Localized deep slab buckling during stepwise convergence deceleration

Abstract

Non-steady convergence is a fundamental feature of subduction–collision systems, yet how its temporal distribution is recorded in deep-lithosphere geometry remains poorly constrained. We use two-dimensional thermomechanical models to compare constant convergence with plate-reconstruction-inspired, three-stage early-fast–late-slow histories. Four reference cases are analyzed; additional experiments vary deceleration amplitude, transition timing, and intermediate-stage duration, and a constant-velocity model with the same 50 Ma cumulative convergence as the principal stepwise model isolates the effect of velocity path. The analysis focuses on the ProContinentalMantle (PCM) component at depths of 300–660 km and quantifies its projected geometry using thickness-, amplitude-, oscillation-, hinge-, and scale-based metrics. In the Case 1–Case 2 and configuration-matched Case 3–Case 4 comparisons, stepwise convergence reduces L _f from 440 to 100 km and from 280 to 90 km, increases N _h from 2 to 4 and from 5 to 6, and raises I _osc from 0.148 to 0.304 and from 0.205 to 0.368, respectively. Across seven stepwise-history models, L _f remains 82–155 km and N _h is 6–7, indicating a robust shift toward shorter-scale, multilobed geometry. At identical cumulative convergence, the principal stepwise model retains a shorter L _f (90 vs. 174 km) and a higher I _osc (0.368 vs. 0.252) than its constant-velocity counterpart. Mechanical diagnostics show that boundary deviatoric reaction adjusts near velocity transitions, followed by redistribution of deep stress, strain rate, and flow, and later by multisegment buckling. Thus, the temporal path of convergence, beyond total convergence alone, modulates deep continental-lithospheric-mantle geometry, while response magnitude depends mainly on deceleration amplitude and stage duration.