PUNCTUATED MELT-ENHANCED DEFORMATION AND TECTONIC REACTIVATION ABOVE A LONG-LIVED SUBDUCTION ZONE, COASTAL ANDES, CENTRAL CHILE

The coastal Andes of Central Chile record a >300 Myr history of convergent margin tectonics since the Carboniferous, including multiple phases of arc magmatism, shortening, extension and margin-parallel shear. We present the results of an integrated field, structural, and thermochronological pilot study that documents four phases of late Paleozoic–early Mesozoic melt-enhanced deformation within the forearc region between San Antonio and Valparaiso. Three of the phases are principally associated with the formation (~305 Ma) and/or subsequent reactivation (~200 and ~160 Ma) of NW-striking structures. In samples that record each deformation phase, the integrated ⁴⁰Ar/³⁹Ar data coupled with the preservation of dislocation creep microstructures, rather than recovery microstructures, require relatively rapid cooling to or near the frictional-viscous transition (~350–275°C) following deformation in the presence of melt. These observations argue against regionally-elevated geothermal gradients and suggest that the early Mesozoic melt-enhanced deformation events were discrete in both time and space. A fifth brittle phase of deformation has also been observed as NW-striking cataclastic faults in outcrop and as microfaults in thin section. It is inferred to have occurred at ~100 Ma based on the minimum ages of argon-loss profiles and may correlate with high-strain ductile and cataclastic shear zones documented further north. Studies of active faulting in the region have determined that deep-seated NW-striking structures pose the greatest seismic hazard. Based on those maps and our pilot study, the strike of active faults mimic basement structural trends associated with the deformation events described above. Comparison of our results with published data from further south indicate our study area may provide an analog for the present-day mid–lower crust in the Pichilemu region, where recent studies of the great 2010 Maule earthquake highlight the importance of the reactivation of NW-striking structures in the middle crust during coseismic deformation in the upper plate of the Chile subduction zone. Thus, the results of this work are pertinent to a holistic understanding of seismic hazards because the rheology of the crust is deeply influenced by its kinematic and thermal history.