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Revised Tectonic Boundaries in the Cocos Plate Off Costa Rica Implications for the Segmentation of the Convergent Margin And

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Revised Tectonic Boundaries in the Cocos Plate Off Costa Rica Implications for the Segmentation of the Convergent Margin And JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B9, PAGES 19,207-19,220, SEPTEMBER 10, 2001 Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models Udo Barckhausen,• Cesar R. Ranero, 2 R. von Huene, 2,• Steven C. Cande,4 and Hans A. Roeser t Abstract. The oceanicCocos Plate subductingbeneath Costa Rica has a complexplate tectonichistory resulting in segmentation.New lines of magneticdata clearlydefine tectonicboundaries which separatelithosphere formed at the East PacificRise from lithosphereformed at the Cocos-Nazcaspreading center. They also define two early phase Cocos-Nazcaspreading regimes and a major propagator.In addition to these sharply definedtectonic boundaries are overprintedboundaries from volcanismduring passageof CocosPlate over the Galapagoshot spot.The subductedsegment boundaries correspond with distinctchanges in upper plate tectonicstructure and featuresof the subductedslab. Newly identifiedseafloor-spreading anomalies show oceanic lithosphere formed during initial breakupof the FarallonPlate at 22.7 Ma and openingof the Cocos-Nazcaspreading center. A revisedregional compilation of magneticanomalies allows refinement of plate tectonicmodels for the earlyhistory of the Cocos-Nazcaspreading center. At 19.5Ma a major ridgejump reshapedits geometry,and after -14.5 Ma multiplesouthward ridge jumps led to a highly asymmetricaccretion of lithosphere.A suspectedcause of ridgejumps is an interactionof the Cocos-Nazcaspreading center with the Galapagoshot spot. 1. Introduction continentalslope [von Huene et al., 1995;Hinz et al., 1996], and even forearc uplift nearshore[Fisher et al., 1998]. The down- The Central America convergentmargin off CostaRica and going slab along Central America changesdip significantly Nicaraguahas been an area of concentratedstudy during the [Prottiet al., 1995a],and the geochemistryof arcvolcanic rocks past decadebecause of its variable characterin a relatively and the alignmentof volcanoeschanges similarly [e.g., Carr small area and its well-imagedsubduction zone. Recent pub- and Stoiber,1990; Patino et al., 2000].A recentstudy shows the licationsreport a distinctivesegmentation of the upper plate degreeto which characterand relief of the subductinglower tectonicstructure and relate much of this to a corresponding plate relatesto upper plate tectonismand arc volcanism[von segmentationof the subductingCocos Plate. This segmenta- Huene et al., 2000]. However, precise age information and tion was recognizedin a progressionof studieseach contrib- identification of tectonic boundaries of the Cocos Plate are uting to an increasingunderstanding of the tectonicorigin of lacking.In this study,we focuson the integrationof new data each plate segment.The existenceof a rough and a smooth with previouslypublished compilations [Barckhausen et al., morphologicaldomain on the CocosPlate was noted in the 1998] that answersome of thesequestions. We presenta de- early 1960s [Fisher,1961] (Figure 1). In the early 1990s a tailed magneticanomaly map including-8000 km of new data comprehensivemultibeam bathymetric survey of ocean crust and analyzethe tectonicsetting of the studyarea in the frame- was made off Costa Rica [vonHuene et al., 1995]. This study work of the regionalmagnetic seafloor-spreading anomalies. showedsharp boundariesbetween three morphologicalseg- This constrainscrustal age and preciselocation of major tec- ments on the oceanicplate: (1) smoothseafloor facing the tonicboundaries. The crustalages permit us to revisethe plate tectonichistory of the Cocos-Nazcaspreading center (CNS) Nicoya Peninsula,(2) a segmentwith abundant(40%) sea- from the breakupof the Farallon Plate at 23 Ma to 10 Ma. mountsto the southeast,and (3) CocosRidge enteringthe subduction zone off Osa Peninsula on the southern Pacific coastof CostaRica. It becameclear that the roughnessof the 2. Previous Work seafloorsignificantly affects the shapeand the tectonicsof the The first consistent models of CNS evolution and the for- mation of the aseismicCocos and Carnegieridges were de- •Bundesanstaltfiir Geowissenschaftenund Rohstoffe,Hannover, rivedby Hey [1977]and Lonsdale and Klitgord [1978]. From the Germany. 2GeomarForschungszentrum fiir marine Geowissenschaften, Kiel, analysisof magneticand bathymetricdata they concludedthat Germany. the FarallonPlate broke into the Cocosand Nazca Platesalong 3Alsoat Departmentof Geology,University of California,Davis, a preexistingfracture zone in equatorial regionsat •27 Ma. California, USA. Accordingto this model the newlyformed CNS interactedwith 4ScrippsInstitution of Oceanography,University of California,San the Galapagoshot spot,which simultaneouslydeposited vol- Diego, La Jolla, California, USA. canic material on both sides of the CNS to feed the Cocos and Copyright2001 by the American GeophysicalUnion. Carnegie Ridges on the Cocos and Nazca plates. Magnetic Paper number 2001JB000238. seafloor-spreadinganomalies had been identifiedin the inner 0148-0227/01/2001JB000238509.00 regionof the CNS alongthe activespreading axis and southof 19,207 19,208 BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 105øW 100øW 95øW 90'W 85'W Ill IllIll Ill Ill Ill Ill I Ill I III 15'N =• • -.•-•. ß .....ß"•' -.:'i: '":i: .... •.... I .....:•L.;:ß ,.'.' .:'• •..,;.....:.:..•.-/."• ..:.•}:•........•..... :..::..;.• •...• • " " 10'N Plate, ß"' Nazca Plate I / 105'W 100'W 95øW Figure 1. Bathymetricmap of the Cocos-Nazcaspreading region based on satellite altimetry of Smith and Sandwell[1997]; EPR, East PacificRise; CNS, Cocos-Nazcaspreading center; PFZ, PanamaFracture Zone; MAT, Middle America Trench. Arc volcanoesin Central America are shownas triangles.Arrows indicate absoluteplate motion vectors[DeMets et al., 1990].The rough-smoothboundary is expressedclearly only in the westernpart of the CNS region. The small rectangleoutlines the area in Plate 1 and Figure 4; the large rectangleindicates the area coveredby Plate 2. the CarnegieRidge. In the areasof the submarineridges that rough-smoothboundaries are geneticallydifferent and do not were overprintedby hot spot related volcanicactivity no lin- necessarilycoincide. Near the Middle America Trench (MAT) eated anomaliescould be identified.North of CocosRidge the the fine-scale topography of the oceanic basement is fairly identificationof seafloor-spreadinganomalies was also impos- smooth, and the rough-smoothboundary was defined at the sible at that time becauseof the paucity of data and compli- limit between an oceanic domain with numeroushotspot re- cated anomaly pattern [Hey, 1977]. Later, Wilson and Hey lated volcanicedifices (ridges, conical volcanoes, and guyots) [1995] revisedthe magneticanomalies of the inner part of the [vonHuene et al., 1995]and a domainwith smoothtopography. CNS and carefully documentedanomalies younger than 10 Barckhausenet al. [1997] showedthat magneticanomalies of Ma, includinga pattern of propagatorsand small ridge jumps. the CNS continue north of the morphologicalrough-smooth Today,oceanic crust along the westand southboundaries of boundary, but the exact position of the boundary between the CocosPlate is generatedby activespreading along the East EPR- and CNS-generated crust was not clear from the mag- PacificRise (EPR) and the CNS. Oceaniccrust formed at the netic anomalydata. The magneticanomaly map compiledfrom EPR has the featurelessmorphology and low-amplitudemag- data acquired during cruise SO-76 [Barckhausenet al., 1998] netic anomaliescommon to fast-spreadingridges [Hey, 1977; showedtwo different patternsof seafloor-spreadinganomalies Wilson,1996]. Oceaniccrust currentlygenerated at the CNS off Costa Rica, both being attributed to the CNS. However, near the triple junctionwith the EPR is formed by slowspread- since the survey area was relatively small and the magnetic ing. It has a rough topographyand high-amplitudemagnetic signaturesof numerousseamounts are superimposedupon the anomalies[Wilson and Hey, 1995]. Hey [1977] mapped the linear anomalies,it was still impossibleto identify seafloor- resulting"rough-smooth boundary," separating two provinces spreadinganomalies and clearly define tectonicboundaries in formed at two different spreadingcenters within the Cocos the oceaniccrust. Wilson [1996] analyzedseafloor-spreading Plate (Figure 1). Hey [1977] projected the rough-smooth anomalieson the EPR-derived part of the CocosPlate along boundary from the seaward area where it is well expressed, the rough-smoothboundary between 94øW and 88øW but did landward to the southerntip of the Nicoya Peninsula.How- not extend the identificationof anomaliesand the triple junc- ever, sinceno seafloor-spreadinganomalies could be identified tion trace eastward to the MAT. in the area off Costa Rica, the location of the boundarywas The bathymetricrough-smooth boundary has been widely defined from bathymetricobservations [Hey, 1977]. In addi- usedby different authorsas the trace of the triple junction of tion, Hey [1977] pointed out that in the older part of the EPR and CNS off Costa Rica and inferred as a major lithos- CNS-derived CocosPlate crust the magneticand bathymetric phericfeature that explainspatterns in the configurationof the BARCKHAUSEN ET AL.: COCOS PLATE TECTONIC BOUNDARIES 19,209 subductingslab along the continentalmargin [e.g.,Pratti et al., following terms for the boundarybetween the EPR- and CNS 1995a;Marshall et al., 2000;Patina et al., 2000]. The boundary derived provinces:The younger part of the boundarywhich
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