CU-Boulder GEOL 5700 - Structure of the Me´rida Andes, Venezuela

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Structure of the Mérida Andes, Venezuela: relations with the South America-Caribbean geodynamic interactionIntroductionPrevious crustal modelsPresent geodynamic settingStrain partitioningChain segmentation along strikeThe Bócono faultThe Mérida Andes foothillsThe northwestern foothillsThe southeastern foothillsSeismicity and focal mechanismsShortening vs. wrenchingAmountActivation ageDiscussionConclusionsAcknowledgementsReferencesStructure of the Me´rida Andes, Venezuela: relations with the SouthAmerica–Caribbean geodynamic interactionFelipe E. Audemarda, Franck A. Audemardb,*aPDVSA Exploracio´n y Produccio´n, Caracas, VenezuelabEarth Sciences Department, Venezuelan Foundation for Seismological Research, FUNVISIS, Apartado Postal 76880,Caracas 1070-A, VenezuelaReceived 28 March 2000; received in revised form 5 April 2001; accepted 11 April 2001AbstractFor over 50 years, several models based on diverse geologic concepts and variable quality of data have been proposed toexplain the major structure and history of the Me´rida Andes (MA), in western Venezuela. Lately, this chain growth andassociated flexural basins deepening have been related to incipient type-A subductions of either polarity, accounting for theacross-chain asymmetry. However, these recent models have not well integrated the present tectonically active setting driven byneighboring major plate interactions. At present, this chain exhibits ongoing strain partitioning where cumulative right-lateralslip along chain axis is as much as half of, or about the same, as the transverse shortening since late Miocene, thus implying thatthe NNE-directed Maracaibo block extrusion with respect to the South America (SA) plate is not a secondary feature.Consequently, this paper discusses some limitations exhibited by the SE-directed continental subduction models—Maracaibocrust underthrusting the Me´rida Andes — in the light of available geological and geophysical data. Besides, it is herein proposedthat the Me´rida Andes structuration is related to a NW-directed, gently dipping, incipient type-A subduction, where chaingrowth and evolution are similar to those of a sedimentary accretionary wedge (i.e., Barbados), but at crustal scale and withongoing strain partitioning. This continental subduction is the SE portion of a major orogenic float that also comprises the Perija´range and the Santa Marta block. D 2002 Elsevier Science B.V. All rights reserved.Keywords: triangular zones; strain partitioning; type-A subduction; crustal models; Me´rida Andes; Venezuela1. IntroductionThe Me´rida Andes (MA) is in the northeastwardtopographic prolongation of the Eastern Cordillera(EC) of the Colombian Andes. The latter belongs tothe main Andes chain that extends all along thePacific coast of South America (SA) . However, theMA an d EC do not keep direct genetic relationshipbetween them (Fig. 1) since the present NE–SWtrending Venezuelan (or Me´rida) Andes are not relatedto direct interactions between the SA craton and eitherarc terrains or oceanic domains, as the rest of the SAAndes does. Besides, both chains are separated by thesouthern termination of the le ft-lateral strike-slip(LLSS) Santa M arta–Bucaramanga fault (SMBF)and the NW–SE trending Santander Massif (SM).As a matter of fact, the MA formation has always0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.PII: S 0040-1951(01)00218-9*Corresponding author. Tel.: +58-212-2577672; fax: +58-212-2579977.E-mail address: [email protected] (F.A. Audemard).www.elsevier.com/locate/tectoTectonophysics 345 (2002) 299 – 327been linked to indirect plate interactions. For instance,an older less-prominent MA chain formed in theMiocene (Fig. 9.11 of Audemard, 1993; Audemard,1998), probably related to the early stages of collisionof the Panama´Arc (PA) against western SA. This firstpulse in the MA was probably coeval with uplift in theEC reported by Taboada et al. (2000). This firstorogenic pulse drove deposition of molassic depositsalong the southern and northern MA flanks named asPara´ngula and Isnotu´formations, respectively. On theother hand, the present chain build-up results fromPliocene–Quaternary transpression due to obliqueconvergence be twee n t wo independe nt continentalblocks. This Plio –Quaternary compression is super-posed to the first Miocene compressional phase.Therefore, Jurassic (half-) grabens have been invertedby these two consecutive compressional phases, thusexposing Precambrian and Paleozoic rocks of the SAcontinental crust.2. Previous crustal modelsSeveral models have been proposed to explain themajor structure of the MA during the last 50 years.They may be essentially gathered in two major con-ceptual models. On one hand, some models haveconceived the MA as an essentially symmetric chainto a major axial strike-slip fault, with both sidesbounded by reverse faults— responsible for chainvertical growth— (Gonza´lez de Juana, 1952; Rod,1956b). Consequently, the MA would resemble apositive flower structure. This model was also sharedby Dallmus (1957, in Rod et al., 1958), at least for theupper crustal level; later on, Stephan (1985) s tillmaintained this interpretation. On the other hand,several other models incorporated more recently theasymmetry of the MA revealed by the gravimetricsurvey of Hospers and Van Wijnen (1959), though thechain asym metry was first evoked by De Cizancourt(1933) and later by Buc her (1952)—before the surveywas performed — but their models did not display thecrustal-scale structure. Among the asymmetric mod-els, there are two trends depending on the majorunderthrusting vergence. For instance, Audemard(1991) indicates that major structuration of the MAresults from a NW-vergent crustal-scale wedgingrooted on top of a crustal NW-gently dipping detach-ment. Instead, Ja´come et al. (1995) propose an ‘‘oro-genic float’’ model. On the contrary, Kellogg andBonini (1982), De Toni and Kellogg (1993), Sa´nchezet al. (1994) and Colletta et al. (1997) favor the SEpolarity models. Additional to the conceptual appli-cation of very incipient type-A subduction or crustaldelamination to the MA, most recent papers on thechain and its related basins (Kellogg and Bonini,1982; Audemard, 1991; De Toni and Kellogg, 1993;Sa´nchez et al., 1994; Castrillo, 1997; Colletta et al.,1997; Funvisis, 1997; Duerto, 1998; Duerto et al.,1998) also incorporate new modern concepts at crustalor lithospheric scale:


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CU-Boulder GEOL 5700 - Structure of the Me´rida Andes, Venezuela

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