Non-volcanic rifted margins, continental break-up and the onset of sea-floor spreading: some outstanding questions

Abstract

During the last 20 years, regional studies on the West Iberia margin and on the former margins of the Tethys have considerably advanced the understanding of processes related to continental break-up and the onset of sea-floor spreading. However, some questions remain outstanding. To tentatively answer these, a coherent interpretation of available data is proposed, based on the detachment fault concept applied to the continental as well as the oceanic lithosphere, and on the hypothesis of a multi-staged rifting process. The interpretation addresses the nature of the lower crust beneath non-volcanic passive margins, the origin of ophicalcites, the probable time gap between syn-or post-rift crystallization of gabbros and extrusion of basalts on the sea floor, and the significance of dipping reflectors within oceanic lithosphere adjacent to non-volcanic passive margins. The interpretation also considers the symmetry v. asymmetry of continental rifting and break-up, the location of the ocean-continent boundary, and the possible association of magnetic quiet zones with ultramafic sea floor (serpentinized peridotite) bordering non-volcanic passive margins.

A moderate melting model for the Vøring margin (Norway) based on structural observations and a thermo-kinematical modelling: Implication for the meaning of the lower crustal bodies

“…Planke et al, 1991). These values are also difficult to interpret as either magmatic underplated or serpentinised Monday, 04 July 2005 VERSION FINALE mantle, because both features are generally focused close to the break-up axis Boillot and Froitzheim, 2001). …”

“…However, such rocks are only expected to occur within highly stretched extensional crustal domain (pre-breakup terranes) in highly-saturated water conditions (Boillot and Froitzheim, 2001). …”

Diachronous evolution of Late Jurassic–Cretaceous continental rifting in the northeast Atlantic (west Iberian margin)

“…Relative crustal stretching (b), rift-related faulting and igneous activity vary significantly in each of the crustal segments [Welsink and Tankard, 2009] imposing disparities in regional thermal regimes prior to, during, and after continental breakup [Boillot and Froitzheim, 2001;Michon and Merle, 2003]. As a result, passive margins are commonly classified as ''volcanic'' or ''nonvolcanic,'' depending on the relative expression of riftrelated igneous processes along them Tucholke and Sibuet, 2007].…”

“…When variations in the structure of crustal segments are considered across conjugate pairs of margins, the concepts of ''upper plate'' and ''lower plate'' margins are commonly evoked [Wernicke, 1985;Lister et al, 1986Lister et al, , 1991Manatschal and Bernoulli, 1998;. On lower plate margins as west Iberia, sets of oceanward dipping extensional faults appear to detach downward into a pervasive detachment zone, the S or H reflector (e.g., Galician margin Boillot and Froitzheim, 2001;Reston and Pérez-Gussinyé, 2007]). Lower plate margins are generally highly structured and preserve most of the rotational normal faults, tilt blocks and half grabens developed during continental rifting.…”

“…Nonetheless, the detailed evolution of continental rifting for the different crustal segments composing the west Iberian margin has not yet been documented. This comprises a major gap in knowledge, as the geometry of the margin, together with the degree and relative angle of stretching, are directly related to variations in basement subsidence rates during synrift extension [e.g., Boillot and Froitzheim, 2001;Sayers et al, 2001] and to the degree of deformation experienced in subsequent episodes of tectonic reactivation [Neves et al, 2009]. Also, the timing of continental rifting, the relative locations of continental breakup and the significance of synrift and postrift tectonics along the west Iberian margin are key issues not yet fully addressed, posing serious problems to the correct assessment of its hydrocarbon potential [Tankard and Welsink, 1987;Masson and Miles, 1986;Hiscott et al, 1990a;Alves et al, 2006].…”