Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Abstract

We present the results of a seismic refraction survey conducted offshore Costa Rica near the Nicoya Peninsula. A dip profile and three strike profiles were carried out over 22 ocean bottom hydrophones and seismographs and were also recorded by land receivers. These data are used to construct a crustal structure model of the convergent margin from 20 km seaward of the Middle America Trench onto the Nicoya Peninsula. The best constrained portion of our model is the velocity at the top of the margin wedge immediately below the slope apron. Velocities increase from 3.5 to 4.2 to 4.6 km/s at distances of 10, 20, and 30–50 km landward of the trench. These velocities are higher than observed within margin wedges at other well‐studied convergent margins but lower than the velocities within the adjacent Nicoya Complex, which are ∼5.5 km/s at similar depths below the surface. We interpret the margin wedge velocities as indicating that material similar to the Nicoya Complex extends seaward to near the lower slope but that fracturing, alteration, or accretion processes have lowered the velocity of the margin wedge with respect to the Nicoya Complex. The seismic refraction data cannot constrain the exact thickness or velocity of a possible low‐velocity zone (LVZ) overlying the subducting plate; however, geologically reasonable structures are only produced with a LVZ<400 m thick. Velocities in the upper part of oceanic layer 2 are ∼3.5–4.0 km/s within the subducting slab. These velocities are unusually low for oceanic crust of this age and may correlate with a proposed highly permeable zone at the top of the subducting crust. The top of the subducted slab is well resolved, and deepens from 5 km depth at the trench to 15–16 km depth at the Nicoya Peninsula coastline. The dip angle of the subducting plate increases from 6° to 13° at a distance of ∼30 km from the trench. Interplate seismicity appears to become common ∼55 km from the trench where the plate boundary is at ∼14 km depth.