Constructing forearc architecture over megathrust seismic cycles: Geological snapshots from the Maule earthquake region, Chile

Abstract

We present new field structural data from the Chilean Coastal Cordillera located above the northern and central parts of the interplate contact ruptured by the A.D. 2010 Mw 8.8 Maule earthquake. The northern study area contains the northwest-striking Pichilemu normal fault, an intraplate structure reactivated after the megathrust event by crustal earthquakes up to Mw 7.0. The structural style of this region is dominated by kilometer-scale normal faults that have been active at least throughout the Quaternary. The orientations of these main faults define three structural systems: (1) northeast- and (2) northwest-striking margin-oblique faults, and (3) north- to north-northeast–striking margin-parallel faults. From north to south, these three systems vary in their predominant occurrence, starting with bimodal orientations of groups 1 and 2, followed by predominantly single north to north-northeast orientations of group 3. Reverse faults coexist in time and space with the normal structures, but are scarce and display variable, apparently random orientations. The shallow crustal normal faults, including the Pichilemu fault, show a persistent kinematic history probably spanning thousands of subduction seismic cycles. Though historically smaller in magnitude than those of the triggered normal faults, interseismic forearc thrust events were recorded above the rupture area prior to the Maule earthquake. The Quaternary reverse faults identified in our study regions may be preserving interseismic, slow-strain-rate, permanent deformation signature in the structural grain. Analogous observations along the A.D. 2011 Tohoku earthquake rupture in Japan imply that such a link between the short- and long-term deformation patterns of the forearc is not exclusive of the Maule earthquake region.