Feedback between cataclasis and interface-coupled reactions in ultracataclastic veins: insights from the Naxos granodiorite, Greece

Pseudotachylytes, quenched melts from frictional heating, and ultracataclasites, comminution of host rock, are considered direct evidence of coseismic slip. In hydrated systems, fluid-rock interactions can influence the nucleation and propagation of these earthquake-induced structures by facilitating element mobility and fault zone weakening. We conducted 2D microstructural and geochemical analyses on a series of ultracataclastic veins hosted in a deformed granodiorite on Naxos, Greece, to investigate potential interactions between physical and chemical processes along rupture paths. Naxos is a classical Miocene Cycladic metamorphic core complex, defined by a central migmatite core, with fluids introduced during peak metamorphism and subsequent brittle deformation. An I-type granodiorite was syn-tectonically emplaced, cooling rapidly from crystallization (650-680°C) at c. 12 Ma to <60°C by c. 9 Ma. The extensional Naxos-Paros Detachment System, active between c.12-9 Ma, dissects the pluton, producing a strong N-S stretching lineation and SCC' fabric generating top-to-N kinematics. Host rock from the immediate footwall of the detachment is composed of a coarse-grained (50 μm-2 mm) matrix, primarily composed of albite (35%), quartz (25%), orthoclase (16%), and biotite (12%). Fine-grained (5-60 μm) anastomosing ultracataclastic veins of the same composition intersect the host rock, with the thickest veins (7 cm) occurring sub-parallel to host rock foliation. Electron backscatter diffraction (EBSD) mapping of albite, orthoclase, and quartz targeted foliation-subparallel veins tips and host porphyroclasts crosscut by the veins. Evidence for minor crystal plasticity is observed as continuous to heterogeneous lattice distortion with an average misorientation of 03° within the host clasts, increasing to 15° towards clast rims and microfractures. The localization of microfractures emanating from the vein tips coupled with the spatial relationship between lattice distortions and microfractures, indicates that strain accommodation via crystal plasticity is linked to brittle deformation. This suggests that cataclasis is the primary deformation mechanism related to the propagation of ultracataclastic veins, which is supported by EBSD orientation data of fine-grained (<60 μm) fragments surrounding clasts (80-120 μm) of the same phase. The fine-grained populations are randomly oriented, with low internal misorientations up to ~10°, and no crystallographic relationship to the host porphyroclasts. Scanning electron microscopy (SEM) imaging highlighted aggregates of fine-grained albite (2-35 μm) along the vein margins with patchy zonation near microfractures and grain rims. A cuspate phase boundary between the albite grains and bordering orthoclase host clasts (2 mm) is characteristic of a dissolution-precipitation reaction front. Electron microprobe mapping of the phase interface reveals a K-depleted rim, 3 μm wide, along orthoclase clast margins, decreasing from ~13.6 wt% to 9.5 wt%. Inclusions of albite grains and Na-enriched zones, increasing to 2.6 wt% from 0.4 wt%, related to microfractures within the orthoclase clasts are present up to 55 μm away from the interface. Based on these observations, we propose that the interplay between cataclasis and interface-coupled reactions localized weakening, creating a feedback loop that promoted fracture propagation and drove continued injection of cataclastic material within the granodiorite. Our results demonstrate the impact of fluid-rock interactions on fault zone evolution and rupture conditions.