There is a massive ongoing reorganization of North Atlantic seafloor spreading geometry occurring at present along the Mid-Atlantic Ridge (MAR) south of Iceland. The previous orthogonal ridge/transform staircase geometry typical of slow-spreading ridge systems is being progressively changed to the present non-segmented oblique spreading geometry on the Reykjanes Ridge as transform faults are successively eliminated. This fundamental plate boundary reorganization, obvious in gravity maps (Fig.1; Sandwell’s website: http://topex.ucsd.edu/marine_grav/jpg_images/grav4.jpg; Sandwell & Smith, 2009) & magnetic anomaly maps (Fig. 2; Vogt et al., 1969; Vogt & Avery, 1974; Macnab et al., 1995; Merkouriev et al., 2008; Maus et al., 2009), initiated south of Iceland ~40 Ma (chron 17) & slowly extended south (Vogt, 1971; White, 1997; Jones et al., 2002). The tip of this reorganization phenomenon presently appears to be at or near the northernmost remaining transform fault south of Iceland, the Bight transform fault near 56.8°N (Fig. 1). This reorganization is presently interpreted as a thermal phenomenon, with a pulse of 50°C warmer mantle expanding away from the Iceland plume causing a progressive change in subaxial mantle rheology from brittle to ductile, so that transform faults can no longer be maintained (White, 1997). This explanation is nearly universally assumed to be true (White, 1997; White & Lovell, 1997; Smallwood & White, 1998, 2002; Ito, 2001; Albers & Christensen, 2001; Jones et al., 2002, 2010; Jones, 2003; Poore et al., 2006, 2009, 2011; Parkin & White, 2008; White et al., 2010) & is used to infer important conclusions about deep Earth & lithospheric geodynamics that are also nearly universally assumed to be true.
Fig. 1: Satellite gravity near Iceland (Sandwell & Smith, 2009), with oversimplified “big V” reorganization wake separating young oblique seafloor structures parallel to the present Reykjanes Ridge (RR) axis from older orthogonally-segmented ridge- transform fabric, & proposed survey polygon at its tip. The RR axis has become an axial valley in our proposed survey area. The Bight FZ is the E-W trending structure near 57°N extending west toward the failed Greenland-North America rift. The V-shaped ridges & troughs (VSRs) are enclosed by the reorganization V.
However, the thermal/ductile hypothesis has never been rigorously tested, although it makes testable predictions about the seafloor structural & magnetic fabric characterizing the reorganization tip & wake, and about the tip location. It is important to do this test because of the far-reaching implications of the thermal model for fields ranging from geodynamics (e.g. Ito, 2001; Jones et al., 2002) to hydrocarbon exploration (e.g. Parkin et al., 2007; Rudge et al., 2008) to global climate change (e.g. Wright & Miller, 1996; Poore et al., 2006, 2009, 2011), & because there is an alternative mechanism for seafloor spreading reorganizations, rift propagation, which does not include a brittle-ductile transition required by the thermal hypothesis. Given that the Reykjanes Ridge is certainly the most obvious & arguably the type-example of active plate boundary reorganization, it is somewhat surprising that a thermal mechanism has near universal acceptance here whereas most if not all other reorganization examples are thought to result from the tectonic rift propagation mechanism. These other well studied examples of plate boundary reorganization by ridge propagation include the Northeast Pacific “Zed” area, Juan de Fuca, Galapagos, Australia-Antarctic Discordance, South Pacific, Easter and Juan Fernandez microplates, Southeast Indian Ridge, and Mid-Atlantic Ridge (MAR) (e.g. Shih & Molnar, 1975; Hey & Vogt, 1977; Hey, 1977; Atwater, 1981,1989; Delaney et al., 1981; Johnson et al., 1983; Searle & Hey, 1983; Vogt et al., 1983; Wilson et al., 1984; Schilling et al., 1985; Caress et al., 1988; Hey et al., 1986, 1988, 1989; Macdonald et al., 1988; Wilson, 1988, 1991; Atwater & Severinghaus, 1989; Kleinrock & Hey, 1989a,b; Kleinrock et al., 1989, 1997; Searle et al., 1989; Cande & Haxby, 1991; Carbotte et al., 1991; Naar & Hey, 1991; Larson et al., 1992; Schouten et al., 1993; Cormier & Macdonald, 1994; Phipps Morgan & Sandwell, 1994; Gente et al., 1995; Wilson & Hey, 1995; Christie et al., 1998; Kruse et al., 2000; Conder et al., 2000; Scheirer et al., 2000; Briais et al., 2002; Marjanovic et al., 2011; Dannowski et al., 2011). Rift propagation is primarily a tectonic mechanism, and makes quite different predictions from those of the thermal model about the reorganization tip and wake structures. In particular, only the propagating rift hypothesis predicts the presence of a zone of lithosphere transferred from one plate to the other. The obvious test between these hypotheses is not presently possible because of the surprisingly large data gap at the tip of this reorganization.
Fig. 2: Compiled magnetic data available through Geological Survey of Canada (Macnab, et al., 1995). The proposed survey will be along seafloor spreading flowlines parallel to the top and bottom of the survey polygon at the tip of the broad reorganization V at the Bight transform fault. Data gaps are shown in gray, partially filled by too sparse Russian data (Merkur'ev et al., 2009).
We will conduct a month-long marine geophysical expedition to collect the multibeam, magnetics and gravity data that would provide a definitive test between the fundamentally different thermal and tectonic hypotheses for exactly how the Iceland plume (or whatever form of mantle convection or heterogeneity creates Iceland) caused the reorganization of the MAR south of Iceland. The data will be collected along best-estimate flowline profiles and will be sufficiently closely spaced for three-dimensional analysis. An important result of this test will be that modelers will either be confident that their thermal Reykjanes Ridge reorganization models are providing accurate information about Earth's behavior, or that they can confidently begin to model the correct mechanism instead. An additional benefit of this project will be the first accurate map of the Bight transform fault/fracture zone complex, known to be an important pathway for westward flow of North Atlantic mid-water circulation across the Reykjanes Ridge boundary (Bower et al., 2002), an important control on global climate change (Siedler et al., 2001).