Squeezing and lifting – modes of long-term forearc deformation in the Northern Chile subduction zone
Bernd Schurr, GFZ German Research Centre for Geosciences
Subduction zone forearcs deform transiently and permanently due to the frictional coupling with the converging lower plate. Transient stresses are mostly the elastic response to the seismic cycle. Permanent deformation is evidenced by forearc topography, upper plate faulting and earthquakes; its relation to the megathrust seismic cycle is debated. Here we study upper plate seismicity in the northern Chile subduction zone as a proxy for forearc brittle deformation. We find that seismicity is distributed unevenly and a dramatic increase correlates both with a break in the morphology and tectonics of the Coastal Cordillera and the onset of a change in subduction obliqueness. Earthquakes in the South American crust under the sea and under the Coastal Cordillera show a remarkably homogenous north-south, i.e., trench-parallel, compressional stress field. Earthquake fault mechanisms are dominated by trench-perpendicular thrusts. Further inland, where the lower plate becomes uncoupled, the stress field is more varied with direction east-west to southeast-northwest (approx. convergence parallel) dominating. The stress-regime above the plate-coupling-zone, almost perpendicular to plate convergence direction, may be explained by a trench-parallel strain component from a change in subduction obliqueness due to the concave shape of the plate margin, which we demonstrate by investigating inter-plate earthquake slip vectors. From these, we derive a strain rate estimate and compare it to one derived from upper plate earthquakes. In the southern part of our study area, where convergence obliqueness is constant, upper plate seismicity is sparse, but Coastal Cordillera elevation and the uplift rate are higher than in the north. A new relocation of two megathrust earthquake aftershock sequences reveals a flat-ramp-flat structure that we interpret as underplating of a 3 km thick slice of eroded material and/or oceanic crust. This structure is also visible in the residual gravity field and in a tomographic model of the region. Our study demonstrates thus two very different mechanisms of forearc thickening active on two neighboring segments of a subduction zone.