Cenozoic lithospheric faulting in the Asunciَn Rift, eastern Paraguay

The Eocene magmatism of the western segment of the Asunción Rift (Asunción alkaline province) was controlled by deep NW–SE-trending faults with magnetic and gravimetric expressions that served as conduits for the ultra-alkaline magma coming from depths of more than 60 km and bearing mantle xenoliths to the surface. Petrologic data suggest that a short interval of time elapsed between the magma ascension and its emplacement at shallow crustal levels. Cooling and fracturing in relatively restricted areas indicate that the NW–SE-trending structural alignments represent deep faults whose activity caused great energy loss in the asthenosphere, leading to melting of the lithospheric mantle by decompression during a relatively short time interval. Families of fractures associated with intrusions in the ultra-alkaline bodies showed a stress regime with σ1 horizontal, in the NW–SE direction, σ3 horizontal, in the NE–SW direction, and σ2 vertical, all of them compatible with the action of a right-lateral strike-slip shear couple oriented E–W. The validity of this regime during the Eocene is corroborated by the NW–SE orientation of the nephelinitic dykes.