Preprints
https://doi.org/10.5194/se-2020-142
https://doi.org/10.5194/se-2020-142

  21 Aug 2020

21 Aug 2020

Review status: a revised version of this preprint was accepted for the journal SE and is expected to appear here in due course.

The Piuquencillo Fault System: a long-lived, Andean-transverse fault system and its relationship with magmatic and hydrothermal activity

Jose Piquer1, Orlando Rivera2, Gonzalo Yañez3,4,5, and Nicolas Oyarzun1 Jose Piquer et al.
  • 1Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Valdivia, 5090000, Chile
  • 2Minera Peñoles de Chile, Santiago, 8320000, Chile
  • 3Departamento de Ingeniería Estructural y Geotécnica, Pontificia Universidad Católica de Chile, Santiago, 8320000, Chile
  • 4Centro de Excelencia en Geotermia de los Andes (CEGA), Santiago, 8320000, Chille
  • 5Núcleo Milenio Trazadores de Metales (NMTM), Santiago, 8320000, Chile

Abstract. Lithospheric-scale fault systems control the large-scale permeability in the Earth’s crust and lithospheric mantle, and its proper recognition is fundamental to understand the geometry and distribution of mineral deposits, volcanic and plutonic complexes and geothermal systems. However, their manifestations at the current surface can be very subtle, as in many cases they are oriented oblique to the current continental margin and to the axis of the magmatic arc, can be partially obliterated by younger, arc-parallel faults, and can also be covered by volcanic and sedimentary deposits, through which the fault might propagate vertically. The Piuquencillo Fault System (PFS) is a proposed lithospheric-scale fault system, located in the Main Cordillera of Central Chile. Here we present the results of the first detailed field study of the PFS, based on structural data collected at 82 structural stations distributed across all the Western Main Cordillera. The first published U-Pb zircon ages for the La Obra batholith, which is bounded to the south by the PFS but it is also affected by younger reactivations of it, were obtained. They yielded 20.79 ± 0.13 Ma (granodiorite) and 20.69 ± 0.07 (monzogranite). Statistical analysis of fault plane data shows that the presence of the PFS is reflected on a strong preferred NW to WNW strike, with variable dip directions, evident from the analysis of the total fault plane population and also from individual segments of the PFS. In some segments, the presence of major NE to ENE-striking faults which intersect the PFS is also reflected in the preferred orientation of fault planes. Preferred orientations of hydrothermal veins, breccias and dikes show that both the PFS and some ENE-striking faults were capable of channeling hydrothermal fluids and magma. Kinematic and dynamic analysis of fault-plane data reveals that most of the PFS was reactivated with sinistral ± reverse kinematics during the Neogene, under a strike-slip to transpressive regime with E- to ENE-trending shortening direction (σ1). Detailed kinematic and dynamic analyses were completed for various segments of the PFS and also for the different rock units affected by it. This study supports the concept that the PFS is a lithospheric-scale fault system, which strongly controlled deformation and the flow of magmas and hydrothermal fluids during the Neogene. The PFS forms part of a larger, margin-transverse structure, the Maipo Deformation Zone, a continental-scale discontinuity which cut across the entire Chilean continental margin, and which has been active at least since the Jurassic.

Jose Piquer et al.

 
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Jose Piquer et al.

Jose Piquer et al.

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Short summary
A proper recognition of deep, long-lived fault systems is very important for society. They can produce potentially dangerous earthquakes. They can also act as pathways for magmas and hydrothermal fluids, leading to the formation of volcanoes, geothermal systems and mineral deposits. However, the manifestations of these very old faults in the present-day surface can be very subtle. Here we present a detailed, multi-disciplinary study of a fault system of this type in the Andes of central Chile.