QUATERNARY FAULTS AND STRESS REGIME OF · PDF fileject, "Major Active Faults of the World",...

Revista de la Asociación Geológica Argentina 61 (4): 480-491 (2006)480

INTRODUCTION

This paper presents an updated compila-tion of the Quaternary faults of Venezuela,

generated in the frame of an ILP II-2 pro-ject, "Major Active Faults of the World",led by Michael Machette of the USGS, res-ponsible for the Western Hemisphere, and

Vladimir Trifonov of Russia, responsiblefor the Eastern Hemiphere. The SouthAmericam maps and data compilationefforts were coordinated by Carlos Costa of

QUATERNARY FAULTS AND STRESS REGIME OF VENEZUELA

Franck. A. AUDEMARD M1, André SINGER P., Jean-Pierre SOULAS and the Neotectonics Section of the Funvisis EarthSciences Department*

¹ Funvisis, Apartado Postal 76.880, Caracas 1070-A, Venezuela. E-mail: [email protected].

ABSTRACTSpatial configuration of Quaternary active tectonic features along the southern Caribbean plate boundary suggests that the region is subjectto a compressive strike-slip (transpressional senso lato) regime, characterized by a NNW-SSE maximum horizontal stress (σH=σ1) and/or anENE-WSW minimum (σh=σ3 or σ2) horizontal stress. Stress inversion applied to fault-plane kinematic indicators measured essentially inPlio-Quaternary sedimentary rocks confirms this tectonic regime. Accordingly, this stress regime is responsible for the Quaternary activityand kinematics of six sets of brittle features along northern Venezuela (from Colombia in the west to Trinidad in the east): (1) east-west right-lateral faults, (2) NW right-lateral faults -acting as synthetic Riedel shears-, (3) ENE to east-west dextral faults -P shears-, (4) NNW normalfaults, (5) almost north-south left-lateral faults -antithetic Riedel shears- and (6) mostly subsurface ENE reverse faults associated with foldingof the same orientation. Brittle deformation conforms to the simple shear model, although not all the deformation can be accounted for itsince strain partitioning is also taking place because regional folding and thrusting are due to the normal-to-structure component of the rela-tive slip vector between the Caribbean and South America plates. On the other hand, the maximum horizontal stress in western Venezuela,particularly in the Maracaibo block and south of the Oca-Ancón fault, progressively turns counter-clockwise to become more east-west orien-ted, producing left- and right-lateral slip along the north-south striking and NE-SW striking faults, respectively. The orientation and spatialvariation of this regional stress field in western Venezuela results from the superposition of the two major neighboring interplate maximumhorizontal stress orientations (σH): roughly east-west trending stress across the Nazca-South America type-B subduction along the pacificcoast of Colombia and NNW-SSE oriented stress across the southern Caribbean boundary zone.

Keywords: Quaternary tectonics, fault-slip data, stress tensor, Caribbean, Venezuela.

RESUMEN: Fallas y campo de esfuerzos cuaternarios de Venezuela.La configuración espacial de los rasgos tectónicos activos cuaternarios en el Caribe meridional sugiere que esta región está sujeta a un régimen tec-tónico transcurrente compresivo (transpresivo senso lato), caracterizado por un esfuerzo horizontal máximo (σH=σ1) de dirección NNO-SSE y unmínimo (σh=σ3 or σ2) orientado ENE-OSO. La aplicación de métodos de inversión a datos microtectónicos determinados sobre planos de fallasafectando rocas sedimentarias esencialmente de edad plio-cuaternarias confirma este régimen tectónico. Consecuentemente, este régimen es res-ponsable por la cinemática y actividad cuaternaria de seis familias de estructuras tectónicas frágiles a lo largo del norte de Venezuela (desdeColombia en el oeste hasta Trinidad en el este): (1) fallas dextrales este-oeste, (2) fallas dextrales NO que actúan como "Riedels" sintéticos-, (3) fallasdextrales ENE a este que se comportan como fracturas P, (4) fallas normales NNO, (5) fallas sinestrales submeridianas consideradas como fractu-ras "Riedels" antitéticos y (6) fallas inversas ENE - sub-paralelas al plegamiento de la misma orientación. En esta región en particular, la deforma-ción frágil obedece el modelo de cizalla simple, aunque no toda la deformación puede atribuírsele porque simultáneamente ocurre partición de ladeformación porque el plegamiento y fallamiento inverso se debe esencialmente a la componente ortogonal a las estructuras del vector de despla-zamiento relativo entre las placas Caribe y Suramérica. Por otra parte, el esfuerzo máximo horizontal en Venezuela occidental, particularmente enel bloque de Maracaibo y al sur de la falla de Oca-Ancón, progresivamente tuerce en sentido anti-horario para orientarse este-oeste, permitiendola activación simultánea de fallas sinestrales y dextrales de orientación norte y NE respectivamente. La orientación y variación espacial del campode esfuerzo regional en Venezuela occidental resulta de la superposición de las orientaciones de los dos esfuerzos horizontales máximos imperan-tes en el norte de Suramérica (σH): el esfuerzo de orientación este-oeste producto de la subducción de la placa Nazca por debajo de la costa pací-fica de Colombia y el esfuerzo orientado NNO-SSE a través del límite de placas del Caribe meridional.

Palabras clave: Tectónica cuaternaria, microtectónica, tensores de esfuerzos, Caribe, Venezuela.

0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina

* The following professionals have once belonged to (or worked for) the FUNVISIS Neotectonics section: Luis Acosta, Amalfi Arzola, Carlos Beltrán, ChristianBeck, Olivier Bellier, Didier Bonnot, Jean-Claude Bousquet, Eduardo Carrillo, Antonio Casas-Sainz, Raymi Castilla, Carlos Costa, Feliciano De Santis, HansDiederix, Claudio Gallardo, Carlos Giraldo, Rogelio González, Antoine Mocquet, Reinaldo Ollarves, Carlos Alberto Rivero, Eduardo Rodríguez, José AntonioRodríguez, Carlos Rojas, Bernard Sauret, Carlos Schubert and Tomas Subieta.

Quaternary faults and stress regime of Venezuela 481

the University of San Luis in Argentina.The Quaternary fault map data of Vene-zuela, updated until 1998, are compared tostress tensors derived from fault-plane kine-matic indicators measured essentially insedimentary rocks of Plio-Quaternary ageand compiled by Audemard et al. (2005). Inthis paper these will be discussed in termsof their significance with respect to theinteractions among the Caribbean, SouthAmerica and Nazca plates and the smallercontinental blocks of northwestern SouthAmerica.

GENERAL GEODYNAMICSETTING

Western Venezuela and northern Colombiaare situated in a very complex geodynamicsetting involving a number of tectonicblocks or microplates. Conversely, northernVenezuela essentially lies in the direct inter-action zone between the South Americaand Caribbean plates (Fig. 1) and there is ageneral consensus on the eastward motionof the Caribbean plate relative to South

America (Bell 1972, Malfait and Dinkelman1972, Jordan 1975, Pindell and Dewey1982, Sykes et al. 1982, Wadge and Burke1983, among others). Lately, this has beenstrongly supported by results of recentGPS surveys (Freymueller et al. 1993,Kaniuth et al. 1999, Weber et al. 2001, Pérezet al. 2001, Trenkamp et al. 2002). Never-theless, this Caribbean-South America pla-te boundary -which drives and defines acti-ve tectonics along northern Venezuela(from Colombia to Trinidad)- can not beclassified solely as being of the simple dex-tral type (Soulas 1986, Beltrán 1994) since itcomprises a more than 100 km wide activetranspressional zone occurring partly offs-hore and partly onshore northern Ve-nezuela (Audemard 1993, Singer and Aude-mard 1997, Audemard 1998, Ysaccis et al.2000). Important positive relief featureswithin the onshore portion of this plateboundary zone, such as the Coastal and In-terior ranges (I, J and Q in Fig. 2), line theentire northern and part of the eastern Ve-nezuelan coast. This would seem inconsis-tent with the Caribbean plate motion vector

in a direction 086º ± 2º with respect to theCentral range of Trinidad predicted byWeber et al. (2001), and N084º ± 2ºE withrespect to South America (Canoa site) byPérez et al. (2001) from GPS data, whichsuggest almost pure wrenching with onlyslight transtension in eastern Venezuela, anissue which shall be discussed in moredetail later in this paper. This wide trans-pressional boundary (in its widest defini-tion, signifying coexistence of strike-slipand compression but not necessarily acco-mmodated jointly on one single structure)extends southwestward into the MéridaAndes (MA in Fig. 1). The plate boundaryin western Venezuela covers a 600 km widezone and comprises a set of discrete tecto-nic blocks or microplates (Fig. 1), whichmove independently among the surroun-ding larger plates (Caribbean, South Ame-rica and Nazca). The Maracaibo block(MTB in Fig. 1) which stands out amongthose blocks for its perfect triangular shape,is bounded by the left-lateral strike-slipSanta Marta-Bucaramanga fault (SMBF) inColombia and the right-lateral strike-slip

Figure 1: Simplified general geodynamic setting of the southern Caribbean (modified from Audemard, 2002). Region is subject to a complex blocktectonics. Vector decomposition of the convergence vector at the Nazca subduction may explain the along-strike slip change of the Romeral faultsystem. Equivalence of used acronyms is as follows: Bonaire (BB), Chocó (CB), Maracaibo (MTB), North Andean (NAB) and Panamá (PB) blocks;Mérida Andes (MA) and Pamplona indenter (PI). Some major faults are also reported: Algeciras (AF), Boconó (BF), El Pilar (EPF), Guaicaramo(GF), Romeral (RFS), Santa Marta-Bucaramanga (SMBF), San Sebastián (SSF) and Oca-Ancón (OAF); and other features as well: Leeward Antillessubduction (LAS), Los Roques Canyon (LRC), North Panamá deformation belt (NPDB), and Southern Caribbean deformation belt (SCDB).

F. A . AUDEMARD M, A . SINGER P. , J.P. SOULAS AND OTHERS482

Boconó fault (BF) in Venezuela and separa-ted in the north from the Bonaire block(BB in Fig. 1) by the right-lateral strike-slipOca-Ancón fault (OAF). Besides, both theMaracaibo and Bonaire blocks are beingextruded northward with the Bonaire blockmoving slightly east. Together they are over-riding the Caribbean plate north of theLeeward Antilles islands, where a youngsouth-dipping, amagmatic, flat subduction(LAS) has developed in recent times (main-ly during the last 5 ma). The tectonic extru-sion of these blocks is induced by the colli-sion of the Panamá arc (PB in Fig. 1)against the Pacific side of northern SouthAmerica and its subsequent suturing(Audemard 1993, 1998). Recent resultsfrom GPS plate motion studies (Freymue-ller et al. 1993, Kellogg and Vega 1995,Kaniuth et al. 1999, Trenkamp et al. 2002),such as the CASA project, confirm thisnortheastward escape of both blocks,which in the process generate the SouthernCaribbean Deformation Belt (SCDB) northof the Netherlands Leeward Antilles.The present Caribbean-South American

geodynamic configuration results from atranspressive evolution that has occurredthroughout the Tertiary and Quaternary,initiated as an oblique type-B subduction.The NW-dipping oceanic lithosphere, atta-ched to northern South America, subduc-ted obliquely under Caribbean plate islandarc. This plate boundary zone later evolvedinto a long-lasting east-younging oblique-collision, where Caribbean-affinity nappesoverrode towards the SSE, thus scrappingoff passive-margin-affinity nappes andincorporating them into the tectonic pile,which in turn was all overthrusted onto theundeformed South America passive mar-gin. More recently, this plate boundary con-verted into a partitioned transpressionwhen and where collision became unsustai-nable (for more details refer to Audemard1993, 1998). Its latest evolutionary stage isstill active in Eastern Venezuela andTrinidad and marks the shift from obliquesubduction to present-day partitioned obli-que collision. This east-younging obliqueconvergent margin has acted diachronicallythroughout the evolution of this entire nor-

thern portion of the plate boundary(Audemard 2000) and clearly indicates that,through time, the plate boundary zone hasprogressively changed from a predominantcompressive to a more wrenching type.This change started at around 17-15 Ma innorthwestern Venezuela (Audemard 1993,1998). The tectonically complex southeas-tern Caribbean is undergoing two majorgeodynamic processes: (a) strain partitio-ning characterized by NNW- trending shor-tening across the entire region from northof La Blanquilla island (O in Fig. 2) to thesouthernmost active thrust front of theInterior range and right-lateral strike-slipalong the main east-west striking El Pilarand NW-SE striking Los Bajos-El Soldadofaults, as well as along other minor paralleland/or synthetic Riedel shear faults (Fig. 2);and (b) slab detachment associated with anincipient type-A subduction (e.g., Russoand Speed 1992, Russo 1999; responsiblefor the largest onshore negative Bougueranomaly of the world, located south of thesouthern edge of the Interior range). Theseprocesses concur with a plate boundary

Figure 2: Schematic map of Quaternary faults of Venezuela (simplified from Audemard et al. 2000). Faults and toponyms used throughout thiscontribution are identified. Very few localities are only reported in figure 4.


geometry such as the present one, wherethe El Pilar fault transfers its slip eastwardto one of its synthetic Riedel shears (LosBajos and El Soldado faults), which is ac-ting as a "lithospheric tear fault", thus se-parating the transpressional boundary onthe west from the conventional type-BLesser Antilles subduction zone on the east.These two distinct seismotectonic domainswere originally identified by Pérez andAggarwal (1981). The different seismicitypatterns support contrasting geodynamiccontexts (oblique collision and oblique sub-duction roughly west and east of the LosBajos-El Soldado fault system, respective-ly). Recent GPS findings by Weber et al.(2001) indicate that most of the strike-slipmotion of the El Pilar fault is transferredonto the Warm Springs fault system of theCentral Range of Trinidad rather than ontoa fault south off Trinidad (Fig. 2). Thiswould imply that the Paria gulf is functio-ning at present as a pull-apart basin boun-ded by the El Pilar and Warm Springs faultson the north and south respectively, at areleasing stepover. Both faults exhibit asimilar slip rate, in the order of 8-10 mm/a(compare Audemard et al . 2000 and Saleh etal. under review). However, this crustaldeformation does not exclude that a majorplate boundary is underlying this region, asevidenced by the instrumental seismicity, aconclusion originally drawn by Pérez andAggarwal (1981) and later confirmed bySobiesiak et al. (2002).As in eastern Venezuela, strain partitioningis also a common feature along the rest ofthe transpressional boundary zone. In theMérida Andes, strain is nicely partitionedbetween the right-lateral strike-slip Boconófault (BF) running slightly oblique along theaxis of the chain and thrust faults boundingthe chain on both flanks (Audemard andAudemard 2002). The north-central coastalranges also exhibit strain partitioning,where dextral slip in the range core isaccommodated by both the San Sebastiánand La Victoria faults and other minorsynthetic Riedel shears whereas transverseshortening is mainly taken by uplift and thefrontal thrust faults bounding the chainalong its southern edge, such as in the Gua-rumen basin (Cojedes state; Audemard1999a).

A large portion of the dextral slip along thiscomplex boundary seems to be presentlyaccommodated by the major right-lateralstrike-slip Boconó-San Sebastián-El Pilar-Los Bajos fault system or segments of it(Molnar and Sykes 1969, Minster andJordan 1978, Pérez and Aggarwal 1981,Schubert 1980a and b, 1982, Stephan 1982,Aggarwal 1983, Schubert 1984, Soulas1986, Beltrán and Giraldo 1989, Singer andAudemard 1997, Audemard et al. 2000,Weber et al. 2001, Pérez et al. 2001). Thisslip is now known to be transferred farthereast onto the Warm Springs fault system ofcentral Trinidad (after Weber et al. 2001'sresults). It is still matter of debate whetherthis is either a transcurrent or transformsystem and depends on the authors andtheir proposed geodynamic models. Mostauthors accept or postulate this major right-lateral strike-slip fault system to be the plateboundary between the Caribbean andSouth America plates (e.g., Hess andMaxwell 1953, Schubert 1979, Aggarwal1983, among many others), whereas fewothers propose different plate boundarymodels along this right-lateral strike-slipplate boundary zone: (a) orogenic float typefor the Andes (Audemard, 1991, Jácome,1994, Audemard and Audemard 2002, Fig.3) or eastern Venezuelan Coastal and In-terior ranges (Ysaccis et al. 2000), thus beingflanked by both an A- and B-type subduc-tions; and (b) SE-directed A-subduction orunderthrusting of the Maracaibo blockunder the Mérida Andes (Kellogg andBonini 1982, De Toni and Kellogg 1993,Colletta et al. 1996, 1997). However, theincorporation of the Boconó fault into thismajor right-lateral strike-slip fault system isa rather recent event, that relates to thenorthward extrusion of the Maracaiboblock, because the former transcurrentboundary used to comprise the east-west-striking Oca-Ancón fault system (OAF)located farther north in northwesternVenezuela (Audemard 1993, 1998). The-refore, the Oca-Ancón fault system belon-ged to this major right-lateral strike-slipplate boundary zone along the southernCaribbean from when transpression startedat around 17-15 Ma to when essentially cea-sed or considerably slowed down at around5-3 Ma. Although the Caribbean with res-

pect to South America is moving at ratesbetween 12 mm/a in the west (geologic sliprate from Nuvel 1-A model) and 18 ± 2mm/a in the east (GPS-derived slip rate byWeber et al. 2001), the present major strike-slip (Boconó-San Sebastián-El Pilar-WarmSpring faults) boundary slips at 1 cm/a(Pérez and Aggarwal, 1981; Soulas, 1986;Freymueller et al., 1993; Audemard et al.,2000; Pérez et al., 2001, Weber et al. 2001,Trenkamp et al. 2002). Secondary faultshowever have slip rates one order of mag-nitude less and in fact most of them exhibitslip rates under 0.5 mm/a, with the excep-tion of the Oca-Ancón (2 mm/a, estimatedfrom a paleoseismic assessment performedby Audemard, 1996), the Burbusay ( <3mm/a), Valera and La Victoria ( <1 mm/a)faults (for more details, refer to Audemardet al. 2000, and to figure 2 for relative loca-tion).

NEOTECTONIC MAPPING INVENEZUELA

Active tectonics in Venezuela at present aredriven by tectonic plate and microplate in-teractions, as explained in the introduction.Knowledge about the southern Caribbeanplate boundary zone has much evolved fromthe original ideas of Hess and Maxwell(1953), when a rather simple dextral wren-ching system had been proposed. Overtwenty years of neotectonic analysis com-prising studies in the disciplines of surfacegeology, geomorphology, microtectonics,seismotectonics and paleoseismology, com-bined with data from conventional geologicstudies and seismic reflection surveyingboth onshore and offshore, now give us amore complete understanding of the com-plex active geologic setting in Venezuelaand surrounding areas.Neotectonics in Venezuela, as for every-where else in the world, is defined as thetectonics resulting from the most recentand still active stress field. On this basis,Venezuelan neotectonics refer to the tecto-nics of the Quaternary (after Soulas 1986),implying that tectonic features that eithershow no evidence of activity during thattimeframe or have an orientation not sus-ceptible to reactivation in the near futureunder the present stress regime, are not in-

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F. A. AUDEMARD M, A. SINGER P. , J .P. SOULAS AND OTHERS484

cluded in this compilation. The latest ver-sion of the neotectonic (Quaternary fault)map depicted in figure 4 is the third versionof this type of maps (Audemard et al.2000), the first version having been elabora-ted by Soulas in 1985, and published in1986 (Soulas 1986), which was later upda-ted by Beltrán (1993) as the second version.The latest version differs from the previousones in three important aspects: (a)- Newneotectonic data corresponding to onshoreareas studied by Funvisis between 1993 and1999 have been incorporated. They includedata on the southern frontal thrusts of theInterior range in eastern Venezuela and thesouthern foothills of the Mérida Andes, aswell as a preliminary assessment of theinner active deformation of the triangularblock defined by the Oca-Ancón, Boconóand Valera faults; (b)- A reinterpretation ofthe San Sebastián-El Pilar relay at theCariaco trough in accordance with the in-terpretation of Blanco and Giraldo (1992);and (c) The map has been simplified interms of the number of faults (imposed bythe aim of the ILP-II-2 project), particu-larly in the Colombia-Venezuela borderarea, known as the Pamplona indenter -PI-.On the other hand, faults exhibit moredetail both with respect to their Quaternarykinematics as to their discrimination by age

of latest activity and slip rate, which allowseasy and quick identification of the majoractive features. The map is complementedwith a database that contains relevant infor-mation about each fault and/or fault sec-tion (length, attitude, age, sense of slip, sliprate, geomorphic expression, latest activityfrom geologic data, and so on) and its seis-mogenic potential (maximum credibleearthquake and its recurrence). For moredetails, the reader is referred to the USGSweb page.

QUATERNARY TECTONICS OF VE-NEZUELA

As mentioned earlier, most of northern andwestern Venezuela is situated in a complexplate boundary zone. Due to this comple-xity, we shall discuss in this paper only themost relevant aspects of the QuaternaryVenezuelan tectonics. Figure 2 portrays allnames of the active tectonic features usedthroughout this paper, while figure 4(Audemard et al. 2000) represents the ver-sion of the map of the ILP project andgives further details on the Quaternary acti-vity and relevance of only the major Qua-ternary features of Venezuela. In the follo-wing, we shall present the main issuesdrawn from these two figures.

The Quaternary deformation does notoccur all over the country. Instead, strain isconcentrated along localized strips or belts(Singer and Audemard 1997); all of themlocated in western and northern Venezuela.Most of these mobile belts exhibit highpositive relief, implying an importantamount of shortening along the plateboundary. It does not exclude however thatsome belts show negative relief but suchfeatures are always of more local extent.The most conspicuous mobile belt is at

least 100 km wide and runs over 1200 km inlength from the Colombian border near SanCristobal in the southwest into Trinidad inthe east. This major belt -which seems to bethe prolongation of the Eastern Cordilleraof Colombia- corresponds to an alignmentof mountain chains that comprises fromsouthwest to east: the NE -trending MéridaAndes and the E-W-oriented Coast andInterior ranges both in north-central andnortheastern Venezuela. This belt exhibits afirst-order dextral fault system that mostlyruns along the chain's backbone over a dis-tance of more than 1200 km and comprisesthe NE-trending Boconó fault in the An-des section (VE-06 in Fig. 4), and both theE-W-striking San Sebastián fault (VE-16 inFig. 4) in central Venezuela and the El Pilarfault (VE-13 in Fig. 4) in eastern Venezuela

Figure 3: The orogenic float model applied to the Mérida Andes. Cross-section extends from the northwestern tip of the Santa Marta Block (SMB;location in Fig. 2) to the Llanos Basin, across the southernmost Mérida Andes, at the Pamplona indenter. Bottom figure displays major geologicunits and structures, whereas top figure only exhibits major structures -brittle thrust and strike-slip faults, detachments and triangle zones- to givea more legible view of their interplays (after Audemard and Audemard 2002).


(Singer and Audemard 1997). Most of thedextral slip between the Caribbean andSouth America plates is accommodated bythis fault system. In fact, this fault systemmoves at about 1 cm/a when most otherfaults slip slower than a 1 mm/a(Audemard et al. 2000, Pérez et al. 2001).The combination of this rather fast slip rateand the significant length of the Boconó-San Sebastián-El Pilar fault system transla-tes into an associated seismicity of modera-te frequency and magnitude and is conside-red to be responsible for most large (Ms >7) historical earthquakes that have struckthe country since the first ever reportedevent in 1530. Fault rupture is concentratedin some particular segments of this plateboundary (e.g., Rod 1956, Cluff andHansen 1969, Aggarwal 1983, Audemard1997a, Pérez et al. 1997, Audemard 1999b,Audemard 2002). Some secondary mobilezones branch off from the major one. Themost important of these are: the E-W-stri-king dextral Oca-Ancón fault system (VE-01 in Fig. 2) in northwestern Venezuela andthree NW trending normal-dextral faultsystems respectively located from west to

east in eastern Falcón, along the submarinecanyon of Los Roques and in the gulf ofParia. The three latter ones constitute theprincipal areas of negative relief due to thesignificant normal component involved.Another deformation belt is the NNE-tren-ding Perijá range in westernmost Venezue-la, whose crestline defines the borderlinewith Colombia.The major Boconó-San Sebastián-El Pilarfault system displays several structural com-plexities of kilometric to several-kilometrescale, either in transtension (pull-apartbasins such as those of La González/SanJuan de Lagunillas -south of Mérida-,Cabudare, Cariaco trough and the gulf ofParia among others; refer to Fig. 2 for rela-tive location), or in transpression (Caigüirehills at Cumaná, and Las Manoas andGuarapiche -west and east of the 1997 epi-centre, respectively, in Fig. 4-; all in Sucrestate, in eastern Venezuela and related tothe El Pilar fault). In fact, this major wren-ching fault system has no major complica-tion of regional scale, except for both ends(Singer and Audemard 1997). In the west, atthe Colombia-Venezuela border, the Bo-

conó fault bends into the Pamplona inden-ter (as defined by Boinet 1985). Here, theBoconó fault sharply changes orientation toconnect with the N-S-striking Chitagá fault,which shows a strong reverse slip when joi-ning the Boconó fault, but progressivelybecomes a dominant left-lateral fault to-wards the south. Farther south, slip is trans-ferred to the NW-trending sinistral Pam-plona-Chucarima-Morro Negro fault sys-tem (CO-37 in Fig. 4). On the other extre-me in the east, in the Paria gulf andTrinidad, the major right-lateral strike-slipfault system also bends rather sharply, 45ºclockwise this time. The fault system splaysinto several NW-striking offshore faults -among which are the Los Bajos (VE-15 inFig. 4), El Soldado and Bohordal faults (inFig. 2)- that exhibit both normal and dextralcomponents. Los Bajos-El Soldado faultsystem is claimed to link the major transcu-rrent fault system to the southern tip of thetype-B Lesser Antilles subduction; hypothe-sis that is not necessarily shared by Weber etal. (2001), who postulate that slip transfertakes place along the Warm Springs fault ofthe Central Range of Trinidad from repea-

Figure 4: Map of Quaternary faults of Venezuela (after Audemard et al. 2000). Also accessible as a pdf file from the USGS web page in open filereports (ofr-00-0018). Line thickness is proportional to fault slip rate: the thickest indicates > 5 mm/a and the thinnest < 1mm/a. Shown faultshave proven Quaternary activity.


ted GPS measurements. At this eastern tip,the El Pilar fault also shows a splay in thenorth compartment that branches off to-wards ENE. This splay, named the Tuna-puy fault, exhibits a dominant reverse com-ponent with minor dextral slip. Beltrán(1998) proposes that the reverse Tunapuyfault in the Paria peninsula prolongs intothe reverse Arima fault of northern Tri-nidad that bounds the southern foothills ofthe Northern range of Trinidad (Fig. 2).Weber et al. (2001b) have demonstrated thatthe Arima fault, based on calcite and quartzgeothermometry applied to south-dippingshear bands and cataclastic zones studiedalong the southern border of the Northernrange in Trinidad, is a south-dipping range-bounding normal fault, whose activity assuch could only be constrained between 12and 1 Ma. However, the results of studiesby Saleh et al. (under review), based on geo-detic data (triangulation and GPS data mer-ging), argue for a reverse slip along theArima fault. This would seem to confirmthe Arima fault slip determined from aerial-photo interpretation based on landforms ofQuaternary activity performed by Beltrán(1998).Several second-order faults of considerablelength diverge obliquely from the majorright-lateral strike-slip fault system. Many ofthese are considered to function simply aslarge synthetic Riedel shears related to theprincipal deformation zone marking thedirect Caribbean-South America plate inter-action in north-central and northeasternVenezuela. The best known faults of thistype are: the Río Guárico (VE-09 in Fig. 4),Tacagua-El Avila (VE-10 in Fig. 4), Tácata(VE-11 in Fig. 4), Píritu (VE-12 in Fig. 4),San Mateo (Jose; VE-14 in Fig. 4), Uricaand San Francisco faults, among severalothers. In the Mérida Andes, along theBoconó fault, there is no manifestation ofsuch large Riedel shears, although there aremany branching faults. Among them deser-ve mention the north-south-striking sinis-tral Valera (VE-04 in Fig. 4) and Burbusayfaults and the subparallel dextral Caparofault (SE of San Cristobal; Fig. 2). The pre-sence of R shears, in this case in associationwith the Oca-Ancón fault system, also hap-pens in the Falcón range. Dextral slip is alsoaccommodated by sub-parallel-trending

faults to the main right-lateral strike-slipfault system. For instance, the San Simónand Caparo faults in the Andes run parallelto the Boconó fault. In north-centralVenezuela, the La Victoria fault acts as a Pshear with respect to the San Sebastiánfault. The North Coast in eastern Vene-zuela is also dextral and runs north of andparallel to the el Pilar fault.In the north-central and eastern mobilebelts, few faults exhibit left-lateral strike-slip. The Quebrada Chacaito fault trendsalmost north-south, north of Caracas, butother slightly oblique faults to the maindextral system in northeastern Venezuelaalso exhibit sinistral motion. A case in pointconstitutes the Punta Charagato fault in theCubagua island, and the Laguna Grandefault in the Araya peninsula (Giraldo andBeltrán 1989). The orientation and slip ofthese left-lateral strike-slip faults seem aty-pical with respect to their tectonic setting innortheastern Venezuela. These ENE-stri-king faults, in combination with the east-west striking dextral faults, bound wedge-shaped tectonic blocks, whose acute anglepoints to between ENE and east. Someblocks in this region show a slip vector thatslightly diverges to ENE (N084º ± 2ºEafter Pérez et al. 2001), which matchesrather well with the orientation of thebisecting line between both fault trends.This could argue for the occurrence ofblock extrusions to some extent in thiscomplex southeastern corner of the Cari-bbean in order to reduce mass excess. Thiswould give a satisfactory explanation to theapparent transtension postulated by otherauthors from GPS vectors (e.g., Weber et al.2001), when transpression is actually theoperating mechanism at plate-boundaryscale.Active thrust faults are present along mostchain fronts, although they may be blind orhidden behind triangular zones or intracu-taneous wedges. They have been detectedonshore along the northern edge of thepresently inverted Falcón basin (south ofCoro), along the foothills on both flanks ofthe Mérida Andes and along the southernfront of the Interior range both in central(e.g., Cantagallo overthrust) and eastern(e.g., Pirital and sub-parallel thrusts) Vene-zuela (Figs. 2 and 4). Other manifestations

of compression are chain uplift and build-up, and intense folding in sedimentarysequences like in the Falcón basin ("D" inFig. 2), the eastern Interior range ("Q" inFig. 2), the Andes foothills, and even in theNeogene-Quaternary foreland sequence ofthe central Interior range.Normal faults are also common and wides-pread within the deformation belts. Tuña-me in the Mérida Andes (VE-05 in Fig. 4),Los Médanos (north of Coro), Cabo SanRomán and Puerto Escondido (north tip ofthe Paraguaná peninsula in northwesternVenezuela) and the Río San Juan graben (ineastern Venezuela) faults are some exam-ples of normal faulting (refer to Fig. 2 forlocation). Except for the Tuñame fault loca-ted in the Andes (Figs. 2 and 4), all othernormal faults in northern Venezuela strikeNNW. Instead, the normal slip of the Tu-ñame fault -that roughly strikes ENE and islocated at the convergence of the Valeraand Boconó faults- is a consequence of avoid effect induced by a bookshelf rotationmechanism produced by simple shear bet-ween the Oca-Ancón and Boconó faults.Dhont et al. (2005) propose instead that theaxial extension within the central Andes,involving the Tuñame fault, responds to thenorthward escape of the Trujillo block,bounded between the north-south-strikingleft-lateral Valera fault on the west and theNE-SW-trending right-lateral Boconó faulton the east (Figs. 2 and 4).In western Venezuela, the mobile belt ismuch wider than in northern Venezuela,reaching up to 600 km in width, and com-prising the entire Maracaibo block (MTB)that covers also part of northern Colombia(Audemard, 2000; Audemard et al., 2000).This block is bounded by the sinistralBucaramanga fault (SMBF in Fig. 1) on thesouthwest and the dextral Boconó andOca-Ancón faults. Other authors considerthe northern boundary of the block to bethe flat subduction (LAS in Fig. 1), respon-sible for the Southern Caribbean Defor-mation Belt (SCDB) established from seis-mic tomographic studies by Van der Hilst(1990), rather than the Oca-Ancón faultsystem (OAF in Fig. 1). In Venezuelan terri-tory, this block contains two deformationbelts with positive relief: the Perijá rangeand the western part of the Mérida Andes,


between which the Maracaibo basin is beingsqueezed and shortened in NW-SE direc-tion. The Perijá range happens to be theleast studied area until the present-day interms of active tectonics because of pooraccessibility and personal security pro-blems.The triangular Maracaibo block (MTB) showsan intense internal fragmentation whereblocks are individualized by north-south toNE-SW trending faults. Most of thesefaults are essentially left-lateral in slip, witha minor thrust component in many cases,such as from west to east: Icotea, PuebloViejo, Valera, Burbusay, among others. Thisstructural and geometrical configurationmay result from a bookshelf rotation me-chanism induced by simple shear generatedbetween the dextral Boconó and Oca-Ancón faults (Audemard et al. 1999, 2005).But the two faults are not parallel and are atan angle of 45º, which is an atypical confi-guration that must result in particular com-plex deformations, as well as switching thesense of slip along faults through time.The spatial configuration of the faults andfolds at regional scale and their kinematics,as deduced solely from the neotectonicmapping, points out that the NorthernFalcón basin (northwestern Venezuela) isundergoing a stress tensor with a NNW-SSE to N-S maximum horizontal stress andan ENE-WSW minimum (or intermediate)horizontal stress (Audemard 1993, 1997b,2001; Audemard et al. 2005). This wouldmean that the simple shear model associa-ted with strike-slip faulting proposed byWilcox et al. (1973) applies. Regional foldingin this region does not only result fromwrenching, as proposed by many authors(e.g.: Stephan 1982), but also from regionalcompression. Both folding due to wren-ching in close association with the Oca-Ancón fault system (transpression s.s.) andto regional compression occur together(transpression s.l.). The neotectonic map-ping in this region also permits the determi-nation of active brittle structures in five dis-tinguishable families based on their orienta-tion and kinematics (Audemard, 1993,Audemard and Singer 1996, Audemard1997b, 2001; Figs. 2 and 4): (1) East-Westright-lateral faults (Oca-Ancón Fault Sys-tem -VE-01-, Adícora Fault); (2) NW-SE

right-lateral faults, as synthetic Riedel shearsto the east-west faults (Urumaco -VE-02-,Río Seco -VE-03-, and La Soledad faults);(3) NNW normal faults (Western Para-guaná, Cabo San Román, Puerto Escon-dido and Los Médanos faults, all fault-bounding the Paraguaná peninsula); (4)North to NNE left-lateral faults, antitheticto the east-west faults (Carrizal, El Hatilloand other minor faults); and (5) ENE rever-se faults, parallel to fold axis of similarorientation (Araurima, Taima-Taima/Chu-chure and Matapalo faults). In terms of sliprate, most Quaternary tectonic features inNorthern Falcón are rather slow, showingslip rates generally below 0.4 mm/a. Themajor Oca-Ancón fault system is the excep-tion to this, with a maximum slip rate closeto 2 mm/a (Audemard 1993, Audemardand Singer 1994, Audemard et al. 1994,Soulas and Giraldo 1994, Audemard 1996,Audemard and Singer 1996, Audemard et al.2000).The Coast and Interior ranges of north-central and eastern Venezuela is the onlyportion of the plate boundary zone toresult from rather simple direct interactionbetween the Caribbean and South Americaplates. The same applies to northwesternVenezuela as registered in the Falcón basinand applicable at larger scale to the Bonaireblock confined between the Leeward An-tilles subduction and the Oca-Ancón faultsystem. Therefore, this region also complieswith the simple shear model, at least as faras the brittle tectonics is concerned, withstrain partitioning also occurring along thisplate boundary segment. Within the Me-sozoic metamorphic SSE-overthrusted na-ppes of central Venezuela, six families ofactive tectonic features are distinguishableon the basis of their orientation and pre-sent kinematics (Figs. 2 and 4): (1) East-West right-lateral faults (San Sebastián, LaTortuga and El Avila faults); (2) NW right-lateral faults, synthetic to the east-westfaults (Río Guárico -VE-09-, Tácata -VE-11- and Aragüita faults; (3) NW to NNWnormal-dextral to dextral-normal faults(Píritu -VE-12- and Tacagua -VE-10-faults); (4) North to NNE left-lateral faults,antithetic to the main east-west faults(Quebrada Chacaito faults); (5) ENE rever-se faults, parallel to fold axis (Nappe fronts,

among which the Cantagallo overthrust);and (6) ENE to E dextral faults -P shears-(La Victoria fault -VE-08-). This is actuallythe only case of active P faulting recogni-zed in the plate boundary zone.For eastern Venezuela, the configuration ofthe active structures is as follows (Figs. 2and 4): (1) East-West right-lateral faults (ElPilar -VE-13-, El Yaque and North Coastfaults); (2) NW right-lateral faults, syntheticto the main east-west faults (San Mateo -VE-14-, Urica and San Francisco faults); (3)NW to NNW normal-dextral to dextral-normal faults (San Juan Graben, Bohordal,Los Bajos and El Soldado -VE-15- faults);(4) ENE left-lateral faults, which do notobey the simple shear model (LagunaGrande and Punta Charagato faults; issuediscussed earlier in this paper); and (5)ENE reverse faults, parallel to fold axisorientation (frontal thrust of the Interiorrange, among which the Pirital thruststands out).All present deformations along northernVenezuela are related to the WNW orientedoblique convergence vector of the Cari-bbean plate with respect to South America(e.g.: N 75º W, after Minster and Jordan1978; later confirmed by many others fromGPS data, as indicated earlier). Meanwhile,western Venezuela presents a more com-plex scenario that shall be discussed later inthis paper that reflects the influence of theconvergence vector between the Nazca andSouth America plates to the southwest inColombian territory.

QUATERNARY STRESS TENSORSOF VENEZUELA

Audemard et al. (2005) have recently publis-hed a nationwide stress tensor compilationderived from fault-plane kinematic indica-tors. In order to ascertain the characteriza-tion of the latest and still active tectonicphase, this compilation has only integratedstress tensors derived from Plio-Quaternarysedimentary rocks (refer to Table 1 inAudemard et al. 2005). This approach hasrelied on an inversion method, throughwhich the stress tensor has been obtainedfrom the measured strain. Those tensorswere calculated with either the Etchecopar


et al. (1981)'s method or the right dihedralmethod designed by Angelier and Mechler(1977). This latter method is less accuratethan that of Etchecopar et al. (1981), whichis also capable of establishing the qualitati-ve shape of the stress tensor ellipsoidthrough the value of "R", being R definedas (σ2 - σ3)/(σ1- σ3). The principal stress invertical position indicates the dominant tec-tonic regime but R helps to better define it(refer to Ritz 1991, for more details). Fromthis tectonic stress inversion, Audemard etal. (2005) reach the following general con-clusions:(1)- northern Venezuela -covering theFalcón basin and the central and easternCoast and Interior ranges from west toeast- is characterized by a Plio-Quaternarystress tensor of rather uniform and cons-tant orientation throughout. The prevailingorientations of this tensor are NW toNNW (145º to 170º) for the maximumhorizontal stress (σH) and NE to ENE forthe minimum horizontal stress (σh). The-refore, there is a good consistency amongstress tensors at regional scale. This geolo-gically derived tensor mostly represents atranscurrent regime (intermediate stress invertical position). Where the Etchecopar etal.'s method was applied, as in the case ofthe Falcón region, the stress regime can beconstrained better and is of the compressi-ve transcurrent type (Audemard 1991,1993, 1997c and 2001). Some authors des-cribe this tectonic regime as transpressiveand refer to the enlarged sense of the termand do not restrict its use only to the loca-lized stress changes introduced by strike-slip motion in proximity of or along thewrench fault. This stress tensor is highlyoblique to the general east-west trend ofthe major wrench faults of northern Ve-nezuela (Oca-Ancón, San Sebastián and ElPilar faults). This strong obliquity is respon-sible, on the one hand for the occurrence ofstrain partitioning (right-lateral strike-slipalong east-west trending faults and trans-verse shortening in NNW-SSE direction).On the other hand, it is also responsible forsimultaneous left-lateral strike-slip motionon faults that are slightly oblique to theeast-west trending dextral faults, as is thecase of the WSW-ENE-striking Punta Cha-ragato and Laguna Grande faults in eastern

Venezuela (Fig. 2). The consistency of thestress-tensor orientation along northernVenezuela from west to east (from north-western Colombia to Trinidad) permits theidentification of local variations of thestress regime. Most of these local stresschanges coincide with known transtensio-nal geometries, or occasionally with trans-pressional geometries (refer to Audemard etal. 2005 for further details). These changesare easily identifiable because the stress ten-sor orientation remains essentially constantbut their principal stress magnitudes vary(stress tensor permutation, where maxi-mum and intermediate stresses may inter-change positions).(2)- The stress tensor calculated by eitherthe Etchecopar et al. (1981)'s automatedmethod or the right dihedral method ofAngelier and Mechler (1977) well repre-sents the stress field for the present-daykinematics of seven major families of acti-ve faults along northern Venezuela: (a) east-west right-lateral faults; (b) NW right-lateralfaults, synthetic to the east-west faults; (c)NNW normal faults; (d) NW to NNW nor-mal-dextral to dextral-normal faults; (e)North-South to NNE-SSW left-lateralfaults, antithetic to the east-west faults; (f)ENE to east-west right-lateral faults -P she-ars-; and (g) ENE reverse faults, parallelingthe axis of active folds. Spatial configura-tion of these brittle tectonic structures indi-cates that the region is undergoing a trans-pressional s.l. (compressive-transcurrent)regime that complies with the simple shearmodel proposed by Wilcox et al. (1973).Therefore, this regional configuration isrelated to the slightly oblique convergencebetween the Caribbean and South Americaplate in the west and almost perfect wren-ching in eastern Venezuela with some"apparent" transtension (Pérez et al. 2001and Weber et al. 2001), that is directly res-ponsible for east-west trending dextralwrenching along northern Venezuela. Onthe other hand, in northwestern Venezuelathere is convergence between the Bonaireblock (BB) and the Caribbean plate in aprocess of flat subduction located offshoreof the Leeward Antilles islands (LAS),which operates in concurrence with theNNE directed extrusion of the Maracaiboand Bonaire blocks. Overall there is good

agreement between stress tensors obtainedfrom microtectonic data and large scaleneotectonic mapping.(3)- σH in the northern Mérida Andes (Larastate), in proximity to the Boconó fault,tends to become east-west oriented, whichallows the simultaneous functioning of theNE-striking dextral (e.g.: Boconó, Caparo,San Simón) faults, the equally-trendingthrust faults along both Mérida Andes foo-thills and the north-south striking sinistralfaults (e.g., Valera and Burbusay, amongseveral others; Fig. 2).(4)- As a result of this, the stress field onthe Maracaibo block (MTB) progressivelyturns counterclockwise from a NNW trendin the north (Beltrán and Giraldo 1989,Audemard 2001, Audemard et al. 2005) toeast-west oriented to the south (Audemardet al. 1999), making it resemble a folding fanwith vertex pointing to the SE (Giraldo1990, Audemard and Audemard 2002). Theorientation of this regional stress field inwestern Venezuela results from the super-position of the two major neighbouringinterplate maximum horizontal stress orien-tations ( σH ): (1) a roughly east-west tren-ding stress across the Nazca-South Americatype-B subduction along the pacific coastof Colombia and (2) a NNW-SSE orientedstress across the southern Caribbean plateboundary (Audemard 2000, Fig. 1). As aresult, the Maracaibo block is simultane-ously being shortened along the NW direc-tion (expressed by the vertical growth ofthe Santa Marta block and Perijá andMérida ranges) and extruded to the NNE(Audemard 1993, 1998, 2000b, Audemardand Audemard 2002).

DISCUSSION AND CON-CLUSIONS

The stress tensors derived from microtecto-nic data collected in Plio-Quaternary rocksand those derived from the spatial configu-ration of Quaternary active faults are ingood agreement. On the one hand, defor-mation along the southern Caribbean coastresults from a compressive strike-slip(transpressional s.l.) regime characterizedby a NNW-SSE maximum horizontal stress(σH=σ1) and/or an ENE-WSW minimum


(σh= σ3 or σ2) horizontal stress. This con-figuration gives rise to the present activityand kinematics of seven sets of brittle andductile features: (1) east-west right-lateralfaults, (2) NW right-lateral faults or synthe-tic Riedel shears, (3) ENE to east-west dex-tral faults or P shears, (4) NNW normalfaults, (5) almost north-south left-lateralfaults or antithetic Riedel shears , (6) ENEreverse faults -sub-parallel to fold axes andmostly in the subsurface- and (7) associatedENE trending folds (well-developed in theFalcón basin -northwestern Venezuela- andthe Interior range in the east). The mainexceptions to this general configuration arefound in eastern Venezuela: the PuntaCharagato and Laguna Grande faults thatdisplay left-lateral slip along the ENE direc-tion. This "apparent" inconsistency may beexplained by using a ENE-directed blockescape model. In most of northernVenezuela, brittle deformation obeys thesimple shear model.On the other hand, the stress field on theMaracaibo block progressively turns coun-ter-clockwise to become more east-westoriented, allowing left- and right-lateral slipalong north-south and NE-SW strikingfaults, respectively. This regional stress fieldin western Venezuela results from thesuperposition of the two major neighbou-ring interplate maximum horizontal stresses(σH) related to the Nazca plate type-B sud-uction and NNW-SSE oriented stressacross the southern Caribbean plate boun-dary. This results in the Maracaibo blocksimultaneously being shortened on the NWdirection and extruded roughly towards theNNE.Wrenching is the dominant geodynamicprocess, but it is always accompanied bycompression of variable magnitude alongstrike of the major strike-slip fault system,proving that stress (or strain) partitioning isthe actuating mechanism. Only locally,transtension becomes significantly impor-tant, such as in the Cariaco trough and gulfof Paria in association with the El Pilarfault. It can now be firmly established thatthe GPS slip vectors do not only supportthe hypothesis of ongoing transpressionalong most of this complex plate boundaryzone, but also tend to confirm most of theactive fault slip rates derived from geologic

and neotectonic studies carried out inVenezuela for almost 25 years by the Fun-visis staff.

ACKNOWLEDGEMENTS

This long-term research would have notbeen possible without the continuous fun-ding from Funvisis that allowed most ofthe dataset collection and generation.Perseverance of the personnel of the EarthSciences Department of FUNVISISthroughout the years made possible thisachievement. The Venezuelan Oil Industry(PDVSA and former affiliates) is also gre-atly acknowledged for providing additionalfunding. Marina Peña is thanked for herdrawings. This paper is partly a contribu-tion to FONACIT projects G-2002000478and PI-2003000090, although funding wasprovided by other sources.

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Recibido: 30 de junio, 2006Aceptado: 15 de noviembre, 2006

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