The Role of Slab Steps on Tectonic Loading Along Subduction Zones: Inferences on the Seismotectonics of the Sunda Convergent Margin

How seismotectonics of convergent margins reconciles with the force balance of subduction is contentious. The comparison of seismotectonics and available slab pull forces along the Sunda convergent margin shows an enigmatic inverse relationship: upper plate thickening and seismicity magnitude are highest along Sumatra and Andaman, where the slab is shorter than ∼300 km; conversely, these are negligible along the Java segment, where the slab reaches deeper, ∼660 km. Using numerical models, we test the role of such slab pull gradients in the force balance of subduction in three-dimensions, where the slab depth, and therefore its net pull, varies along the trench. We show that in the presence of a “slab step,” the deeper slab drives the convergence of the rigid plate, causing upper plate compression and trench advance in the neighboring trench segments, where a short slab may have no pull to subduct the incoming plate. While neglecting convergence obliquity, the simplified models show relevant along-trench variations of coupling, trench rotations, and minor strike-slip shearing due to the slab step, providing a diagnostic strain pattern, with compression/extension atop the short/long slab and minor strike-slip, increasing in magnitude with depth difference. The modeled tectonic patterns are compared to Sunda margin deformation across scales, from the Cenozoic tectonics to the seismic strain rates, showing remarkable consistency with deformation gradients from Sumatra to Java, potentially illustrating the contribution of the slab step to the seismotectonics of the region.