EARLY PALEOZOIC TECTONISM OF THE WESTERN MARGIN OF GONDWANA AND ITS RELATIONSHIP TO THE TECTONICS OF EASTERN LAURENTIA

The Eastern Sierras Pampeanas of central Argentina contain evidence that constrains the style and timing of the principal orogenic events that impacted the margin of western Gondwana during the early Paleozoic. Several major tectonic events in the area are broadly coeval with tectonic events in eastern Laurentia. East-directed subduction (present-day coordinates) initiated along the western margin of Gondwana in the late Paleozoic to early Cambrian and preceded the accretion and deformation of accretionary prism sediments. Middle Cambrian high temperature, low pressure, post-tectonic metamorphism affected the entire accretionary prism. This was followed by late Ordovician to early Devonian oblique collision of a series of small, distinct, tectonothermal terranes, some of which may contain Ordovician arc-related tonalites and granites. The timing of accretion is constrained by crosscutting relationships and existing U/Pb and 40Ar/39Ar data.

Several significant NNE-trending ductile shear zones, with true thicknesses that range from hundreds of meters to several kilometers, separate the terranes. The shear zones crop out within and along the margins of the Sierras Pampeanas. Detailed analyses of the metamorphic and deformation fabrics suggest that at least some of the terrane-bounding shear zones were predominantly dextral oblique transpression zones, and all contain a strong east over west component. Major deformation culminated with the suture of the Precordillera terrane, considered by many workers to be a microcontinent that originally detached from southeastern Laurentia during the early Cambrian.

The shear zones of the Sierras Pampeanas likely represent major sutures related to terrane emplacement during the Famatinian orogeny (late Ordovician to early Devonian) and collision of the Precordillera terrane (middle Devonian), and may have been reactivated during the accretion of Chilenia. Similarly timed tectonic events associated with the development of eastern Laurentia include the Ordovician accretion of an arc system (Taconic Orogeny) and Devonian suture of a microcontinent (Acadian Orogeny).