THE EAST PACIFIC RISE CRUSTAL THICKNESS, MOHO TRANSITION ZONE CHARACTER AND OFF-AXIS MAGMA LENS MELT CONTENT FROM 9°37.5’N TO 9°57’N: RESULTS FROM THREE-DIMENSIONAL MULTICHANNEL SEISMIC DATA ANALYSIS
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This thesis discusses the results from the first multi-source and multi-streamer three-dimensional multichannel seismic experiment conducted over a mid-ocean ridge environment. Prestack time migration was applied to the dataset resulting in the most detailed reflection images of a spreading center and its flanks to date. The key products from this work are maps of crustal velocities, crustal thickness, and Moho transition zone (MTZ) reflection character for a section of the fast-spreading East Pacific Rise (EPR) from 9°37.5’N to 9°57’N, excluding the area from 9°40’N to 9°42’N where no data were collected. Moho reflections were imaged within ~92% of the study area. The derived average crustal thickness and average crustal velocity for the investigated ~880 km2 area are 5920±320 m and 6320±290 m/s, respectively. The average crustal thickness varies little from Pacific to Cocos plate suggesting mostly uniform crustal production in the last ~180 Ka. Detailed analysis of the crustal thickness and MTZ reflection character shows that the third-order segmentation is governed by melt extraction processes within the uppermost mantle while the fourth-order ridge segmentation arises from mid- to upper-crustal processes. This analysis also suggests that both the mechanism of lower-crustal accretion and the volume of melt delivered to the crust vary along the investigated section of the EPR. More efficient mantle melt extraction is inferred at latitudes from 9°42’N to 9°51.5’N, with greater proportion of the lower crust accreted from the AML than for the rest of the study area. Larger volume of melt is delivered to the crust from 9°37.5’N to 9°40’N than to the investigated crust further north. At some locations, the Moho reflections are for the first time unambiguously imaged below the AML away from any ridge discontinuity suggesting that the Moho is formed at zero age at least at some sections of the spreading centers. The first study of the melt content of mid-crustal off-axis magma lenses (OAML), done using amplitude variation with offset technique calibrated for a magmatic plumbing system, shows that these magma bodies contain 0 to 20% melt. This suggests that OAMLs likely contribute little to the overall crustal formation.