WESTWARD EXPANSION AND COALESCE OF THE CORDILLERAN WEDGE WITH THE COASTAL WEDGE, SOUTHERN CENTRAL ANDES

The South Central Andes are underlain by an active orogenic wedge, formed by accretion from its eastern side, in association with westward subduction of South American basement. Eastward subduction of the Nazca plate at the Chile subduction zone is paralleled by a well-defined forearc high, corresponding to the Coast Range of Chile.

Santiago, Chile (~33.5 S) marks an important transition in the morphology of the margin. South of Santiago, the Coast Range and Andes are separated by the Central Valley, which is known, based on its geology, to have remained stable since ~30 Ma. North of Santiago, the Andes and the Coast Ranges merge together, and rise upward to merge with the Altiplano.

We propose that this transition provides a space-for-time representation of the evolution of orogenic topography in this part of the Andes. The area south of Santiago shows the initial configuration of the margin, with two orogenic wedges, separated by a tectonically stable Central Valley: the Coastal wedge to the west, formed by Nazca subduction, and the Cordilleran wedge to the east, formed by South America subduction. The Cordilleran wedge has grown at a faster rate due to accretion of a thick crustal section from the subducting South American plate. The accretion rate increases northward, and accounts for westward expansion and coalescence of the Cordilleran wedge with the Coastal wedge to the north of Santiago.

We have run sandbox experiments that demonstrate this hypothesis. An important conclusion is that the locus and rate of orogenic deformation is controlled not by decollement stress (aka coupling), but rather by accretionary flux. The Coastal wedge has a fast subduction velocity (~50 km/Ma) but only a very thin accreted section (<0.5 km), so wedge growth is slow there. In contrast, the Cordilleran wedge is underlain by a slowly subducting plate (~5 km/Ma, and increasing northward) but a much thicker accreted section (~25 km) , so wedge growth is faster there.