Abstract
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; be partially obliterated by younger, arc-parallel faults; and 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 also affected by younger reactivations of it, were obtained. They yielded 20.79 ± 0.13 Ma (granodiorite) and 20.69 ± 0.07 Ma (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 channelling 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 has been active at least since the Jurassic.
Highlights
Large-scale permeability of the crust and lithospheric mantle is controlled by the presence of lithospheric-scale fault systems (McCuaig and Hronsky, 2014)
As they correspond to ancient features, in many cases they are oriented oblique to the axis of younger magmatic arcs and can be obliterated by younger, arc-parallel faults. They might be covered by volcanic and sedimentary deposits through which the fault has to propagate vertically (Fig. 1). These difficulties can be overcome with a multidisciplinary approach, combining detailed, field-based structural mapping with geologic observations at different scales and various types of geophysical datasets; such an approach has led to the recognition of this type of lithospheric-scale structures in different geological settings, improving our understanding about their complex reactivation histories and their relationship with magmatic–hydrothermal activity (e.g. Chernicoff et al, 2002; Gow and Walshe, 2005; Cembrano and Lara, 2009; Acocella et al, 2011; Lanza et al, 2013; Richards et al, 2013; Fox et al, 2015; Febbo et al, 2019)
Data were collected during three field campaigns between 2016 and 2018
Summary
Large-scale permeability of the crust and lithospheric mantle is controlled by the presence of lithospheric-scale fault systems (McCuaig and Hronsky, 2014) This implies that their characterization is fundamental to understand the distribution and geometry of magmatic–hydrothermal systems and the mineral deposits, volcanic complexes and geothermal sys-. The surface expression of these types of structures is often very subtle As they correspond to ancient features, in many cases they are oriented oblique to the axis of younger magmatic arcs and can be obliterated by younger, arc-parallel faults. They might be covered by volcanic and sedimentary deposits through which the fault has to propagate vertically (Fig. 1). These difficulties can be overcome with a multidisciplinary approach, combining detailed, field-based structural mapping with geologic observations at different scales and various types of geophysical datasets; such an approach has led to the recognition of this type of lithospheric-scale structures in different geological settings, improving our understanding about their complex reactivation histories and their relationship with magmatic–hydrothermal activity (e.g. Chernicoff et al, 2002; Gow and Walshe, 2005; Cembrano and Lara, 2009; Acocella et al, 2011; Lanza et al, 2013; Richards et al, 2013; Fox et al, 2015; Febbo et al, 2019)
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