Structural Evolution of a Crustal‐Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)

How major crustal‐scale seismogenic faults nucleate and evolve in crystalline basements 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 ≤ 300°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 geologic 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 optimal to 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 the hard linkage of these anisotropy‐pinned segments during fault growth.