Nejapa Tephra: The youngest (c. 1 ka BP) highly explosive hydroclastic basaltic eruption in western Managua (Nicaragua)

Beschreibung

Nejapa maar (2.5 x 1.4 km, c. 120 m deep), the largest of 2 prominent maars along the Holocene Nejapa Miraflores Lineament (NML), is the source of a widespread basaltic tephra blanket (Nejapa Tephra: NT) in western central Nicaragua as shown by isopachs and isopleths. The NT, the youngest tephra deposit of the newly defined Nejapa Miraflores Group (NMG), covers an area >10 km² large in western/northwestern Managua. The minimum total magma volume erupted is estimated as >0.1 km³. Juvenile, dominantly slightly vesicular (20-40 vol.%) basically tachylitic cauliflower-shaped lapilli with an average density of 2.1 g/cm³, make up >90 vol. % of the NT, while lithics generally comprise <1 vol. % except in the proximal facies of the southern and northern fan. This, the paucity of fine-grained tuffs and the dominant plane-parallel bedding all suggest fragmentation by shallow interaction of a vesiculating magma with external water, eruption in moderately high eruption columns (~ 7-10 km) and dominant deposition as dry to damp warm to cool fallout. Surge transport is reflected in fine-grained locally crossbedded tephra beds that are most common in an elongate depression just west of Asososca maar (AM). Synvolcanic faulting along the NML is inferred. Faults mapped in the study area indicate that the activation of the N-S trending Nejapa Miraflores Fault (NMF), representing the western flank of Managua graben, preceded deposition of NT and underlying Masaya Tuff (1.8 ka BP), Chiltepe Pumice (1.9 ka BP) and Masaya Triple Layer (2.1 ka BP). Dike intrusion and faulting along the NML are probably related processes and faulting may have facilitated diking and vice versa. The NT-deposit is underlain by a regional paleosol and topped by a soil. The basal paleosol contains pottery sherds made by the Usulután negative technique during the Late Formative period (2.7-1.7 ka BP). The soil overlying NT contains pottery related to the Ometepe technique dated as between c. 1 350 and 1 550 AD.

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Juanita Rausch

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Geology Hazards Hydroclastic Nejapa Miraflores Lineament Nejapa Tephra Nejapa maar Western Managua

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