Marine Tephrochronology and Quaternary Explosive Volcanism in the Lesser Antilles Arc

Abstract

Explosive volcanism in the Lesser Antilles arc has been tackled by combined tephrochronologic studies of land deposits and 100 deep-sea piston cores from the adjacent Atlantic and Grenada Basins. Volcanic production in the arc during the late Quaternary has been estimated from deep-sea core data for a 105 year period as 527 km3 (285 km3 D.R.E.), with the majority of this material (445 km3) deposited in the marine environment. Marine tephra deposits are principally of two types: ash-fall layers and a variety of subaqueous pyroclastic gravity flow deposits. Ash-fall layers and associated air-borne dispersed ash represent about 1/3 of total production. They have been deposited almost exclusively in the Atlantic east of the arc and their dispersal is entirely controlled by westerly high-altitude winds. A variety of pyroclastic sediment gravity flow deposits account for approximately 2/3 of the volcanic production. Most important are deposits which form as a result of the entry of ignimbrites directly into the sea during major eruptions. They have travelled up to 250 km from source along the back-arc basin and form massive, poorly graded deposits up to 5 m thick. Pyroclastic gravity flow deposits occur predominantly in the Grenada Basin, west of the arc and their distribution is aided by the steep (9°) slopes of the western arc flank. Microprobe analyses of glass shards in tephra deposits from the deep-sea and land show that rhyodacites are the dominant products of volcanism in this arc.