Geomorphology, morphometry, spatial distribution and ages of mafic monogenetic volcanoes of the Peinado and Incahuasi fields, southernmost Central Volcanic Zone of the Andes

Abstract

The back-arc southern Puna region (south of ~24°S) in the Central Volcanic Zone of the Andes contains hundreds of Miocene to Quaternary mafic monogenetic volcanoes roughly distributed in several volcanic fields. Analysis of the geomorphology, morphometry, spatial distribution and age of 36 mafic monogenetic centers in the Peinado and Incahuasi fields, at the southwestern margin of the Puna plateau, gives insights on the links that mafic monogenetic volcanism can have with main and secondary structures, and the effects of degradation on scoria cone shape. Degree of preservation, morphometry and six new unspiked K-Ar ages indicate two distinct age groups, old centers > 2 Ma and young centers ≤ 1 Ma. The older mafic centers consist of degraded scoria cones and lava flows with an estimated cumulative bulk volume of 2.2 km3. The younger mafic centers display a variety of well-preserved landforms (scoria cones, lava flows, maars, tuff rings, domes) and have an estimated cumulative volume of 6.2 km3. Their distribution in a main NNE-SSW and secondary ENE-WSW trends suggests a control by the regional NNE-SSW striking Peinado and San Francisco fault systems in a dextral strike-slip configuration. Scoria cone and lava flow morphometries are similar in both fields, but a marked difference indicating hydrovolcanic activity is the presence of tuff rings, maars and tephra fallout deposits only in the Peinado field. Morphometric analysis of dated pristine and degraded scoria cones of the southern Puna region shows that parameters related tocrater depth and inner crater steepness decrease consistently with age, suggesting that crater infilling is the degradation- related process that is most sensitive to age at scoria cones. In the southern Puna, high values of crater depth/crater width, crater depth/cone height and mean inner crater slope are indicative of a young (<1 Ma) age, but low values are not necessarily indicative of an old (>1 Ma) age, as young cones can have initially low values.