TECTONICS, VOL. 20, NO. 5, PAGES, 649-668 OCTOBER 2001

Tectonic erosionand consequentcollapse of the Pacific margin of Costa Rica: Combined implications from ODP Leg 170, seismicoffshore data, and regional geology of the Nicoya Peninsula

PaolaVannucchi, 1David W. Scholl,2Martin Meschede, 3 andKristin McDougall-Reid 4

Abstract. The convergentmargin off the Pacific coastof the subduction erosion, similar to those described for the Nicoya Peninsula of Costa Rica exhibits evidence for scientifically drilled Japan, Tonga, and Peru margins, best subductionerosion caused by the underthrustingCocos plate. account for offshore and onshore evidence for a post- Critical evidencefor efficacyof this processwas recoveredat Paleogenehistory of crustal thinning and landward trench the Ocean Drilling Program(ODP) drilling Site 1042 (Leg migrationof CostaRica's Pacific margin.During the past 16- 170), positioned-7 km landward of the Middle America 17 Myr the calculatedmass removal and landward migration trench axis off the Nicoya Peninsula.The primary drilling ratesare 34-36 km 3 Myr'l km4 of margin,and 3 km Myr'l, objectiveat this site was to identify the age and origin of a respectively.These values are similarto thosefound for other regionallyextensive and prominentseismic discontinuity, the Pacific margins dominatedby nonaccretionarysubduction so-calledbase-of-slope sediment (BOSS) horizon or surface. zone processes. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastalarea and parallel to it for 1. Introduction hundredsof kilometers,separates a low-velocity(- 2.0-2.5 km s-1) sequenceof slopesediment, from an underlying The Pacificmargin of CostaRica haslong beenconsidered sequenceof much higher-velocity (> 4-4.5 km s-1) rock. Site a favorableplace to studyprocesses of subductionaccretion 1042 reachedthe acousticallydefined BOSS horizon at a (Figure 1). In part for this reason a large quantity of below sea level depthof- 3900 m and yieldeda carbonate- geophysicaldata have been gatheredin this offshoreregion. On the basis of this information, and Deep Sea Drilling cementedcalcarenitic breccia of early-middleMiocene age. Sedimentological,geochemical, paleontological, and cement Project(DSDP) Leg 67 and 84 drilling results,two dominant, paragenesisdata documentthat the brecciaaccumulated in a but very different, models have emerged during the past shallow water depositionalenvironment. On the basis of decaderegarding the structureof the CostaRica margin and coastal exposures,the BOSS horizon, as a margin-wide the tectonic mechanismsshaping its form and evolution geologic interface, can be temporally and [Shipleyet al., 1990, 1992; VonHuene and Flueh, 1994; Hinz lithostratigraphicallycorrelated to a regional angular et al., 1996; Ye et al., 1996]. One model ascribesmargin unconformity.This unconformity,known as the Mal Pals evolution to subductionaccretion, in particular to massive unconformity,separates Neogene and younger shelf-to-littoral underplatingof oceanicsediment since the early Tertiary. A bedsfrom the underlyingmafic unitsof the MesozoicNicoya contrasting or nonaccretionarymodel contends that the Complex and Cretaceousand early Tertiary sedimentary margin is mostly underlainby Costa Rican basementrocks sequences.At Site 1042 it is inferred that tectonismcaused (i.e., chiefly mafic ocean crustal units of Cretaceousage), the vertical subsidenceof the early Neogenebreccia from a which extend seaward from coastal outcrops to near the shallowto a deepwater setting.The Mal Paisunconformity of trench axis. Figure 2, adaptedfrom Von Huene and Flueh the BOSS horizon thus connects the rock fabric of the [1994], diagrammatically contrasts the accretionary and outermostpart of marginto that of coastalNicoya and implies nonaccretionarymodels. It is importantto recognizethat for CostaRica the accretionaryand nonaccretionaryconcepts are that in the early Neogenethe Nicoya shelf extendedseaward based on the interpretationsof seismic reflection and to near the presenttrench axis. This circumstancerequires refraction information; that is, they are effectively that the earlyNeogene trench axis was at least50 km seaward of where it is now located. The long-term effects of unconstrainedby geologic observations.As emphasized below,this paper focuses new geologicinformation on testing the viability of the nonaccretionarymodel. •EarthSciences Department, University of Modena and Reggio A major scientific objective of Ocean Drilling Program Emilia,Modena, Italy. (ODP) Leg 170 was thus to supplygeologic information off 2Departmentof Geophysics, Stanford University, Stanford, the Nicoya Peninsulaof Costa Rica. Drilling was targeted California, USA. 3Instituteof Geology,University of Tuebingen,Tuebingen, along a traversefrom the trenchto the midslopearea. Beyond Germany. drilling to investigatethe relative importanceof different 4U.S.Geological Survey, Flagstaff, Arizona, USA. tectonicmechanisms forming the margin'srock and sediment framework, Leg 170 sciencealso focusedon mass balance Copyright2001 by theAmerican Geophysical Union. issuesdealing with the volume of solid and fluid material Papernumber 2000TC001223. addedto, subtractedfrom, or that bypassedthe marginduring 0278-7407/01/2000TC001223512.00 much of Cenozoictime [Kimura et al., 1997].

649 650 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN

Nt".CARAGUA

SANTA ELEN/: CARIBBEAN SEA

PACIFICOCEAN ...... • ..:•! •.•:•!•. Leg! FISHER .....

CocosPlate •

87 ø 86" 85 ø 84 ø 83 ø 82"

Active volcano • NicoyaComplex • MiddleAmedcaTrench e ODP Site 565 00DP Leg170 study area

Figure 1. Simplifiedmap of CostaRica and surroundingregions showing bathymetric features and ODP Leg 170 location.A schematictectonic map is shownin the inset.

Theinitial attempt to investigatethe geologic history of the Meschedeet al., 1999]. In this paper we assembleand CostaRica marginwas at DSDP Site 556, Leg 84. This site interpret the geologic and tectonic implications of a wasdrilled in responseto the findingsof thetransect of holes combinationof newlyacquired Leg 170 findingsand onshore drilled by DSDP Leg 67 acrossthe Guatemalamargin mappingand rock information.These data sets are focusedto northwestof Costa Rica [Von Huene et al., 1980]. The test the viability of the nonaccretionarymodel, which can be objectiveof Site 556 drilling,located on the middleslope, approachedbecause this model predicts that the margin'srock was to reach a prominent,subsurface reflection horizon, flameworkor wedgeunderlying the BOSShorizon is exposed variablycalled the rough,smooth, or base-of-slopesediment along the Nicoya coastand extendsseaward to near the base (BOSS) surface(Figure 3). At the time of the drilling the of the slope.We conclude,on the sedimentary,structural, and surfacewas, on geophysicalgrounds, thought to be eitherthe micropaleontologicalevidence described and interpretedin top of a subductioncomplex buried beneatha 500-1000-m this paper,that the predictionsof the nonaccretionarymodel thick apronof slopedeposits, or a regionalunconformity are consistentwith our fmdings.With respectto the larger beneath which Costa Rican basement rock would be implicationsof our conclusionsthe Cenozoicevolution of the encountered.Unfortunately, drilling did not penetratethe Costa Rican margin can be viewed as a manifestationof the overlyingapron [Von Huene et al., 1985]. A decadelater, in tectonic processesof subduction erosion and sediment 1994, visual data were collectedduring 20 Alvin dives subduction. exploringfor fluid vents [Silver, 1996]. Thus, in effect, geologicaldata resulting from Leg 170drilling are among the 2. Tectonic Setting and Previous first to providephysical observations about the natureof the Geophysical Studies BOSShorizon and thus to decipherthe geologicevolution of the CostaRica margin.Crucial informationabout the nature The Nicoya Peninsula,which rises above the leadingor of the BOSS horizon was recovered near the bottom of Site western edge of the Caribbean plate, is underthrustat a 1042, which, located near the base of the landward trench velocityof-90 mm yr-• (90 km myr-•) by the eastward slope,was sitedto penetratethis surface. subductingCocos plate [DeMets et al., 1990]. A detailed The existence of the BOSS horizon has been cited as bathymetricswath map of the Pacific side of Costa Rica evidencefor subductionerosion [Vannucchiet al., 1998; reveals that relief of the incoming Cocos plate determines a) ACCRETIONARY MODEL 651

0 10 20 30 40 km 50 Middle America '"Slope apron 2

Pelagic Oceanic 6 deposits

"' 10 AfterVon Huene and Flueh (1994) 10

b) NONACCRETIONARY MODEL •. 0 10 20 30 40 km 50 Middle c America ._o 2 ..... ' i Tr•ch ...... o• 4

x Pelagic O •u .... illE ..... -'lie ..... --'lie ..... ii,• ..... ii"E = • = :i -• 6 deposits ß-•illllC Crus•- ":---iLi.=.--i•::-•::-z•i•:•JJ:•i•L;•:•i•:"--:•,ui•:-:iig.,u•:=:ii•,u:::--rim, .'".'-'-u:•:-:ila•r:i•:-• 6 Accretionary prism '":,,• .....-',,• .....-',,• .....-i,,• .....i ß 8 ":•itl;i[:--:'i•.11L'.i;:--:i E '-' 10 AfterVon Huene and Flueh (1994)

c) Velocity model CostaRica Coast MiddleAmerica SIo.p.eapron O -- Pelagic Trench • __2.5• ..:-Ill . r-••-•"•• •:tl ..:=::: -- deposits •.u• n•-----a•.... w., ---..-.:i!=!i:-,-',• : ß:: ,n-..--- _•-•!!:ß, ß;,,., ;i'•.i!!•i=iii•,._-:::=ii:,•,.-_ "'.•i .... •.i!!• •!:,_=.= -- • • =_'::'..i m_-.m.. • :'..: im_-.m :•:: O.U ::::-'••m- ;n• ::'•,,..-..::-'::: Ira-- ...... -•"'.'. -- •'""'"'•'•:;0 "•:;' liE:::'•'1'i 5. IL.,.:•ii5.0r::" 'ILL.'..... ll.:i'::;ilie!!:=i•i;i;iei•11L":i:=:l•.11L::!: MARGIN ROCK ß Oceaniccrust•••••----••.•ø'•-5.5 ' ::::-' =',;•...... •""' ::".i•',;.• '.::'_':::=-'., :: '-" FRAMEWORK --

VE=2.1 • /.Z -•• ---•--?.='...-•' •':•',;•:..':i••. •!"=::•:-'•"' 5.4 IP-wavel 20 Iveløcityl -20 0 20 km40 60 80 AfterYe et al. (1996)

Figure2. Accretionaryand nonaccretionary models for the submerged Costa Rica margin, adapted from Von Hueneand Flueh [ 1994],and velocity structure of Nicoyaforearc after Ye et al. [ 1996].

much of the morphologyof the continentalmargin [Von A numberof seismicinvestigations of theNicoya offshore Huene et al., 1995, Fisher et al., 1998; Von Huene et al., havebeen completed during the pastdecade. Stoffa et al. 2000] (Figure 1). Southeastof the Nicoya Peninsulathe [1991] and Shipleyet al. [1992] first conducteda three- Cocosplate is populatedby seamounts,that inboard of the. dimensional(3-D) seismicsurvey in thisarea. Figure 3 shows Middle'America trench (MAT) penetratethe lower trench a 2-D seismicreflection profile off the Nicoya Peninsula.In slopeand leave subduction tracks appearing as embayments thisprofile the margiftssubsurface velocity structure is well [ VonHuene and Scholl, 1991, Von Huene et al., 1995,Fisher constrained from -•20 km landward of the trench axis to the et al., 1998](Figure 1). The topographyof the Cocosplate coastalarea (Figure 2). Four velocity domainscan be seawardof the Nicoya Peninsulais, in contrast,rather distinguished:that of thelower or incomingCocos plate, the smooth,and the correspondingCosta Rican continental lowermostcontinental slope, below the BOSS horizon,the marginhas a nearlyuniform sedimentary cover [Mcintosh et inner massof the marginof the middle-uppercontinental al., 1993]. Along the Nicoya margin slumpsand other slope,and, above this horizon, the apronof slopesediment gravitationalprocesses erode the middle and upper slope and [Ye et al., 1996]. drive materialsto the lower slopeand ontothe trenchfloor The incomingCocos plate, which, at thelocation of Leg 170, [Fisheret al., 1998; VonHuene and Ranero,1998] (Figure was generatedat the East PacificRise [Lonsdaleand 1). Klitgord,1978, Meschede et al., 1998; VonHuene et al., 652 VANNUCCHI ET AL.: TECTONIC EROSIONOF THE COSTARICA MARGIN "BOSS" Reflector

UTIGCR-20 • :.•.... / Site1041 Site565 / (projected)(Pr ' '-• '• - -4.00 (projected)••••i•Site1042 '".•'•

Site Site 1040Site ....Forearc bsement

..... Oceanic basement

2 km BOSS Reflector

;...... :...... Site1042 -

ream basemen"-.t

Figure3. Seismicsections across the forearcwedge front of the CostaRica convergentmargin showing locationsof ODP andDSDP sites [Shipley et al., 1992].

2000], has a 450-600 m thick coverof oceanicpelagic Beneaththe region of the middle to upper continental depositsoverlying an igneouscrust of low or smoothrelief. In slope,at a waterdepth range of 3500-1000 m andfrom 55 to thevicinity of thetrench, normal faults vertically displace the 25 km from the shoreline,three main sedimentaryand rock oceaniccrust from 50-150 m, where it entersthe subduction units are recognized on seismic reflection and refraction zone. profiles(Figures 2 and 3). From top to bottomthese are (l) The baseof the lowercontinental slope is a wedge-shaped the sedimentarydeposits of the slopeapron, (2) the landward massof sedimentcharacterized by low acousticvelocity, thickening,wedge-shaped unit below the BOSS horizonthat -•1.5-2.0km s '], thatabruptly terminates landward against an formsthe margin'sforearc basement, and (3) beneaththe base irregular,landward dipping reflector (Figures 2 and 3). This of the margin,the underthrustingsurface of the oceanicslab. outerwedge is 3-5 km wide and < 1 km thick. A few coherent Beneaththe middle slope, poor reflectivityand incoherent or continuous reflectors, either subhorizontal or landward stratificationcharacterize the apronsequence, but below the dipping,characterize its internalstructure. Beneath the outer upper slope lateral continuityof internal reflectivity,i.e., wedgea strongreflector of reversepolarity, the interplate stratification,is well expressed.The thicknessof the apron d6collement or megathrust, is recorded. Beneath the sequenceranges from 0.5 to 2.6 km, thickeninglandward d6collementthe subducting ocean crust and overlying blanket suchthat the apronof slopesediment beneath the upper slope of pelagicsediment is well definedfor a downdipdistance of is more than twice that of the lower slope. A bottom- -3-4 km. Underthrustingis well imaged,with duplex simulatingreflector (BSR) is presentat a subsurfacedepth of structuresof-1 km in lengthinterpreted by K. Mcintosh -150 m (Figure3). (personalcommunication, 1998). Slumpsand gravitational As previouslyemphasized, the lower boundaryof the slideslocally determine the morphologyof the lowerslope apronsequence is coincidentwith the strongreflector called producingscarps that are sharply outlined in theswath map of BOSS horizon or surface, beneath which is the forearc VonHuene et al. [ 1995,2000]. basementwedge. The basal sedimentof the slopeapron VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 653

drapesthe BOSS horizon, and locally it isoffset by a fewtens igneousoceanic crust (Sites 1040 and 1043); and two mid-to- of metersof reverseand normalfaults [Meschede et al., lower slopesites (Sites 1041 and 1042). The referenceCocos 1999].Stratification within the apronsequence, as noted,is plate section at Site 1039 and the comparisonunderthrust weak,and correlationsfrom the upperslope to the lower sediment at Site 1043 document that no more than 5-10 m, if slopeare unreliable. Locally, the slope apron sediment onlaps any at all, of the top of the underthrustingreference section the BOSS surface[Hinz et al., 1997], documentinga are missing at Site 1043. This observationimplies either a downslope,time-transgressive burial of the BOSShorizon. negligible to zero volume of frontal accretion, and that The BOSShorizon marks an abruptchange in velocity sedimentrecovered at the toe of the lower slope is mass- from2.0-2.5 km s '• forlower beds of theslope sediment to wasted debris from the lower slope [Kimura et al., 1997; 4.5km s -1 and higher, with a maximumof 5.9km s -1, in the Vannucchiand Tobin, 2000]. It is possiblethat erosionof the underlyingforearc basement wedge (Figure 2). Below the uppermostsediment by trench axis bottomcurrents occurred lower and middle slopethe surfaceof the BOSS horizon prior to underthrusting[Silver et al., 1997]. However,it is exhibitsan irregular morphology, but beneath the upper slope evidentthat the d6collement,which is an overpressured the profile of the BOSS surface changesto a smooth, horizondocumented by itsnegative polarity reflection (Figure continuoussurface (Figure 3). What is importantabout the 3), allowsthe lower plateto slip with little frictionbeneath BOSS horizon is that it can be traced landward to near the theupper plate of thecontinental margin. Nicoya coast and parallel to it along much of Costa Rica The discoverythat, effectively,frontal accretion does not [Barbozaand Zucchi, 1994; Hinz et al., 1996] and northward occuralong the drillingtransect has important meaning for off at leastsouthern Nicaragua [Ranero et al., 2000]. theobservation that inboard of theNicoya Peninsula, løBe is The nature of the forearc basementwedge underlyingthe not detectedin young arc eruptiverocks, althoughthis BOSS reflector has been a matter of debate for decades. The radiogenicisotope, which indicatessubduction of young interpretationof the high-velocityand indistinctstructures oceansediment to the depth(- 100 km) of arc magmatism, imaged on seismic profiles is the focal point of the occursin abundancein sedimententering the CostaRica controversy.Shipley et al. [1992] and Hinz et al. [1996] subductionzone [ Valentineet al., 1997]. agreedthat below the middle slope the main fabric of the forearc basement wedge consists of landward dipping horizonsthat generally do not offset the BOSS interface. 3.2. Stratigraphy of Middle Slope Sites Beneaththe upper slope they recognizedthat landwardthe ODP Site 1041 was drilled into the middle slope of the internalreflectors become more gently dippingapproaching Nicoya marginto reach,sample, and determinethe geologic horizontalattitudes. Shipley et al. [ 1992] interpretthe seaward natureof the BOSS surfaceand to determinethe stratigraphy part of the interior wedge as an accretionarystructure, of the overlying sedimentaryapron. Unfortunately,after a constructedof larger coherentunits separatedby landward goodrecovery of the slopeapron sediment, because of poor dipping,low-angle thrust. This hypothesisstates that packets hole conditions,the BOSS horizon (-550 m below sea floor of offscrapedand underplatedoceanic sediment, now highly (mbsf)) and underlyingrock units of the forearc basement consolidated, form the margin's interior wedge, with wedge couldnot be reached.Hence site 1042 was drilled near accretionarybuildup of the forearcbasement wedge since the the BOSS horizon oceanwardclosure (-7 km landwardfrom Eocene (Figure 2). Accretionary buildup of the forearc the trench),where it was expectedat only 300 mbsf (Figure basementwedge has been under way sincethe Eocene. 3). At this location the setting of BOSS horizon can be Von Huene and Flueh [1994], Hinz et al. [ 1996] and Ye et distinguishedby the terminationof the landwardthickening, al. [1996] confinethe accretionaryprocesses (i.e., the tectonic wedge-shapedunit that formsthe margiftsforearc basement, addition of lower plate material to the upper plate) to the and, in fact, a thrustsurface was drilled [Kirnuraet al., 1997]. lowermost slope of the margin. More importantly, they The slopeapron sequence at site 1042 was coredevery 50 m. considerthe bulk of the forearc basementwedge to be the Thus Site 1041 is the referencesite for the apron section offshore extension of the so-called Nicoya Ophiolite (Figure4). Site 565 of DSDP Leg 84 was devotedto the same Complex.This effectivelynonaccretionary model explains the objectivesas Sites1041 and 1042, and,even though Site 565 few indistinctlandward dipping reflectors beneath the middle- did not reachthe basement,a continuousapron sequence of upper slope as part of the basement'sfabric of rock and latestMiocene to Quaternaryage was recovered[l/on Huene structuralunits, and it explainsthe BOSS surfaceas a major et al., 1985]. unconformity separatingtwo very different stratigraphic At Leg 170's Site 1041, the complete Pleistoceneto sequences.More recently,enhanced seismic processing has Mioceneslope apron sequence was penetrated(Figure 4). The imagedthick (1-2 km) lenticularbodies along the interplate sequenceconsists of olive green clay(stone)and silt(stone) surfacethat are interpretedby Raneroand VonHuene [2000] with minor sandstone,limestone, and volcanic ash (Figure as displayingupper plate materialbeing transported downdip 5a). A relative abundanceof coarser material, medium to by processesof subductionerosion. coarsesandstone containing granule to pebble-sizedclasts of claystone,is presentin the lower part of the unit. Ash layers 3. Leg 170 Results are common in the upper part of the apron sequence,and granules of volcanic glass are common throughout this 3.1. Lower Slope section. Apron sedimentation seems to have been Leg 170 drilledfive sites(Figure 1): onereference site on characterizedby episodesof mass flow intercalatedwith the subductingCocos plate (Site 1039); two sitesthrough the accumulationof pelagic/hemipelagicsediment, especially in toe of the landward trench slope, the d6collement,the the upperpart of the column.The coresare generallyof poor underthrustsedimentary section, and into the underlying quality,with intensedrilling disturbance.Also, dissociationof 654 VANNUCCHI ET AL ßTECTONIC EROSION OF THE COSTA RICA MARGIN

Sealevel Site1042 •.3592 rn 0 -

•oo- .• •,

_ •592 rn 200-•'•.:•.. .O -- E - •

• _ Site 1041 oo Bedding Dip 90 ø o

ß 3oo - '• • '• ß - Subunit 2A ß ß ß - E •:• Carbonate•ement• • ,• breccia • , Thrust- • ©• • 2B

lOO o - Subunit lB* • oE ß 400 - •= •= •••Che•-basalt Sil•claystone breccia Gas Irates - -- recoveP

ß

200

E v , :

300

400

Figure 4. Schematicrepresentation of Sites1041 and 1042lithostratigraphic columns. The beddingdip of the slopeapron sequence of site 1041 is alsosummarized. Sea level is the referenceframe.

gashydrate, found in the intervalbetween l 16 and 184 mbsf, tectonism is concentrated in two intervals where two faults contributesfurther to coredisturbance. Although complicated cuttingthrough the apronsequence are suspected. by thesedisrupting factors evidence for low-gradetectonic Benthieforaminiferal assemblages identified in Site 1041 deformationwas only recognizedas microfaults,minor (S. Hasegawa,unpublished data, 1998) indicatea general fracturesand changesin bedding dip. This evidence for shallowingtrend from upper middle bathyal depths (500-1500 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 655

23 •2 125 2{• OCEANDRILLING 3• '3.3. 34 ...... :-tJ....•";•...::i::•!'i::.....l'...• •.:::.;.:. i '-l LEG I HOLE jCORE ! SECT ...... •...... -•..... ;:•;{...... :...:.:.•....•...... :.,,..:c4;a...... •.::,.:....?...... ?.:...y...:•½..•7;½½..?.?..... ca. a...... ;.:....' ,'ii'.i.Bii!!i'j-jj:ii.i'i'l.iij:j...... -,.... •.?:..;•.::•,;....::.'...,:;r.•.t.•:"L.:...... :.: i

....:;"::';: ?.;'• ...... ;...... >.. .

• '-:-.,.i:-'::-.'ß i,11i•:;':i.. '4' ;?'::-::.•: . * . •...-.•;:... :.:: .:.:. :, .:.: :. . ..:..;: :..•.> ....,.:.,{:-

:•.:•. •"'*' '"" .•.•;•½.... ;:•....

.i:.:. •.•:..•.½,•:•.,• ...... ::..•.:•.,•?• :.,r%?"•' :...... ,,::: •

b

Figure 5. (a) Photographof Site 1041 siltstone.(b) Photographof the Site 1042 carbonate-cementedbreccia.

m) in the middle Mioceneto abyssaldepths (>_4000 m) in the Miogypsina, Archaias, and Amphistegina have been Pleistocene.The Hole C (samplesfrom 405 mbsf to 424 recognizedamong the benthic foraminifera(E. Robinson, mbsf) and the lower part of Hole B (from 280 mbsfto 386 written communication,2000). Approximately5% to 25% of mbsf) are dominatedby specieswhich have upper depth the calcarenitedebris are volcaniclasticcomponents, mostly limits in the upper middle bathyal(500-1500 m) and upper plagioclasefeldspar, pyroxene, amphibole, and volcanic bathyal(150-500 m) biofaciesas defined by Ingle [1980]. fragments, and <2% are siliciclastic componentsand These species include Cassidulina laevigata carinata, glauconite (percentageswere calculated by N. Lindsay- Epistominella exiqua, E. californica, E. pacifica, and Griffith, written communication, 1998). All the components Uvigerina basicordata.Stratigraphically higher in Hole B are unalteredand thusfresh in appearance.Angular claystone (from 155 mbsfto 260 mbsf)and in Hole A (from 23 robsfto clasts were also observed. 155 mbsf) benthic foraminiferal assemblagesindicate The calcareniteclasts constitutingthe breccia are well depositionoccurred in the lower bathyalto abyssalbiofacies cemented without showing evidence of compaction. (> 2000 m) of Ingle [ 1980]. Theseassemblages are dominated Sedimentarystructures, such as laminationsand vadosesilt by Bulimina rostrata, Pullenia bulloides,Stilstomella spp., filling of cavities (Figure 6a), are preservedin the clasts. Uvigerinaprobiscidea, and Uvigerinasenticosta. The high Petrographic,X-ray, and isotopic analyses were used to abundanceof uvigerinidsin samplesfrom Hole B at 222-250 identifythe depositionaland cementationenvironment of the mbsf accompaniedby an increasein the abundanceof other clasts.The cementis composedboth of fibrous to bladed lower slopespecies suggests an increasein organicmatter and calcitecrystals grown aroundthe clastsand of dirty micrite lower oxygenconditions which accompanyrising sea levels filling the pores(Figure 6a). The originalmicrite of someof and an increase in corrosive bottom water. The peak the breccia fragmentsis completelyrecrystallized to a abundanceof uvigerinidsin a samplefrom Hole A at 23 mbsf neomorphicsparite. Polygonal sutures are not observablein suggestseither a glacialinterval or risingsea level. the pore spaces.Pellets and microfossilchambers in the clasts Site 1042 penetratedthe uppermostpart of the BOSS commonlybear brownishto greenglauconite (Figure 6b). reflectionhorizon as definedas an acousticdiscontinuity, i.e., Rarely,glauconite is presentas intergranularcement. a downsectionbreak from •-2.0 to at least3.5 km s -I (Figures Secondaryporosity is widespread,and calcitefilling of 2 and 4). Spot coring of the apron recovereda sedimentary fractures(veins) took placeafter primarycementation of the sectionsimilar to the deeperpart of Site 1041. However, at calcarenite. These calcite veins are preserved as Site 1042, from 316 mbsf down to 342.9 mbsf, a carbonate- prebrecciationstructures together with microfaultsand mud cementedlimestone breccia was cored(subunit 2A). Between injections(Figure 6c). The calcite veins are constructedof 342.9 and 353 mbsf a different breccia was encountered, syntaxial(grown from the edgesto the centerof the vein) mainly composedof fragmentsof red chert,doleritic basalt, fibrouscrystals that are strain free except for raretwinning on and serpentinizedmafic rock, togetherwith fragmentsof the e planes,a patternthat reflects growth twinning [Laurent sandstoneand claystonein a clayeysilty matrix (subunit 2B). et al., 1990]. Veins do not exhibita preferredorientation, The carbonate-cementedbreccia consists mainly of angular even thoughtruncations are observed.The only consistent calcareniteclast of varioussizes, from granuleup to > 10 cm relationis that thickerveins, up to 1 cm, are youngerthan (Figure 5b). Someof the recoveredmaterial may comefrom thinner veins of submillimeter width. The thickest veins are continuouscalcarenite beds (as the interval 1042B-1R-2, 90- constructedof drusy calcitethat incompletelyfills the void 98 cm given by Kimura et al., [1997]), but the limited (Figure6d). The structuresin the clastslost their persistency diameterof the coresprecludes verification. The calcareniteis becauseof the brecciation,and few postbrecciationstructures 70% to 90% composedof neritic plus subordinatepelagic cutthrough both clasts and matrix. components,including planktonic and benthicforaminifera, The matrix of the carbonatebreccia is composedof shell, algae and sponge fragments. Lepidocyclina, carbonatemudstone containing microfossils and bloclasts. 656 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN

lmm b)

c) lmm d)

•{.•.::•'•'•'•c•:: '"":•$.Figure 6, (a)Photomicrograph offibrous and mosaic cement (crossednicols). •e fibrouscement is preferentially developedin meniscus•pical of vadoseconditions. (b) Commonglauconite grains. The a•ow pointsto a microfossil withgeo•ml filling (parallel nicols). (c)Photomicrogr•ph of an injectionin a clastof the carbonate-cementedbreccia (parallelnicols). (d) •ick vein with dmsy calcite.(e) Asymmetricvein synmxially grown at a clasffmatrixbounda• e) ...... -• lmm of the car-•.nate-cementedbreccia (parallel nicols).

Patchycalcite aggregates and common glauconite testify to a particularlyMn andFe, the principal activator and quencher, rudimentarycementation process (Figure. 6e)? Syntaxial veins respectively,in calci.te. The observationscarried out on the and healedfractures are commonand preferentiallylocated at cementand veins show low luminescencerelative to all other the boundarybetween clasts and matrix, or theyoccur entirely carbonatephases. within the matrix(Figure 6e). Veins locatedat the.boundaries The br.eccia•has been dated using both planktonicand of the clasts exhibit asymmetricalgrowth: They have. a benthonic foraminifera. The planktonic foraminifera in syntaxialedge linked m thec!ast wall andthey show euhedra! sedimentimmediately overlying the brecciaare assignedto crystalterminations in the m.atrix (Figure, 6e), Boundaryveins the N8-N10 zone,i.e., •12.6-16.4 Ma by Ibaraki [2000]. On are also present at. the edges of clayey clasts. the basisof his studyof thin sectionsof the breccia,E. Cathodoluminescenceobservations were madeto distinguish Robinso:n(written communication, 2•000)• estimates a similar calcitegenerations on the basisof traceelements distribution, agefor-the limestone. To moredefinitely date the carbonate- VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RlCA MARG1N 657

Table 1. Sr IsotopeRatio in Samplesfrom Core 1042B l R, Around the vein are clastswith evidenceof shear.The type 4 Section 2W vein is the only one with a clear tectonic origin. Interval,cm Depth,mbsf Sr isotoperatio Cathodoluminescence of the veins of subunit 2B shows 10-13 317.17 0.708740+2xl 0-s luminescencerelated to calcite,even though they lack relative 80-87 317.85 0.708715+2xl 0-s intensitydifferences. 91-94 317.98 0.708699ñ2xl0 -s Within the chert-basaltbreccia, at around 343.42 mbsf, a 5 allere mbsf is metersbelow seafloor cm thick band of well-indurated silty clay was recovered. Optical analysisof this materialreveals a uniform lithology and a complexhistory of deformation(Figure 10), where cementedbreccia, three samples were analyzed for 87Sr/86Sr hydraulic brecciation,similar to what has been found in ratio by A. Paytan (written communication,2000). The sedimentfrom the CascadiaMargin [Clennelland Maltman, averageratio is 0.70872, which correspondsto an age of 16- 1995],and shearing predate vein formation. The oldermineral 17 Ma, the top of the early Miocene [Paytanet al., 1993], the precipitationconsists of quartz pods, cut by extensional ageof the samplesincreases slightly with depth(Table l). calciteveins perpendicular to the top of the core.The younger Transition sedimentto subunit 2B, the chert-basaltbreccia, veinsare calcite,quartz, and composite calcite-quartz bearing, was not recovered;hence the change is stratigraphically and theyoccur with no preferredorientation and crosscutting abrupt. Subunit 2B is a polymictic breccia of clasts of relationship.Type 4 veinscan be linkedto this siltyclay level doleriticbasalt, palagonite, devitrified pumice, red and white and its deformation. The bottom of the chert-basalt breccia chert, claystone,siltstone and sandstone,serpentinized mafic, showsevidence of a severeshearing (Figure 11). Cataclasis and quartz-micaschist. Clasts of micritic limestone,feldspar, affects the clasts. There are no veins in the sheared horizon, glauconiteand olivine occurin traceamounts (Figure 7). All but the clastsand the matrix are very altered, especiallythe the lithologiccomponents show a mediumto high degreeof glass fragments.Below 353 mbsf the silty claystoneof alteration. The clasts constitute about 40-50% of the breccia; subunit lB, highly fracturedand with the developmentof they are tightlypacked but rarely in contact.Brecciated clasts scaly fabric, has been interpretedto mark a thrust fault are also present.The matrix is coloredgreen by chloritized [Kimura et al., 1997] (Figure 11). clay, and a minor amountof calciteoccurs within the matrix (Figures 8a and 8b). The chert-basaltbreccia is a poorly 4. Discussionand Interpretations sortedaggregation of angularand subangularclasts (Figure of Offshore Data 8b). The shapesof the clastsare various,but the serpentinized mafic rocksare spherical.The size of the clastsranges from Site 1042, positioned-7 km landwardof the trenchaxis submillimetersup to 5 cm. off the Nicoya Peninsulaof Costa Rica, recovered a The clastsof the chert-basaltbreccia contain prebrecciation calcareoussedimentary breccia at a depth of-3900 m. The structuressuch as laminationsin the sedimentarycomponents brecciais datedpaleontologically as just older than -15 Ma, and commonveins. Veins are quartzin the chert,calcite in the and directlyvia its strontiumisotope ratio at 16-17 Ma. The limestone,and zeolite in the volcanicfragments. Zeolite veins brecciais representativeof a basalwell-cemented unit of the exhibit both ribbon-shapedcrystals alternating with clay mineralsand amoeboidcrystals (Figure 9). In both examples the zeolite is clinoptinolite.Veins are presentalso in the matrix within which a complexhistory of mineralizationcan be traced.All of the carbonateveins sampledin the chert- ' ...... basaltbreccia can be classifiedinto four typesbased on their .•..•:.:• i•':•© .... morphology,structural association with the host sediment, :...•...... •, [:..•....•..• ..... and intersections(Figure 9). Type l veins occur in all the ...... examinedsamples and cross-cuttingrelations document they are the olderveins (Figure 9). Type 1 veinsare constructedof micritic mosaic calcite incorporatingclasts of the breccia, .... they are anastomosing,up to 0.1 mm thick, and have diffuse edges.These veins have no preferredorientation, and where ,....:,.• they surroundclasts their thicknessis greatestand the calcite becomesclean and fibrous. Type 1 veins resemblecement precipitationin irregularfractures, implying an earlyorigin. Type 2 veinsalso occur in all the examinedsamples; they consistof dirty, pervasiveveinlets with mosaicand antitaxial .....• :: ....• calcite (Figure 9). These veins do not show a preferred orientation.Type 3 veins are zeolite, calcite, or composite ...... :...... calcite-zeoliteveins that cut through the type 1 and type 2 , OCEAN DRIL.LtNG CO'e SeCZ veins as straightfracture fillings (Figure 9). Type 3 veins are mosaicor fibrous in texture, but always extensionalwith no pressure-solutionseams. A type 4 vein occursin one sample at 343.39 mbsf. This vein consists of fibrous calcite with extensional,antitaxial growth (Figure 9). It is anastomosing, 0.5 mm thick, and perpendicularto the top of the core. Figure 7. Chert-basaltbreccia of Site 1042. 658 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARG1N

Figure 8. (a) Chloritizedclay (dark) and calcite (light) in the matrix of the chert-basaltbreccia (crossed nicols).The grain in the top right comer is a zeolitizedgrain. (b) Photomicrographof the chert-basaltbreccia (crossednicols).

apronsequence of slopesediment and its velocityis high,3.5- exhibitsfew inclusionsof fine material,such as clay particles. 4.0km s -•. Thus, from a velocityperspective the basal breccia High-energy environmentsare associatedwith oxidizing beds are part of the prominentvelocity jump representedby conditionsthat, precludingincorporation of Fe and Mn in the the acousticallydefined BOSS horizon. However, the basal carbonatelattice, provide a good explanationfor the lack of breccia units are temporallycongruent with the overlying cathodoluminescenceof the brecciacarbonate grains [Moore, bedsof the slopeapron. As a consequence,and in our view, 1989]. the greatergeologic significance of the BOSS horizon,which At leastthree generations of calciteveins testify to fracture underliesvirtually the width and breadthof the Costa Rica of the calcarenite soon after cementation,but prior to its margin, is that it is a regionally extensive unconformity brecciation.Other fractures in the clasts are producedby separatingrock unitsof contrastinglithology and age. Drilling sediment injection. These fractures reveal interplay of at Site 1042 thuspenetrated into the top of the acousticBOSS overpressuringand displacementoperating in the calcarenite. horizon but not the more important geological or age- The sedimentarysuite and the kineticfactors necessary for contrastinginterface included within the velocitytransition. cementationof the clastslimit their depositionalenvironment The carbonate-cemented breccia contains neritic shell to shallow marine conditions. Distinguishing the many fragments, all well preserved. The Archaias and the possible settings is, however, difficult. The intertidal- Amphisteginaare abundantin the sedimentand they are both shorefacedzone, in associationwith beach sedimentationand shallow inner to middle shelf generaat the presentday and the shelf margin are both possiblehabitats for the Archaias frequentcomponents of carbonatebeach sands (E. Robinson, and the Amphisteginaand are high-energyplaces where written communication,2000). The sedimentarysuite of the depositionand cementationof the calcarenitecould have clasts of the carbonate-cemented limestone breccia includes taken place. Brecciation also occurred in an environment fine-laminated, unbioturbated and well-sorted calcarenite, where high-energy cementationwas able to operate, as claystonechips, sediment injections, and a rich photic-zone indicatedby the syntaxialasymmetric veins developedat the fauna. These sedimentarycharacters are associatedwith grain matrix boundariesand the patchycalcite aggregatesin thoroughlithification. DSDP and ODP samplestestify that the matrix. The angular boundaries of the clasts, the deep-seacarbonate units are weakly induratedexcept for intrabasinal nature of the breccia, and the lack of exotic, Cretaceous and Jurassic sediment [Larson et al., 1992]. younger fauna suggest a low level of reworking and Moreover, deep-seacarbonate beds display evidence of transportationof the clastsafter they were formed. Also, the widespreaddissolution as the main diageneticprocess. In Sr isotoperatio data indicatethe carbonate-cementedbreccia contrast, the carbonate-cementedbreccia recovered at Site gets older downward (Table 1). This is an evidencethat the 1042 is 16-17 Myr old, and dissolutionis not observed.The sectionis stratigraphicallyordered and thusmost likely intact. cementparagenesis in the carbonate-cementedbreccia, with The age progressionis also not consistentwith the possibility fibrous to bladed calcite surroundingthe grains and micrite that the breccia sectionis part of a slide massof nearshore filling the pores, suggests the circulation of waters deposits that ended up in deep water. The high-energy supersaturatedwith respectto CaCO3 and vigorous fluid settingsinvoked for the depositionand cementationof the fluxing in order to maximize CO2 degassing.High-energy calcareniteforming the clasts may well have producedthe settingsare favorable places where both the sedimentary brecciationas well. Processesthat can disrupt lithified bands facies and cementationprocesses displayed by the 1042 are associatedwith shorefaceor shelf margin environments, calcarenite characteristicallyform. This recognition is where storm action or unstableslopes can brecciatebeds of consistentwith the observationthat the originalcalcarenite of calcarenitedeposits. the breccia clasts, although both porous and permeable, Our stratigraphic,sedimentological, cementation, and rock VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 659

Type 1 Presentin all the samples,they are the older veins.

Micritic mosaiccalcite incorporating clasts of the breccia,up to 0.1 mm thick with undefined edges.The calcite is clean and fibrous around

clasts.

No preferredorientations, anastomosing. Type 1 veinsresemble cement precipitation in irregularfractures implying an earlyorigin. Type 2 Presentin all the samples. Dirty, pervasiveveinlets with mosaicand antitaxial calcite. 0.5 mm

No preferredorientation. Type 3 Zeolite,calcite or compositecalcite-zeolite veins cuttingthrough the formerones as straight fractures.

Mosaicor fibrouscalcite, but alwaysextensional with no pressure-solutionseams.

0.5 mm Type 4 Only in onesample at 343.39 mbsf. '.,•:*, ..:'•-•*• Fibrouscalcite with extensional,antitaxial growth. --• Anastomosing,0.5 mm thick,perpendicular to the top of the core. Around the vein there are clasts with evidence of

shear.

Type 4 vein is the only onewith a cleartectonic origin. 0.5 mln

Figure 9. Synopticdiagram of texturesand relationshipsof veinsin the chert-basaltbreccia. structure observationsare incompatible with downslope intercalatedwith thebreccia, (3) thepronounced angularity of transportationof the brecciafrom shallow to deeperwater, the clasts,and (4) the lack of tractive featuresin the matrix and in particularto a recoverydepth that would have been and size gradationof the clasts. below the Miocene calcite lysoclinewhere interclastcalcite The occurrenceof commonglauconite implies a low to cementationcould not havetaken place. Related evidence that restricted sedimentation rate or restricted sediment movement. distantdownslope transport of the calcareniteclasts did not For glauconiteto form, in fact, the sediment-waterinterface take place includes (1) cementation was active after mustbe maintainedfor an appropriatelength of time to assure brecciation,(2) hemipelagicand pelagic sedimentare not stabilization.From the samplesexamined it is not possibleto 660 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN

carbonate-cementedbreccia is thusthat of a shallowwater, high-energyenvironment, where the depositedand cemented bioclasticsediment could be brecciatedand redepositedin the sameplace. The passagefrom the carbonate-cementedbreccia to the underlying chert-basaltbreccia marks a prominent change from bioclastic to coarse detrital. The clasts of the chert-basalt breccia define an assemblageof material derived from outcrops of well-lithified source or parental rocks that includes chert, limestone, sandstone, doleritic basalt, and serpentinizedmafic units. The sourcerock sequenceof the chert-basalt breccia is similar to that of an oceanic or ophiolitic assemblage of igneous basement units and overlying sedimentarycover. The basalt clasts have zeolite veins implying ocean floor alteration and fluid circulation, while the brecciated clasts and the quartz-mica schists indicate tectonic deformationand metamorphism.On the Figure 10. Photomicrographof the well-induratedsilty-clay Nicoya Peninsulaa suitablesource sequence, the ophiolitic recoveredat 343.42 mbsf of Site 1042 (parallel nicols). Nicoya Complex, is widely exposed [Dengo, 1962; Hydraulicbrecciation and shearingpredate vein formation. Baumgartneret al., 1984]. The Nicoya Complex,possibly a The oldermineral precipitation consists of quartzpods, cut by fragmentof an oceanicplateau, is largely of Cretaceousage extensionalcalcite veins perpendicular to the top of the core. [Sintonet al., 1997], althoughchert masses of Jurassicage are The youngerveins are calcite, quartz, and composite calcite- reported. quartzbearing, and they occur with no preferredorientation andcrosscutting relationship. The chert-basaltbreccia recovered at Site 1042 is very immatureboth in lithologyand in texture: The clastsdo not exhibit evidenceof distanttransport. The clay-richmatrix and determineif glauconiteformed prior to or after cementation, the numerous veins argue for an undrained deposit with but the mineral has either a brown limonitic or green episodesof high fluid pressure.Mud and sandinjections and appearance, suggestingdifferent degrees of oxidation. type 1 veins suggestescape paths as consequenceof fast Becauseglauconite is readily weathered,it is not likely that compaction.The describedfeatures are compatiblewith a individualgrains were transportedunder subaerial conditions, debris flow mechanism where the sediment mobilization was but a marinereworking may well haveoccurred. helpedby the fluid overpressurethat allowedmass movement The scenarioenvisaged by the diageneticfeatures of the of material even on a low dipping plane. The silty clay-

OCEAN DRILLING

Figure 11. (a) Site 1042shear zone at thebase of thechert-basalt breccia. (b) Photomicrographof the shear zone fabric. VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 661

induratedhorizons recovered as interlayersin the chert-basalt Nicoya Complex consistsof pillow and flow basalt brecciaconfirm the presenceof highfluid pressureand help representingoceanic basement [Wildberg etal., 1981;Gursky definethe sedimentaryenvironment. In fact, eventhough the et al., 1984; Frisch et al., 1992]. It is associatedwith nature of the silty clay level is difficult to infer, this band plagiogranite,diorite, and pelagicsediment, mainly suggests pelagic sedimentation following events that radiolarianchert of earlyJurassic to Santonian age [Schmidt- emplacedthe chert-basaltbreccia. The presenceof this fine- Effing, 1979; Tournon,1984; DeWeveret al., 1985; grainedintercalation within the chert-basaltbreccia suggests a Baurngartner,1987]. submarineenvironment located between the proximaland the In the field,the imbricatearchitecture of the Nicoya intermediateslope, where pelagic, hemipelagic,and debris Complexand structuralas well as petrographicfeatures flow alternate. definea southvergent stack, interpreted as an intraoceanic Summarizing,the sedimentaryunits corresponding to the accretionaryprism of mostly mafic igneousrocks acoustic BOSS horizon are indicative of an intermediate- [Baurngartneret al., 1984;Frisch et al., 1992].An overlap upperslope environment, the chert-basaltbreccia passing to a sequenceof youngersedimentary deposit of late Cretaceous shallow water depositionalenvironment for the carbonate- andearly Tertiary age unconformably overlies the imbricate cemented breccia. The overlying apron sequencemarks structureof theNicoya Complex [Baumgartner et al., 1984]. another variation in the sedimentaryregime; in fact, the The overlapsequence starts with the GarzaSupergroup brecciatedcarbonate sandstone of the subunit2A passes [Sprechmann,1984] (Figure 12). The lowestformation of the upward to the olive green clay(stone)and silt(stone)of unit GarzaSupergroup is a discontinuousbasal breccia, mainly 1A. The bottompart of the slopeapron sequence, unit 1A, is formedby basalticclasts. The GarzaSupergroup can be characterizedby coarsermaterial than the top, with layers subdividedinto two groups [Baumgartner et al., 1984].The interpretedas debris flow deposits.These have different lowestunit, the pelagic sedimentary deposits of the Sabana charactersthan the basal chert-basaltbreccia because they GrandeGroup, rests unconformably onthe breccia or directly lack an assemblageof hard-rockclasts and only include onthe basement (Figure 12). The Sabana Grande Group is pebble-sizedclasts of claystonehaving the samecomposition composedof limestone,radiolarian chert, and fine to coarse as the matrixmaterial. This partof the apronsequence is thus distalturbiditic beds ranging in agefrom Santonianto the envisaged as an accumulationof remobilized continental Maastrichtian-earlyPaleocene [Baumgartner et aL, 1984]. slopesediment. Theoverlying Samara Group [Sprechmann, 1984] represents The up-sectionpassage from the carbonate-cementedbreccia the highestpart of the GarzaSupergroup (Figure 12). Clastic to the slopeapron sedimentis a major changeboth in the sedimenttypical of gravity flows characterizesthe Samara lithologicaltypes suppliedby the sourceareas and in the Group. Conglomerateand proximal turbiditic beds both sedimentation environment. Data on the benthic foraminifera siliceousand calcareous,and rangingin age from upper revealthat the apronsequence deepens upward. Santonianto upper Eocene, formed the SamaraGroup [Baumgartneret al., 1984]. 5. GeologicalSetting of the Nicoya Peninsula Startingfrom the middleEocene, angular unconformities and changesin the depositionalenvironment marked the end of The carbonate-cemented and chert-basaR breccias the Garza Supergroupsedimentation [Lundberg, 1982] recoveredat Site 1042 placethe sourceareas on the upperor (Figure 12). Carbonateand siliciclasticsediment, lying Caribbeanplate and suggestthat equivalentunits might be unconformablyon the GarzaSupergroup or directlyon the exposedon the nearbyNicoya Peninsula.Fundamentally, basement,form the heterogeneousMal Pais Supergroup, CostaRica is an igneousmassif of arc volcanicrocks of which is constructedof neritic to nonmarinedepositional Cenozoicage built overa basementof Mesozoicoceanic crust sequences(Figure 12). Baumgartneret al. [1984] recognized and superimposedsedimentary sequences of Cretaceousand at leastthree shallow water events recorded by the Mal Pals early Tertiary age. Basementuplift, i.e., accretion or depositsin the Nicoya Peninsula.The first event in south obduction, occurred at an intraoceanic subduction zone Nicoyaoccurred during the middle-upperEocene with burial sometimeduring or after the late Cretaceous[Lundberg, of the SabanaGrande Group in the CaboBlanco-Montezuma 1982; Meschedeand Frisch• 1998; Sintonet al., 1997]. The area.The secondevent is recordedin the centralpart of the accretedmafic clndst,the Nicoya Complexof Dengo [1962] NicoyaPeninsula from Nosara, to CaboBlanco, to Paquera, (Figure12), is well exposedon the SantaElena, Nicoya, and where shallow water sedimentof upper Oligocene-lower OsaPeninsulas just seawardalong the Pacificcoast of Costa Mioceneage accumulatedon t_heSabana Grande Group. The Rica (Figures1 and 12). thirdand youngest event records deposition of shallowmarine The Nicoya Complex is subdived into two beds in the Cabo Blanco-Paqueraarea duringthe middle lithostratigraphicsub-complexes: the Lower and the Upper Miocene-Pleistocene. NicoyaComplex [Wil-dberg et al., 1981;Gursky et al., 1984] The Cenozoicshallow water depositsof the Mal Pals (Figure12'). The UpperNicoya Complex consists of pillow Supergroupdocument uplift of much of the area of the shelf, and flow basaltthat includessmall masses of pelagicsediment which is underlainat depthby folded and faultedMesozoic of Jurassic-Cretaceousage [Bart and Escalante,1969]. The urtitsof the maficNicoya Complex and Cretaceous •and earl. y igneousunits of the UpperNicoya Complex are mainly those Tertiary pelagic and turbiditicsedi.ment of the overlying of a primitiveisland arc and oceanicplateau flood basalt GarzaSupergroup. Toward the endof Eocenetime these deep [Wildberg,1984; Sinton et al., 1997]. In northwesternCosta waterdeposits accumulated in areasi_mmediately adj'acent to Rica (S. ElenaPeninsula area, Figures 1 and 12), the Lower where depositionof the shallowwater Mal Pals sequences Nicoya Complex_structurally overlies the Upper Nicoya was gettingunder way. The tectonic.scenario envisaged by Complexas a nappe [Bourgoise! al., 1984]. The Lower the sedimentaryrecord of the Nicoya coast is thus one 662 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN

Generalized column of the Oceanic Sediment above the Nicoya Complex

GARZA SamaraGroup SUPERGROUP(Upper Santonian-Upper Eocene) •--•-•--•-•--•i•- .... :.•. (Santonian-Maastrichtian/EarlyPaleocene) ...... Ni-•-OYA; ...... I•.v:-;I ...... COMPLEX

10 Legend ::::-._._.---:•Alluvialplanes

/ .... •i}iii!i!i!: 0 10 20 30 • "::•.:•iiiiiiiiiii? Mal Psis Supergroup

.[-•.:, Shallow(Eocene-Quaternary)wateroverlap sequences.

Garza Supergroup

MALPAlS Supergroup !ii:i •iiiiiiiiiii•: ii• (Campanian-Eocene)Deepwatersequences Garza -] .'.-.• calcareous ii::;iiii•j::i?___i•ii!i:':'::•i•iiiiii?•ili:iiii:•?i ---"-':-'-'"•i:ii:iiiil;:...... , / --I:::q•silt/sandstones •ii!i!•:-,•iiiiiii•!ii:•ii!iiiii••;:"'---"-::•:---:•i•1111ii:"::"•"•-:•••:•}iiiiiiiiiiiii!iil i• r•coya uomp•ex ß•n ,,,,,/ '-'-'[• L' Oligocene •iiii!i.•"!!i!:i!i::'•""•'""'•'::••••:''•'•::"'"":"-':'''" '•'•'"i•!J• ' """"-••!iiiiii•;ii?,2• • Lower Nicoya Complex: basaltand _ --•_=::_=...... •--("'•-•....••'."i[ii•...... t•' '9-'•'),/ -•'•••••"-' -r--•-:lUpper Nicoya Complex: basalt, dolerite 20m-J -----• slopeu/•r•/_.• and •upergroupbasin deposit of ....•11•11::-'•:.--i!:.:•-:-- }',•','-I an d ch ert (Albian-Santoni) an / ::• siltstonesand sandstones • '-••;::'"•':•'X••--••••• ---- ......

SAMARA-r •'•?-:••!:• •:-'•_•.--'-.• PAQUERA SITE PUNTA % TA NGO 1042 ,042 C'•RURNtTLALOCARRILLO ...... %"' ' ' :•••;NoO MAR 0m ...... ß '•- MAR 0m- I•/'-----:•i•l Clay(stone),silt(stone), CABUYA I::--":•i•i!ilcalcareous sandstones 100m- '-:.:-:-'•'i?-.: sand(stone) ...... 10 m - I':---"•:...... :•':..:•l Plio-Pleistocene I•-:-'• ::•{I LATEMIOCENE

F-----:'-:-'-<-•1 ...... F• .•v I NICOYA COMPLEX I-::•:_:i•l /Carbonate-cemented...... 30m •.."-..1basalts 300rn_ I:j_ii;I•-:--.:. i• ,,,/ ...... breccia I=J4KLY-IVIIUULE - L'",'I cretaceous ...... •/'Chert-basalt breccia MIOCENE lii•i•-•--•i•;•t•;•i;...... -•ii-6i5• ......

Figure 12. Summarygeological map of the NicoyaPeninsula and lithostratigraphiccolumns at the passage betweenNicoya Complex-deepwater sedimentand shallow water sediment.Columns refer to the Punta Pelada,Tango Mar, and ODP Site 1042 sections. involvingthe verticalmovement of adjacentcrustal blocks 1.7-4.5 km Myr-') since the middle to late Pleistocene [Baumgartneret el., 1984](Figure 12). Differentialvertical [Marshall,1991; Marshall and Anderson, 1995]. These high tectonismcontinues today as the late Holocene deposits of the rates of uplift and the laterally discontinuousnature of Cabuya-Montezumacoast of southernNicoya (Figure 12) exposedshallow water depositsof middleand late Cenozoic suggestnet uplift .rates in thisarea of 1.7-4.5m Kyr'l (i.e., age open questions about the underlying mechanisms VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 663 involved. Becauseof the high rates and spatial limitations rectangular-shapedfracture network perpendicularto the observed,for the southeasternNicoya peninsulaattention has beddingand typical microfaults developed at high angleto the been focused on vertical tectonism linked to the bedding, a style of deformation that indicates extension underthrustingof seamountsand seamountgroups [Marshall, parallelto the bedding. 1991; Gardner et al., 1992; Marshall and Anderson, 1995]. Fartherto the southeast,in the Montezumaarea at Tango The oversteepenedtrench slope offshore of Cabo Blanco Mar, southernNicoya (Figure 12), the MontezumaFormation [Von Huene et al., 1995; Fisher et al., 1998; Von Huene and of Miocene-Pleistoceneage directly overlies the Nicoya Ranero, 1998] (Figure 1) may have developedbecause of Complex along a highly irregular unconformity [Dengo, localized rapid uplift and linked mass slumpingrelated to 1962; De Boer, 1979; Lundberg, 1982, 1991; Mora, 1985] subductionof the Fisher Seamount immediately to the (Figure 13b). Where exposedalong sea cliffs, the calcareous southeastof the peninsula(Figures 1 and 12) [Marshall, sandstone of the Montezuma Formation reaches a maximum 1991;Marshall and Anderson,1995]. thickness of 120 m and includes a 1-3 m-thick basal conglomerateconstructed directly on the Nicoya Complex 6. Outcrops of the BOSS Horizon basalt (Figure 13b). The conglomerateis composed of along the Nicoya Coast subroundedto subangularbasalt and chert clasts set in a coarse sandstone matrix. The basal conglomerate is At Site 1042 the top of the acousticallydefined BOSS fossiliferous,massive, and generallymatrix supported.The horizon was evidently reached. However, a prominent remainder of the overlying Montezuma Formation is unconformityseparating rocks of contrastingage and sandstonewith pebbly beds and horizonsrich in gasteropod, lithologywas not penetrated.What this unconformity might echinoderm,and barnacle debris. The upper part of the separateis impliedby the contrastinglithologic composition MontezumaFormation is fine grainedand highlybioturbated. of the brecciaof shallowwater limestoneand the underlying The Montezuma Formation is demonstrablya transgressive chert-basaltbreccia of Nicoya complexdebris. Because the shallow marine deposit,with the lower part representinga BOSS horizon can be traced landward,without interruption, high-energy,wave baseenvironment [Lundberg, 1982, 1991]. to nearthe Nicoya coast, we searchedthe Pacific margin of Conglomerateswith the same charactersas the Montezuma the peninsulafor outcropsof neffticbeds of the Mal Pals basal part occur in severallocalities along the Montezuma- Supergroupunconformably overlying Nicoya basement Cabuyacoast to the SW (Figure 12; Lundberg,[1982]). complexor temporallyrelated beds of theGarza Supergroup. Neritic sedimentarydeposits of the Mal Pals Supergroup At Tango Mar, the Montezuma-Nicoyacontact is exposed cropout in restrictedareas along the southwestcoast of the near sealevel at the baseof the modem seacliff (Figure 13b). NicoyaPeninsula (Figure 12), mostnotably at PuntaPelada Just inland, at an elevation of 160-180 m, at the base of a (Figure13a) northwest of Samaraand in the Mal Pais-Cabo Pleistoceneterrace of marine origin, the Cobano surface, Blanco-Montezumaarea along the southwesternand southern Nicoya Complex cropsout under the MontezumaFormation edgesof thepeninsula (Figure 13b). At PuntaPelada the Mal (J. Marshall,personal communication, 1998). The trendof the PaisSupergroup, represented by thePunta Pelada Formation, Montezuma-Nicoya contact implies that the Montezuma unconformablyoverlies the Garza Supergroup.The Garza Formationwas depositedover an emergentearly Miocene Supergroupis here representedby the Barco Quebrado erosionalsurface that had appreciablerelief (•- 10-100 m). Formation,a distal facies of massiveturbiditc beds of The Montezuma Formation was depositedon this surface Maastrichtian-earlyPaleocene age [Baumgartneret al., during its submergencein late Miocene to Quaternarytime, 1984]. The Barco QuebradoFormation thinly overliesthe interruptingthe upliftingtrend [Marshall, 1991;Marshall and Nicoyabasement complex, which is exposedjust inland(1-2 Anderson,1995]. A returnto uplift beginningin the middleto km) of Punta Pelada. At Punta Pelada an angular late Pleistocene, possibly, as earlier noted, caused by unconformitymarks the contact between the Punta Pelada and underthrusting seamounts, led to emergence of the Montezuma Formation and erosion of the Cobano surface BarcoQuebrado Formations. A 1-2-mthick basal breccia of the Punta Pelada Formation directly overlies the Garza during the late Pleistocene[Marshall, 1991; Marshall and Supergroupsediment (Figs. 12 and 13a). The brecciais Anderson,1995]. composedof subroundedto angularsandstone clasts set in a The depositionof the neriticand shelfal Punta Pelada and sandstone matrix. The remainder of the Punta Pelada MontezumaFormations over mucholder and deepwater rock Formation,late Oligocenein age [Lundberg,1982, 1991; sequencesimplies at leasttwo cyclesof uplift-subsidence- Baurngartneret al., 1984],is characterizedby 10-15-mthick upliftof coastalNicoya since the early Tertiary. The recovery sectionof biocalcarenite,with crosslaminations (dipping up of evidencefor depositionally,temporally, and lithologically to 30ø),superimposed by-30 m of alternatingsiltstone and similarrelations at the baseof Site 1042 impliesthereby that a sandstone beds. The Punta Pelada Formation is rich in majorunconformity of thefirst cycle, i.e., the geologic BOSS macroforaminifera,echinoderms, and biogenicdebris. Thin horizon,was closelyapproached at the bottomof this hole sectionanalysis shows extensive cementation as the well- nearthe modemtrench floor. Projectedshoreward, the BOSS lithified nature of the Punta Pelada Formation suggests. horizon can thus be tied to the Punta Pelada unconformity Bladedto fibrouscement is arrangedaround the clasts,and buried by late Oligocene shelfal beds of the Mal Pais pore spacesare filled with clean mosaiccalcite. These Supergroupor to the Montezumaunconformity buried by late microscopicobservations imply deposition took placein a Cenozoic shallow beds of youngerMal Pais deposits.The medium-to-highenergy environment typical of anupper shelf circumstanceof similar geologicrock sequencesexplored at environment[Lundberg, 1982, 1991; Baumgartneret al., the baseof Site 1042 and alongthe nearbycoastal area of the 1984]. The Punta PeladaFormation exhibits deformation Nicoya Peninsulais thus in keepingwith the predictionsof features such as extensionalsyntaxial calcite veins, a the nonaccretionarymodel of Von Huene and Flueh [ 1994] 664 VANNUCCHIET AL.:TECTONIC EROSION OF THE COSTA RICA MARGIN

Mal Pais/Garza a) PUN TA PELADA angular unconformity Mal PaisBeds: ...... : :..•..-.-: .: ..:...... ,.....•::.•:::.::.::•.:.::.:-:::....:.:..,.:...... -::.:. :...... : . . . shallow shelf detrital limestones (Early Neogene)

. GarzaBeds: ..

deep water/slope .. ..•;,-, .:., basin deposits,

C:

Section (10 m) ,-, South ,,,, North-East :/_ •?.• •_L----c"_L•.___•-----

b) TANGO MAR shallowshelf,beach detritallimestones (Miocene-Pleist.) B: NicoyaComplex: basalts (Late Cretaceous) C: Beachtalus

•;•,,.;•...... *•''-"?:'*;½** -- - • ...... ' ::;/::•;•;:,::½•::½•:4:. -..•,:;,-•.:<•:-:t;c•:.;;;;•a•;t•-...... '*:•-':?: '".•: •::,;.•::::•,*{•;½•...... ,.-.:•,,:-•**....½:"•--; •'"•'""•••'•'••••••;:-:,,**':"'•, ...... ,•:½'"'•:•'•:-.:•-•"*-•:•.' ".f.•'; "' :4"•:-•'-•-•.-,-•' Mal Pais/Nicoya Complex

•-.- . ... ß . . -• -•;:•:•:..__• •:;:•'• :• ::'•:' -•'" -• ';•'• •;•'•'•:--• "' • conglomerate

• •;s•;-?½ '.,•>;,• •'%c'•L.."• ':•' ":':::"::•:•;½/•.::.:•..:•...... "'-

Figure 13. (a) Photographand interpretativediagram of field relationswhere the PuntaPelada Formation (Mal PaisSupergroup) unconformably overlies the BarcoQuebrado Formation (Garza Supergroup) at Punta Pelada.(b) The MontezumaFormation (Mal PaisSupergroup) unconformably overlies basalts of the Nicoya Complexat TangoMar. VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA RICA MARGIN 665 and as elaboratedon more recently by Vannucchiet al. accretion,and where drilling reachedbasement rock beneatha [1998], Meschedeet al. [1999], and Raneroand VonHuene BOSS-typehorizon, have been affected by subsidenceinstead [2000]. of uplift, as, for example,the Japan[Von Hueneet al., 1982], Mariana [Hussongand Uyeda, 1981], Peru [Von Huene and 7. Subsidenceof the Costa Rica Margin' $choll, 1991], and Tongaforearcs [Cliff and MacLeo& 1999]. A Case For Subduction Erosion Drilling of the Guatemalamargin on Legs67 and 84 -500 km northwestof Leg 170 sitesalso recoveredno evidenceof net Subduction erosion has been described as the major accretionarythickening or uplift sinceat leastthe beginning tectonicprocess involved in shapingthe rock fabric of many of the Eocene,if not sincethe pre-Campanian[Von Huene et convergentmargins, as, for instance,the Japan,Tonga, Chile, al., 1985]. Accretionarythickening is also missingfrom the and Peru margins[V on Huene et al., 1982; Von Huene and frontal part of the Costa Rica margin. The amount of Scholl, 1991], and on the basis of regional coastal and underplatingoccurring beneath the BOSS surfaceremains, offshore information, the margin bordering the Middle however, a matter of debate [Meschedeet al., 1999]. America trench [Scholl et al., 2000]. The process of However, the recoveryof shallowwater sedimentat Site 1042 subductionerosion is definedas the removaland transportof documentsmargin-wide subsidence during the past 16-17 upperplate material toward subcrustal and mantle depths. The Myr, the oppositeobservation of that predictedby modelsof primary evidencefor this processis the documentationof massiveunderplating of oceanicsediment beneath the BOSS substantial(i.e., by 3-5 k_m)subsidence of the outer forearcs horizon, a processthat would have thickened and thus duringrelatively short periods (20-50 Myr) of geologictime. uplifted the margin across most of its width. However, Coastal uplift can be paired with offshore subsidenceas consistentwith an accretionaryunderplating model is the lack documentedby the coastalobservations of Marshall [1991], of løBein young Costa Rica eruptive rocks. The occurrence of Gardner et al. [ 1992], and Marshall and dnderson [ 1995]. IøBein arcmagmas [Brown et al., 1982;Tera et al., 1986] Subductionerosion is also documentedby vertical subsidence providesquantitative information on the amountof sediment of the submergedmargin in conjunctionwith progressive that is bothaccreted to and subductedbeneath the marginto seawardtilting towardthe trench [Cliff and MacLeo& 1999]. mantle depths(see review by J.D. Morris in Scholl et al., Other fundamentalevidence testifying to the efficacy of [1994]).løBe is a cosmogenic,relatively short-lived isotope subductionerosion is the landwardmigration of the coastline (half-lifeis 1.5 Myr) that is stronglyadsorbed by fine-grained and the arc magmatic front over substantialperiods of sedimententering the subduction zone. For this reason, løBe geologictime [Rutland,1971; VonHuene and Scholl, 1991]. enrichmentsare commonlyobserved in modern arc volcanic For CostaRica, Alvaratio [1993] has documenteda landward rocks,but not in the eruptivesof othertectonic settings [Tera shift of the Costa Rica volcanic arc of •40-50 km since the et al., 1986;Morris and Tera, 1989;Morris et al., 1990]. middle Miocene. With respectto theseprimary observations Northof CostaRica, efficient subduction of løBe-enriched concerning the consequencesof subduction erosion at sedimentand thus the lack of any significantmeasure of convergentmargins, Leg 170 drilling resultscombined with frontaland underplating accretion, is documentedby the high coastalstudies of the adjacentNicoya Peninsulacan be cited contentof løBe in arc volcanicrocks of Guatemalaand as matching evidence for substantialamounts (3-4 km) of Nicaragua[Carr and Rose,1987; Cart et al., 1990]. But the ocean margin subsidenceduring Neogene and Quaternary Guatemalaand Nicaragua IøBe peak dramatically decreases to time. The carbonate-cemented breccia drilled at Site 1042 the southeastin Costa Rica volcanic units [Carr and Rose, exhibits shallow water sedimentologicalfeatures typical of 1987;Cart et al., 1990],despite the fact that the IøBe content those described for the Punta Pelada and the Montezuma of Cocosplate sedimententering the Costa Rica subduction Formations. The chert-basalt breccia recovered underneath zone is sufficiently high to produce a strong arc signal showsaffinities with the Nicoya Complexrocks. Studieson [Valentineet al., 1997].Thus the absence of løBein young the benthicforaminifera in the slopeapron sediments show a Costa Rican eruptive rocks seeminglygreatly limits the deepeningupward succession. The situationdescribed at Site volumeof youngoceanic sediment that is subductedto mantle 1042 is thus similar to that for the Mal Pais Supergroup, depths. BecauseLeg 170 drilling establishedthat frontal where this shallow water sediment rests on oceanic crust or accretion is, effectively, not taking place, a rational depositionallykindred deep water sedimentaryunits along the explanationfor thelack of a prominentIøBe signal in Costa Nicoya coast.The evidenceof ocean floor alterationin the Rica eruptiverocks is that youngunderthrusting sediment is chert-basaltbreccia of Site 1042 arguesfor a sourceterrane of sequestered,at depth,by underplatingbeneath the baseof the altered and brecciated oceanic crust. margin.This conjecturedoes not squarewith evidencefor as The presentposition of the carbonate-cementedbreccia is much as -4 km of margin subsidencenear the trenchfloor in •3900 m below sea level, which is thusroughly equal to the the face of, at the most,-100 m of coastaluplift. The answer amount of subsidence that affected the middle and lower to theenigma of theabsent løBe signal is unknownto us, slope since 16-17 Ma. Sedimentologicaland geochemical other than to suggestthat dilution causedby the flushingof analyseson the carbonate-cementedbreccia imply that the subductionerosion debris to the mantle might be involved. BOSS horizon is geologicallya deep submergedregional Alternatively,løBe-bearing fluids and sedimentmight be surface of wave base and subaerial erosion. The erosional transferredto the mantleat depthstoo shallowand cold for surfaceis time transgressiveor diachronicas arguedon the the generationof arc magmas[Leeman and Cart, 1995]. basisof the burialage of its exposedmanifestation as the Mal ODP sites 1040 and 1043 show that at the Costa Rica Paisunconformity. Thus uplift and subsidenceof the Mal Pais margin, effectively all the incoming oceanic depositsare unconformityare discontinuousin time and spacealong the subductedbelow its high-velocityrock framework[Kimura et Nicoya Peninsula.Other marginslacking evidence for frontal al., 1997]. Bathymetric and seismic reflection data also 666 VANNUCCHI ET AL.' TECTONIC EROSION OF THE COSTA RICA MARGIN

Early Miocene (-- 16-17 Ma)- Uplift Nicoya Dry Land 1042 BOSS • MAT ...... /• •- 6km • CaribbeanPlate t CocosPlate UPLIFT CostaRican Miocene Arc

Middle Miocene (-,-14 Ma) - Subduction Erosion and Subsidence MAT Normal • faults BOSS

S.L. 1042

6km1 /•' •--•//ff/'Y? --•'•'••'•'••/.•ff-ff.ff?r/v• .,...... •ff ff_/',)' ?•>-•-Miocene CostaRicanArc SU•DUCT•ON .,• "-

Volume of material removed •.ff-ff.d.•... . by subduction erosion since Early-MiddleMiocene MalPais Supergroup li!iiii:iiiiGarza Supergroup •"'i"-•tNicoya Complex

Wedge front retreat RATES OF SUBDUCTION EROSION from middle Miocene to Present Volume of early Miocene margin less 50 km Volume remainingbeneath modern margin I I = 560-600 km• Vertical exaggeration 1:2 EROSIONRATE = 560-600km3/16.5 Myr= 34-36 kma/MydkmTRENCH

TRENCH ADVANCE = 50-60 km= Present 3-3.5 km/Myr

Compressional Extensional Domain Domain BOSS Nicoya

S.L. 1042•,/ .• Peninsula•, • 6km1 Cocos Plate ,._••, •a_ ' dbbeanPlate CostaModernRican MAT ModemRemains of \• Arc Early-MiddleMiocene •• Forearc

Figure14. Tectonicevolution of thePacific margin of CostaRica offNicoya Peninsula since early Miocene. VANNUCCHI ET AL.' TECTONIC EROSION OF THE COSTA RICA MARGIN 667

documentthat subductedseamounts thin the upper plate of fossiliferousbeds of the largelyNeogene and youngerMal the Costa Rica margin as they cut tectonicgrooves through PaisSupergroup. Subsidence of the Mal Paisunconformity in the base of the landward trench slope [Von Huene et al., Neogeneand Quaternarytime to trenchdepths requires the 1995, 2000]. Although tracts of seamountsare not now landwardmigration of the trenchaxis and coastline,relative enteringthe sectorof the Nicoyamargin drilled by Leg 170, to a fixed interiorCosta Rica, by at least40-50 km (Figure theymay have done so in thepast to, in part,account for the 14). The shift is aboutthe sameas the landwardmigration of high rate of crustalthinning recorded by the shallowwater the volcanicchain [Alvarado, 1993]. The Mal Pais erosion limestonebreccia recoveredat Site 1042. More importantly, surfacecan thus be laterallytied to the BOSS horizonat Site Raneroand Von Huene [2000] infer from seismicreflection 1042, which, near the base of the landwardtrench slope, imagesthat along the Costa Rica margin upper plate material separatesGarza and Mal Paisdepositional units. is transferredto the lowerplate by the removalof largerock As recordedat Site 1042, and at leastsince the top of the lenses(as much as 1-2 km thick and tens of kilometersin early Miocene (-16-17 Ma), the seawardedge part of the length)from the baseof the margin.Seamount subduction submergedforearc has subsidedto a depthof 3900 m below does not need to be involved in this processof basal the sea level--(Figure 14). Subsidenceof the Mal Pais subduction erosion. unconformityis inferredto recordcrustal thinning caused by processesof frontal and basal subduction erosion. The 8. Summary and Conclusions removalof upperplate material may be linkedto the missing løBesignal in the CostaRica arc lavas and with the arcs' The Cenozoicevolution of the CostaRica marginmay be landwardshift. The recoveryof shallowwater carbonate beds interpretedfrom the combinedimplication of Leg 170 results, at -4000 m below seallevel at the baseof the forearcimplies regional seismic reflection and refraction data, and the -50-60 km of forearctruncation during the past 16-17 Myr. regionalgeology of the Nicoya Peninsula(Figure 14). These Thetrench axis landward migration isthus ---3 km Myr '•. The data form a coherentpicture from the early Miocene to the volumeloss during subduction erosion is ---560-600km 3 per present,but the Mesozoichistory is basedonly on the rock kilometeralong the trench, equivalent to a rateof 34-36knl 3 record exposedin the Nicoya Peninsulawhere the stacked Myr-• km-•. and shortenedmarie rocksof the Nicoya Complexdocument the convergenceof the Farallonand Caribbeanplates. Acknowledgments. The authorsthank Tom Gardner,Don Fisher, By the early Tertiary, convergentprocesses uplifted and and PeterSack for guidingus to the Mal Paissections exposed along carriedthe Nicoya Complexand overlyingdeep water beds of the Nicoya coast,our Leg 170 colleaguesfor sharinginformation. the Garza Supergroupto shallow water depthsand subaerial We particularwish to expressour thanksand gratitudeto Adina Paytan and Edward Robinsonfor providingcritically neededage elevations(Figure 14). Subsidenceof this regionalerosion data on the carbonate-cementedbreccia. Finally Brian Wernickeand surfacebelow wave base began at least by late Paleogene Kelin Whipple are thankedfor their stimulatingreview. This paper time, resulting in its burial by the calcareousand richly was supportedby the Italian CNR fundsto the ODP.

References

Alvarado, G.E., Land of Volcanoes. Gallo Pinto implicationsfor subduction,Nature, 299, 718- palcomagnetic data and a revised Press,San Jose, Costa Rica, 181 pp., 1993. 720, 1982. tectonostratigraphicsubdivision of Costa Rica Aubouin, J., J. Bourgois,J. Az6ma, and R. Von Carr, M.J., and W.I. Rose, CENTAM - A database andwestern Panama, in Geologicand Tectonic Huene, Guatemala margin: A model of of Central America volcanic rocks, d. of Developmentof the CaribbeanPlate Boundary convergentextensional margin?, Initial Rep., Volcan. Geotherm.Res., 33, 2239-2240. 1987. in Southern Central America, edited by P. DeepSea Drilling Project,84, 911-917, 1985. Carr, M.J., M.D. Feigenson,and E.A. Bennet, Mam• Geol. Soc. Am. Spec. Pap. 295, 1-27, Barboza,G., and H. Zucchi, Sismoestratigrafiade Incompatibleelement and isotopicevidence for 1995. algunascuencas en el antearcodel sector tectonic control of sourcemixing and melt Fisher, D.M., T.W. Gardner, J.S. Marshall, P.B. centralde CostaRica. Proill, 7, 161-169, 1994. extraction along the Central American arc, Sak, and M. Protti, Effect of subductingsea- Barr, K.W., and G. Escalante, Contributi6n al Contrib. to Mineral. Petrol., 105, 369-380, floor roughnesson fore-arckinematics, Pacific esclarecimentode la edad del complejo de 1990. coast,Costa Rica, Geology,26, 467-470, 1998. Nicoya, CostaRica, Publ. Geol., 2, pp. 43-47, Clennell, B., and A. Maltman, Microstructuresin Frisch,W., M. Meschede,and M. Sick, Origin of Ist. Centroam. Invest. y Tecnol. Ind., accretedsediments of the CascadiaMargin, the CentralAmerica ophiolites: Evidence from GuatemalaCity, Guatemala,1969. Proc. OceanDrill. Program,Sci. Results,146, paleomagneticresults, Geol. Soc. Am. Bull., Baumgartner,P.O., Tect6nicay sedimentaci6ndel 201-209, 1995. 104, 1301-1314, 1992. Cretficicosuperior en la zonapacifica de Costa Cliff, P.D., and C.J. MacLeod, Slow rates of Gardner, T.W., D. Verdonck, N. Pinter, R. Rica (America Central), In El Cretcicicode subduction erosion estimated from subsidence Slingerland,K. Furlong,T.F. Bullard, and S.G. Mexicoy AmdricaCentral, Actas de la Fac. de and tilting of the Tonga forearc,Geology, 27, Wells, Quaternaryuplift astridethe aseismic Cienc.de la Tierra, vol. 2, pp. 251-260, edited 411-414, 1999. CocosRidge, Pacific coast of CostaRica, Geol. by C. Barbarin,H.-J. Gursky,and P. Meiburg, DeBoer,J., The outerarc of the CostaRica orogen Soc.Am. Bull., 104, 219-232, 1992. Univ. Aut6n. de Nuevo Leon, Linares, Mexico, (oceanicbasement complex of the Nicoya and Gursky,H. J., M. Gursky,R. Schmidt-Effing,and 1987. Santa Elena peninsulas),Tectonophysics, 56, H. Wildberg, Karten zur Geologie yon Baumgartner,P.O., C.R. Mora, J. Butterlin, J. 221-259, 1979. Nordwest-Costa Rica (Mittelamerika) mit Sigal, G. Glaqon,J. Az6ma,and J. Bourgois, DeMets, C., R.G. Gordon, D.F. Argus, and S. Erlauter-lungen,Geol. Paleontol.,18, 173-182, Sedimentaci6ny paleogeografia del Cretficicoy Stein,Current plate motions,Geophys. d. Int., 1984. Cenozoicodel litoral pacifico de Costa Rica, 101,425-478, 1990. Ibaraki, M., Planktonic foraminifers off Costa Rica Rev. Geol. de Am. Centr., 1, 57-136, 1984. Dengo, G., Estudio geologicode la region de in the East Pacific Ocean-biostratigraphicand Bourgois, J., J. Az6ma, P.O. Baumgartner,J. Guanacaste,Costa Rica, pp. 1-112,Inst. Geogr. chronostratigraphicanalyses, Proc. Ocean Toumon, A. Desmet, and J. Aubouin, The Nac., San Jos6,Costa Rica, 1962. Drill. Program,Sci. Res.,170, 1-58, 2000. geologichistory of the Caribbean-Cocosplate DeWever, P., J. Az6ma, and A. Desmet, Hinz, K., R. Von Huene,H. Meyer, S.Y. Ye, H.A. boundarywith specialreference to the Nicoya D6couvertede mat6riel oc•anique du Lias- Roeser, C. Reichert, Marine 3-D seismic ophiolitecomplex (Costa Rica) and D.S.D.P. Dogger inf6rieur dans la p6ninsulede Santa reflectionstudies at the active pacific margin results(legs 67 and 84 off Guatemala):A Elena (Costa Rica, AmeriqueCentrale), C. R. off Costa Rica Middle America, (Abstract syntesis,Tectono'•hysics, 108, 1-32, 1984. Acad Sci., 2, 759-764, 1985. supplement1 EUG 9 Strasbourg23-27 March Brown, L., J. Klein, R. Middleton, I.S. Sachs,and Di Marco, G., P.O. Baumgartner,and J.E.T. 1997), Terra Nova, 9, 319, 1997. F. Tera, løBe in islandarc volcanoesand Charmell, Late Cretacous-early Tertiary Hinz, K., R. Von Huene, C. R. Ranero, and 668 VANNUCCHI ET AL.: TECTONIC EROSION OF THE COSTA PdCA MARGIN

PACOMAR Working Group, I ectomc Implications for sediment subduction, Vannucchi, P., and H. Tobin, Deformation structure of the convergentPacific margin Geochim.et Cosmochim.Acta, 53, 3197-3206, structuresand implicationsfor fluid flow at offshore Costa Rica from multichannel seismic 1989. CostaRica convergentmargin, ODP Sites1040 reflection data, Tectonics,15, 54-66, 1996. Morris, J.D., W.P. Leeman,and F. Tera, The and 1043, J. Struct. Geol., 22, 1087-1103, Hussong,D.M., and S. Uyeda,Tectonic processes subductedcomponent in island arc lavas: 2000. and the historyof the Marianaarc. A Synthesis constraintsfrom Be isotopesand B-Be Vannucchi,P, M. Meschede,and D.W. Scholl, of the resultsof DSDP Leg 60, Initial Rep. systematics,Nature, 344, 31-36, 1990. Tectonicerosion of the Pacificmargin of Costa DeepSea Drilling Project,60, 909-929, 1981. Paytan, A., M. Kastner, E.E. Martin, J.D. Rica: combined implicationsof offshore Ingle, J.C., Jr., Cenozoic paleobathimetryand MacDougall, and T. Herbert,Marine Barite as drilling, a regional"base-of-slope-sediment- depositional history of selected sequences a monitor of seawaterStrontium isotope BOSS"reflector and a regionalunconformity within the southern Califomia continental composition,Nature, 366, 445-449, 1993. on theNicoya and Osa Peninsulas, in European borderland, Studies in Marine Ranero, C,R. and R. Von Huene, Subduction Ocean Drilling Forum Programme and Micropaleontologyand Paleoecology- A erosionalong the MiddleAmerica convergent Abstracts,edited by A. Robertsonand J. Pierce, memorial volume to Orville L. Bandy, Spec. margin,Nature, 404, 748-752, 2000. p. 102, Univ. of Edimburgh,Scotland, UK, Publ. 19, edited by W.V. Sliter, 163-195, Ranero,C.R., R. Von Huene,E. Flueh,M. Duarte, 1998. CushmanFound. for ForaminiferalRes., 1980. D. Baca, and K. Mcintosh, A cross-sectionof Von Huene, R., and E.R. Flueh, A review of Kimura, G., et al., Proceedingsof the Ocean the convergentPacific margin of Nicaragua, marinegeophysical studies along the Middle Drilling Program, Initial Reports,Leg 170, Tectonics,19, 335-357, 2000. America Trench off Costa Rica and the 458 pp., OceanDrill. Program,College Station, Rutland,R.W.R., Andean orogeny and ocean floor problematic seaward terminus of continental Tex., 1997. spreading,Nature, 233, 252-255, 1971. crust,Proill, 7, 143-159,1994. Larson, R.L., et al., Proceedingof the Ocean Schmidt-Effing,R., Alterund Genese des Nicoya- Von Huene, R., and C. Ranero, Analogous Drilling Program. Scientific Results 129, Komplexes, einer ozeanischen Pal'•okruste segmentation,ocean crust, continental margin, OceanDrilling Program,College Station, Tex., (Oberjura bis Eoz'an) im siidlichen and volcanoes in Costa Rica. Eos Trans. AGU 1992. Zentralamerika,Geol. Rundsch.,68, 457-494, 79 (45), Fall Meet.Suppl., F395, 1998. Laurent, Ph., C. Tourneret, and O. Laborde, 1979. Von Huene, R., and D.W. Scholl, Observationsat Determiningdeviatoric stress tensors from twin SchollD.W., T. Plank,J.D. Morris, R. Von Huene, convergent margins concerning sediment lamellae in calcite. Applications to andM.J. Mottl,Science opportunities in ocean subduction,subduction erosion, and the growth monophasedsynthetic and natural polycrystals, drillingto investigaterecycling processes and of continentalcrust, Rev. Geophys.,29, 279- Tectonics,9, 379-389, 1990. materialfluxes at subductionzones, 84 pp., 319, 1991. Leeman, P.W., and M.J. Cart, Geological Proceedingsof a JOI/USSACworkshop, Joint Von Huene,et al., Leg 67: The DeepSea Drilling constraintson subductionprocesses in Central Oceanogr.Inst., Avalon, Calif., June 12-17, Project Mid-America Trench transect off Americanvolcanic arc: Implications for Boron 1994. Guatemala,Geol. Soc. Am. Bull., 91, 421-432, geochemistry, Geologic and Tectonic Scholl,D.W., P. Vannucchi,and M. Meschede, 1980. Developmentof the CaribbeanPlate Boundary The Pacific margin of Costa Rica recordsa Von Huene, R., M. Langseth,N. Nasu, and H. in Southern Central America, edited by P. long historyof subductionerosion - Evidence Okada,Summary of Cenozoictectonic history Mann, Geol. Soc. Am. Spec.Pap., 295, 57-73, and thoughts about tectonic processes along the IPOD JapanTrench transect, Geol. 1995. governingthe evolutionof the Middle America Soc.Am. Bull., 93, 829-846, 1982. Lonsdale,P., and K.D. Klitgord, Structureand Margin, paper presentedat Union Geofis. Von Huene,R., et al, Initial Reportsofthe Deep tectonic history of the easternPanama Basin. Mex., PuertoVailart& Mexico, 2000. Sea Drilling Program,vol. 84, 967 pp., U.S. Geol. Soc. Am. Bull., 89, 981-999, 1978. Shipley,T. H., P. L. Stoffa, and D.F. Dean, Gov. Print. Off., Washington,D.C., 1985. Lundberg,N., Evolutionof the slopelandward of Underthrustsediments, fluid migrationpaths, Von Huene, R., et al., Morphotectonicsof the the Middle AmericaTrench, Nicoya Peninsula, and mud volcanoes associated with the pacific convergentmargin of Costa Rica, in Costa Rica, in Trench-Forearc Geology: accretionarywedge off Costa Rica: Middle Geologic and TectonicDevelopment of the Sedimentation and Tectonics on Modern and AmericaTrench, d. Geophys.Res., 95, 8743- CaribbeanPlate Boundaryin SouthernCentral Ancient Active Plate Margin, edited by J.K. 8752, 1990. America, edited by P. Mann, Geol. Soc. Am. Leggett,Geol. Soc. Spec.Publ., 10, 131-147, Shipley,T. H., K. D. Mcintosh,E.A. Silver,and Spec.Pap., 295, 291-307, 1995. 1982. P.L.Stoffa, Three-dimensional seismic imaging Von Huene,R., C.R. Ranero,W. Weinrebe,and K.

Lundberg,N., Detrital recordof the early Central , of theCosta Rica accretionary prism: structural Hinz, Quaternaryconvergent margin tectonics American magmaticarc: Petrographyof the diversityin a smallvolume of the lower slope, of CostaRica, segmentation of the Cocos plate, intraoceanic forearc sandstone, Nicoya J. Geophys.Res., 97, 4439-4459, 1992. and Central American Volcanism, Tectonics, Peninsula,Costa Rica, Geol. Soc. Am. Bull., Silver,E.A., Introductionto the specialsession on 19, 314-334, 2000. 103, 905-915, 1991. fluid flow in the CostaRica accretionary prism, Wildberg,H.G.H., Der Nicoya-Komplex,Costa Marshall, J.S., Neotectonicsof the Nicoya Geophys.Res. Lett., 23, 881, 1996. Rica, Zentralamerika.Magmatismus und Peninsula, Costa Rica: a look at forearc Silver, E., and the Leg 170 Scientific Party, Geneseeines polymagmatischenOphiolith- responseto subductionat the MiddleAmerica Complete sediment subduction and Komplexes,Miinstersche Forsch. Geol. Trench,M.S. thesis,196 pp. Univ. of Calif., implicationsof fluid flow in the Middle Palaontol,62, 123pp., 1984. Santa Cruz, 1991. P,anericaTrench off Costa Rica, doidesJ., 23/2, Wildberg,H., H.J.Gutsky, R. Schmidt-Effing,and Marshall, J.S., and R.S. Anderson,Quatemary 1-3, 1997. M. Strebin, Der Ophiolith-Komplexder uplil•and seismic cycle deformation, Peninsula Sinton,C.W., R.A. Duncan,and P. Denyet,Nicoya HalbinselNicoya, Costa Rica, Zentralamerika, de Nicoya,Costa Rica, Geol. Soc. Am. Bull., peninsula,Costa Rica: a single suite of Zentralbl. Geol. Palaontol. Teil I, 1981, 195- 107, 463-473, 1995. Caribbean oceanic Plateau magmas, J. 209, 1981. Mcintosh,K., E. Silver,and T. Shipley,Evidence Geophys.Res., 102, 15507-15520,1997. Ye, S., J. Bialas,E.R. Flueh,A. Stavenhagen,R. and mechanismsfor fore-arc extension at the Sprechmann,P. (Ed,), Manual de Geologia de Von Huene, G. Leandro, K. Hinz, Crustal accretionaryCosta Rica convergentmargin, Costa Rica, 320 pp. Ed. de la Univ. de Costa structureof the Middle AmericanTrench off Tectonics,12, 1380-1392,1993. Rica, San Jos6, 1984. Costa Rica from wide-angleseismic data, Meschede,M., and W. Frisch,A plate tectonic Stoffa, P.L., T.H. Shipley, D.F. Dean, W.K. Tectonics,15, 1006-1021,1996. modelfor the Mesozoicand early Cenozoicof Kessinger,R. Elde, E. Silver,D. Reed,and A. theCaribbean plate, Tectonophysics, 296, 269- Aguilar,Three-dimensional seismic imaging of K. McDougall-Reid,U.S. GeologicalSurvey, 291, 1998. the Costa Rica accretionary wedge: Field 2255N. GeminiDrive, Flagstaff, AZ 86001,USA. Meschede,M., U. Barckhausen,and H.U. Worm, programand migrationexamples, J. Geophys. (kris•usgs.gov) Extinctspreading on the CocosRidge, Terra Res., 96, 21693-21712, 1991. M. Meschede,Institute of Geology,University Nova, 10, 211-216, 1998. Tera, F., et al., Sedimentincorporation in island- of Tuebingen,Sigwartstr. 10, D-72076 Tuebingen, Meschede, M., P. Zweigel, and E. Kiefer, arc magmas:Inferences from WBe, Geochim. Germany.(meschede•uni-tuebingen.de) Subsidenceand extensionat a convergentplate Cosmochim.Acta, 50, 535-550, 1986. D. W. Scholl, Depatmentof Geophysics, margin:evidence for subductionerosion off Toumon, J., Magmatismesdu Mesozoique a MitchellBuilding, Stanford University, Stanford, CostaRica, TerraNova, 11, 112-117, 1999. l'actuelen Am6riqueCentrale: L'exemple de CA 94305,USA. (dscholl•usgs.gov) Moore, C.H., Carbonatedialgenesis and porosity, Costa Pica, des ophiolitesaux andesite,Mere. P. Vannucchi, Earth Sciences Department, Der. Sedimentol.,46, 338 pp., 1989. Sci. Terre, Univ. Pierre Marie Curie, 84, 335 Universityof Modenaand Reggio Emilia, Piazzale Mora, C.R., Sedimentologiay geomorfologiadel pp., 1984. S. Eufemia 19, 1-41100, Modena, Italy. sur de la Peninsulade Nicoya (Provinciade Valentine, R.B., J.D. Morris, and D. Jr. Duncan, (paolav•geo.unifi.it) Puntarenas,Costa Rica), Tesisde licentiatura, Sedimentsubduction, accretion, underplating, 148pp., Escuela Centroam. de Geol., Univ. de arc volcanismalong the marginof CostaRica: CostaRica, San Jos&Costa Rica. 1985. Constraintsfrom Ba, Zn, Ni, and løBe (ReceivedMarch 31, 2000; Morris,J.D., and F. Tera, WBe and '9Be in mineral concentrations,Eos Trans.AGU, 78 (46), Fall revisedDecember 14, 2000; separatesand whole rocks from volcanic arcs: Meet. Suppl.,F673, 1997. acceptedApril 17,2001.)