Northern South America basement tectonics and implications for paleocontinental reconstructions of the Americas
Introduction
Pre-Jurassic paleocontinental reconstructions are largely built from circumstantial geological evidence given the absence of contiguous oceanic crust between intervening continental fragments. In the words of the late Peter Coney, it is akin to reconstructing an automobile accident with only a headlamp for evidence. It becomes even more difficult to gather such evidence from rocks that have undergone strong and/or recent orogenic overprints as in the case of the Andes. A particular reconstruction of interest here is that of Hoffman (1991) depicting a proto-Andean orogen fringing the Amazonian craton. This reconstruction has stimulated a long-standing geological debate regarding the interactions of the margins of Amazonia and Laurentia from late Proterozoic to early Paleozoic time (Bond et al., 1984, Kent and Van der Voo, 1990, Hoffman, 1991, Keppie et al., 1991, Keppie, 1993, Dalla Salda et al., 1992a, Dalla Salda et al., 1992b, Park, 1992; Dalziel et al., 1994 and others). Recently published U–Pb geochronologic data from basement rocks in the central Andes (Chew et al., 2007), confirms the existence of a proto-Andean orogenic system in Peru and Ecuador and its consolidation during the Grenvillian–Sunsas (∼1.0 Ga) and Famatinian (∼0.47 Ga) orogenic cycles. Furthermore, this data indicates that the rocks involved in this proto-Andean belt were largely authochtonous i.e. pericratonic sequences as suggested by Hoffman (1991). We present geological, geochronological and paleontological data that supports the existence of such proto-Andean orogen into the Northern Andes (Colombia–Venezuela) and its extension into southern Mexico, as initially suggested by Yañez et al. (1991), Restrepo-Pace et al. (1994) and Ruiz et al. (1999).
Section snippets
Basement of Colombia–Venezuela
Differential uplift and denudation resulting from Andean (Meso-Cenozoic) tectonics has left sparse basement exposures in the northern Andes. Basement rocks in northern South America crop out in the eastern Orinoquian foreland basin and in the western Andean domain (Fig. 1). The term basement here understood as the assemblage of rocks which make up the craton or metamorphic units underlying Paleozoic sediments in the Andean realm (Etayo-Serna and Barrero, 1983, Cediel and Caceres, 1988, Cediel
Extent of the proto-Andean orogen
Despite the multitude of geochronological data recently acquired (Restrepo-Pace, 1995, Restrepo-Pace et al., 1997, Ruiz et al., 1999, Cordani et al., 2005, Jimenez-Mejia et al., 2006, Ordonez-Cardona et al., 2006, Cardona-Molina et al., 2006, Chew et al., 2007), many details of the of late Proterozoic–early Paleozoic paleocontinental reconstructions of northern South America remain elusive. Geochronological data together with faunal affinities from sequences covering the basement massifs,
Constraints on the relative position of NW South America from Paleozoic faunal assemblages
The controls exerted by the paleo-environment on faunal provinciality or cosmopolitanism of a given species or assemblage is still a matter of debate (Fortey and Cocks, 2003). Nonetheless, the relative paleo-positions of continental fragments derived primarily from paleomagnetic studies can be refined by comparing correlative fossil assemblages. In the case of northern South America in Paleozoic time, the first order conclusion is that early Cambro-Ordovician fauna is dominantly Gondwanan and
Paleogeographic implications
A variety of paleogeographic models have suggested a close link between the Appalachian orogen and the proto-Andes. Some models depict opposing orogens separated by active subduction and/or shear boundaries throughout Late Proterozoic–Paleozoic times (e.g. Bond et al., 1984, Van der Voo, 1988, Kent and Van der Voo, 1990, Hoffman, 1991 and others). Other researchers have taken these models further to suggest that transfer of continental terranes from either side had occurred (e.g. Dalla Salda et
Acknowledgements
The authors would like to thank Dr. Victor Ramos and an anonymous reviewer for their detailed and constructive comments and suggestions done on an early version of this manuscript.
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