In July–August 2000, >3000 km of multichannel seismic (MCS) reflection, magnetic, gravity, and multibeam bathymetric data and 1000 km of wide-angle reflection/refraction data were acquired along three transects across the eastern Grand Banks and Newfoundland Basin during the Studies of Continental Rifting and Extension on the Eastern Canadian Shelf (SCREECH) survey (Fig. F1). This was a two-ship program, with MCS, magnetic, gravity, and multibeam bathymetric data acquired by the Maurice Ewing (Cruise 00-07) and wide-angle reflection/refraction data acquired by ocean-bottom seismometers/hydrophones (OBS/Hs) deployed and retrieved by the Oceanus (Cruise 359-2). The northern two transects were collected conjugate to seismic and drilling transects on the Iberia margin (Ocean Drilling Program [ODP] Legs 103, 149, and 173) based on the reconstruction of Srivastava et al. (2000). SCREECH transect 1 is conjugate to the ODP Leg 103 transect, and SCREECH transect 2 is conjugate to the ODP Leg 149/173 transect. Taken together, the geophysical data sets collected on the Newfoundland and Iberia margins constitute the most complete information available for conjugate margins of a nonvolcanic rift.
The purpose of the SCREECH program was to (1) distinguish between competing hypotheses for the origin of "transitional" crust lying between unambiguous oceanic crust and continental crust on the Newfoundland margin, (2) compare the crustal structure of the Newfoundland margin to the conjugate Iberia margin in order to learn more about the processes that extend and ultimately rupture continental crust, and (3) obtain site survey data that could be used to select and justify ODP drill sites in the Newfoundland Basin. In a larger context, the SCREECH program was germane to investigating broader issues related to continental rifting, such as the relative importance of pure and simple shear during margin formation (Lister et al., 1986; McKenzie, 1978; Wernicke, 1985) and the transition from late-stage rifting to early seafloor spreading (Cochran and Martinez, 1988; Hopper et al., 2004; Taylor et al., 1995).
Nonvolcanic rifted margins are produced by rifting that is not accompanied by significant magmatism, which is in contrast to the voluminous magmatism that commonly covers and masks extensional structures on volcanic margins (e.g., Louden and Chian, 1999). Thus, nonvolcanic margins are excellent places to image the structures associated with rifting and initial seafloor spreading. The lack of magmatism on nonvolcanic margins is often attributed to very slow and cold rifting, where conductive heat loss may suppress melt generation (Bown and White, 1995), although alternative theories call for rapid strain localization and continental rupture (Harry and Bowling, 1999) or depressed subcontinental geotherms (Reston and Phipps Morgan, 2004).
Seismic and drilling investigations of nonvolcanic margins, particularly the Iberia margin, have identified zones of serpentinized peridotite between thinned continental crust and "normal" oceanic crust (Beard and Hopkinson, 2000; Boillot et al., 1992; Dean et al., 2000; Pickup et al., 1996; Whitmarsh et al., 2001). Geochemical studies of samples recovered during ODP Legs 149 and 173 suggest that this altered mantle is subcontinental in origin (Abe, 2001; Hébert et al., 2001). Exhumed, serpentinized mantle has been emplaced over a width of as much as 100 km on some sections of the Iberia margin (Dean et al., 2000; Pickup et al., 1996). Additionally, bright, subhorizontal reflections have been observed in seismic sections from the Galicia Bank and the southern Iberia Abyssal Plain margin, and they are interpreted to be mechanical structures (detachment surfaces) related to final thinning of continental crust, unroofing of subcontinental mantle, or both (Chian et al., 1999; de Charpal et al., 1978; Krawczyk and Reston, 1995; Manatschal et al., 2001; Reston et al., 1996, 2001).
The identification of exhumed subcontinental mantle on the Iberia margin immediately raises the question of the origin and characteristics of crust on the conjugate Newfoundland margin. Previous geophysical studies recognized a zone of crust of disputed affinity between oceanic crust and continental crust on the Newfoundland margin (Keen et al., 1989; Reid, 1994; Srivastava et al., 2000; Tucholke et al., 1989). Here, this is termed "transitional crust." The presence of low-amplitude magnetic anomalies, the unusual reflection characteristics of basement in MCS sections, and a previous lack of extensive wide-angle reflection/refraction data allowed three possible explanations for the origin of this transitional crust: (1) slow-spreading oceanic crust (Srivastava et al., 2000; Sullivan and Keen, 1978), (2) thinned, possibly intruded continental crust (Tucholke et al., 1989; Tucholke and Ludwig, 1982), and (3) exhumed, serpentinized mantle (Reid, 1994). A significant part of the motivation for collecting seismic reflection and refraction data during the SCREECH survey and for drilling during ODP Leg 210 was to investigate the origin of the transitional crust on the Newfoundland margin and thus to better constrain the evolution of the Newfoundland–Iberia rift. Site 1276 was located within the zone of transitional crust, and Site 1277 was drilled seaward of transitional crust near magnetic Anomaly M1 on crust that is interpreted to be oceanic.
In this contribution, we present prestack time-migrated seismic reflection sections, together with the coincident magnetic and gravity data, to place Leg 210 drilling results into a regional context. SCREECH line 2MCS is conjugate to the ODP Leg 149/173 drilling transect across the Iberia Abyssal Plain on the Iberia margin (Srivastava et al., 2000). Interpretations of prestack depth migrations of the SCREECH transect 2 survey will appear in a forthcoming paper.