One of the outstanding discoveries of the Ocean Drilling Program (ODP) in the North Atlantic Ocean is that a broad strip of seafloor parallel to both rifted margins can no longer be considered simply as oceanic crust formed by spreading at a mid-ocean ridge. This seafloor instead originated, at least partially, from subcontinental mantle lithosphere that was exhumed during the latest stages of continental breakup. This seafloor is equivalent to the Zone of Exhumed Continental Mantle of Whitmarsh et al. (2001). Basement ridges drilled within the ocean–continent transition zone on the Iberia-Galicia margin were initially identified as serpentinized peridotite of apparently subcontinental origin, based on geophysical, petrological, geochemical, and dating studies (e.g., Boillot et al., 1980). The zone of exhumed subcontinental mantle lithosphere between definite continental crust and inferred "normal" oceanic crust is estimated as 150–180 km wide for the restored Iberia-Newfoundland conjugate (Tucholke et al., 2007). This zone may include thinned continental crust on both the Iberia margin (Reston et al., 1995) and the Newfoundland margin (Van Avendonk et al., 2006). Possible ocean crust on this conjugate traverse is located in the vicinity of magnetic Anomaly M3 on the Iberia margin and probably oceanward of this on the Newfoundland margin. The age of the oldest "normal" oceanic crust on the Iberia-Newfoundland conjugate is inferred to be latest Aptian–earliest Aptian (Tucholke et al., 2007).
The zone of subcontinental mantle lithosphere on both margins of the Atlantic is presently one of the most poorly known regional tectonic settings on Earth. ODP Leg 210 provided an opportunity to sample this intermediate zone between continental crust and "normal" oceanic crust at Site 1277. This site is located directly oceanward of a magnetic anomaly identified as M1 (late Barremian) and is assumed to lie between the rifted Newfoundland margin and the oldest "normal" oceanic crust (Shillington et al., 2006; Tucholke et al., 2007). A basement composed of serpentinized peridotite is overlain by a cover of serpentinite-rich mass flows interbedded with basaltic lava flows. This succession was recovered near the summit of a basement ridge, known as the Mauzy Ridge (Fig. F1). Drilling at Site 1277 recovered coherent lava flows for the first time within the ocean–continent transition zone, with important implications for continental breakup and the initiation of seafloor spreading.
This paper provides a detailed description and interpretation of the sedimentary, volcanic, and tectonized rocks overlying exhumed mantle at Site 1277. It also compares these with the previously well-documented recovery of lithologies and structures from the Iberia-Galicia conjugate and with the classic Jurassic land exposure in the Alps-Apennines region.