In an active volcanic arc, magmatically sourced fluids are channeled through the brittle crust by structural features. This interaction is observed in the Andean volcanic mountain belt, where volcanoes, geothermal springs and the locations of major mineral deposits coincide with NNE-striking, convergent margin-parallel faults and margin-oblique, NW/SE-striking Andean Transverse Faults (ATF). The Tinguiririca and Planchón-Peteroa volcanoes in the Andean Southern Volcanic Zone (SVZ) demonstrate this relationship, as both volcanic complexes and their spatially associated thermal springs show strike alignment to the outcropping NNE oriented El Fierro Thrust Fault System. This study aims to constrain the 3D architecture of this fault system and its interaction with volcanically sourced hydrothermal fluids from a combined magnetotelluric (MT) and seismicity survey. The 3D conductivity model and seismic hypocenter locations show correlations between strong conductivity contrasts and seismic clusters in the top 10km of the crust. This includes a distinct WNW-striking seismogenic feature which has characteristics of the ATF domains. As the surveyed region is characterized by high heat flow regimes, volcanic activity and hydrothermal systems related to the volcanic arc, the conductivity contrast suggests that magmatically derived fluids meet an impenetrable barrier, most likely the sealed core of the fault. The resulting increase in hydrostatic fluid pressure facilitates seismic activity on this WNW oriented structure. These results provides the first observation of the mechanism behind the reactivation and seismogenesis of ATF. The study also uncovers the role of the ATF the compartmentalization of magmatic-derived fluids that accumulate to form hydrothermal reservoirs in the SVZ.