Abstract:
This thesis investigates the evolution of lithospheric deformation and crustal structure from
continental margins to mid-ocean ridges. The first part (Ch. 2) examines the style of
segmentation along the U.S. East Coast Margin and investigates the relationship between
incipient margin structure and segmentation at the modem Mid-Atlantic Ridge. The second
part (Chs. 3-5) focuses on the mechanics of faulting in extending lithosphere. In Ch.
3, I show that the incorporation of a strain-rate softening rheology in continuum models
results in localized zones of high strain rate that are not imposed a priori and develop in
response to the rheology and boundar conditions. I then use this approach to quantify the
effects of thermal state, crustal thickness, and crustal rheology on the predicted style of
extension deformation. The mechanics of fault initiation and propagation along mid-ocean
ridge segments is investigated in Ch. 4. Two modes of fault development are identified:
Mode C faults that initiate near the center of a segment and Mode E faults that initiate
at the segment ends. Numerical results from Ch. 5 predict that over time scales longer
than a typical earhquake cycle transform faults behave as zones of significant weakness.
Furthermore, these models indicate that Mode E faults formed at the inside-comer of a
ridge-transform intersection wil experience preferential growth relative to faults formed at
the conjugate outside-comer due to their proximity to the weak transform zone. Finally, the
last par of this thesis (Ch. 6) presents a new method to quantify the relationship between
the seismic velocity and composition of igneous rocks. A direct relationship is derived
to relate V p to major element composition and typical velocity-depth profiles are used to
calculate compositional bounds for the lower continental, margin, and oceanic crust.
