GEOMORPHOLOGY REVEALS LONG-TERM (MYR) UPLIFT ABOVE THE SEISMOGENIC ZONE, NICOYA PENINSULA, CENTRAL AMERICA SUBDUCTION ZONE (Invited Presentation)

Understanding the processes that lead to vertical tectonism in the forearc above seismogenic zones is important for unraveling the mass and energy balance of subduction zones and the potential coupling between megathrust and forearc processes. Although classic subduction zone models depict long-term subsidence of a submarine forearc above the seismogenic zone, there are cases where the seismogenic zone instead experiences uplift, such as at the Nicoya, Osa, and Burica Peninsulas in Central America. At the Osa and Burica peninsulas, uplift above the seismogenic zone occurs in rapid, short-lived pulses due to the upper plate overriding lower plate bathymetric relief.

In contrast to the rapid and short-lived uplift at Osa and Burica, geomorphic evidence on the central Nicoya Peninsula, which lies above the seismogenic zone ~200 km to the northwest of Osa, suggests longer-term uplift at more moderate rates. Long-term uplift on Central Nicoya is recorded by a series of paleo-abrasion surfaces located at ~500 m elevation. The rock uplift rates deduced from these surfaces, based on topographic analyses and field observations, imply a history of rock uplift that extends back to ~1 Ma.

The longer duration of uplift at more moderate rates suggests that long-term uplift on Central Nicoya is not solely related to the subduction of bathymetric relief. Alternate models that could explain the moderate uplift include underplating, uplift above a coastal thrust fault, or wholesale uplift of the upper plate along the megathrust. Longer-wavelength uplift along the megathrust is supported by along-strike changes in residual bathymetry and evidence for significant regional arc and backarc extension. Sustained and permanent forearc uplift above seismogenic zones, such as Nicoya and at the Mejillones Peninsula in Chile, suggests that at least for some margins, the coupled region represents a persistent feature of the subduction interface where a portion of plate convergence is accommodated by upper plate deformation and rock uplift over many seismic cycles.