Understanding Chaos: Fabric-Forming Processes from the Kampos Belt "Mélange" (Syros) and Implications for Megathrust Rheology

The architecture of deep subduction zones governs the mechanical behavior and rheology of the subduction interface, influencing processes from long-term mountain building to earthquake rupture dynamics. Traditionally, mélanges have been considered chaotic assemblages resulting from high-strain tectonic mixing, yet recent studies challenge this view. The Kampos Belt on Syros Island has been regarded as a prime example of a subduction mélange, where high-viscosity eclgoties, blueschsits and metagabbros are embedded in a lower-viscosity metasomatic matrix composed of chlorite tremolite schist. However, the formation mechanisms of this structure remain debated. This has significant implications for understanding how, where and when strain is accommodated along megathrust shear zones.

In this study, we present new high-resolution field mapping, structural and petrological analyses, along with thermodynamic modeling to refine the spatial distribution of lithologies, deformation styles, and metasomatic processes that contributed to the structure of the Kampos Belt. Our findings suggest that rather than a chaotic mélange, the Kampos Belt represents a coherent stack of variably deformed metamafic slivers juxtaposed due to localized deformation along shear zones, where lithological variations largely reflect pre-subduction heterogeneities. These localized shear zones originate from different sources, including: metasedimentary slivers (mica schist), relict peridotitic lenses (antigorite schist), and metasomatic horizons associated with relict mafic lenses (chlorite-tremolite schist). Moderate- to high-strain domains are preferentially localized along metasomatic chlorite tremolite schist shear zones, which formed through fluid-assisted reactions at prograde to early-exhumation conditions. These metasomatic zones played a key role in strain localization, weakening the subduction interface and shaping the observed shear zone architecture. Our results challenge the classical interpretation of Kampos as a mélange. We suggest that the architecture of the belt is unlikely to have formed through large-scale tectonic mixing, instead we support a model where pre-existing lithological heterogeneity and fluid-assisted deformation (e.g., continued metasomatism along fractures) controlled the shear zone fabric. These findings have broad implications for subduction zone rheology, as they highlight the role of lithology dependent strain partitioning and fluid-induced weakening in deep megathrust shear zones.