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

Abstract

Seismic wide-angle measurements across the Pacific margin of Nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots. The structure and the P-wave velocity of the subducting Cocos and overriding Caribbean Plates were determined by modelling wide-angle data and further constrained by coincident seismic reflection, borehole and gravity data. The oceanic crust of the Cocos Plate is 5.5 km thick, with a thin pelagic sediment cover. The plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network. 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 middle of the shelf. This wedge-shaped unit is 15 km thick beneath the shelf edge and is composed of a 5-km-thick upper part with velocities increasing from 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 densities 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 offshore. Landward, the basement underneath the Sandino Basin is formed by older igneous rock drilled beneath upper Cretaceous sedimentary rock onshore Nicaragua. Beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12–18 km and coincides with the down-dip limit of the seismogenic zone. Mantle densities are required for the sliver to match the gravity data. In a tectonic reconstruction, the suture of an oceanic plateau on the Farallon Plate against the Chortis Block in upper Cretaceous time is suggested. Suturing left the former trench and margin in deep water, consistent with the 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–Polochic Fault System. The development of a new subduction zone in Eocene–Oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the Chortis Block and shifted the volcanic arc about 70 km southwestwards, close to its present position.