Interaction entre un chevauchement imbrique et une zone transcurrente; le flanc nord-ouest de Andes venezueliennes

Abstract

Abstract Geological framework; Geological setting: The Venezuela Andes or Merida Andes (fig. 1) extend from the Colombian border in the SW to Barquisimeto in the NE, and constitute a basement uplift exceeding 5,000 m near Merida (Pico Bolivar). This young chain is bordered to the W by the Maracaibo foredeep basin, and to the E by the Barinas-Apure foreland basin. The Bocono fault divides the Andean Belt in two parts along a NE-SW direction. This shows that the uplift of the Andes is contemporaneous with an oblique translation. In the study area, located on the northwestern flank near Maracaibo basin, three major structures are present: in the E, the N-S senestral strike slip Valera-Rio Momboy fault, in the S the E-W dextral strike slip Pinango fault and, in the center, the SW-NE striking Las Virtudes thrust verging toward NW. Lithologic and stratigraphic formations (fig. 4): The Las Virtudes Fault separates two different structural zones. In the SE, overthrust units are made of crystalline basement, Paleozoic substratum and preorogenic sedimentary formations (Cretaceous-Eocene). The foredeep flexural basin, located NW, is filled by synorogenic molasses (Neogene and Quaternary), largely developed within the Betijoque Fm. (Upper Miocene to Pliocene in age) which reaches a thickness of 5000 m. Structure of the northwestern Andean flank; Las Virtudes Fault and its thrust slice zone: Near Las Virtudes village (fig. 5, 6-2), this thrust is systematically associated with a narrow overturned foredeep depobelt (Cretaceous to Neogene in age). These slices are unknown elsewhere in the Andean Chain and represent the terminal faulted part of the thrust drag. However, where this slice zone is missing (central and northeastern part of the study area), the Las Virtudes Fault is not clearly documented: its throw decreases rapidly and it is possible that the fault disappears northeastward. Andean unit: Near the main strike slip faults, NE trending SE verging reverse faults develop (fig. 6-5). In central and northeastern parts, the throw of the reverse faults increases toward the Valera Fault. It seems that reverse faults are horsetail of this major strike slip fault (fig. 5). Internal part of the northwestern Andean foredeep basin: The foredeep sedimentary formations generally dip toward the NW. Associated to the lack of some formations, tilted anticlines toward the SE are observed (fig. 6-3 and 6-7), and indicate the vicinity of decollement levels in the foredeep, located in Luna-Colon, Pauji and Betijoque Fm.. Seismic profiles show (fig. 7) that the major decollement level of the foredeep is located in La Luna and Colon Fms. [Audemard, 1991; De Toni and Kellogg, 1993; Colletta et al., 1997]. Crustal architecture and timing of the deformation: Several stages can be distinguished in the building of the Andes. Development of an intracutaneous thrust wedge: The first effects of the Andean phase during Miocene are the development of an intracutaneous thrust wedge [Price, 1986]. The lower flat is located in the basement and the upper one in Cretaceous formations. The transport direction is NW. The foredeep develops on the forelimb of this structure and collects detrital products coming from erosion of the first (oldest) reliefs. Decollements in the foredeep basin could be contemporaneous with this major overthrust. Their origin could be due to radius of curvature differences within the thick sedimentary formations (fig. 8). Las Virtudes Fault and backthrusting: Las Virtudes Fault is one of the last events of this part of the Andean Belt. During Plio-Pleistocene, the continental crust breaks with a dip of 35 degrees SE. The Andean unit overthrusts the foredeep basin. Some of the foredeep decollements could still be active and form, together with Andean basement, a triangle zone. Las Virtudes Fault throw reaches 5 km between Las Virtudes and Monte Carmelo villages (fig. 8A). It decreases southwestwards and the back thrusts are probably younger. Northeastwards the throw decreases and eventually disappears (fig. 8B). In the same time the back thrust throws increase. Both seem to be contemporaneous. Conclusions: This structural model explains the basement occurrence in front of the Las Virtudes Fault on seismic profiles and allows to restore correctly the different northwestern flank structures of the Venezuela Andes. These structures can be explained by the conjugate movements of a NW verging intracutaneous thrust wedge and strike slip faults which create a SE verging triangular area (fig. 5). The Andean overthrust is transferred in the Falcon zone along the Valera fault. In the northeastern part of the Maracaibo block, the Valera and Bocono strike slip faults limit the Trujillo block (fig. 10) which moves towards the North during Neogene and Quaternary times.