The inferred setting of rifting and final continental breakup at Site 1277 is summarized here (Fig. F15), building on comparisons with the Iberia-Galicia margin and the Alps-Apennines and adopting the preferred interpretations.
Rifting of the future Newfoundland continental margin and the conjugate Iberia margin (Sawyer, Whitmarsh, Klaus, et al., 1994; Beslier, Whitmarsh, Wallace, and Girardeau, 2001) took place during a prolonged period of pulsed crustal extension (e.g., Jansa and Wade, 1975; Tucholke, Sibuet, Klaus, et al., 2004), which culminated in the exhumation of subcontinental mantle lithosphere to create new seafloor at Site 1277 by late Barremian time. Upper-plate crustal rocks were removed from the immediate area, leaving an extensional detachment exposed on the seafloor. This detachment zone and underlying mantle peridotite together with minor gabbroic intrusives then underwent submarine erosion on a gently sloping seafloor (possibly <20°), giving rise to mainly coarse grained clastic sediments. Individual debris flows include up to meter-sized blocks of gabbro and serpentinized peridotite, together with derived clasts of serpentinite mylonite and cataclasite.
Three basaltic lavas flows were erupted locally, interspersed with the mass flows (Fig. F14). Chemical analysis showed that the basaltic rocks are of MORB composition, with a marked relative depletion of Nb. The negative Nb anomaly is suggestive of an inherited subduction influence on mantle composition. Eruptions were episodic and of low volume at Site 1277.
The basaltic lavas and interbedded clastic sediments later experienced extensional fracturing. Subvertical cracks opened, allowing carbonate silt to filter downward, together with hyaloclastite and basalt fragments. After cementation, refracturing took place, allowing further sediment infill and a later phase of precipitation of hydrothermal spar, as extension continued. These features may have developed during late-stage, high-angle normal faulting that uplifted the Mauzy Ridge.
Many lithologies are extensively altered and replaced, mainly by calcite. Some individual clasts in the polymict sediments have a wide variety of degrees of alteration within a single sample, showing that the alteration was variable and began at a relatively early stage before the rocks were exposed at the seafloor. On the other hand, the alteration (e.g., carbonate veining) that affects sediment infill of the neptunian fissures occurred later. Extension coupled with alteration, therefore, persisted for an extended period at Site 1277. The sedimentary-volcanic succession at Site 1277 was finally covered by a ferromanganiferous and fossiliferous crust that capped the Mauzy Ridge, above which terrigenous deep-sea sediment eventually accumulated.