Interpretation of the North Atlantic rifted margins can benefit significantly from comparison with the rifting and early stages of opening of the Jurassic Tethys Ocean in the Alps and the Apennines where there is abundant evidence that mantle rocks were exposed and reworked during rifting. However, these are exposed within far-traveled thrust sheets, making palinspastic reconstruction difficult.
Much early evidence came from the Italian Apennines where Alpine ophiolites are well exposed (e.g., northern Apennines and Elba). These rocks experienced relatively little emplacement-related deformation and metamorphism during contractional orogenesis, making it relatively easy to recognize seafloor features (e.g., Abbate et al., 1980). Different sections expose a variety of mafic and ultramafic rocks depositionally overlain by lithologies that include carbonate-rich serpentinite breccia (i.e., ophicalcites; see below) intercalated with polymict breccia and basalts of MORB type (e.g., Barrett and Spooner, 1977; Abbate et al., 1980; Lagabrielle et al., 1984). During the 1970s and early 1980s the generally favored view was that exposure of plutonic rocks on the seafloor and the formation of related polymict sediments was largely along transform faults within the opening Jurassic ocean. Faulting of the oceanic basement was attributed to transcurrent faulting (e.g., Gianelli and Principi, 1977), transtension in a transcurrent rift (Weissert and Bernoulli, 1985; Ishiwatari, 1985), or rifting at a spreading center (Barrett and Spooner, 1977). Additional important evidence came from the French, Italian, and Swiss Alps; however, only a few relatively small ophiolitic remnants in these areas have escaped emplacement-related deformation and metamorphism sufficiently to allow primary ocean floor relationships to be restored. During the 1980s and 1990s individual ophiolitic "massifs" of the "internal" Alps were investigated. In all these areas the "ophiolite" association of mafic and ultramafic rocks overlain by polymict clastic sediments and basic volcanics differs strongly from the established concept of a Penrose-type ophiolite (Anonymous, 1972) that was modeled largely on the Jurassic Coast Range ophiolite in California, the Jurassic Vourinos ophiolite in Greece, and the Upper Cretaceous Troodos ophiolite in Cyprus. These ophiolites have since been shown to have formed in subduction-related settings based mainly on chemical evidence (e.g., Pearce et al., 1984; see also Robertson, 2002).
It was long assumed that ocean crust in the modern oceans should have a Penrose-type ophiolite structure. However, evidence from the Mid-Atlantic Ridge has shown that mantle can be exposed on the seafloor, especially in slow-spreading settings (e.g., Tucholke and Lin, 1994). The Apennine and Alpine Jurassic ophiolites were interpreted as a rifted slow-spreading ridge (Tricart and Lemoine, 1983; Lemoine et al., 1987; Lagabrielle and Cannat, 1990; Lagabrielle, 1994). However, the sediments overlying the exhumed plutonic basement locally include relatively coarse proximal terrigenous clastic sediments (e.g., with granite, mica schist, or dolomite), showing that the crust was in a proximal-margin position rather than in the setting of an open-ocean spreading ridge. Thus, some of the plutonic rocks could represent the exhumation of subcontinental mantle associated with the transition from a rifted continental margin to seafloor spreading. This interpretation was supported by geochemical investigations of both the plutonic ophiolitic rocks (e.g., gabbro and serpentinite) (Pognante et al., 1986; Piccardo et al., 1990) and the overlying lavas of transitional or mid-ocean-ridge type (Beccaluva et al., 1984).
In recent years much progress has come from detailed comparisons of the results of ocean drilling on the Iberia and Newfoundland margins with several ophiolite sections in the Alps (Manatschal and Bernoulli, 1998; see Manatschal et al., in press, for a review) coupled with modeling studies (e.g., Lavier and Manatschal, 2006). Different sections document the exhumation of both middle crust and subcrustal mantle. For example, exhumed crust (e.g., gabbro or amphibolite) is preserved in several tectonic units, notably the Platta nappe (e.g., Piz d'Err-Piz Bial areas) (see Manatschal and Nievergelt, 1997, and references therein). This aspect is not discussed further here because similar crustal lithologies were not recovered at Site 1277.
Critical field relations similar to Site 1277, notably basaltic lavas covering detachment faults, are well exposed in eastern Switzerland (Manatschal and Nievergelt, 1997; Desmurs et al., 2001, 2002). Specific examples include the Totalp, the Err-Platta, and the Malenco units in the eastern Central Alps, in Graubünden (Switzerland), and also in northern Italy (Manatschal et al., 2001, 2003). Also, within the Lanzo metaophiolite of northern Italy, serpentinized peridotites document exhumed mantle that is overlain by basaltic rocks related to final continental breakup (Lagabrielle et al., 1989; Pelletier and Müntener, 2004).
One of the best-exposed examples, the Chenaillet ophiolite, straddles the French/Italian border and was examined on a field trip by members of the Leg 210 Scientific Party, who were impressed by the close similarities with the core recovery at Site 1277 (Fig. F13). At this location, serpentinized mantle peridotites and intrusive gabbros are topped by a prominent shear zone, interpreted as a large-scale oceanic detachment fault (Manatschal and Müntener, 2005). The detachment surface is directly overlain by dark tectonic-sedimentary breccias, interpreted as fault gouge and cataclasite reworked from within the zone of crustal extension. Basaltic pillow breccias and pillow lavas overlie the coarse clastic sediment or lie directly on the detachment. An important observation, demonstrated by detailed mapping, is that the detachment is cut and offset in a domino-like fashion by a number of high-angle normal faults. In places, these faults are covered and sealed by lava breccias and pillow basalts, showing that high-angle faulting was active at an early stage following basement exhumation (Manatschal and Müntener, 2005). These faults also cut the fault gouge and serpentinite-rich mass flows, confirming that the faulting took place after exhumation and mass wasting of the surface of the exhumed basement.
Other well-exposed reference sections are provided by the Platta nappes in southeast Switzerland, examined by participants in the interMARGINS workshop, 2004 (Manatschal and Müntener, 2004). Individual thrust sheets preserve remnants of the ocean–continent transition zone that is interpreted as part of the southern margin of the Jurassic Tethys (Froitzheim and Manatschal, 1996; Manatschal and Nievergelt, 1997). The Platta nappe records a zone of exhumed subcontinental mantle into which gabbros and basaltic dikes were locally intruded. Serpentinites of the exhumed mantle are directly overlain by tectonic-sedimentary breccias. Mid-ocean-ridge–composition pillow basalts and postrift sediments contain continentally derived material, indicating a proximal setting. Serpentinite-rich calcite-cemented breccias, known as ophicalcites, range from one type of lithology, in which the serpentinite is fissured and the fractures are infilled by fine-grained red carbonate and calcite spar, to another type that is dominated by clast-supported fragmented breccia clasts (Bernoulli and Weissert, 1985). These lithologies, known as the Type 1 ophicalcite of Lemoine et al. (1987), are very similar to those drilled at Site 1277 and they include comparable geopetal infills that confirm that the ultramafic rocks were exposed on or near the seafloor. The clasts include foliated gabbro that was deformed under high-temperature conditions before being eroded and emplaced within the breccias. In places (e.g., in the Falotta area) the tectonic-sedimentary breccias are stratigraphically overlain by mid-ocean-ridge–type pillow basalts, which thicken oceanward in restored sections (Schaltegger et al., 2002). The basalts are interpreted to have been derived from a depleted mantle source in a setting of seafloor spreading immediately following continental breakup that opened the Jurassic Tethyan Ocean. The eruption of basalts appears to have been controlled by high-angle faults postdating the low-angle faulting that exhumed serpentinized peridotite; however, basaltic dikes are also locally cut by detachment faults. This implies that low-angle and high-angle faulting and basaltic volcanism were all closely associated in space and time (Manatschal et al., in press).
The field studies also confirm that different fault zones are present at different levels of the restored ocean–continent transition, and these can be interpreted as multiple detachments. The serpentinite breccias were formed by both tectonic processes (i.e., ductile shearing to brittle fragmentation) and sedimentary processes (i.e., as multiple mass flows) in different parts of individual sections. The breccias were deposited across the former detachment surface and were not restricted (e.g., to local fault scarps). The overlying bedded deep-sea sediments (e.g., radiolarite) are inclined at a low angle (<20°) relative to the underlying inferred detachment faults. This shows that these faults were active at a low angle at least during the later stages of their movement and that they were directly overlain and sealed by deep-sea sediments (Manatschal and Nievergelt, 1997).