Evidence for magmatic-related induced seismicity along misoriented faults in the Southern Andes

In the Andes, the oblique convergence/subduction between the Nazca and South American plates, is not only accommodated through partitioned thrust and strike-slip deformation along major arc-parallel structures, but also through Andean Transverse Faults (ATFs). These are thought to be inherited pre-Andean structures serving as current crustal weak zones which favour arc-related fluid release. They are categorized into: ENE-striking, sub-parallel to regional ~N70°E s ₁, thus favourably oriented to allow fluids migration and 2) highly misoriented NW to WNW-striking faults. These NW to WNW-striking ATFs seem to control quaternary volcano and hot springs locations in the Southern Andes while their intersections with margin-parallel faults host major ore deposits in the north, indicating they potentially serve as magmatic and hydrothermal pathways.

In this study we show seismicity constrained from a 20-month-long seismic deployment, from April 2017 to December 2018, in the Southern Volcanic Zone of the Andes. We locate over 700 crustal earthquakes and show that previously unknown WNW ATFs are seismogenic. Focal mechanisms show dominant strike-slip deformation along the main ATF seismogenic zone observed in our study. A second area of high seismicity is located beneath the recently active Planchón-Peteroa Volcanic Complex in the south of our study area. A coeval magnetotelluric study shows these regions of high seismicity bound an area of high conductivities, interpreted as either hydrothermal or magmatic fluids reservoirs in the upper crust.

We suggest that fluid migration, possibly driven by magmatic processes lead to local pore fluid pressure increases that trigger seismicity along the pre-existing ATF structure. This fault may act as a conduit for fluids along-strike, while being a barrier to northwards fluid flow. This supports the hypothesis that these features play a key role in controlling fluid flow in the upper crust.