We revise commonly accepted models explaining long-term stratigraphic trends along Atlantic-type passive margins
by including the impact of complex lithosphere deformation at depth and it’s coupling with surface processes.
To achieve this, we simulated the evolution of a passive margin basin using a cascade of three modeling tools: a
thermo-mechanical model of the syn-rift stretching of the lithosphere, a flexural and thermal model of the post-rift
stage that includes coupling with surface processes and, finally, a stratigraphic model of the associated sedimentary
basin architecture. We compare two necking styles that lead to different margin geometries: wide and narrow margins
that form by heterogeneous stretching. Wide margins, forming thinner and wider sedimentary wedges, show
significantly larger aggradation component and longer preservation duration, in more continental/proximal depositional
facies. Narrow margins are characterized by enhanced erosion and by-pass during transgression. Through
a parametric analysis we constrain the relative contribution of lithosphere deformation and surface processes on
the stratigraphic trends and show that both may contribute equally to the stratigraphic architecture. For example,
enhanced erosion in narrow margins impacts the volume of sediments delivered to the basin, which, in turn, significantly
increases the subsidence. Our simulations also underline the importance of the assumed sediment transport
length, which controls whether the main depocentres remain in the necking zone or reach the more distal parts of
the margin.
Abstract The study of a dense network of high resolution seismic profiles in the bay of Vilaine, INSU-CNRS cruise Geovill, have led to the characterization of the architecture of the sediment wedge preserved between the coast and the 50 m isobath. This wedge lies on a substratum composed of three seismic units, U1, U2 and U3 respectively attributed to metamorphic and magmatic rocks, Lutetian and Ypresian sandy carbonates and post-Eocene sediments. The coastal sediment wedge comprises three major units. A basal unit (U4), dated around 600 to 300 ky BP, interpreted as braided river sandy conglomerates. A median unit (U5) corresponding to estuarine and fluvial sandstones and clays that give way to the west to mouth bar sandstones. A sommital unit (U6) attributed to marine argillites and barrier island sandstones dated from 8110+ or -200 years at the base. These three units are bounded by two major surfaces: an unconformity between U4 and U5 and a marine (wave and tidal) ravinement surface between U5 and U6. The unconformity is interpreted as a sequence boundary between two depositional sequences: a lower one with U4 seismic unit and a topmost one with U5 and U6 seismic units. Based on the available datations, the lower sequence is attributed to the Saalian and/or Elsterian glacial cycles and, the upper sequence to the Weichselian (lowstand systems tract) and to the Holocene marine transgression (transgressive systems tract). The passage from one sequence to the other corresponds however to a drastic shift in the paleoflow directions (60 degrees ) in the Bay of Vilaine closely related to the main faults orientations. The tectonic activity in Brittany during the Pleistocene, linked to intraplate stress, seems to exert a control on sediment architecture in the coastal wedge. Indeed, the tilt of the Armorican Massif during that period has caused a complete rejuvenation of the fluvial profiles in land and the separation of the paleo-Vilaine from the Paleo-Loire river courses.
These animations provide the data for the analyses and figures of the main manuscript. Data is available for the five main models (Model M1 - M5) and the seven supplementary models (Models SM1 - SM7). In each animation the evolution of the model output over the model duration (i.e., 25 Myr) is displayed. In the four panels of each animation, we show: strain rate on top of hillshade (top, left panel) FastScape topography and sediment thickness (top, right panel) Accumulated plastic strain (Bottom, left panel) Material colors from thermo-mechanical model (Bottom, right panel) We refer to Table 3 of the main manuscript for model parameters. The original model output is available upon request.
Within the Afar depression (Djibouti) the stratiform basaltic lava flows (emplaced between 4 Ma and 1 Ma) cover the two thirds of the depression. Because of the desert climate, they are almost non eroded. The topography can then be used as a reference surface to measure the vertical offset of younger faults. Using a detailed map the southeastern part of the depression showing the fault pattern and the vertical offset on each fault (obtained from stereoparts of SPOT images), we have restored the topography in map view. We have computed the fields of finite displacement, rotation and strain. The field of horizontal displacements shows vectors trending to the NE on average. This direction is compatible with the displacement to the NE of the Arabian plate with reference to the African plate. In detail however, displacement vector trend from NNE‐SSW to NE‐SW. Major axes of strain ellipse trend NE‐SW suggesting that major faults (Nl20) are dominantely extensional. Strain intensities and magnitudes of block rotations are increasing from southwest to northeast of the restored area. Magnitudes of block rotations are very small compared with those obtained from paleomagnetic measurements. Therefore we computed a second restoration that covers a smaller area where the true vertical offset of the major faults taking into account the sedimentary fill of basins were avalaible. This second model gives rotation magnitudes in better agreement with paleomagnetic data but still underestimates. We discuss these results with the available kinematic models of the southeastern part of the depression.
