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Structural evolution of a crustal-scale seismogenic fault in a magmatic arc: The Bolfin Fault Zone (Atacama Fault System)
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  • Simone Masoch,
  • Rodrigo Gomila,
  • Michele Fondriest,
  • Erik Jensen,
  • Thomas Matthew Mitchell,
  • Giorgio Pennacchioni,
  • Jose Cembrano,
  • Giulio Di Toro
Simone Masoch
Dipartimento di Geoscienze, Università degli Studi di Padova

Corresponding Author:[email protected]

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Rodrigo Gomila
Dipartimento di Geoscienze, Università degli Studi di Padova
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Michele Fondriest
Institut des Sciences de la Terre (ISTerre), Université Grenoble-Alpes
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Erik Jensen
CIGIDEN
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Thomas Matthew Mitchell
Department of Earth Sciences, University College London
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Giorgio Pennacchioni
Dipartimento di Geoscienze, Università degli Studi di Padova
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Jose Cembrano
Departamento de Ingeniería Estructural y Geotécnica, Pontificia Universidad Católica de Chile
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Giulio Di Toro
Dipartimento di Geoscienze, Università degli Studi di Padova
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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.
Aug 2021Published in Tectonics volume 40 issue 8. 10.1029/2021TC006818