Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings

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 × 10 6 m 3 , with ~24 × 10 6 m 3 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. Many thanks to my project advisors and cooperators, Brian Tucholke, RalphStephen, and Maurice Tivey, who have worked with me during the past five and a half years. These research experiences have greatly broadened my horizons on marine geology and geophysics, synthetic seismology, and geomagnetism. Brian introduced me into the geological world of detachment fault system, and his patience and insightful comments on our scientific paper on Kane oceanic core complex are greatly appreciated.I am very thankful for Ralph who introduced me to the world of time-domain finite difference (TDFD) synthetic seismology and Tom Bolmer for setting up the workstation for my TDFD calculations. Ralph's encouragement, enthusiasm, and interactions with me have inspired me to dig out more intere...

Variations in axial magma lens properties along the East Pacific Rise (9°30′N–10°00′N) from swath 3‐D seismic imaging and 1‐D waveform inversion

“…Based on those tests, we restrict our analysis to traces within a slowness window from 0.01 to 0.158 s/km (mainly constraining the AML structure) for the 1‐D waveform inversion. For p > 0.16 s/km, the transform produces artifacts with slope similar to that of the AML‐reflected phases [ Xu , ].…”

“…Incompleteness of field data (finite time, offset, and bandwidth) results in transform artifacts, which must be minimized for successful waveform inversion [Korenaga et al, 1997]. Details of the tests performed to find the appropriate parameters for the τ À p transformation are given by Xu [2012]. Based on those tests, we restrict our analysis to traces within a slowness window from 0.01 to 0.158 s/km (mainly constraining the AML structure) for the 1-D waveform inversion.…”

“…The full waveform inversion method is described in detail by Kormendi and Dietrich [1991], and further information on the inversion procedure can be found in Collier and Singh [1997], Korenaga et al [1997], and Minshull et al [1994]. We give only a brief outline here; more details can be found in Xu [2012]. The waveform inversion scheme was designed to find the 1-D velocity structure that minimizes the misfit between the observed and predicted seismograms in frequency-slowness (ω À p) domain.…”