Weakening of the lithosphere beneath Scotian basin: Prelude for initiation of subduction of Atlantic Ocean

While plate tectonic theory has been developed considerably in the last four decades, the of initiation of subduction remains an enigma. In this thesis, I investigate the possible initiation of subduction of North Atlantic oceanic lithosphere at the Scotian basin off east coast of Canada.;On the basis of the relationships between topography, gravity and magnetic anomalies, in both space and Fourier domains, a crustal model is proposed to explain the long- and intermediate-wavelength components of the gravity anomalies of eastern Canada which takes into account the topography, density perturbations in the crust and upper mantle and Moho undulation with certain degree of compensation. No pronounced correlation is observed between the reduced-to-pole magnetic anomalies and topography, as well as between the magnetic anomalies and the vertical gradient of gravity anomalies. The low magnetic susceptibility beneath suture zones indicates thermal demagnetization of the high-magnetic crustal root.;The above topography and gravity analysis method is used to determine the compensation state of the Scotian basin. A 3-D density model that incorporates seawater, sediments, oceanic and continental crust and the lithosphere down to 35 km depth is established, which together with density perturbations inside the crust and mantle of the model, produce the observed gravity anomalies. Regional compensation is required to explain the observations. A 2-D thin elastic plate model is adopted to investigate the flexural response of the lithosphere subjected to the loads due to the sediments and excess density perturbations. The rigidity of the lithosphere beneath the basin is estimated to be about one to two orders of magnitude smaller than elsewhere. The weakening is due to thermal blanketing effects of the thick sediments and the temperature-dependent rheology of lithosphere. The weak lithosphere would have a controlling effect on reactivation of normal faults at the ocean-continent boundary. The possible re-orientation of the spreading ridge of North Atlantic ocean may produce tensional stresses that would decouple the oceanic lithosphere from the continental one at the weak zone, allowing the lithosphere beneath the basin to bend more. Subduction initiation would be facilitated when taking into account further weakening of the lithosphere by extra sediments and subsequent compression. (Abstract shortened by UMI.).