Abstract Despite a number of published papers focusing on the geodynamic implications of the recent Southern Puna mafic magmatism, there have been fewer studies of the volcanology and stratigraphy of this outstanding volcanism. This paper presents a detailed map of two well-preserved Quaternary scoria cones showing their complex stratigraphy. Complementary morphometric, morpho-structural, petrographic and geochemical data were used to reconstruct the evolution of both volcanoes. The occurrence of more than one eruption at each volcano was inferred by the recognition of temporal hiatuses using morpho-stratigraphic criteria. The polycyclic nature of both scoria cones could be related to a combination of a high input magma in response to lithospheric delamination, a favourable regional stress field and the interaction of rising magma with pre-existing faults. The youngest eruptions in both volcanoes were complex, with shifts in the eruptive style from violent strombolian to hawaiian/strombolian phases, and probably lasted for a few years. The explosive activity was accompanied by the emission of lava flows from lateral vents. Phreatomagmatic activity was triggered during the waning stages of the eruptions. The occurrence of more than one eruption in a single scoria cone and the changes in the eruptive style during long-lasting eruptions are important topics for volcanic hazard assessment in the Southern Puna.

Abstract Anisotropy of magnetic susceptibility (AMS) data reveal heterogeneous pyroclastic flow processes and variable flow directions within the intra-caldera setting of the Permian rhyolitic welded Ora ignimbrite. Magnetic fabric is primary, orientated during the pyroclastic flow emplacement, and is controlled by paramagnetic and ferromagnetic mineral phases. The ignimbrite has typically weak mean magnetic susceptibilities (1.32–21.8×10 −4 SI) but with a large spread and low anisotropy degrees (1.003–1.023), which vary in different parts of the caldera. The intra-caldera magnetic fabric provides significant information on the dynamics of the intra-caldera setting, relating to changing vertical and lateral flow emplacement processes. AMS shape ellipsoids range from oblate to prolate; these are interpreted to reflect the heterogeneous nature of the flow resulting from the influence of underlying topography, constraints of the caldera walls, primary welding and post-emplacement mineral growth. We have identified different depositional units and possible eruptive source regions, indicating that more than one source fissure vent was active during eruption within this caldera system. The lateral variations demonstrate a meandering of flow pulses. The caldera margin acts as an obstacle in preventing and rebuffing certain flows from scaling the caldera margin.