Structural Geology of the Active Forearc above the Maule Megathrust: Traces of a Long-lived Subduction Segment

Abstract

We present new structural data of the Coastal Cordillera from the northern end of the 2010 Mw8.8 Maule earthquake rupture in central Chile. This area contains the Pichilemu normal fault, an intraplate structure reactivated after the megathrust with earthquakes up to Mw7.0. The structural style of the region is dominated by kilometerscale normal faults which have been active at least throughout the Neogene. The strikes of these main structures define three structural systems: (1) a NE and (2) a NW sets of margin-oblique faults, and (3) a ~NS, marginparallel set. The Pichilemu fault belongs to the second group. From north to south, these three systems vary their occurrence starting with bimodal orientations of groups (1)and (2), followed by unimodal structures of group (2) and ending with single NS orientations. Reverse faults exist but are scarce. We propose that this extensional structural grain represents the long-term legacy of repeated great subduction ruptures. The northward transition from NS- to NW-trending faults outlines the northern end of a semielliptical pattern of major structures along the Maule rupture area, which are likely enclosing a long-lived subduction segment. The bimodal domain of NW and NE structures may represent the boundary of two adjacent segments.