PRELIMINARY PALEOMAGNETIC RESULTS FROM LOWER MESOZOIC SEDIMENTARY ROCKS EXPOSED IN THE NORTHERN ANDES, COLOMBIA

The NW margin of South America is the result of the interaction between the Nazca, Caribbean, and South American plates with continual processes of subduction, obduction, and accretion of oceanic terranes in the west and northwest margins of Colombia and is characterized by three principal cordilleras (Western, Central, and Eastern Cordillera). The genesis and evolution of these cordilleras have been discussed and evaluated using several different kinds of evidence, such as plate motion reconstruction, geology and tectonics of the individual chains, geochronology, geochemistry and paleomagnetism. At present, there is no consensus on many details of Triassic to Jurassic plate tectonic processes that shaped the far northwest part of South America. Two hypotheses have been proposed for the evolution of early Mesozoic terranes in northwest South America, the allochthonous model that is based on paleomagnetic data from Jurassic rocks suggests along-margin northward translation of terranes during the Early Jurassic. Translations of those terranes took place from more southerly latitudes, corresponding with Peru and Ecuador. The autochthonous model is based on thermochronology, U-Pb zircon geochronology, and geochemistry and invokes an essentially in-situ accretion in the western margin of South America, and the proponents disagree with the allochthonous model because it is based on K/Ar dates, which rarely record crystallization ages and probably do not relate to the time that the remanence was acquired, and that the zircon U-Pb dates from the Jurassic arc in Colombia show that arc rocks progressively young to the west, and that the Jurassic arc is not duplicated in Colombia or Ecuador. We present preliminary paleomagnetic results from Jurassic to Lower Cretaceous strata exposed in the Yariguies and Arcabuco anticlines to test the two major hypotheses for early Mesozoic blocks forming the NW margin of South America. Preliminary results of one section of Giron Group strata, from 47 sites in the Norean, Jordan, Girón, Los Santos, Palermo Montebel and La Rusia formations indicate that characteristic magnetization components (ChRM) are well-defined and of normal and reverse polarities and the study area has been affected by clockwise rotation (about 40°), which could be related to displacement along the Suarez Fault.