Seismological insights into the building of the Lesser Antilles Arc

• Main Author: Schlaphorst, David
• Published: University of Bristol 2016
• Subjects: 551.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686191

Subduction is a key process in the formation of continental crust. However, the interaction of the mantle with the subducting slab is not fully understood and varies between subduction zones. In this thesis I use seismological techniques to investigate subduction beneath the Lesser Antilles Arc (LAA) and eastern Greater Antilles. I first investigate the earthquake-magnitude distribution along the LAA, which shows "bulls-eyes" of elevated b-values, implying a relative increase in smaller magnitude earthquakes. They coincide with subducted fracture zones, suggesting they serve as conduits for increased amounts of water, which acts as a lubricant. I then investigate the LAA crustal structure at 10 islands using receiver functions (RF). Synthetic modelling shows that increased amounts of crustal water content can weaken the Moho (at about 30km depth) and a mid-crustal discontinuity (about 20km) can dominate the RF result. ID velocity-depth profiles, constrained from petrological studies, reveal a dichotomy in Moho strength; it is weaker south of Martinique due to higher crustal water content. On average, both discontinuities are deeper in the north. Finally, I use shear-wave splitting analyses of local and teleseismic phases to investigate anisotropy. Local event delay times (0.21 ± 0.12s) do not increase with depth, indicating a crustal origin and an isotropic mantle wedge. Teleseismic delay times are larger (1.34 ± 0.47 s), indicating sub-slab anisotropy. The results suggest trench-parallel mantle flow, with the exception of trench-perpendicular alignment in narrow regions east of Puerto Rico and south of Martinique, suggesting mantle flow through gaps in the slab. The RF and anisotropy results show a change in arc properties between Martinique and St. Vincent, possibly marking the North-South American plate boundary. Dissimilarity in the RF study and b-value patterns indicates that crustal water does not necessarily correspond with water released at the subducted plate, but may correlate with sediment dewatering at shallow levels. Cumulatively, the three investigations offer insights into the dynamic behaviour of a region of slow subduction.