LOCKING, MASS FLUX AND FOREARC RESPONSE – THE CHILEAN CASE

On the long term convergent plate boundaries have been shown to be controlled by either accretion/underplating or by subduction erosion. Vertical surface motion is coupled to convergence rate - typically with an uplift rate of the coastal area ranging from 0 to +50% of convergence rate in accretive systems, and -20 to +30% in erosive systems. Offshore forearcs tend to show a higher fraction of subsidence with subduction erosion. Vertical kinematics, however, are not necessarily linked to horizontal strain mode, i.e. upper plate shortening or extension, in a simple way. This range of kinematic behaviors – as well as their acceleration where forearcs collide with oceanic ridges/plateau – is well expressed along the Chilean plate margin.

Towards the short end of the time scale, we here find that fault motion as well as vertical motion along the coastal cordillera may exhibit reversal of motion from the preseismic to the postseismic stage, and may show extensional as well as compressional deformation to temporally coexist next to each other. Similarly, kinematics may vary vertically within the upper plate exhibiting a pattern in the upper crust different from that near the plate interface. This entire complexity is not only controlled by gravitational and plate boundary forces, but is suggested to be also controlled by seismic cycle related forearc bending and unbending causing fault triggering in the upper plate, a mechanism unique to forearcs.

In addition, the short term deformation time end appears to exhibit a close correlation with the frictional properties and geodetic locking at the plate interface. Corroborating analogue experiments of strain accumulation during many earthquake cycles, forearc deformation and uplift focus above the downdip and updip end of seismic coupling and slip and are related to the stage of the seismic cycle, but with opposite trends for both domains. Similarly, barriers separating locked domains along strike appear to accumulate most upper plate faulting interseismically. Hence, locking patters are reflected in topography and from the long-term memory contained in the forearc topography the relief of the Chilean forearc seems to reflect long term stability of the geodetically observed heterogeneity of locking at the plate interface, independent of the long-term mass flux style.