Tethyan closure, Andean orogeny, and westward drift of the Pacific Basin

The deep-sea trenches that circumscribe the Pacific basin are moving westward relative to the deep mantle by a few cm/yr, together with the Pacific seafloor. While the westward drift of the Pacific basin dominates the observed net westward rotation of Earth's tectonic plates relative to the deep mantle source of the Pacific hotspots, its cause remains uncertain. We demonstrate that the source of this westward drift is tectonic in origin: the westward push of the high Andean topography along the eastern Pacific margin is large enough to shear the entire Pacific upper mantle with a surface velocity of similar to 30 mm/yr. in addition, because the Pacific upper mantle is confined between vertically coherent slabs of subducted lithosphere on both sides of the Pacific, slabs in the western Pacific are forced to advance toward upper plates because the eastern Pacific slabs are pushed westward. This cross-Pacific interaction provides an explanation for trench advance, which is observed in the western Pacific and is not easily explained by the local dynamics of subduction in which dense sinking slabs essentially tend to retreat from overriding plates. Thus, the basic asymmetry of the Pacific system, which has been noted for almost all tectonic features including slab dips, trench motions, and upper plate tectonics, is the result of one-sided Andean forcing on the volume of Pacific upper mantle. The net rotation of the lithosphere is then the result of surface tectonics: westward motion is an accident resulting from the current distribution of tectonic plates, and was triggered by the mid-Cenozoic closure of the Tethys Ocean, via a westward expansion of the Atlantic and Andean cordilleran building. (C) 2008 Elsevier B.V. All rights reserved.