CHANGES IN SUBDUCTION DIRECTION AND RIDGE COLLISION AGAINST A CURVED OROGEN: THE CASE OF THE SOUTHERNMOST ANDES

A recently updated chronology of deformation in the southernmost Andes is combined with plate tectonic reconstructions to reveal the connection between deformational events and ridge collision in combination with changes in subduction speed and direction. Late Cretaceous-Tertiary evolution of the southern tip of the Andes and its present structural conformation are explained as due to a convergent subduction system on a curved plate boundary. This configuration implies that an eastward oriented convergence vector for the Farallon – Aluk plates resulted in an oblique subduction against the Fuegian Andes and an orthogonal subduction against the Patagonian Andes, whereas a northward convergence vector produced the inverse effect.

Three deformational events are differentiated in relation with changes in the subduction direction: (1) Late Cretaceous - Paleocene (circa 80 to 47 Ma) eastward subduction: Oblique subduction in the Fuegian Andes produced deformation partitioning and broadly distributed orogen-parallel left-lateral strike-slip faults in the basement domain. Orthogonal subduction against the Patagonian Andes caused intense deformation and the onset of the Austral foreland basin. (2) Eocene – Oligocene (circa 47 – 28 Ma) northward subduction: Collision of a seismic ridge followed by perpendicular subduction against the Fuegian Andes produced the major propagation of the deformational front during the Eocene (Río Bueno thrusting, 49 to 34 Ma). In the Patagonian Andes, Eocene oblique subduction produced block rotation in the internal-basement zone and a contractional component that caused shortening in the thin-skinned fold-thrust belt. (3) Neogene: A regional tectonic plates rearrangement disconnected the Fuegian Andes from the subduction zone, and the Patagonian cordillera experienced the period of major contractional deformation, when the convergence vector rotated to a relatively more orthogonal position.

The results support previous theories associating ridge collisions and the subduction direction of Farallon-Aluk-Nazca plates with deformational events in the southernmost Andes.