Vestiges of an Ordovician Ocloyic west-vergent, thin-skinned thrust belt in the Argentine Precordillera, southern central Andes

THOMAS, W.A.; ASTINI, R.A.

Mendoza

Congreso; Gondwana 1º2; 2005

Academia Nac. de Ciencias

The Middle Ordovician Ocloyic orogeny, the earliest accretionary event along the western margin of Gondwana, is interpreted to represent collision of the down-going, Laurentia-derived Argentine Precordillera terrane with an upper-plate segment of the Gondwanan margin. Evidence for the Ocloyic orogeny includes Ordovician magmatism/volcanism and deformation in Famatina and metamorphic ages in the Sierra de Pie de Palo and Sierra de Valle Fertil. Within the Precordillera, an upward transition from passive-margin carbonates (San Juan Limestone) to black shale (Gualcamayo Formation) indicates initial subsidence of a foreland basin in earliest Middle Ordovician time. Coarser clastic sediment (conglomerate, sandstone, and mudstone of the Las Vacas Formation and Trapiche Group), filling the foreland basin, includes extrabasinal igneous and quartzite clasts and intrabasinal limestone clasts. Rounded clasts generally are <35 cm; however, 10-m blocky olistoliths of limestone are common locally along with rare olistoliths of black shale. No certain Ocloyic structures have been recognized in the Precordillera, and the lack of a preserved west-vergent Ocloyic thrust belt on the lower plate has been used to question the hypothesis of Ordovician accretion of the Argentine Precordillera.  In contrast, however, some observations can be assembled to suggest vestiges of a thin-skinned Ocloyic thrust belt.