The C3-SI Framework: A Newtonian Model for Compositional-Contrast-controlled Subduction Initiation

Subduction initiation (SI) remains one of the most fundamental unresolved problems in solid Earth geodynamics. Building on the hypothesis of Niu et al. (2003) that SI requires lateral compositional buoyancy contrasts within the lithosphere, we develop here the first fully quantitative C3–SI (Compositional-Contrast-Controlled Subduction Initiation) framework based on transparent Newtonian mechanics. The framework integrates two components: (1) the Subduction Initiation Index (SII), which quantifies lithospheric rupture driven by compositional buoyancy contrast, and (2) the Fault-Block Sequential Sinking (FBSS) mechanism, which explains the transition from rupture to self-sustaining subduction. We show that ocean-continental (O–C) and ocean-oceanic plateau (O–P) boundaries generate lateral horizontal buoyancy compressive stresses (σO-C ∼ 120 MPa andσO-P ∼ 60 MPa, respectively), far exceeding ridge push (σRP≈ 22 MPa). This stress-equivalent term represents the horizontal compressive stress generated by the lateral gravitational potential energy (GPE) gradient, analogous to ridge push. These stresses are strongly localized within ∼50 km of the boundary and decay rapidly toward plate interiors, explaining why SI is spatially restricted. The SII successfully predicts where rupture occurs, but rupture alone cannot produce subduction. Lithosphere-scale bending is mechanically implausible because required stresses (∼1–3 GPa) exceed available tectonic stresses by an order of magnitude. The FBSS resolves this by treating the oceanic lithosphere as a fault-segmented system, where inherited abyssal-hill faults enable sequential block sinking. As a 100-km-wide slab segment sinks to ∼50–60 km into the asthenosphere, slab-pull reaches a threshold of ∼3 × 1018 N, driving acceleration to >50 mm/yr and enabling self-sustaining subduction. The predicted spatial localization, force balance, and timescales (1–3 Myr) are consistent with global observations. The C3–SI framework provides the first fully Newtonian and transparent mechanical description of SI, resolving the long-standing problem of how lithospheric rupture occurs and how rupture evolves into subduction, without invoking ad hoc weakening mechanisms in opaque numerical models.A global survey of ∼30 subduction zones (50 segments) confirms that most SI events occur at O–C or O–P boundaries. Even the so-called intra-oceanic systems (e.g., Mariana and Tonga) are underlain by compositionally highly depleted, physically buoyant forearc lithospheric mantle, consistent with the original hypothesis of Niu et al. (2003).