STRUCTURAL AND FLUID SOURCE EVOLUTION DURING THE GROWTH OF A CORDILLERAN INTRA-ARC SEISMOGENIC FAULT SYSTEM (ATACAMA DESERT, CHILE)

Among the many factors controlling earthquake mechanics, the three-dimensional architecture of fault systems and fluids contribute to the seismic vs. aseismic behavior of faults. Because crustal faults are hardly well-exposed at the Earth's surface, little is known about their architectural evolution in space and time. Here, we describe the architectural evolution in space and time and the deformation environment of the >40-km-long, exhumed, seismogenic Bolfin Fault Zone (BFZ) of the Early Cretaceous, intra-arc, strike-slip Atacama Fault System (Northen Chile). The BFZ is arranged into three main segments and has a sinuous crustal-scale geometry cutting through Jurassic to Early Cretaceous diorite and tonalite-granodiorite plutons. The BFZ consists of pseudotachylyte (i.e., solidified frictional melt)-bearing cataclastic strands linked by hybrid-extensional, epidote-cemented fault-vein systems, and formed in a fluid-rich, seismically active environment at 3-7 km and 200-300 °C. The sinuous crustal geometry of the BFZ resulted from (i) exploitation of precursory geometrical anisotropies (i.e., magmatic foliation of plutons and dyke swarms) during fault nucleation and (ii) hard linkage of these anisotropy-pinned fault segments during fault growth. Stable hydrogen and oxygen isotopes indicate that the early stage pseudotachylyte-cataclasite fault strands formed in a rock-buffered environment with limited circulation of external fluids. Conversely, the later epidote-cemented faulting was modulated by the conspicuous ingression of external, basin-derived fluids along the fault system. We conclude that the complex pattern of heterogeneities in the magmatic arc played a key role in controlling the architectural evolution of the BFZ, and the change in deformation environment was coupled with the progressive larger infiltration of basin-derived fluids within the BFZ, which represents an exhumed analogue of upper-crustal, seismically active hydrothermal systems.