The ∼ 1245 yr BP Asososca maar: New advances on recent volcanic stratigraphy of Managua (Nicaragua) and hazard implications

Abstract

Asososca maar is located at the western outskirts of Managua, Nicaragua, in the central part of the active, N–S trending and right-lateral Nejapa–Miraflores fault that marks an offset of the Middle America Volcanic Arc. It constitutes one of the ∼ 21 vents aligned along the fault, between the Chiltepe Volcanic Complex to the North and Ticomo vents to the South. Asososca consists of an East–West elongated crater filled by a lake, which is currently used for supplying part of Managua with drinking water (10% of the capital city demand). The crater excavated the previous topography, allowing the observation of a detailed Holocene stratigraphic record that should be taken into account for future hazard analyses. We present a geological map together with the detailed stratigraphy exposed inside and around Asososca crater aided by radiocarbon dating of paleosols. The pre-existing volcanic sequence excavated by Asososca is younger than 12,730 + 255/− 250 yr BP and is capped by the phreato–plinian Masaya Tuff (< 2000 yr BP). The pyroclastic deposits produced by Asososca maar (Asososca Tephra, in this work) display an asymmetric distribution around the crater and overlie the Masaya Tuff. The bulk of the Asososca Tephra is made of several bedsets consisting of massive to crudely stratified beds of blocks and lapilli at the base, and superimposed thinly stratified ash and lapilli beds with dune structures and impact sags. Coarser size-fractions (>− 2 ϕ) are dominated by accidental clasts, including basaltic to basaltic–andesitic, olivine-bearing scoriae lapilli, porphyritic and hypocrystalline andesite blocks and lapilli, altered pumice lapilli and ash, and ignimbrite fragments. Juvenile fragments were only identified in size-fractions smaller than − 1 ϕ in proportions lower than 25%, and consist of black moss-like, fused-shape, and poorly vesiculated, fresh glass fragments of basaltic composition (SiO 2 ∼ 48%). The Asososca Tephra is interpreted as due to the emplacement of several pyroclastic surges originated by phreatomagmatic eruptions from Asososca crater as suggested by impact sags geometry and dune-crest migration structures. According to radiocarbon dating, these deposits were emplaced at 1245 + 125/− 120 yr BP. The stratigraphic position of the Asososca Tephra and the well-preserved morphology of the crater indicate that Asososca is the youngest vent along the Nejapa–Miraflores fault. Explosive eruptions might therefore occur again along this fault at the western outskirts of Managua. Such kind of activity, together with the strong seismicity associated to the active fault represents a serious hazard to urban infrastructure and a population of ca. 1.3 million inhabitants.