Structure of the Costa Rica convergent margin, offshore Nicoya Peninsula

Abstract

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 ø to 13 ø 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.

Along‐strike variations in underthrust sediment dewatering on the Nicoya margin, Costa Rica related to the updip limit of seismicity

“…The slope of the seafloor on the wedge is 5.4°. The subducting plate dips 6°for the first 30 km of the subduction zone, after which the dip angle increases to 13° [ Christeson et al, 1999].…”

Subduction dynamics at the middle America trench : new constraints from swath bathymetry, multichannel seismic data, and 10BE

“…Seismic velocities of 4-6 km/s beneath the lower and middle slope of the Nicoya Peninsula forearc are interpreted as igneous rock, presumably the ophiolitic Nicoya Complex (Chrsteson et al, 1999;Sallares et al, 2001). Therefore, Ranero and von Huene (2000) concluded that the eroded material transported into the subduction zone is igneous basement from the forearc.…”

“…The throws on these extensional faults are equal to or greater than the thickness of the sediment column (--400 m) . Offshore of Costa Rica, normal faulting in the subducting plate is not as pronounced (Shipley and Moore, 1986;Hinz et al, 1996;Ye et al, 1996;, Chrsteson et al, 1999. It was therefore thought possible that beneath Nicaragua, large basement faults facilitated efficient subduction of the shallow, lOBe-bearing sediment layer to the arc volcanic roots, whereas this absence of fault relief offshore of Costa Rica could lead to removal of the uppermost sediment layer through underplating (Kelly and Driscoll, 1998).…”

From seamount accretion to tectonic erosion: Formation of Osa Mélange and the effects of Cocos Ridge subduction in southern Costa Rica

“…The Caribbean overriding plate, for example, shows a dramatic thinning from 16 km at the coastline in Guatemala [Ye et al, 1996], far and not influenced by Cocos Ridge subduction , to 12-14 km in Nicoya [Christeson et al, 1999;Sallares et al, 2001], where the sharp increase in subduction erosion rate is in agreement with the beginning of thickened crust subduction , to 3 km in Osa Peninsula [Walther, 2003]. A general increase in erosional rates along the margin toward Cocos Ridge can also be inferred from the trench morphology, where a sudden landward shift of the trench axis strike occurs coincident with the area of Figure 13.…”