Orogenic-wedge deformation and potential for great earthquakes in the central Andean backarc

Abstract

Subduction of the Nazca plate beneath South America has driven the growth of the Andes Mountains. Subduction has routinely generated earthquakes larger than magnitude 8.0 along the western margin of the mountain belt 1 , but the potential size of less frequent earthquakes in the eastern, backarc margin is unknown. Continued support of the high Andean Plateau at the centre of the Andes can be explained only if deformation of the backarc margin is ongoing 2‐4 . Here we present GPS data that record surface motions in the Subandean ranges that are part of the backarc margin. We find that the velocity of surface movement decreases sharply from west to east across the Subandean ranges. We suggest that a subhorizontal fault underlying the ranges slips freely at depth in the west, but is locked for up to 100 km in shallower sections further east. Analysis of fault scarps formed where the subhorizontal fault intersects the surface indicates that the fault has generated repeated large earthquakes. We suggest that rupture of the entire locked section of the fault could generate an earthquake of magnitude 8.7‐8.9. We attribute the large seismic potential to the unusual width of the Subandean ranges, and suggest that deformation of the Subandean ranges, at a rate unmatched by erosion, causes the mountain range to widen. The central Andean Plateau (AP) is a bilaterally symmetric plateau of 4;000m average elevation (Fig. 1). Although AP uplift scenarios are currently debated 5,6 it is generally accepted that the backarc orogenic wedge on the AP’s east flank, comprising the Eastern Cordillera (EC), Inter-Andean zone (IAZ), and Subandes (SA), has contracted by more than 250km since 45Myr ago (refs 79). Wedge deformation has been facilitated by the presence