Abstract
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.
Highlights
Most continental crustal deformation is localized into ductile shear zones and brittle, commonly seismogenic, faults (e.g., Snoke et al, 1998)
We show that the large-scale sinuous geometry of the seismogenic Bolfin Fault Zone (BFZ) is imposed by the local pinning of fault orientation on magmatic structures related to the precursory history of the magmatic arc
We propose that magmatic-related structures, such as foliated plutons whose magmatic foliation can extend for several kilometers and dyke swarms, play a pivotal role in controlling the geometry of crustal-scale faults within magmatic arcs, as do cooling joints at the scale of mesoscale faults within a single pluton (e.g., Di Toro & Pennacchioni, 2005; Pennacchioni et al, 2006; Segall & Pollard, 1983; Smith et al, 2013)
Summary
Most continental crustal deformation is localized into ductile shear zones and brittle, commonly seismogenic, faults (e.g., Snoke et al, 1998). The evolution in space and time of crustal-scale, seismogenic faults remains poorly known
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