Abstract:
The structure of the oceanic lithosphere results from magmatic and extensional
processes taking place at mid-ocean ridges (MORs). The temporal and spatial scales of
the variability of these two processes control the degree of heterogeneity of the oceanic
lithosphere, represented by two end-member models: the classical Penrose Model
exemplified by layered magmatic crust formed along fast-spreading MORs, e.g., East
Pacific Rise (EPR); and the recently defined Chapman Model describing heterogeneous
mafic and ultramafic lithosphere formed in settings of oceanic detachment faulting
common along slow-spreading MORs, e.g., Mid-Atlantic Ridge (MAR). This thesis is
using advanced marine geophysical methods (including finite-difference wave
propagation modeling, 3D multi-channel seismic reflection imaging, waveform inversion,
streamer tomography, and near-bottom magnetics) to study lithospheric accretion
processes in MORs characterized by contrasting tectono-magmatic settings: the
magmatically dominated EPR axis between 9°30'-10°00’N, and the Kane Oceanic Core
Complex (KOCC), a section of MAR lithosphere (23°20’-23°38’N) formed by
detachment faulting. At the EPR study area, I found that the axial magma chamber
(AMC) melt sill is segmented into four prominent 2-4-km-long sections spaced every ~5-
10 km along the ridge axis characterized by high melt content (>95%). In contrast, within
the intervening sections, the AMC sill has a lower melt content (41-46%). The total
magma volume extracted from the AMC sill was estimated of ~46 × 106 m3, with ~24 ×
106 m3 left unerupted in the upper crust as dikes after 2005-06 eruption. At the KOCC, I
used streamer tomography to constrain the shallow seismic velocity structure.
Lithological interpretation of the seismic tomographic models provides insights into the
temporal and spatial evolution of the melt supply at the spreading axis as the KOCC
formed and evolved. Investigation of a magnetic polarity reversal boundary in crosssection
at the northern boundary of KOCC suggests that the boundary (representing both
a frozen isotherm and an isochron) dips away from the ridge axis along the Kane
transform fault scarp, with a west-dipping angle of ~45° in the shallow (<1 km) crust and
<20° in the deeper crust.
