Resolving crystallisation ages of Archean mafic¯ultramafic rocks using the Re¯Os isotope system

Travel-time and amplitude modeling were carried out on data recorded by three ocean bottom seismometers along two strike profiles, located 10¯20 km landward of the Middle America Trench offshore Costa Rica. These two profiles bracket a region of rapid change in physical properties and structural styles. The travel-time modeling indicates little structural change along the profiles, and therefore the data are suitable for amplitude modeling with the reflectivity method. For the profile 10 km from the trench, amplitude modeling indicates a margin wedge with velocities of 4¯4.4 km/s and Qp=25¯50. For the profile 20 km from the trench, the amplitude modeling indicates a margin wedge with velocities of 4.4¯4.8 km/s and Qp=50¯75. The primary new result of this analysis is the constraint that Qp=25¯75 within the margin wedge at both locations. These high attenuation values are consistent with fracturing of the material making up the margin wedge, either ophiolitic Nicoya Complex rocks, dewatered and cemented sediments, or a tectonic mixture of both.