Structural evolution of a crustal-scale seismogenic fault in a magmatic
arc: The Bolfin Fault Zone (Atacama Fault System)
Abstract
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.