Publication Date:
2022-05-25
Description:
Submitted in the partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1996
Description:
Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic
processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge,
are discontinuous and composed of ridge segments. Segments are thus fundamental units
of magmatic accretion and tectonic deformation that control the evolution of the crust. The
objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading
segments, to identify the factors controlling segment propagation, and to provide
constraints on lithospheric strength with laboratory deformation experiments.
In chapter 2, bathymetry and gravity from various areas along the global mid-ocean
ridge system are analyzed to quantify systematic variations at the scale of individual
segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of
segment offsets. We develop a model of faulting to explain these observations. Low-angle
faults appear to accommodate tectonic extension at the inside corners of ridge-offset
intersections, and result in substantially uplifted terrain with thin crust with respect to that at
the outside corners or centers of segments.
Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not
maintained off-axis. This transition is revealed by both statistical and spectral analyses of
bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting
in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between
bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic
crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and
seafloor morphology off-axis.
Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of
ridge segmentation, and provides constraints on the processes driving the propagation of
segments. The pattern of segmentation is controlled mainly by the geometry of the ridge
axis, and secondarily by hot spots. Segments migrate primarily down regional gradients
associated with hot spot swells. However, the lack of correlation between gradients and
propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform
offsets are short-lived and migrating, while transform offsets are long-lived and stable.
Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2
and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from
laboratory deformation experiments on serpentinite. These experiments demonstrate that
serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant
style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites
may weaken the lithosphere, enhance strain localization along faults, and control the style
of faulting.
Description:
A fellowship from Caixa de Pensions "La Caixa" in Barcelona provided me with all the
required financial support to come to WHOI. The work presented in this thesis was also
supported by the National Science Foundation grants OCE-90l2576, OCE-930078, OCE-9313812, and Office of Naval Research grant N00014-9l-J-1433.
Keywords:
Sea-floor spreading
;
Plate tectonics
;
Structural geology
;
Mid-ocean ridges
;
Plumes
;
Submarine geology
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
Permalink