T6[Topic] Subduction zones and on-land accretionary complexes

11:15 AM - 11:30 AM JST(2:15 AM - 2:30 AM UTC)

[T6-O-4] Evolution of Rheological Heterogeneity During the Slow-to-Fast Deformation Transition in an Accretionary Fault Zone

*Yoshitaka Hashimoto¹, Takahiro Hosokawa¹, Taizo Uchida¹ (1. Kochi University)

Keywords:

accretionary complex,Rheological heterogeneity,strain localization

ccretionary fault zones preserve a wide variety of deformation structures spanning diverse temporal and spatial scales, including pressure solution, mineral veining, cataclasis, and earthquake slip accompanied by frictional heating. These structures have traditionally been discussed as products of distinct deformation mechanisms. However, they commonly coexist and overprint one another within a single fault zone, indicating the need for a unified framework to understand their relationships. In this study, we propose a conceptual framework for the transition from slow to fast deformation based on the evolution of rheological heterogeneity, integrating geological observations with a deformation localization phase diagram derived from numerical modeling.Finite-element simulations demonstrate that stress and strain rate concentrate along material-property boundaries, where repeated yielding, strain softening, and stress redistribution progressively promote deformation localization. Furthermore, when the bulk strain rate is normalized by the model size, localization behavior becomes independent of the absolute scale and can be systematically described by two governing parameters: the rheological contrast and the bulk strain rate. Based on these results, we construct a deformation localization phase diagram using these two parameters. The phase diagram predicts that localization develops gradually under low rheological contrast but accelerates rapidly once the contrast exceeds a critical level. These results suggest that fast deformation emerges as a consequence of sufficiently developed rheological heterogeneity, implying that geological processes capable of modifying rheological structure through time play a fundamental role in controlling deformation style. Rheological contrast should therefore be regarded not as a fixed material property but as a state variable that evolves together with deformation.From this perspective, we present two natural examples illustrating the evolution of rheological heterogeneity. First, cathodoluminescence imaging combined with Al concentration mapping of cataclasite-hosted cements reveals the coexistence of rapidly precipitated and slowly precipitated cement generations. This observation indicates that fluid migration and mineral precipitation are heterogeneous in both space and time, suggesting that slow chemical processes progressively generate rheological contrasts within fault zones. Second, magnetic mapping of cataclasites recording frictional heating demonstrates that high-temperature alteration is localized along discrete fluid pathways rather than being distributed throughout the entire fault zone. This indicates that rapid localization concentrates fluid flow, heat, and mineral reactions into restricted regions, suggesting that fast deformation further modifies and reorganizes the pre-existing rheological structure.These observations imply a positive feedback between deformation and rheological heterogeneity. Slow chemical processes progressively build rheological contrast, and once a critical threshold is reached, rapid deformation localization is triggered. In turn, fast deformation localizes fluid flow, thermal effects, and mineral alteration, thereby reorganizing the rheological architecture of the fault zone and influencing subsequent deformation. We propose that the Slow–Fast deformation transition in accretionary fault zones can be understood as the co-evolution of deformation and rheological heterogeneity. The proposed deformation localization phase diagram provides a unified framework for interpreting diverse fault-rock fabrics and fluid-related processes and offers a new geological perspective on the long-term evolution of fault zones toward seismic slip.