  Shortening of the continental lithosphere is generally accommodated by the
growth of crustal wedges building above megathrusts in the mantle lithosphere.
We show that the locus of shortening in the western margin of South America has
largely been controlled by the geometry of the slab. Numerical models confirm
that horizontal subduction favors compression far from the trench, above the
asthenospheric wedge and steeply dipping segment of the subducting slab. As a
result, a second crustal wedge grows in the hinterland of the continent, and
widens the Andes. In the Bolivian orocline, this wedge corresponds to the
Eastern Cordillera, whose growth was triggered by a major episode of horizontal
subduction. When the slab returned to a steeper dip angle, shortening and
uplift pursued, facilitated by the structural and thermo-chemical alteration of
the continental lithosphere. We review the successive episodes of horizontal
subduction that have occurred beneath South America at different latitudes and
show that they explain the diachronic widening of the Andes. We infer that the
present-day segmented physiography of the Andes results from the latitudinally
variable, transient interplay between slab dynamics and upper plate tectonics
over the Cenozoic. We emphasize that slab flattening, or absence thereof, is a
major driving mechanism that sets the width of the Andes, at any latitude.
