Rifting of a continent and birth of a new oceanic spreading center are fundamental yet poorly understood parts of the plate tectonic cycle. Rifted margins are commonly classified as two types, volcanic and nonvolcanic (White et al., 1987; Mutter et al., 1988; White and McKenzie, 1989), although a relatively continuous range of margin types that vary in character according to tectonic stress, lithospheric strength, and mantle conditions is likely (Mutter, 1993). Two principal models of lithospheric extension have been proposed for nonvolcanic margins. In the pure shear model (McKenzie, 1978), crustal thinning is relatively uniform across a rift; brittle deformation causes thinning and faulting of the upper crust and ductile deformation thins the lower crust. This model predicts that conjugate margins will have generally similar crustal thickness, structure, composition, and subsidence history. Progress in modeling of continental rift structure and extensional tectonics together with observations of significant asymmetries in conjugate margins, however, suggests that many rifts may develop by a simple shear mechanism (e.g., Lister et al., 1986, 1991; Rosendahl, 1987; Wernicke, 1985). Simple shear predicts an upper plate margin consisting of weakly structured upper continental crust with a rift-stage history of uplift and a lower plate margin dominated by highly structured lower continental crust and a history of subsidence. Melt generation and attendant volcanism, compared to a pure shear environment, is probably minimal (Latin and White, 1990; Buck, 1991).
As continental plates separate, crustal thinning, volcanism, faulting, uplift, subsidence, and sedimentation profoundly modify the structure of the rifted margins. To understand these processes, we need detailed information on the resulting geological record, particularly the basement architecture and the overlying sedimentary framework. Furthermore, in order to evaluate the role of pure shear, simple shear, or other mechanisms of rift extension, it is essential to examine the geological record of conjugate rifted margins. This is best done by acquiring and analyzing wide-angle reflection/refraction and vertical-incidence reflection data along carefully chosen conjugate margin transects and then by sampling critical sections by drilling.
In the early 1990s, the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES) North Atlantic Rifted Margins Detailed Planning Group recommended the Newfoundland and Iberia conjugate margins as high-priority drilling targets to understand the evolution of nonvolcanic rifts (Fig. F1). These margins presented several advantages for such a study:
By design of the JOIDES advisory and planning structure, extensive drilling (Ocean Drilling Program [ODP] Legs 149 and 173) was conducted on the Iberia half of the rift (Fig. F1). This complemented earlier drilling (ODP Leg 103) on the western margin of Galicia Bank, and it was supported by extensive geophysical work (e.g., Whitmarsh et al., 1990, 1996; Pinheiro et al., 1992; Reston et al., 1995; Whitmarsh and Miles, 1995; Reston, 1996; Pickup et al., 1996; Discovery 215 Working Group, 1998). An early drill site from Deep Sea Drilling Project (DSDP) Leg 47B (Site 398) also provided valuable information near the Leg 149/173 transect. These studies, summarized below, provided surprising results about the composition and origin of crust in the transitional zone between known continental and known oceanic crust on the Iberia margin. They also raised major questions about how the Newfoundland–Iberia rift developed and whether rifting was symmetrical or asymmetrical. To answer these questions, it is necessary to investigate the structure and evolution of the conjugate Newfoundland margin, which was the major objective of Leg 210. The following section outlines the geological setting of the Newfoundland–Iberia rift and summarizes the results of a site survey that was conducted in preparation for drilling on the Newfoundland margin.