Abstract

Introduction

The Mérida Andes (Venezuela) formed in the middle Miocene due to oblique convergence between the South American plate and the Maracaibo block [Audemard et al., 2002] (figs. 1A and 1B). The study area corresponds to the so-called “Barbacoas platform” [Renz, 1960], which constitutes the northeastern termination of the belt, NE of Valera (fig. 2). It is located in the northeastern part of Trujillo block [Hervouët et al., 2001], considered as an independent block separated from the main Maracaibo block along the Valera fault.

According to Stéphan [1982], the N170°E-trending Caribbean compression developed in this area from late Cretaceous to Eocene. It was followed by a N105°E-trending compression older than middle Miocene, and finally by the NW-SE Andean compressional stage that lasted till now in most of the chain. However, east of El Empedrado fault, a NNE-SSW compression presently occurs that is oblique to the classical Andean stage.

The tectonic evolution of the Andean stage is not well understood. The Mérida Andes are mainly composed of Precambrian and Paleozoic rocks. The northern part of the belt only comprises a complete and continuous Jurassic to Paleogene cover. This lithologic pattern is probably a consequence of the tectonic escape of the Maracaibo block, and more particularly of the smaller Trujillo block. The Mérida and Carribean belts being close to each other, the influence of the Andean deformation on the Caribbean allochthonous must be taken into account.

In order to make a structural analysis at regional scale, we privileged the use of remote sensing data (Landsat, Spot and Radar images) and aerial photographs. This was complemented by structural data obtained in the field, allowing the study of geometric and chronological relationships between the tectonic structures.

Lithostratigraphy of the northeastern Andes

Andean formations

The first Mesozoic deposits rest unconformably upon the marine Permian (Palmarito Fm.) [Gonzalez de Juana et al., 1980]. During the Jurassic, continental deposits of La Quinta Fm. were trapped in NE-SW grabens [Gonzalez de Juana et al., 1980] that opened due to rifting of the northern margin of the South-American plate. The first marine sediments are composed of thick sandstone layers at the bottom and limestone at the top, Barremian-Albian in age (Peñas Altas Fm). It is followed by Cenomanian-Campanian (La Luna Fm.) composed of euxinic black limestone and clay. Cretaceous ends with the Maastrichtian (Colon Fm.) composed of clay and limestone lenses with the intercalation of a white sandstone layer (Cujisal Member) [Renz, 1959; Pierce, 1960; Gonzalez de Juana et al., 1980].

Paleogene layers correspond to low depth deposits, such as the Paleocene-Eocene (Humocaro Fm.), and the upper Paleocene-early Eocene (Quebrada Arriba Fm).

The Caribbean allochthonous

The Caribbean series outcrop in the El Tocuyo area (fig. 3). The Barquisimeto Fm. (late Cretaceous) is composed of clay, clayey schist, marl, dark gray and black limestone, and phtanite [Bellizia, 1985; Stéphan, 1982; Lexico Estratigra-fico de Venezuela, 1997]. The Matatere Fm. (late Cretaceous-Paleocene) is composed of sandy turbidites involving sandy conglomerates.

Tectonic structures of the northeastern Andes and adjacent plains

The study area (figs. 3, 4) forms reliefs that can reach 3000 m. It is westerly and easterly surrounded by flat lying plains (La Pastora plain to the west ; El Tocuyo “synclinorium” [Stéphan, 1982] to the east) with elevations less than 500 m. To the north and east, it is overlain by the Lara overthrusts belonging to the Caribbean orogen. To the south, the Andean reliefs are mainly composed of Precambrian and Paleozoic rocks. This Barbacoas platform is westerly and easterly bounded by the San Pedro and Humocaro anticlines respectively, separated by the “Barbacoas synclinorium” [Renz, 1960] forming a flat-lying area.

The Humocaro anticline: inversion of a Jurassic graben

The 9 km width and 16 km length NE-trending Humocaro Bajo anticline is composed of a complete Meso-Cenozoic sedimentary sequence. It is overturned toward the SE (fig. 4). The Jurassic layers outcrop in the San Pedro and Humocaro Bajo anticlines whereas they are missing in the Barbacoas synclinorium, showing that the area was organized in horsts (Barbacoas) and grabens during the Jurassic. The western boundary of the Jurassic layers is not defined (El Empedrado fault?) compared to the eastern border where the layers overstep the Humocaro area. We interpret that the Humocaro anticline formed during inversion of a preexisting graben in the Andean stage. This led to formation of a wide anticline overturned outwards of the eastern graben. This is not the case of the San Pedro anticline overturned inwards of the western graben.

The Barbacoas synclinorium: formation of flower structures

Both the morphology and trend of the folds in the Barbacoas synclinorium (fig. 4) differ from those of the Humocaro anticline. Folds, trending ~N030°E, are narrow and well displayed on the remote sensing imagery. There are bounded by N030°E to N045°E trending left-lateral strike-slip faults juxtaposing contiguous anticlines. Numerous tectonic structures of various sizes affect these folds. In the field, we observed tension fractures striking N050°–060°E, N110°E and N150°E, an high-grade N030°–040°E deformation nearby strike-slip faults, fractures in various directions, and a field of flattened ammonites stretched in the N010°– to N030°E direction associated with a N010°E-striking cleavage.

A N-S to NNE-SSW compression is well individualized (fig. 5, VE01–14, VE01–26, Ve02–30). It is responsible for left-lateral strike-slip motions along N030–045°E directed faults. Narrow anticlines, parallel to these faults, can be interpreted as flower structures. Nearby these tectonic irregularities, the compression turns from N-S to N110°E.

The western area : associate pull-apart and buckling

The western plain can be considered as a pull-apart basin that opened locally at the step-over of a NE-SW left-lateral fault (figs. 3, 5, VE03–12). The San Pedro anticline is parallel to and has the same length of the La Pastora plain. Since it is overturned toward the Jurassic graben (fig. 4), it cannot be related to a positive tectonic inversion. We interpret the San Pedro anticline as an extensional forced fold [Cosgrove and Ameen, 2000; Maurin and Nivière, 2000] because of (1) its position relative to the La Pastora graben; (2) the lack of internal flank; and (3) the 1500 m lowering between San Pedro and La Pastora. This deformation is associated to the NNE-SSW compressional stage (fig. 5; Ve03–12), which appears also in other areas of the northeastern Andes (figs. 5, 9).

The allochthonous deformation

West of El Tocuyo (figs. 3, 6), the Caribbean allochthonous, Upper Cretaceous in age (Barquisimeto Fm.), associated to the Paleocene-Eocene Morán Fm. is juxtaposed to the Andean autochthonous along vertical or sub-horizontal faults (fig. 6, 7). The Andean series are composed of the Peñas Altas and La Luna Fms. The Morán Fm. is highly folded comparatively to the underlying Cretaceous deposits. Moreover, since the intermediate deposits (Colon, Humocaro and Quebrada Arriba Fms.) do not outcrop, we interpret that the Morán Fm. is allochthonous in this area. To the south, in the Humocaro area, the Morán Fm. is slightly deformed and rests conformably upon the Quebrada Arriba Fm., showing that it is autochthonous in this southern area.

In the El Tocuyo area, the Barquisimeto Fm. is represented as olistolites involved in the Morán Fm. (fig. 7B). In the Barquisimeto Fm., we observed N070°E- to N140°E-trending folds (fig. 8) with sub-horizontal axes that formed during the emplacement of the Caribbean overthrusts. These folds were reactivated during the Andean stage into recumbent folds trending N175°E to N050°E. In both the Barquisimeto and Morán Fms., we observed N000°– to N035°E-trending folds that can be sorted into two units corresponding (1) to sub-horizontal axes (plunges from 5°to 30°to the north) associated to a fracture or crenulation cleavage; and (2) to upright (45°to 64°to the north) axes. High-grade deformation develops at the bottom of the sedimentary units, showing that folding is related to ESE-ward displacement of these two formations. The upright fold axes, only observed on the borders of the El Tocuyo plain, characterize strike-slip motion of sub-meridian faults bounding this basin.

Flat lying plains develop east and west of the northeastern Andes (figs. 3, 6, 7) and constitute an abrupt change in the topography. Our analysis of satellite imagery complemented by field observations show that the El Tocuyo plain (fig. 6) may be interpreted as a pull-apart basin that initiated along a left-lateral fault relay trending N000°E. The synclinal-like morphology of the basin results from the progressive breaking of the western fault, that locally buckles before vanishing in the south (fig. 7). This geometry greatly resembles to that of extensional forced folds initiating at the borders of grabens. Here, this deformation is associated to the NW-SE compressionnal Andean stage (fig. 9, VE01–49, VE02–54).

Conclusions

The northeastern part of the Mérida Andes recorded several tectonic stages that can be described as follows:

  • – (1) the Jurassic rifting, corresponding to the formation of NE-trending grabens filled by La Quinta Fm.;

  • – (2) the Caribbean orogen (upper Cretaceous-early Oligocene) associated with the emplacement of Caribbean overthrusts. The front of some units (Barquisimeto Fm.) can be partly involved into the Paleocene-Eocene Morán Fm. This stage is characterized by N070°E- to N150°E-trending folds;

  • – (3) an ESE-WNW compression older than the middle Miocene, that may correspond to the N105°E-trending stage of Stéphan [1982], and related to ESE-ward displacement of the Morán Fm. and the Caribbean units;

  • – (4) the NW- to NNW-directed Andean compression lasting from middle Miocene onwards. It is responsible for the formation of NE-SW folds, sometimes related to reactivation of the Jurassic grabens (Humocaro anticline);

  • – (5) a NNE-SSW compression, younger than the Andean compression, and responsible for N-S right-lateral strike-slip faulting and NE-SW left-lateral strike-slip faulting. Relays along strike-slip faults locally created transtensional or transpressional areas, which localized flower structures (Barbacoas area for instance) and pull-apart basins (La Pastora) respectively. Important lowering along the pull-apart basins created the buckling of the sedimentary cover responsible for the formation of extensional forced folds. Near NE-SW strike-slip faults, the stress turns anti-clockwise to strike N100°E.

North of the Boconó fault and east of the Valera fault, the Trujillo block [Hervouët et al., 2001] is composed of several compartments separated by sub-meridian left-lateral strike-slip faults. Our analysis of satellite imagery, structural observations in the field and slip vectors derived from focal mechanisms of earthquakes [Dhont et al., 2002; 2004] show that the Trujillo block tectonically escapes towards the north or north-east. However, between El Empedrado fault and Bocono fault, in front of the Carribean overthrusts, numerous focal mechanisms of earthquakes and structural observations in the field indicate that the Andean compressional stage is relieved by a NNE-SSW compression. North of the Trujillo block, the Caribbean overthrusts act as a barrier for the escape of this triangular block towards the north-east (fig. 1C).

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