  We discuss the relationships between Andean shortening, plate velocities at
the trench, and slab geometry beneath South America. Although some correlation
exists between the convergence velocity and the westward motion of South
America on the one hand, and the shortening of the continental plate on the
other hand, plate kinematics neither gives a satisfactory explanation to the
Andean segmentation in general, nor explains the development of the Bolivian
orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab
segments below South America, arguing that they result from the subduction of
oceanic plateaus whose effect is to switch the buoyancy of the young subducting
plate to positive. We argue that the existence of horizontal slab segments,
below the Central Andes during Eocene-Oligocene times, and below Peru and
North-Central Chile since Pliocene, resulted (1) in the shortening of the
continental plate interiors at a large distance from the trench, (2) in
stronger interplate coupling and ultimately, (3) in a decrease of the
trenchward velocity of the oceanic plate. Present-day horizontal slab segments
may thus explain the diminution of the convergence velocity between the Nazca
and South American plates since Late Miocene.
