PERMANENT DEFORMATION AND RELIEF CONSTRUCTION AT SUBDUCTION FOREARCS

The style and tempo of upper plate deformation along subduction margins are strongly modulated by the forces transferred from the plate interface to the forearc. While plate convergence can be seen as continuous and relatively stable over recent geologic time, the plate contact loading experiences, in turn, its own dynamics. During the coseismic-postseismic phase of the megathrust earthquake cycle, the continent is loaded under tension, producing rapid stretching of the forearc wedge oriented towards the subduction rebound. Interseismically, the upper plate is gradually compressed, generating slow-strain-rate shortening. This drastic alternation of stresses results in particular styles of permanent deformation of the upper plate wedge, which should preserve the distinctive loading signatures of the subduction cycle phases. In this talk, I’ll draw from examples from key locations of the Andean subduction margin in northern and central Chile, to illuminate how that signature leads to the construction of structural relief, probably spanning thousands of subduction seismic cycles. The static deformation field produced by a great subduction earthquake is an effective mechanism for generating permanent extension above the seismogenic zone, reactivating suitably oriented, long-lived normal faults, capable of causing large, shallow earthquakes. These first-order extensional faults populate and build up the relief of the Coastal Cordillera along the outer forearc wedge, displaying semi-elliptical patterns, parallel to and concave towards the trench, which may define the location of long-lived megathrust segments. Reverse, upper plate faults coexist in time and space with the normal structures, but are generally scarce along the outer forearc wedge, with the exception of northern Chile near the Andean orocline, where they are ubiquitous and highly oblique to the margin. Interestingly, a large earthquake on one of those faults initiated the foreshock sequence that led to the 2014 Pisagua megathrust events near its epicenter. Margin-parallel reverse faults also dominate the architecture of the inner portion of the upper plate wedge along the western slope of the Main Cordillera. We suggest that upper plate reverse faults may be preserving the interseismic, permanent deformation signature in the structural grain. Analogous observations along the Honshu, Japan and Cascadia subduction zones imply that such a link between the short- and long-term deformation patterns of the forearc is not exclusive of the Andean margin.