Imaging a steeply dipping subducting slab in Southern Central America

Subduction of oceanic lithosphere leads to large changes in seismic velocities that are observable through converted and scattered teleseismic waves. In this study, we use teleseismic data from a dense broadband seismic experiment in Central America to image a slab that dips > 60°. Although strongly bent, up going P-to- S mode conversions show a strong signal from 60–200 km depth; reverberations do not reach the surface because the slab dips steeply here. The subducted oceanic plate is successfully imaged to 200 km depth in Nicaragua where station geometry is adequate, but only at periods near 6 s at depths > 100 km, compared with 1–2 s periods for shallower structure. The long-period nature of the deeper conversion indicates a gradational boundary rather than a discontinuity, probably marking the thermal boundary layer at the slab-wedge interface. A sharper discontinuity is imaged near the top of the slab at depths < 80 km, probably denoting the Cocos plate Moho. Seismicity lies above the boundary at depths < 80 km, and below the inferred thermal boundary at greater depth. The change of character of the discontinuity with depth may reflect partial eclogitization of the subducting crust and associated reduction of the velocity contrast at the Moho, with seismicity lying within subducting crust but the dominant velocity change migrating out of it with increasing depth. In the upper plate, images map the Moho in both Costa Rica and Nicaragua, with Moho conversions weakest directly beneath the arc. Complex dipping structure at 25–50 km depth beneath the Nicaragua forearc perhaps reveals the deep roots of an accreted arc terrane.