Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles

Paulatto, M. and Minshull, T. and Baptie, B. and Dean, S. and Hammond, James O.S. and Henstock, T. and Kenedi, C. and Kiddle, E. and Malin, P. and Peirce, C. and Ryan, G. and Shalev, E. and Sparks, R.S.J. and Voight, B. (2010) Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles. Geophysical Journal International 180 (2), pp. 685-696. ISSN 0956-540X.

Abstract

To better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seismic experiment (Seismic Experiment with Airgun-source – Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufriére Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5–5.5 km s-1) beneath the island, with highest velocities beneath the Soufriére and Centre Hills, corresponding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the offshore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s-1, consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ∼1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0–6.0 km s-1 and is inferred to correspond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ∼5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes.

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