Seismic constraints on the volcanoes and upper crustal structure of the Mariana island arc

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(Thesis) Ph.D.
Date created
The Mariana intra-oceanic island arc is imaged using a seismic survey to provide new constraints on the shallowmost crustal structure of the arc and the fore-arc where numerous serpentinite mud volcanoes, which provide valuable samples of mantle-derived fluids, are located. The multi-channel seismic (MCS) survey was acquired in 2002 from 14° N to 19° N using a 6 km long streamer. I developed tomographic velocity models from the inversion of refracted first-arrival traveltimes recorded in the MCS data, but where the refracted arrivals are secondary, a synthetic ocean bottom experiment (SOBE) was created to reorganise the MCS data so that refractions became first-arrivals. Though the resolution of the velocity model from the SOBE data is generally higher than the surface-recorded data, resolution is limited by difficulties in picking first-arrivals on traces containing noise from the downward continuation process. The velocity models are used to provide better constraints on the lithology and degree of compaction beneath the seafloor of the Mariana arc, which includes isolated volcanoes along the modern arc, and the Fantangisña serpentinite seamount on the fore-arc. I also reprocessed the MCS lines to migration to obtain reflection images, which can better characterize subsurface structures. A combined interpretation of the migrated reflection sections and velocity models reveals that the upper crust of the Mariana arc comprises interlayered, less consolidated and consolidated sediments overlying a porous igneous basement. I infer from the presence of the numerous horsts and grabens in the arc massif that the Eocene arc is under extension. Interpretation of the data from the modern arc reveals the volcanoes tend to be younger in the south though some isolated volcanoes may have developed synchronously. I identify some igneous intrusions on the seafloor and buried volcanoes in the sub-seafloor. The novel high resolution velocity model and depth-migrated reflection image of the serpentinite seamount on the fore-arc reveal that the internal structure of the seamount is asymmetric and highly heterogeneous. The serpentinite seamount was formed from repeated episodes of eruptive mudflows from a central conduit, but I infer from its structure that it may have undergone multiple slope failures.
144 pages.
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Thesis advisor: Calvert, Andrew
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