Magnetotelluric imaging of the Merida Andes and surrounding areas in Venezuela

The Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100-km-wide Merida Andes extend from the ColombianNenezuelan border to the Caribbean coast. To the north and south, the Merida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Merida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broad-band tnagnctotelluric profile crossing the Merida Andes and the Maracaibo and Barinas-Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with 1-D or 2-D subsurface structures for the sedimentary basins but also indicate a strong 3-D setting for the Merida Andes. Using a combination of 2-D and 3-D modelling we systematically examined the influence of 3-D structures on 2-D inversions. Synthetic data sets derived from 3-D model ling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2-D inversion models show electrically conductive basins with depths of 2-5 km for the Barinas-Apure and 2 71an for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Bocone and Valera Faults, cross-cut the Merida Andes in NE-SW direction along its strike on a length 400 km and N-S direction at its centre on a length 60 km, respectively. Both faults arc associated with subvcrtical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2-D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the 'floating orogen hypothesis' developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.