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Bernard Monod, University of Pau (France)
Damien Dhont, University of Pau (France)
Guillaume Backe, University of Adelaide (Australia)
Yves Hervouet, University of Pau (France)
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The Mérida Andes in Venezuela are an intracontinental orogen laterally cut by the active NEtrending right-lateral strike-slip Bocono fault. Uplift of this mountain range started in the Miocene as a consequence of the relative oblique convergence between two continental blocks namely the Maracaibo block to the northwest and the Guyana shield to the southeast. Deformation in the Mérida Andes is partitioned between dextral shearing along the Boconó fault, left-lateral strike-slip along the N-S Valera fault and NW-SE shortening perpendicular to the belt. The surficial trace of the Boconó fault is marked by major geomorphic features: it is shifted southward relative to the chain axis and it does not have a continuous and linear trace but is composed of several fault segments of different orientations striking from N35°E to N65°E. The onset of strike-slip motion, the horizontal fault offset and its geometry at depth remains a matter of debate. Our work, based on morphostructural analyses of remote sensing images complemented by field observations, provides new data on both the geometry and the tectonic evolution of this major structure. We build a 3D geological model and argue that the Boconó fault only affects the upper crust and connects to a decollement level probably located at the brittle-ductile transition in the crust. The Boconó fault therefore cannot be considered as a lithospheric or plate boundary but forms the southern crustal boundary of the triangular Trujillo block limited to the west by the Valera. Both faults allow the Trujillo block to escape towards the NE, which is in agreement with the nature of the focal mechanisms showing a regional partitioning of deformation into strike-slip and extension within the Trujillo block and compression along the foothills. Microtectonic measurements collected along the central part of the Boconó fault are characterized by polyphased tectonics, and also support the escape process. Dextral shearing along the fault is superimposed to reverse oblique-slip motion, showing that initiation of transcurrent movement is more likely to have occurred after a certain amount of shortening. The present day strain partitioning along the Mérida Andes seems to be younger than the rise of the chain and coeval with the initiation of right-lateral shearing along the Boconó fault, which would have therefore problably initiated during the Pliocene. The Mérida Andes can be considered as a case study for escape tectonics of crustal units along a major strike-slip fault.
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