Abstract
In many active margins, severe deformation is observed at the front of the overriding plate where seamounts or aseismicridges subduct. Such deformation appears to be a main tectonic feature of these areas which influences the morphologyand the seismicity of the margin. To better understand the different stages of seamount subduction, we have performedsandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismicimages and bathymetry interpretation. We focus, in this paper, on the surface deformation directly comparable with seafloormorphology. Two types of subducting seamounts were modelled: relatively small conical seamounts, and larger flat-toppedseamounts. The indentation of the margin by the seamount inhibits frontal accretion and produces a re-entrant. The marginuplift includes displacement along backthrusts which propagate from the base of the seamount, and out-of-sequenceforethrusts which define a shadow zone located on the landward flank of the seamount. When the seamount is totally buriedbeneath the margin, this landward shielded zone disappears and a larger one is created in the wake of the asperity due tothe elevated position of the de´collement. As a consequence, a section of the margin front follows behind the seamountto greater depth. A ‘slip-line’ network develops concurrently above the subducting seamount flanks from the transtensionalong the boundaries of the shadow zone. In a final stage, normal faults, controlled by the shape of the seamount, develop inthe subsiding wake of the asperity. Swath-bathymetric data from the Costa Rica margin reveal detailed surface deformationof the margin above three subducting seamounts. Shaded perspective views highlight the detailed structure of the seafloorand compare well with surface deformation in the sandbox experiments. The good correlation between the marine dataand experimental results strengthen a structural interpretation of the Costa Rican seamount subduction.