No abstract

No abstract

This data report focuses on Holes U1394B, U1395B, and U1396C located offshore Montserrat. These holes were drilled during Integrated Ocean Drilling Program Expedition 340 and contain deposits associated with the growth and decay of the volcanic island of Montserrat. Hole U1394B dates to ~353 ka and is composed of 17 bioclastic turbidites, 24 mixed turbidites, 55 volcaniclastic turbidites, and 32 tephra fall layers within a background stratigraphy of hemipelagic marine mud. Hole U1395B dates to older than 1 Ma and contains 18 bioclastic turbidites, 27 mixed turbidites, 43 volcaniclastic turbidites, and 52 tephra fall layers. The uppermost 7 m of Hole U1396C covers ~1 My of stratigraphy and contains 1 bioclastic turbidite, 1 mixed turbidite, 9 volcaniclastic turbidites, and 13 tephra fall layers. Tephra fall and some volcaniclastic deposits are associated with episodes of island building, whereas bioclastic turbidites, mixed turbidites, and some volcaniclastic turbidites are associated with large landslide events from Montserrat. During the Expedition a total of nine sites were drilled offshore Montserrat, Martinique, and Dominica in the Lesser Antilles (Expedition 340 Scientists, 2012) (Fig. F1).

Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long‐term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6–10 km3) collapse of the Soufrière Hills Volcano at ~130 ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufrière Hills, estimated to have initiated <100 years after collapse. This basaltic volcanism was a sharp departure from the andesitic volcanism that characterized Soufrière Hills' activity before the collapse. Mineral‐melt thermobarometry demonstrates that the basaltic magma's transit through the crust was rapid and from midcrustal depths. We suggest that this rapid ascent was promoted by unloading following collapse.

Constraining how the physical properties and seismic responses of recently deposited sands change with time is important for understanding earthquake site response, subsurface fluid flow, and early stages lithification. Currently, however, there is no detailed (cm-scale) assessment of how sand physical properties and associated seismic velocities evolve over the first two centuries after deposition. Here, we integrate sedimentation rates with seismic velocity and sediment physical properties data to assess how the vadose zone sands at Port Royal Beach, Jamaica, change within 180 years after deposition. We show that compressional and shear wave velocities increase with sediment age, whereas porosity, grain size, sorting, mineralogy, and cementation fraction remain relatively unchanged during the same period. Rock physics models (constrained by the measured physical properties) predict constant seismic velocities at all sites regardless of sediment age, though misfits between modeled and observed velocities increase with sediment age. We explain these misfits by proposing that shallow sands undergo microstructural grain reorganization that leads to a more uniform distribution of grain contact forces with time. Our results imply that beach sands undergo a previously undocumented lithification process that occurs before compaction.