Slab Thermal Field & Back-Arc Basin Diagram

This is a 2D-native counterpart to the 3D slab-rollback simulator: instead of a rendered block scene, the slab-pull force driving trench rollback is computed by numerically integrating a real thermal boundary-layer field — one independent half-space-cooling solution per downdip column, gridded across distance and depth — rather than multiplying a single thickness value by a capped length. The resulting force still drives the same rollback-velocity and back-arc-spreading formulas, visualized as a live cross-section diagram whose arc position, slab dip and basin width all update from the same numbers shown in the readouts.