A Transient Coastal Uplift Event Revealed by Pleistocene Marine Terraces Along the 2015 M8.3 Illapel Earthquake Rupture, Chile

Abstract

Uplifted Pleistocene marine terraces in the Illapel area along the forearc of central Chile record spatially and temporally variable uplift rates on glacial-cycle timescales. However, the underlying mechanisms for this uplift variability and its potential relationship with the megathrust-earthquake cycle remain poorly understood. The Illapel region was affected by the 2015 M_W8.3 Illapel earthquake and provides an opportunity to assess the relationship between the megathrust seismic cycle and permanent coastal uplift. We combined mapping of late Pleistocene marine terraces along 300 km of coast with stratigraphic and sedimentological analyses, luminescence dating, morphometry, and numerical modeling to constrain the timing and rates of deformation. Our results reveal modest uplift rates of 0.1–0.2 m/ka during Marine Isotope stages (MIS) 9 and 5e that were superseded by rates between 0.5 and 0.9 m/ka during MIS 7, lasting for about 200 ka. The lack of a correlation between terrace-uplift rates and GPS velocities at the decadal scale suggests that permanent forearc deformation is unrelated to the coseismic and interseismic phases of the Illapel earthquake cycle. We also discard the influence of upper-plate normal faulting on patterns of surface deformation, as our dislocation modes suggest that it plays a secondary role. Instead, we propose that the short period of uplift acceleration was linked with changes in the margin boundary conditions caused by subduction of bathymetric anomalies from adjacent sea-mount chains. These results highlight the complexity of mechanisms controlling forearc deformation and the hidden dynamics of deep processes in subduction systems.