Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatic systems and activate mesh-like fault-fracture networks at zone of fault complexity. How these networks develop through space and time along seismic faults is poorly constrained in the geological record. Here, we describe a spatially dense array of small-displacement (< 1.5 m) epidote-rich fault-veins within granitoids, occurring at the intersections of subsidiary faults with the exhumed seismogenic Bolfin Fault Zone (Atacama Fault System, Northern Chile). Epidote faulting and veining occurred at 3-7 km depth and 200-300 °C ambient temperature. At distance ≤ 1 cm to fault-veins, the magmatic quartz of the wall-rock shows (i) thin (<10- µm-thick) interlaced deformation lamellae, and (ii) crosscutting quartz-healed veinlets. The epidote-rich fault-veins (i) include clasts of deformed magmatic quartz, with deformation lamellae and quartz-healed veinlets, and (ii) record cyclic events of extensional-to-hybrid veining and either aseismic and seismic shearing. Deformation of the wall-rock quartz is interpreted to record the large stress perturbations associated with the rupture propagation of small earthquakes. Instead, dilation and shearing forming the epidote-rich fault-veins are interpreted to record the later development of a mature and hydraulically-connected fault-fracture system. In this latter stage, the fault-fracture system cyclically ruptured due to fluid pressure fluctuations, possibly correlated with swarm-like earthquake sequences.

Coarse-grained quartz veins from the Prijakt Nappe (Austroalpine Unit, Schober Mountains, Eastern Alps), that formed under amphibolite facies conditions, were overprinted by lower greenschist facies deformation. During overprinting, subgrain rotation (SGR) recrystallization was the dominant mechanism assisting the evolution from protomylonite to (ultra)mylonite. The initial Ti-concentration [Ti] (3.0-4.7 ppm) and corresponding cathodoluminescence (CL) signature of the quartz vein crystals were reset to different degrees mainly depending on the availability of fluids and their partitioning across the microstructure. The amount of strain played a subordinate role in resetting. In recrystallized aggregates the most complete re-equilibration ([Ti] of 0.2-0.6 pm) occurred in strain shadows surrounding quartz porphyroclasts, acting as fluid sinks, and in localized shear bands that channelized fluid percolation. We applied a correlative multi-analytical workflow using optical and electron microscopy methods (e.g. electron backscatter diffraction and cathodoluminescence) in combination with secondary ion mass spectroscopy for [Ti] measurement. The most efficient [Ti] resetting mainly occurs along wetted high angle boundaries (misorientation angle >10-15°), and to a minor extend (partial resetting) along dry low angle boundaries (<10-15°). This key-study prove for the first time that pure subgrain rotation recrystallization in combination with dissolution-precipitation under retrograde condition is able to provide microstructural sites to apply the TitaniQ geothermobarometer at deformation temperatures down to 300-350 °C provided that information on pressure and Ti-activity is available.

How major crustal-scale seismogenic faults nucleate and evolve in the crystalline basement represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and ≤ 310 °C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geological surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from hard linkage of these anisotropy-pinned segments during fault growth.