ABSTRACT The Iglesia basin, in the San Juan Province, is the northern part of a Cenozoic piggy-back basin (the Iglesia-Calingasta-Uspallata basin), located above of the active flat-slab subduction zone of western Argentina. This basin is located between two mountain ranges, the Cordillera Frontal to the west and the Precordillera to the east, affected by thin and thick skinned tectonics, respectively. It is elongated in a north-south direction (70 km) and has a maximum width of 35 km. We have analyzed 17 seismic reflection lines corresponding to more than 500 km of interpreted sections. A strong reflection at 0.5-2 s is interpreted as indicative of shales and sandstones in the upper part of the Agua Negra Formation (Carboniferous); this represents a seismic basement for the Iglesia basin clearly separating stratified (shallower) from chaotic (deeper) reflectors. Several high angle faults have been recognized in the interpreted seismic lines affecting the Neogene fill and even the basement; some of them correlate with structures observed at the surface such us El Tigre Fault System. A 3D model shows the seismic top of basement smoothly deepening down to 3,650 m beneath the Pismanta area. The basement continues upward to the east with a steeper slope clearly defining an asymmetrical shape for the basin. Moreover, there are three depocenters of similar maximum depths around the geothermal hot spring of the Pismanta center. Some of the interpreted faults may be helping in the outflow process of meteoric water heated by a normal geothermal gradient. Considering earthquake data framework, our observations correlate with transpressive deformation likely associated with El Tigre Fault System, which affects Neogene and Pleistocene strata of the Iglesia Group.