Crustal structure across the Pacific margin of Nicaragua: evidence for ophiolitic basement and a shallow mantle sliver

Seismic wide-angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording mar ine airgun shots. The structure and the P-wave velocity of the subducting C ocos and overriding Caribbean Plates were determined by modelling wide-angl e data and further constrained by coincident seismic reflection, borehole a nd gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic s ediment cover. The plate boundary can be traced to 40 km depth and is gener ally similar to configurations derived earlier from the local seismic netwo rk. A major feature of the upper plate is an 80-km-wide high-velocity, high -density rock unit reaching from the front of the margin to about the middl e of the shelf. This wedge-shaped unit is 15 km thick beneath the shelf edg e and is composed of a 5-km-thick upper part with velocities increasing fro m 3.5 km s(-1) near the trench to 5.2 km s(-1) at the shelf break overlying a 10-km-thick lower part with velocities of 6.0 km s(-1). Analysis of the gravity field requires densities of 2.6-2.7 g cm(-3) for the upper part and 2.9 g cm(-3) for the lower part of this unit. These velocities and densiti es suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore Guatemala and exposed on the Nicoya Peninsula, Costa Rica. The velocity structure of this ophiolitic unit ends about 50 km offs hore. Landward, the basement underneath the Sandino Basin is formed by olde r igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Ni caragua. Beneath the ophiolitic basement we find a sliver with velocities t ypical of mantle material that begins at depths of 12-18 km and coincides w ith the down-dip limit of the seismogenic zone. Mantle densities are requir ed for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Faral lon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with t he late Cretaceous to Palaeocene deep-water sediment of the Rivas Formation at the base of the Sandino Basin. Suture of the Farallon Plate and Chortis Block might have initiated the strike-slip movement along the Motagua-Polo chic Fault System. The development of a new subduction zone in Eocene-Oligo cene times left the ophiolitic basement and a sliver of oceanic mantle atta ched to the Chortis Block and shifted the volcanic are about 70 km southwes twards, close to its present position.