APPARENT TRUNCATION AND JUXTAPOSITION OF CAMBRIAN AND ORDOVICIAN ARC-ACCRETIONARY COMPLEXES, EASTERN SIERRAS PAMPEANAS, ARGENTINA

N-S trending Cambrian (Pampean) and Ordovician (Famatinian) orogenic belts in the Sierras Pampeanas record episodes of convergent tectonism on the Paleopacific margin of South America. We describe here new evidence indicating truncation of the Cambrian belt and juxtaposition against the Ordovician belt along a ~250 km Paleozoic shear zone, approximately following the modern western scarps of the Sierras de Córdoba.

Our recent work in the S. de Córdoba and San Luis indicates that: 1) the high grade Cambrian accretionary prism rocks (~520 Ma) found throughout the S. de Córdoba are absent from nearby San Luis; the latter contains similar rocks but they exhibit only Ordovician (450-485 Ma) metamorphic and igneous signatures; 2) the Cambrian rocks in S. de Córdoba are intruded by a diverse suite of Ordovician (460-480 Ma) plutonic rocks of broadly calc-alkaline character. Some are emplaced syntectonically into localized deformation zones, whereas other plutons remain completely undeformed. Collectively these Ordovician plutons are much more widespread in the S. de Córdoba than previously recognized, and overlap in age the Ordovician plutons of the main Famatinian belt to the west; 3) the Cambrian and Ordovician metasedimentary assemblages of the S. de Córdoba and San Luis, respectively, meet in apparent sharp tectonic contact along a thick, sinuous, but locally discontinuous shear zone of mylonitic, ultramylonitic, and locally pseudotachylytic rocks (see Whitmeyer et al., this meeting).

We interpret this shear zone to be a major crustal-scale tectonic contact along which the extinct Cambrian arc and accretionary prism of the S. de Córdoba was truncated in Early Paleozoic time. This truncation juxtaposed a developing or already developed Ordovician accretionary complex against the Cambrian belt. Kinematic indicators observed to date are predominantly dip-slip (east thrust over west), but the disposition of rock units suggests significant lateral displacements are probable. Present age constraints bracket the shear zone between Ordovician and Devonian time. We argue that an Ordovician arc-accretionary complex developed on the extinct Cambrian arc, and that this composite margin was disrupted by plate motion between Laurentia and Gondwana following the break up of Rodinia.