Upper plate deformation associated with seamount subduction

In many active margins, severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct. Such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin. To better understand the different stages of seamount subduction, we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation. We focus, in this paper, on the surface deformation directly comparable with seafloor morphology. Two types of subducting seamounts were modelled: relatively small conical seamounts, and larger flat-topped seamounts. The indentation of the margin by the seamount inhibits frontal accretion and produces a re-entrant. The margin uplift includes displacement along backthrusts which propagate from the base of the seamount, and out-of-sequence forethrusts which define a shadow zone located on the landward flank of the seamount. When the seamount is totally buried beneath the margin, this landward shielded zone disappears and a larger one is created in the wake of the asperity due to the elevated position of the decollement. As a consequence, a section of the margin front follows behind the seamount to greater depth. A 'slip-line' network develops concurrently above the subducting seamount flanks from the transtension along the boundaries of the shadow zone. In a final stage, normal faults, controlled by the shape of the seamount, develop in the subsiding wake of the asperity. Swath-bathymetric data from the Costa Rica margin reveal detailed surface deformation of the margin above three subducting seamounts. Shaded perspective views highlight the detailed structure of the seafloor and compare well with surface deformation in the sandbox experiments. The good correlation between the marine data and experimental results strengthen a structural interpretation of the Costa Rican seamount subduction. (C) 1998 Elsevier Science B.V. All rights reserved.