| This thesis is devoted to a study of two unique zones of slow, internal deformation occurring in oceanic lithosphere: namely, the Azores-Gibraltar plate boundary in the eastern Atlantic Ocean and the Davie Ridge-Madagascar region between the east African coast and Madagascar. Despite the slow rate of deformation, high levels of seismic activity are present in both regions. The earthquakes were analyzed by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases.;The present-day deformation of the Azores-Gibraltar plate boundary, which separates the Eurasian and African plates, includes a divergent segment near the Azores Islands and a convergent segment near Gibraltar. The zone of ocean-ocean convergence is characterized by scattered seismicity, complex bathymetry, and large positive geoid and gravity anomalies. The dispersed locations of the earthquakes and the lack of consistency in the orientation of their nodal planes and slip vectors all suggest that a single major plate boundary is not present there. Instead a regional stress field of NNW-SSE compression can be inferred from the consistent P-axis directions. The focal depths for earthquakes in this region reach a maximum of 50 ;On a regional scale, normal faulting seems to characterize the scattered seismicity throughout the region between Madagascar and the eastern arm of the East African rift, even though oceanic lithosphere dominates part of the region and no topographic expression of a rift system can be identified. The present-day deformation along the Davie Ridge is apparently unrelated to its past motion and the region seems to be part of a wide zone of diffuse deformation which marks the southern termination of the African-Somalian plate boundary. |
