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

Abstract

The 2010 Mw8.8 Maule earthquake rupture in central Chile produced significant upper and lower plate normal fault aftershocks including some of the largest recorded, the Mw7.0 Pichilemu events 11 days after the main event. Our understanding of the context and significance of these events for permanent deformation of the upper plate has been hampered by poorly known regional geology overlying the northern and central parts of the Maule rupture. We present new structural data of the Coastal Cordillera from the northern end of the rupture which illuminates the relationship between coseimic and long term deformation. We show that the Neogene normal faults along the outer forearc, including the Pichilemu normal fault, can be reactivated by the coseismic stress imposed within the upper plate by great subduction ruptures. The structural style of the region overlying the northern end of the Maule rupture is dominated by kilometer-scale 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, margin-parallel set. The SW-dipping Pichilemu fault, which has at least three flights of uplifted marineterraces in the footwall but only a single low terrace displaying a rollover anticline in the footwall, belongs to the second group. The first two sets characterize the northernmost part of the rupture and spatially overlap, displaying a bimodal orientation; the third set occurs farther south and appears to characterize the central part of the rupture segment. Reverse faults exist but are scarce. Using the slip model of the Maule earthquake by Vigny et al. 2011 we calculate the strikes of optimally oriented normal faults along the Coastal Cordillera from the Coulomb stress increment. Comparing these strikes to the strikes of known faults and our new data, we find that nearly half agree in orientation within 22.5°. The extensional structural grain may represent the long-term signature of repeated great subduction ruptures. The northward transition from NS- to NW-trending faults outlines the northern end of a semi-elliptical 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. This suggests that great earthquakes, such Maule, have ruptured the same segment in the past and may occur in the future