 km. Each line consistsof 650
traces,each33.33 m apart,for a line lengthof 21.631 km. The
line numberingconvention(from 41 to 210) and trace numbers

analyses
withintheslopeapronandstarting
estimates
for the
prism and are consistentwith the limited dat•i available from

DSDP site565. Extensive2- and3-D migrationtrialswereused
to develop a reasonableestimateof the velocity structure
within the prism. That velocity structureremainsan es{imate,
(150 to 799) are shownin Figure3. Elevenadditionaldip lines however,becauseof the relative insensitivityof post,tack
extendout onto the Cocosplate and passthroughthe grid to migration velocities.

the upperslopeand shelf off CostaRica and are referredto as
swath lines. Finally, for conveniencewe will describe the
position of some features relative to the distance from the
trench axis (y axis) and parallel to the trench (x axis). The
seismicsectionswill be labeled as such.The grid coversthe
area between3 km (y) and24.6 km (y) arcwardof the trench.
Stoffaet al. [1991] havepresenteda detaileddescriptionof the
field programandprocessing
procedures.
Derivation of reflection geometry from seismic traveltime
data impliesknowledgeof a velocity field. All illustrationsare
shownwith the vertical dimensionof depth (or thickness)so
that the geometrywill be correctlyrepresented.The velocity
structureusedin the directdepthmigrationof the 3-D grid is a
simplified model based on both conventional velocity

We developed
a smooth
velocity
modelforthema•g'ul
Which
variesonly with positionnormalto the margin.The seafloor,
the top of prism and the top of basementwere usedto define
two variable-traveltime layers throughoutthe 3-D volume

(Figure4). A smooth
velocitymodelis usedsolocalvariaiions
in geometrywill not be causedby localizedvelocityeffectsof
the direct depth migration. Reasonablevelocity•gi'adients
(estimatedfrom Hamilton [1980]) decreaseddiscontinuitiesin

the verticalvelocityfield at the slope/prism
boundaryand the
seafloor/slope
apronboundaryto help reducemigrationartifacts.The basementvelocitywas kept unrealisticallylow to
reducemigration artifactswithin the overlyingprism, the
primaryfocusof this work. The resultingmeanintervalvelocity of the slope apronvaries from 1600 to 1700 m/s across

4442

SHIPLEYET AL.: THREE-DIMENSIONAL
STRU•

(r•)H.Ld•a

OFCOSTARICAPRISM

(lfil)l)H.Ld=lCi

o

!
!

i=

• II

I$

O_

•o

O-

':::5

SHIPLEY ET AL.: THREE-DIMENSIONAL

STRUCTURE OF COSTA RICA PRISM

4443

EXTERNAL STRU•
the grid. The meaninterval velocity of the prism varies from
1650 to 3220 m/s. The seafloordip is about4ø and the slab dip
is about3ø with this velocitymodelwithin the 3-D surveyarea.
OceanicBasementSurface
The data volume was directly migratedto depth in a one-pass
migrationof the completedata set [Stoffaet al., 1991].
The basementis difficult to define preciselyarcwardof 15
km (y) because of the lower signal levels and narrower
bandwidth,but its generalshapeis identified to an uncertainty
of one seismicwavelength(about 10 m at 3 km (y) to 150 m at
25 km (y)) (Figure 5). A well-defined graben structurewith
-150 m of relief is parallel to both the plate fabric and the
trenchat 7 km (y). Justarcwardof the grabenis a horst block
with similar trend and relief (at 8 km (y) between 6.5 and 8.5
km (x)). An unusualdepressionoccursbetween7 and 12 km (y)
at 0 and 2 km (x). This depressionis about 250 m below the
regional basement depth. The basement surface steepens
abruptlyarcwardbetween 13 and 15 km (y) and again at about
45 km (y) (see also Figure 2), giving the downgoingslab an
overall convex curvature. The dip increasesfrom about 3 ø
within the first 20 km of the trench,then to approximately7ø,
and finally to more than 14ø between45 and 55 km (y). We are
confident that the relatively abrupt dip changesportray the
•200m •, ,•
actual structureof the downgoing slab becausethe velocity
field
used for depth conversion (and migration) varies
20 40•60
smoothly acrossthese slope breaks.

Base of SlopeApron/Topof Prism
This top surface of the prism is the most complicated and
roughest in the survey area (Figure 5). A high-amplitude
reflection forms the boundary between the slope apron and
underlying accretionaryprism (Plate 1). We define the slope
apron as a largely depositional sedimentary unit and the
acc•etionary prism as a body formed primarily by tectonic
addition of sediments.The greatly improved images of the
surface, produced by the 3-D acquisition and processing
techniques,show that it is composedof folded and multiplely
faulted thrustsheetsthat impart the observedrough texture (see
Plate 2c and Figure 6). However, due to discontinuousand
overlappingnature of the reflection, detailedcorrelationof the
roughsurfacebetweenlines only 50 m apartis still difficult and
placeslimits our ability to map it precisely.
We have relied on a combinationof continuityand amplitude
to define this surfaceto produce the contouredstructuremap
includedin Figure 5. The roughsurfaceis highly irregular,with
relief of 150 m or more at wavelengthsof a kilometer or less.
The generalstructureis that of a trenchparallel low, spanning
the width of the surveybetween4 km and 8 km (y), then a west
plunging high in the middle third of hhe grid, and irregular
trendsin the arcwardthird. The depressionin the surfaceat 5
km (y), 1 km (x) is offset from the low in the underlying
basement,and there is no evidence for this depressionin the
seafloor.

SeafloorSurface

A seafloorsurfacecontourmap derivedfrom the 3-D grid data
is also shown in Figure 5. Stoffa et al. [1991] reported that
-20
these data define the seafloor with comparable detail to an
existing 20-m contour map produced by Sea Beam which
KILOMETERS
(X)
indicates that the navigation and basic processing of the
Fig. 3. Locationof linesandnamingconventions
usedin thispaper.After seismic data set is correct (Figure 7). The general trend of
processing
thedatasetconsisted
of 170lines50 m apartandsmacked
traces
33.3m apart. Distancesareannotated
normalto thetrenchin kilometers(y contoursis not parallel to the trench except at the trenchward
axis)andparallelto thetrenchin kilometers
(x axis).Thegridcoversthearea end of the grid. The slope of the seafloor is significantly
from 3 km to 24.6 km (y) and 0 km to 8.5 km (x).
gentler between the 2900-m and 3200-m contour. This zone

4444

SHIPLE•ETAL.:THREE-DIMENSIONAL
STRUC'II.
JRE
OFCOSTA
RICAPRISM
MIGRATION VELOCITY MODEL (m/s)

Z

BASEMENT

3.0

km(y)

24.6

Fig.4. Velocity
function
used
indepth
migration.
Theseafloor,
topofprism,
andbasement
surfaces
weredefined,
andthenthis3D velocivy
fieldwasused
indirect
one-pass
depth
migration
ofthewhole
dataset.Thisfunction
isasmoothed
version
based
mainly
on trial migrations.

seemsto represent
a modemdepocenter
andbenchin theslope southwestcomerof the map in Plate 1, are anomalouslyhigh.

sincemanysmallchannels
in theupperslopeterminate
here Sincethe basementstructureis fairly uniformin the trenchward
amplitudes
arenot likely an
and the slopecoveris about150 m thicker(Figure5, slope portionof the area,the anomalous
isopach).
Fiveseafloor
mounds
formmudvolcanoes
or linear effectof basementgeometrynor of variationsin seismicpropagation,becausethe overlyingstructuresare simple. One
mud fissures(Figure7).
explanation
for the anomalous
amplitudes
is thatthe reflection
is not simplythe contactbetweensedimentandvolcanicrocks
but is someapproximation,
includingsedimentswhich smooth
Isopachs
for thetwomainunits,theslopeapronandprism, basementirregularities.Thus the more continuousreflection
axe also shownin Figure 5. The prism thickensaxcward, identifiedas basementreally may be producedin sedimentary

lsopachMaps

varyingfromabout400 m to 2800m. Thethinnest
section
is rocksjust abovebasementwhich smoothsthe interface.The
at about5 km (y), 1 km (x) in a persistent
low,offsetfromthe high reflectivityin the southwest
comerof the surveyedarea,
basement
depression.
The slopeapronis thickabovethethin then,may be relatedto changesin the sedimentdepositional
sectionin theunderlying
prism.Conversely,
a thin slopesedi- patternjust abovebasementand/orrelatedto localizeddiamentapron,