ASOSOSCA MAAR STRATIGRAPHY: A GEOLOGICAL RECORD OF HOLOCENE PHREATOMAGMATIC AND MAGMATIC ERUPTIONS AT WESTERN MANAGUA, NICARAGUA

Asososca maar is the youngest of the clustered and aligned vents related to the active Nejapa-Miraflores Fault (NMF), located at the western skirts of Managua City, Nicaragua. The NMF is a North-South, 15km long, right-lateral fault that has been the locus of intense volcanic activity since the Oligocene. In this study, we present the detailed stratigraphy of the Asososca maar aided by radiocarbon dating of buried paleosols, in order to get an approach to Holocene eruptions related to the NMF. Asososca has an East-West elongated crater filled by a lake which is currently used for Managua water supply. It is excavated through a volcanic basement made of pyroclastic surge deposits interbedded with scoria fallouts. This basement consists of 26 eruptive units dated between 12,730 + 255 yr. B.P. and 6340 + 110 yr. B.P. covered by the phreatoplinian Masaya Tuff (<2000 yr). The Asososca Unit, is younger than 1245 +125/-120 yr B.P. and is capped by a 535 + 70 B.P. paleosol. The Asososca Unit is a well stratified sequence of lapilli and ash beds, characterized by moderately sorted deposits of massive, plane-parallel and cross- stratified, beds, with local impact sags, mainly composed of subrounded accidental basaltic to basaltic-andesitic scorias, lava fragments, ignimbrite fragments and dacitic accidental pumice. Juvenile basaltic-andesitic glass fragments are subordinated to accidental lithics, being less than 25% in volume. These deposits were likely formed by pyroclastic dry surges produced by a phreatomagmatic eruption. Due to its age, Asososca indicates that potential explosive eruptions may occur along the NMF at distances shorter than 5 km from the capital city of Nicaragua. This kind of activity, together with strong seismicity associated to the active fault represents a serious hazard to infrastructure and population of ca. 1,300,000 inhabitants.