Ultracataclastic vein evolution within a granodiorite along the Naxos-Paros Detachment System

Ultracataclastic and pseudotachylytic veins are interpreted as direct evidence of coseismic slip, reflecting the conditions and mechanisms associated with fault rupture. Although the Naxos granodiorite, Greece, is known for its pristine pseudotachylyte exposures, these features remain largely understudied in terms of their microstructural evolution and implications for upper and middle crustal seismicity. We examine a suite of well-preserved veins hosted in the deformed Miocene granodiorite, historically described as examples of melt-origin pseudotachylytes, but here are re-interpreted as dominantly ultracataclasites, produced from the comminution of host rock. The veins occur in the immediate footwall of the fluid-rich, low-angle Naxos-Paros Detachment System, active between c. 20-10 Ma, and possess a similar composition as the surrounding host rock of primarily albite (35%), quartz (25%), orthoclase (16%), and biotite (12%). Structural field observations from the highest strain zone reveal three distinct crosscutting vein sets. Electron backscatter diffraction mapping of host rock porphyroclasts of albite, orthoclase, and quartz crosscut by ultracataclastic veins demonstrates that cataclasis is the dominant deformation mechanism. Variations in microstructural maturity of vein fragments suggest episodic emplacement, with rupture exploiting pre-existing slip planes. Cuspate phase boundaries between orthoclase and albite, initially observed in optical and scanning electron microscopy, are confirmed by electron microprobe analyses as a result from dissolution-precipitation reactions. These features indicate deformation by dissolution-precipitation creep. The results suggest that fluid-mediated reactions localized at vein tips contributed to weakening, generating a mechanical-chemical feedback loop that promoted ultracataclastic vein propagation along the detachment. • Three types of fault-vein sets were defined in the footwall of the Naxos Detachment • Cataclasis, not plastic deformation, facilitated fault-vein emplacement on Naxos • A dissolution-reprecipitation reaction caused localized weakening at the vein tips • Feedback between cataclasis and fluid-induced reactions promoted vein propagation