Morgan, JuliaLevander, Alan2023-01-052023-06-012022-122022-12-02December 2Nguyen, Luan Chan. "From mantle shear-zones to crustal detachment surfaces: Geophysical investigations of rifting at continental margins." (2022) Diss., Rice University. <a href="https://hdl.handle.net/1911/114220">https://hdl.handle.net/1911/114220</a>.https://hdl.handle.net/1911/114220This dissertation contributes to the understanding of the tectonic evolution of the lithosphere as it undergoes extensional deformation during continental rifting. The study consists of two main projects carried out over two rifted margins using distinct geophysical methods to probe the subsurface at different scales. Over the Gulf of Mexico and its bounding margins, Rayleigh surface-wave tomography from the cross-correlation of the seismic ambient noise field was used to construct a 3D velocity model for the top 150 km. The model unveils a significant feature in the region beneath the extended terrain of the northwestern Gulf of Mexico. This anomalous pinch-and-swell structure in the mantle lithosphere is interpreted as geologic boudinage that reflects the deformational history of the continental lithosphere as it was stretched and deformed. Our analysis, in combination with previous findings, demonstrate that geologic boudins play an important role in enhancing localized deformation leading to the well-documented asymmetric geometry of conjugate rifted margins worldwide. We infer that boudinage development is influenced by inherited thickness of the lithosphere and by the process of mantle refertilization triggered by melt infiltration from the ascending asthenosphere during rifting. Lithospheric boudinage proves to be a lasting feature that persists long after the breakup of continents such that it provides a means to investigate other rifted margins around the world. In the second part of this project, we show that crustal variation along the US Gulf Coast margin as well as the structural asymmetry observed across its conjugate Yucatan margin are consistent with the scenarios in which rifting was accompanied by mantle shear-zones. The direction of plate motion is determined to have changed significantly during the transition from rift to drift which is attributed to the development of deep shear-zones in the western part of the rift. Our estimated thickness of the lithosphere indicates that the extended continental lithosphere over the Gulf of Mexico margins has mostly regained its thickness since the time of breakup. The topic of shear-zones, but in the brittle regime, is revisited in the context of rifting at the Galicia margin at depth of less than 12 km below sea level , where 3D active-source reflection seismic imaging clearly shows the morphology of the S reflector, a major detachment surface that facilitated the motion of the overlying crustal blocks during continental extension. The nature of the rock layer above this surface contains a record of block displacements and can yield insight rifting kinematics. Defining the physical properties of this rock layer has been a challenge in previous studies. Here, we applied the analysis of amplitude-variation-with-offset to the S reflector to determine the elastic properties of the rock layer immediately overlying S. We identified a wide distribution of areas of low elastic properties above the S reflector, consistent with the presence of a fault gouge. Our derived rock densities and Vp/Vs ratios indicate that the gouge’s composition is highly heterogeneous throughout the study area, but with systematic distributions that reflect the evolution of the fault system. Our results suggest an increasing level of mantle serpentinization within the fault gouge toward the eastern part of the Galicia margin, an indication of a longer history of mantle hydration attributed to water ingress along early-formed crustal faults. In addition, we infer from the correlation between gouge composition and thickness that heterogenuous fault strength resulting from differential serpentinization of the mantle detachment surface may have affected block displacement and overall geometry of the rifted margin.application/pdfengCopyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.Continental riftingGulf of MexicoGaliciaAmbient noise tomographysurface waveAVOpassive marginlithosphereThesis2023-01-05