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.
Delineation of catchment zones of geothermal systems in large-scale rifted settings
