Continental Rifting and Break-Up at the West Iberia Margin: an Integrated Geophysical Study

The purpose of this work has been to study the processes of continental extension and break-up at non-volcanic margins. In particular, we studied how extension halts leading to failed rifts. For this purpose, I have processed multichannel seismic reflection and modelled wide angle data from the Galicia Interior Basin (GIB), a failed rift located offshore west Iberia (Chapter 1, 2 and 3). Our studies reveal the structure, amount of thinning and timing of extension at the GIB (Chapter 2 and 3). We have integrated the new results on the GIB with those already existent at the rest of the margin and suggested possible causes for rift failure at the basin in connection with propagation of seafloor spreading at the final break-up margins (Chapter 3 and Outlook). Additionally, we studied how extension leads to continental break-up at non-volcanic margins of the west Iberia type. We developed a numerical model based on the seismic, petrological and chronological observations made at the segments of the west Iberia margin where continental break-up occurred (Chapter 4). We have applied this model successfully to other non-volcanic margins such as the south-west Greenland margin, the Rockall Trough and the Porcupine Basin (both located offshore west Ireland)(Chapter 5). We have also found that the differences in the initial thermal structure of the margins may lead to the differences in the structural style of continental break-up between non-volcanic margins of the west Iberia type and those showing more magmatic activity during extension (e.g. the Woodlark basin, located at the eastern tip of Papua New Guinea). Our modelling stresses the importance of the rheological consequences of the process of serpentinisation at non-volcanic margins of the west Iberia type in contrast to those of a more robust magmatic production

Thermomechanical Implications of Sediment Transport for the Architecture and Evolution of Continental Rifts and Margins

International audienceErosion and deposition redistribute mass as a continental rift evolves, which modifies crustal loads and influences subsequent deformation. Surface processes therefore impact both the architecture and the evolution of passive margins. Here we use coupled numerical models to explore the interactions between the surface, crust, and lithosphere. This interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink. If transport is efficient, there are two possible outcomes: (1) Faulting within the zone of extension is longer lived and has larger offsets. This implies a reduction of the number of faults and the width of the proximal domain. (2) Efficient transport of sediment leads to significant deposition and hence thermal blanketing. This will induce a switch from brittle to ductile deformation of the upper crust in the distal domains. The feedbacks between these two outcomes depend on the extension history, the underlying lithospheric rheology, and the influence of submarine deposition on sediment transport. High erosion/sedimentation during early faulting leads to abrupt crustal necking, while intermediate syntectonic sedimentation rates over distal deep submarine hotter crust leads to unstructured wide distal domains. In models where rheological conditions favor the formation of asymmetric conjugate margins, only subaerial transport of sediments into the distal domains can increase conjugate symmetry by plastic localization. These models suggest that passive margin architecture can be strongly shaped by the solid Earth structure, sea level, and climatic conditions during breakup

Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation-including examples from the Norwegian Sea

The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle

Global whole lithosphere isostasy : implications for surface elevations, structure, strength, and densities of the continental lithosphere

The observed variations in the thickness of the conductive lithosphere, derived from surface wave studies, have a first‐order control on the elevation of the continents, in addition to variations in the thickness of the crust—this defines whole lithosphere isostasy (WLI). Negative buoyancy of the mantle lithosphere counters the positive buoyancy of the crust, and together, their respective thicknesses and density contrasts determine elevation of the continents both in their interiors and at their edges. The average density contrasts for lithospheric mantle with crust and with asthenosphere are typically 300 to 550 and 20 to 40 kg m−3, respectively, with a ratio 10 to 16, suggesting moderate average depletion of lithospheric mantle. We show that a crustal model for Antarctica, assuming WLI and using these density contrasts, provides a close fit to estimates of crustal thickness from surface wave tomography and gravity observations. We use a global model of WLI as a framework to assess factors controlling topography, showing that plausible regional variations in crustal and mantle densities, together with uncertainties in the crustal and conductive lithospheric thicknesses, are sufficient to account for global elevations without invoking dynamic topography greater than a few hundred meters. Estimates of elastic thickness Te in the continents are typically 25–50% of the thickness of the conductive lithosphere, indicating that the mantle part supports some of the elastic strength of the lithosphere.Published versio