ALBERT

Description: Combined on- and offshore wide-angle seismic data were collected in southern Costa Rica along a profile that extends from the Middle America Trench and crosses the Pacific continental margin, the isthmus, and reaches to the Caribbean coast. Closely spaced marine airgun sources were used in the Pacific, coincident with the preexisting seismic reflection line P-1600 recorded by SHELL Int. Four large explosive charges were detonated in boreholdes onshore. The combined interpretation of the near-vertical and wide-angle data provides a detailed crustal model. The profile is located on the northern flank of Cocos Ridge, where we have determined that the subducting oceanic plate has a thickness of 11 to 12 km. It dips down with an average angle of 17° and can be imaged to a depth of at least 35 km with the available data. Erosion of the margin wedge is suggested by a thick low velocity zone overlying the downgoing slab. Like elsewhere along the Middle America Trench, the body of rock underlying forearc and slope sediment has high velocities (〉4.0 km/s). At deeper levels (〉5 km), the crustal rocks along the profile generally show high velocities (6.0 to 6.6 km/s). To the southwest, these high velocities can be followed to the mid-shelf area. The inverted Térraba Basin contains intermediate velocity sediment (3.0 km/s) with a total thickness of about 2 km. The Limón Basin has up to 6 km of Tertiary and Quaternary sediment. An intracrustal discontinuity is found at a depth of about 18 km, but cannot be determined in detail due to the limited offset of the available data.