Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments
Williams, Clare M.
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East Pacific Rise
KeywordMagnetic measurements; Petrology; Atlantis (Ship : 1996-) Cruise AT7-4; Atlantis (Ship : 1996-) Cruise AT11-7
The origin of symmetric alternating magnetic polarity stripes on the seafloor is investigated in two marine environments; along the ridge axis of the fast spreading East Pacific Rise (EPR) (9º 25’-9º 55’N) and at Kane Megamullion (KMM) (23º 40’N), near the intersection of the slow-spreading Mid Atlantic Ridge with Kane Transform Fault. Marine magnetic anomalies and magnetic properties of seafloor samples are combined to characterize the magnetic source layer in both locations. The EPR study suggests that along-axis variations in the observed axial magnetic anomaly result from changing source layer thickness alone, consistent with observed changes in seismic Layer 2a. The extrusive basalts of the upper crust therefore constitute the magnetic source layer along the ridge axis and long term crustal accretion patterns are reflected in the appearance of the axial anomaly. At KMM the C2r.2r/C2An.1n (~ 2.581 Ma) polarity reversal boundary cuts through lower crust (gabbro) and upper mantle (serpentinized peridotites) rocks exposed by a detachment fault on the seafloor, indicating that these lithologies can systematically record a magnetic signal. Both lithologies have stable remanent magnetization, capable of contributing to the magnetic source layer. The geometry of the polarity boundary changes from the northern to the central regions of KMM and is believed to be related to changing lithology. In the northern region, interpreted to be a gabbro pluton, the boundary dips away from the ridge axis and is consistent with a rotated conductively cooled isotherm. In the central region the gabbros have been removed and the polarity boundary, which resides in serpentinized peridotite, dips towards the ridge axis and is thought to represent an alteration front. The linear appearance of the polarity boundary across both regions indicates that the two lithologies acquired their magnetic remanence during approximately the same time interval. Seismic events caused by detachment faulting at Kane and Atlantis Transform Faults are investigated using hydroacoustic waves (T-phases) recorded by a hydrophone array. Observations and ray trace models of event propagation show bathymetric blockage along propagation paths, but suggest current models of T-phase excitation and propagation need to be improved to explain observed characteristics of T-phase data.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007
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