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Plate Motions in the North Andean Region Jeffrey T University of South Carolina Scholar Commons Faculty Publications Earth, Ocean and Environment, School of the 12-10-1993 Plate Motions in the North Andean Region Jeffrey T. Freymueller James N. Kellogg University of South Carolina - Columbia, [email protected] Victor Vega Follow this and additional works at: https://scholarcommons.sc.edu/geol_facpub Part of the Earth Sciences Commons Publication Info Published in Journal of Geophysical Research, Volume 98, Issue B12, 1993, pages 21,853-21,863. Freymueller, J. T., Kellogg, J. N., & Vega, V. (1993). Plate motions in the North Andean region. Journal of Geophysical Research, 98 (B12), 21,853-21,863. ©Journal of Geophysical Research 1993, American Geophysical Union This Article is brought to you by the Earth, Ocean and Environment, School of the at Scholar Commons. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B12, PAGES 21,853-21,863, DECEMBER 10, 1993 Plate Motionsin the North AndeanRegion JEFFREYT. FRE•J•-:• ,1 JAMESN. Km•Oc_,G,At, O VICTOR VEGA Departmentof GeologicalSciences, University of SouthCarolina, Columbia Repeated geodetic measurementswith the Global Positioning System (GPS) provide direct measurementsof displacementsdue to plate motionsand active crustal deformation in Central Americaand northern South America, an area of complex interaction of the Nazca, Cocos, Caribbean and South Americanplates. The displacementrates for the period 1988-1991, obtainedfrom the resultsof the first threeCentral AndSouth America (CASA) GPScampaigns, are in generalagreement with the predictionsof the NUVEL-1 plate motion model, but there are differencesin detail betweenthe observationsand the model. The Nazca-NorthAndes convergence rate vector measuredby GPS is differentfrom the NUVF.•I vector at 95% confidence.The differenceimplies that the North Andesare moving northwardrelative to SouthAmerica. The measured convergence between the Caribbeanplate andthe North Andessuggests that the southernmargin of the Caribbeanplate is locatedin the SouthCaribbean deformed belt. The April 1991 CostaRica earthquakeand the Cocos-Caribbeanconvergence rate determinedby GPS suggestthe possibility of significant ongoing deformationbetween Central Americaand the stableinterior of the Caribbeanplate. Our GPS resultsare consistent with deformationof the overriding plates at the convergentmargins of Central andSouth America and confirmthat activeconvergence is occurringaround muchof the southernmargin of the Caribbeanplate, from Colombia west to Costa Rica. Costa Rica and Panam• are not part of the stable Caribbeanplate. Instead,the SouthCaribbean deformed belt andthe North Panam•fold belt probablyrepresent the southernmargin of the Caribbeanplate. INTRODUCTION The rate of change of baseline vectors, three-dimensional vectors between two sites, measurethe rates of motion of the The CASA (Central and South America)Global Positioning major plates, as well aslocal or regional deformationat each System(GPS)project was inauguratedin 1988 to studyplate site. The baseline rate of change vectors are decomposedinto motions and crustal deformation in Central America and components which represent the motion rates in the local northern SouthAmerica, a tectonically active areaof complex north, east and vertical directions. Details of the GPS data interaction between the Nazca, Cocos, Caribbean and South analysis and the precision and accuracyof the GPS solutions American plates. There is general agreementthat the three are given in the appendix. A single baseline is insufficient to plates are converging, but the locations of certain plate computea pole of relative motion, so the observedbaseline boundarieshave long been a subject of controversy [e.g., ratesof changeare compared to modelrates computed from the Dewey, 1972; Shagam, 1975; Aggarwal, 1983]. Diffuse NUVEL-1 plate motion model [DeMets et al., 1990; Argus and seismicity is spreadout over a wide area from the Cocos Gordon, 1991]. Further studies will compute displacement transform fault south of Panamfi to the East Andean and rates for all the sites of the CASA network, to study the Bocon6 fault systems at the eastern edge of the Andes internal deformationof the plates. [Pennington, 1981]. Kellogg et al. [1985] have interpretedthe tectonicsof the region in termsof two additionalmicroplates, REGIONAL •NICS Panatariand the North Andes, both of which are subject to internal deformation(Figure 1). Colombia-EcuadorSubduction System Repeated geodetic measurementswith GPS provide direct Subduction(70 mm/yr) of the oceanic Nazca plate occursat measurementsof the displacementsof the individualGPS sites, the western margin of South America, and the trench is which can be used to compute rates of plate motion and seismicallyactive (Table 1). Four large (Ms > 7) shallow intraplate deformation.The presentCASAnetwork consistsof subduction-relatedearthquakes have occurredwithin this area 30 stations in five countries (Figure 1), most of which have in the last century.The 1906 event rupturedmore than 400 km been occupied in at least two of the three CASA GPS of the trench, from the equatorto the north [Kelleher, 1972] campaigns. Five sites (Isla Baltra, Isla del Coco, Liberia, (Figure 2). Three smaller events, in 1942, 1958, and 1979 Lim6n and Jerusalen)positioned to measurethe velocities of have together rupturedapproximately the same length of the the major plates in the region have been occupiedin all three trench as did the 1906 event (Figure 2), althoughthe seismic campaigns. This paper concentrateson the plate motion momentof the 1906 event is approximately5 times as large as determinationsmade using thesefive sites, and two sites (Isla the sum of the moments of the smaller events [Kanamori and San Andrts and Cartagena) which measurethe convergence McNally, 1982]. Based on the seismic record of the last between the Caribbean plate and the North Andes. The century, Nishenko [1989] estimated the probability of a measurementscover a 3-year period, 1988-1991. magnitude7+ earthquakein these segmentsof the subduction zone within the next 10 years to be greater than 70%. A 1Nowat Departmentof Geophysics,Stanford University, magnitude M ~ 6.6 earthquakeoccurred on November 1991, Stanford, California. roughly 100 km to the north of the northern extent of the Copyright1993 by the AmericanGeophysical Union. 1906 rupture(Figure 2). The preinstrumentalrecord of seismicity in this region is Paper number 93JB00520. incomplete, due to the historically small population on the 0148-0227/93/93 JB-005 20505.00 Pacific coast.Only one eventprior to 1906 can be attributedto 21,853 21,854 FREYMUF•LERET AL.: PLATEMOTIONS IN NORTHANDEAN REGION l CARIBBEAN Isla San Andr•s 10mm/yr • 10 ø N cocos I mm/yr ¾ I / • •s•ade• Coco I I NAZCA o • IslaBaltra 69 90øW 80øW 70øW Fig. 1. ObservedGPS (solid lines) andmodel NUVEL-1 [DeMets et al., 1990] (dashedlines) baselinerates of change,with their 95% confidenceellipses. Baselinerates of changemeasure the relative motions of two GPS sites. The Caribbean-NorthAndes platemotion is basedon only two epochsof GPSdata(1988and 1991), andthe uncertaintygiven is conservative.GPS sites used in this studyare indicatedby squares;other GPS sitesare indicatedby circles. this segment of the trench, an earthquakenear Tumaco late Cenozoic [Bowin, 1976; Mann and Burke, 1984; Pindell (Colombia) in 1798, which was large enough to produce et al., 1988]. notable damage300 km away in Quito, Ecuador[Comit6 del On the east side of the North Andes, right-lateral Afio Geoffsico Internacional del Ecuador,1959]. The damage transpressivemovement is taking place on the Bocon6 and causedin Quito was similar to the damagecaused by the 1979 East Andean fault zones. The Bocon6 fault in Venezuela is Tumaco earthquake,but no other information is available to principally a right-lateral strike slip fault [Schubert,1982] determinethe magnitudeof this event. whichruns throughthe M6rida Andes.Subparallel thrust faults flank the Bocon6 fault, and small-scalegeodetic networks in Motion and Deformation of the North Andes the M6rida Andes indicate significant compressionnormal to the Bocon6 fault in additionto strike-slip motion [Henneberg, The North Andes are boundedby the Colombia-Ecuador 1983]. The East Andean frontal fault system consists of trench and Panamfi on the west, the South Caribbean deformed subparallelwestward-dipping faults. Based on a studyof focal belt to the north, and the Bocon6 and East Andean fault zones mechanisms in the East Andean frontal fault system, to the east(Figures 1 and2) [Bowin, 1976; Pennington,1981; Pennington [1981] proposedthat transpressiveright-lateral Kellogg et al., 1985; Adamek et al., 1988]. Within the slip is occurring on these faults, and that the North Andes northern part of the North Andes, significant Cenozoic block is movingNNE relative to the SouthAmerican plate. displacementshave occurredon the Oca (right-lateral, east- To the south the Ecuadorian Andes include a system of west trending) and Santa Marta-Bucaramanga(left-lateral, subparallel northeast trending right lateral strike slip faults northwest-southeast trending) faults (Figure 2) [e.g., and north trending thrust faults. The relationships between Campbell, 1968; Kellogg, 1984; Mann and Burke, 1984; thesefaults arenot well understood;perhaps the best studiedof Mann et al., 1990]. There areno estimatesof the slip ratesof
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