Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments
Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments
Date
2007-09
Authors
Williams, Clare M.
Linked Authors
Person
Alternative Title
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As Published
Date Created
Location
9°25’-9°55’N
East Pacific Rise
23°40’N
Kane Megamullion
East Pacific Rise
23°40’N
Kane Megamullion
DOI
10.1575/1912/2031
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Keywords
Magnetic measurements
Petrology
Atlantis (Ship : 1996-) Cruise AT7-4
Atlantis (Ship : 1996-) Cruise AT11-7
Petrology
Atlantis (Ship : 1996-) Cruise AT7-4
Atlantis (Ship : 1996-) Cruise AT11-7
Abstract
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.
Description
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
Embargo Date
Citation
Williams, C. M. (2007). Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/2031