Volcano-Tectonic History of the Island of Montserrat, West Indies, From Seismic Reflection Profiles

Seismic reflection profiles provide a cross-sectional view of crustal layers and thus details about local sedimentation rates, chronology, and depositional materials. Based on seismic profiles collected as part of the SEA-CALIPSO seismic experiment, we apply this method to interpreting the volcanic and local tectonic history of the island of Montserrat, in the Lesser Antilles arc. In December 2007, the vessel RRS James Cook towed a tuned, 2600 cubic inch, 8-airgun array along encircling and radial lines around Montserrat. The airguns fired every 60 sec (approx. every 140 m) at a pressure of 2000 psi. The ship also towed a 600 m streamer consisting of 48 hydrophone channels. Over a period of 77 hours, the hydrophones recorded a total of 4414 shots. Onboard the ship, data were stacked to produce 26 seismic profiles. The profiles vary in length up to 15 km and allow interpretation up to a depth of approx. 2.5 km. The profiles from east of Montserrat reveal fans of coarse-grained debris flows and submarine pyroclastic flows that derive from both the older volcanic centers and the active Soufriere Hills Volcano (SHV). The flows form tapering wedges that have been overlain by younger sea-floor sediments. Older ( > 1 Ma) sedimentary rocks, containing multiple reflective layers, deflect downwards towards and beneath Montserrat, forming a moat into which the debris and pyroclastic flows have deposited. A sub-sediment volcanic basement is present offshore at approximately 1.5 km depth. Offshore on the west side of the island the prominent Belham valley fault can be traced trending NW. The new data suggest that the fault line has been active in the recent geological past; the fault has offset submarine deposits offshore and tectonic blocks onshore (Garibaldi Hill, St. Georges Hill, Roches Bluff), has caused the down-warping of ocean sediments on the east side of the island, and likely influenced the location of domes and feeding conduits at and adjacent to the SHV. Montserrat itself appears to be located at the SW boundary of a large half-graben.