Lithospheric structure of northwestern Venezuela: implications for the understanding of continental margins evolution

Abstract

Western Venezuela is located in a complex deformation zone due to the Caribbean-South American plates interactions. Several models regarding the shallow and deep structure have been proposed, nevertheless, most of these studies are based on basin-scale observations. As a part of the GIAME project, onland refraction profiles were acquired across the Guyana Shield, the main orogenic belts, and the Caribbean allochtonous terranes, with a total length of ~2500 km of 2D seismic data. In this work, an analysis of a 560 km long profile is presented. The Northern Andes profile consists of 545 short-deployment seismometers, which recorded 13 land shots. The seismic data was used to generate a two-dimensional P-wave model showing velocities between 3.2-5.7 km/s for sedimentary covers; 6.1-6.5 km/s for upper crust, and 6.7 to 7.5, and 7.8 to 8.9 km/s for lower crust and upper mantle, respectively. The most prominent features imaged were a crustal thinning beneath the FalcÛn basin, interpreted as a back-arc basin. In addition, lateral variability suggests collisional process related to suture zones between both Proterozoic and Paleozoic, and Paleozoic and Mesozoic provinces. Finally, the Caribbean slab was observed in a low-angle subduction context beneath NW South America. Introduction The Caribbean-South American plate interaction has resulted in a wide and complex deformation zone in northwestern South America. During the last 60 years, several models regarding the shallow and deep structure of this region have been proposed. Nevertheless, most of these studies are based on basin-scale observations, and they lack crustal observations in order to support the deep structure interpretation. While the plate-scale architecture of this boundary has been imaged by regional seismology studies (van der Hilst and Mann, 1994; Bezada et al., 2010), a coherent relation between the upper mantle features and the crust has been undermined by the insufficient information. Main surface structures of NW South America have been recognized by fieldwork campaigns and seismic interpretation, but the deepest structures have not been imaged from any active-source seismic experiment. Therefore, all the interpretations proposed to describe these structures might be gathered in two major conceptual models, depending on whether the main orogenic belts in this area are symmetric or asymmetric. If we consider that the main orogenic-belts are symmetric, they might have formed as the result of low- angle subduction (Duerto et al., 2006). In contrast, if the orogenic-belts are asymmetrical, then they could have formed in response to continental subduction to either SE (Kellogg and Bonini, 1982) or NW (ChacÌn et al., 2005) directions. There are also models which involve the orogenic float concept in a transpressional framework (Audemard and Audemard, 2002). As a part of the Integrated Geosciences of the MÈrida Andes project (GIAME) executed by the Venezuelan Foundation for Seismological Research (FUNVISIS), onland deep penetration wide-angle refraction profiles were acquired in 2014 across the main strike of the major orogenic-belts (Figure 1).

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018