The shallow structure beneath Montserrat (West Indies) from new Bouguer gravity data

Abstract

[1] Although the persistently active Soufrière Hills Volcano (Montserrat, West Indies) is one of the most extensively studied active stratovolcanoes, a local Bouguer gravity map of the volcano and the island of Montserrat has yet to be constructed. We collected 157 new gravity data, which we analyzed and inverted in order to constrain the island's subsurface density distribution. Our model results reveal high-density material beneath the centers of the extinct volcanic complexes-presumably related to exposed dome cores-while the volcanic flanks and the active Soufrière Hills Volcano are underlain by low-density material. Volcaniclastic deposits and subsurface melt aggregations, respectively, may explain these negative gravity anomalies. Our results are in good agreement with previous structural observations from seismic tomography; however, a higher spatial density of the gravity survey network has allowed us to additionally capture smaller, shallow-seated anomalies in the gravity field that relate to tectonic structures and fluvial filling deposits. Citation: Hautmann, S., A. G. Camacho, J. Gottsmann, H. M. Odbert, and R. T. Syers (2013), The shallow structure beneath Montserrat (West Indies) from new Bouguer gravity data, Geophys. Res. Lett., 40,[5113][5114][5115][5116][5117][5118]

Magma storage in a strike-slip caldera

“…We interpret these anomalies as dense, remnant magma bodies (likely sheeted dykes), composed of fully crystallized chemically intermediate to evolved (andesitic, dacitic and rhyolitic) subvolcanic rocks. Their anomalous gravity signal and density contrast is similar to that of the dense cores of extinct dome complexes on Montserrat 33 , which have a modelled density of up to 2,600 kg m −3 . The parallel alignment of positive density contrasts with the domes, the NW segment of the main intra-caldera gravity low, and the principal compressive stress orientation, may suggest a progressive north-eastward evolution of magma storage and extraction over time.…”

The effects of thermomechanical heterogeneities in island arc crust on time‐dependent preeruptive stresses and the failure of an andesitic reservoir

“…Finally, the density structure to impose lithospheric stresses for −1≥ z ≥−31 km depth is approximated via where ρ r is in kg/m 3 and z is in meters. An edifice density of 2300 kg/m 3 as deduced from gravity measurements [ Hautmann et al , ] is invoked in all models. The resultant E versus z and ρ r versus z profiles are shown in Figure .…”

The influence of long- and short-term volcanic strain on aquifer pressure: a case study from Soufrière Hills Volcano, Montserrat (W.I.)

“…The remnants of the old andesitic volcanic domes from the extinct volcanic centres SH and CH are today dominating surface expressions on the island. Their flanks are surrounded by aprons of volcaniclastics, which are predominantly pyroclastic flow deposits, with lesser amounts of pumice-and-ash-flow, pumice-fall, lahar, debris-avalanche and fluvial deposits (Shalev et al, 2010;Hautmann et al, 2013). The volcaniclastic beds are intersected by radially incised valleys that are formed by run-off from heavy rainfall (Hemmings et al, 2015a).…”

“…The analysis of geodetic data from different volcanic activity phases of SHV allowed for the identification and characterization of three connected pressure sources in the crust (e.g., Odbert et al, 2014b). Assembling these inferred sources gives a model of a magma plumbing system that consists of two vertically-stacked magma chambers that link to the surface via a dyke-conduit feeder system (e.g., Hautmann et al, 2013Hautmann et al, , 2014. The lower magma chamber (LMC) has been found to be prolate with its centroidal source depth located at ~13 km below sea level (bsl) (Hautmann et al, 2010), while the upper magma chamber (UMC) has been inferred to be spherical, centred at ~6 km depth (Voight et al, 2006).…”

“…The lower magma chamber (LMC) has been found to be prolate with its centroidal source depth located at ~13 km below sea level (bsl) (Hautmann et al, 2010), while the upper magma chamber (UMC) has been inferred to be spherical, centred at ~6 km depth (Voight et al, 2006). Independent studies on magma flow dynamics, seismic tomography and strain data inversion constrained the UMC:LMC magma chamber volume ratio to 1:3, whereof the LMC is estimated to have a size of 8 km3 (Paulatto et al, 2012;Hautmann et al, 2013;Melnik and Costa, 2014). A NW-SE trending dyke that opens at ~1 -1.5 km depth into a small cylindrical conduit connects the UMC with the surface (Mattioli et al, 1998;Costa et al, 2007;Hautmann et al, 2009;Linde et al, 2010;Gottsmann et al, 2011).…”