Reverse Faulting as a Crucial Mechanism for Magma Ascent in Compressional Volcanic Arcs: Field Examples from the Central Andes

The nature of crustal deformation in active arcs and the feedback mechanisms between tectonics and magma transport constitute fundamental problems in the understanding of volcanic systems. Additionally, for geothermal energy exploration, a better understanding of how crustal architecture and stress field controls fluid ascent and heat transfer from deep levels to the surface is crucial. The Central Andes volcanic belt is an excellent, modern example of such systems but, the scarcity of good outcrops has limited our ability to define the relations between structure and volcanism. In the Salar de Atacama Basin of northern Chile, there are good exposures of folded and faulted Neogene units (continental sediments, volcanic rocks and ignimbrites) and reverse faults spatially and temporally related to volcanic edifices. The subsurface of the study area has been interpreted by previous authors as a thin-skinned, 6-8 km-deep, east-vergent compressional belt. We carried out structural mapping, Digital Elevation Models (DEMs) analyses, strain tensor analyses and fault-related fold kinematic modelling to assess the causal relationship between compressional deformation and magmatism in this region. Field observations indicate that the structures deformed progressively Oligocene-Miocene continental sedimentary units, the upper sedimentary infill of the Salar de Atacama basin (Pliocene-Present), and Pliocene-Pleistocene Ignimbrites. The topographic expression of the compressional belt corresponds to a set of subparallel, asymmetric, fault-related-folds, which can be seen in the field as prominent NS-trending ridges with heights ranging between 50 and 400 m. Furthermore, we found evidence of a ~100 km-long structure along the active magmatic arc, so-called Miscanti Fault. This fault represents the easternmost expression of the above mentioned compressional belt. Pleistocene-Holocene monogenetic cones and strato-volcanoes are located either at the hinge zone of fault-propagated folds and/or at the surface trace of concealed thrusts. Strain tensor analyses and the NS orientation of the fold-and-thrust-belt, exhibits a nearly steady EW orientation of the maximum compressional axis. This style of deformation has been active from at least 9 Ma. We propose that volcanism occurred during horizontal, EW compression perpendicular to the arc. The listric and/or flat-ramp geometry of main thrust structures are used as channels for magma to ascend. Magma transport may occurs during the development of structural permeability on the fault plane, in a similar way to that of the fault-valve mechanism. Magma reservoirs could form in flats parts of faults and/or at the core of fault-propagated antiforms. Surface extension perpendicular to the anticlinal hinge results from “buckling”, locally changing the orientation of main stresses (σ1 vertical and σ3 horizontal). This process generates fold-axis parallel vertical weak zones and facilitates magma extrusion.