Flat-Slab Mechanics Transforming Double-Arc Expressions Along the Middle America Trench

The southeast–northwest variation in the trench–volcano distance along the Middle America subduction zone is widely recognised to be due to the horizontal (flat) geometry of the descending Cocos plate (slab) in its northwestern part. In the topographic and gravitational expressions, the southeastern segment forms doubled low–high (i.e. trench–continental shelf edge–oceanic basin–volcanic arc) belts, whereas the northwestern segment forms wide doubled low–high belts. To define their attribution, the surface uplift rates were computed from a two-dimensional dislocation-based subduction model with hypocentre-based plate interface geometries. The trench-perpendicular plate-interface profiles exhibited similar curvature compositions, which were reflected in three (shallow, moderate depth and deep) convexities (downward bends) and one concavity (upward bend) between the moderate-depth and deep convex sections. The steady subduction along the first and second positive curvatures (shallow and moderate-depth convexities) in the southeastern and northwestern segments could serve as mechanical sources of short-wavelength double-arc formation. The latter two (negative and third positive) curvatures (concavity and deep convexity) in the northwestern segment produced distinct features including the narrow continental shelf, large trench–volcanic-arc distance and relevant long-wavelength double arcs. This was attributed to the direct contact of the flat elastic slab with the overriding plate.