Continental rifting that leads to seafloor spreading commonly is classified as either volcanic or nonvolcanic (Mutter et al., 1988; White and McKenzie, 1989; White et al., 1987), although variations in tectonic stress, lithospheric strength, and mantle conditions likely create a range of margin types between these end-members (Mutter, 1993). At nonvolcanic margins, lithospheric extension has been proposed to occur either symmetrically by pure shear (McKenzie, 1978) or asymmetrically by simple shear (e.g., Lister et al., 1986, 1991; Rosendahl, 1987; Wernicke, 1985). Pure-shear extension is envisaged to thin crust relatively uniformly, with brittle deformation occurring in the upper crust and ductile deformation in the lower crust, and it leads to similar crustal thickness, structure, composition, and subsidence history on conjugate margins. In contrast, conjugate nonvolcanic margins formed by simple shear exhibit marked differences in these features, and any attendant magmatism probably is minimal compared to a pure-shear environment (Buck, 1991; Latin and White, 1990).
To understand how rifts develop, it is necessary to obtain and interpret data from a combination of geophysical surveys, geological sampling (e.g., drilling), and modeling of conjugate margins. During the early phases of the Ocean Drilling Program (ODP), a North Atlantic Rifted Margins Detailed Planning Group (NARM-DPG) was formed by the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) Planning Committee to study options and make recommendations for drilling of conjugate-margin transects on volcanic and nonvolcanic margins. For nonvolcanic margins, the NARM-DPG recommended the Newfoundland and Iberia conjugates as high-priority targets. Perceived advantages of drilling these margins included the following:
The earliest drilling in the Newfoundland-Iberia rift preceded the NARM-DPG report and was conducted on Galicia Bank (Fig. F1), first during Deep Sea Drilling Project (DSDP) Leg 47 at Site 398 south of Vigo Seamount (Sibuet and Ryan, 1979) and subsequently on the western Galicia margin during ODP Leg 103, Sites 637–641 (Boillot, Winterer, et al., 1988). Drilling designed to follow the concept of conjugate transects was initiated during ODP Leg 149 (Sites 897–901) (Whitmarsh, Sawyer, Klaus, and Masson, 1996) and continued during ODP Leg 173 (Sites 1065–1070) (Beslier, Whitmarsh, Wallace, and Girardeau, 2001). During both of these legs, sediments and basement were sampled along the southern perimeter of the Galicia crustal block beneath the edge of the southern Iberia Abyssal Plain. These ODP expeditions were supported and complemented by extensive geophysical studies in this area (e.g., Whitmarsh et al., 1990, 1996b; Pinheiro et al., 1992; Reston et al., 1995; Whitmarsh and Miles, 1995; Reston, 1996; Pickup et al., 1996; Discovery 215 Working Group, 1998; Dean et al., 2000).
In contrast, during this period no scientific drilling was done and little geophysical work was conducted on the conjugate Newfoundland margin. In 2000, the SCREECH program (Study of Continental Rifting and Extension on the Eastern Canadian sHelf) acquired a set of three major seismic reflection/refraction transects that crossed the Grand Banks and Flemish Cap into the Newfoundland Basin (e.g., Funck et al., 2003; Hopper et al., 2004; Lau et al., 2006a, 2006b; Shillington et al., 2006; Van Avendonk et al., 2006). The central transect, SCREECH Line 2 (Shillington et al., 2004, 2006; Van Avendonk et al., 2006), was conjugate to Leg 149/173 drilling off Iberia and it provided the survey data necessary to select and drill the first holes in the Newfoundland margin. During ODP Leg 210, two holes were drilled along this transect (Figs. F1, F2, F3). The first hole, at Site 1276, was located ~15 km west of magnetic anomaly M3 and penetrated to 1736.9 meters below seafloor (mbsf) before hole conditions forced abandonment. The hole terminated in the lower of two diabase sills that were intruded into lowermost Albian to uppermost Aptian sediments (Tucholke, Sibuet, Klaus, et al., 2004), and it is estimated to have bottomed only ~100–150 m above basement. Site 1277 was drilled ~40 km to the southeast on a prominent, shallowly buried basement ridge a few kilometers seaward of anomaly M1 (Fig. F3). In this hole serpentinized peridotite basement beneath a thin cover of basalt flows and basaltic, gabbroic, and serpentinite debris flows was recovered (Tucholke, Sibuet, Klaus, et al., 2004).
Drilling on both sides of the Newfoundland-Iberia rift has produced surprising insights into nonvolcanic rifting but at the same time has raised important new questions. Perhaps the single most important observation has been that extensive tracts of mantle ("transition zones") were exhumed seaward of the edges of thinned continental crust and landward of oceanic crust formed by seafloor spreading (we use the term seafloor spreading here to mean generation of new seafloor at a discrete plate boundary and with a major, if not dominant, component of magmatic accretion). Drilling at Site 1277 seaward of anomaly M1 on the Newfoundland margin, and at Sites 897, 899, and 1070 in the area of anomalies M5 to M1 on the Iberia margin, demonstrated that basement there is serpentinized mantle with only very minor indications of magmatism, despite the fact that apparently normal magnetic anomalies are developed in this young part of the M-series reversals. Thus there are major questions not only about the source of the magnetic anomalies but also about whether the prolonged mantle exhumation should be considered to be "seafloor spreading" or just continued rifting, for example, of subcontinental mantle lithosphere. In addition, there are important questions about how expected tectonic events such as separation of continental crust or initiation of seafloor spreading might be manifested in the structural and stratigraphic record along the margins. Finally, drilling at Site 1276 provided the surprising result that substantial and possibly widespread diabase sills were intruded into postrift sediments of the Newfoundland Basin, whereas there is no evidence of similar magmatism on the Iberia margin. Was this magmatic pattern "inherited" because of asymmetries in mantle fertility that developed during rifting, or was it caused by another process, such as plate migration across a mantle plume?
In the following pages, we consider these issues as we summarize the tectonic development of the Newfoundland-Iberia rift through time. We then outline the postrift sedimentary history, and we conclude with thoughts about important issues to be addressed by future drilling.