Project Details (... continued)
North Atlantic Tectonic Reorganization
There have been three distinct phases of North America - Eurasia seafloor spreading south of Iceland (Figs. 1,2; Vogt et al., 1969; Vogt, 1971; Vogt & Avery, 1974; White, 1997; Smallwood & White, 2002; Jones et al., 2002; Jones, 2003; Merkouriev et al., 2009). An initial orthogonally-spreading ridge system without transform faults was established immediately after the Greenland - Eurasia breakup ~55 Ma, forming seafloor without fracture zones. This pattern abruptly changed ~42 Ma into a more typical slow-spreading orthogonal ridge/transform staircase pattern caused either by a change in seafloor spreading direction (Vogt, 1971; Vogt & Avery, 1974), or a decrease in the North Atlantic mantle temperature (White, 1997; Smallwood & White, 2002). Very shortly after, beginning at ~anomaly 17 time, ~40 Ma (Vogt, 1971; Vogt & Avery, 1974; White, 1997; Jones et al., 2002), the most recent fundamental reorganization of this plate boundary initiated south of Iceland. The orthogonal ridge/transform geometry that had just been formed began to be progressively eliminated by the southward-growing oblique Reykjanes Ridge. The tip of this reorganization phenomenon appears to have very recently reached at least to within 20 km of the Bight transform fault, commonly called the Bight fracture zone (FZ) near 56.8°N (Fig. 1). Whether this reorganization is continuing at present or is stopped at the Bight FZ is unknown. This is at the southern tip of the large-scale southwest pointing V shown in Figs. 1&2. This V as drawn is clearly oversimplified, but on a first-order scale separates seafloor formed on the oblique unsegmented Reykjanes Ridge system from older seafloor formed on the more typical orthogonally segmented slow-spreading ridge/transform system, such as those segments still active south of the Bight FZ. Whatever the mechanism involved, this was a major reorganization event in the plate tectonic history of the North Atlantic. Determining exactly how this ongoing major reorganization occurred is the object of this project.
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.
Vogt & Johnson (1975) thought that the earlier transition to orthogonal spreading could readily be explained as the response of the spreading process to a change in spreading direction using the concept of Menard & Atwater (1968). This change in direction of Greenland-Eurasia seafloor spreading from ~125° to 100° was presumably caused by the termination of spreading on the Ran Ridge between Greenland & North America which happened about the same time. They noted that the ongoing transition from the orthogonal staircase geometry to oblique spreading was more problematic, but perhaps was caused by pipelike asthenosphere flow from the Iceland hotspot, either causing a more hexagonal tension regime in the lithosphere or by weakening the sub-axial lithosphere in such a way that new spreading would favor a reorientation to oblique spreading. Fig. 3 (from Vogt & Johnson, 1975) shows a conceptual model for how this evolution would occur if channeled pipelike asthenospheric flow from the Iceland hotspot following the overall oblique trend of the plate boundary heats, erodes, remelts & weakens the axial lithosphere overlying its path. Each progressively farther southwest ridge segment in turn would gradually feel the warmer asthenosphere & begin to gradually rotate into position with the soon to be colinear segment to the northeast, an example of “Zed-pattern” rotation by differential asymmetric spreading (Menard & Atwater, 1968). We describe this geometry in some detail because Fig. 3, although published 36 years ago, is the only figure we've found that actually shows how this ridge reorientation is supposed to have occurred in any existing model.
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).
Vogt & Johnson's (1975) longitudinal flow model was based on Vogt's (1971) channeled “pipe flow” model, which was once the preferred explanation for how the plume interacts with the ridge axis, although it is now thought that his alternative radial plume flow model (Vogt, 1971) is correct. This is because recent geodynamic models strongly argue against channeled flow under the Reykjanes Ridge axis (Ito, 2001; Jones, 2002), as a consequence of melt extraction & dehydration (Hirth & Kohlstedt, 1996) producing a strong compositional lithosphere uniformly above the solidus depth. In these recent models there is no basal lithospheric relief (channel) associated with the ridge axis (or with transform faults); thus the Iceland mantle plume must be affecting the Reykjanes Ridge through radial rather than channeled flow, & Vogt & Johnson's (1975) asthenosphere flow model for the transition to oblique spreading must be modified somewhat.
Fig. 3: Vogt & Johnson (1975) reorganization model, in which Iceland asthenosphere flow (dashed contours) thermally weakens axial lithosphere & progressively rotates each segment by differential asymmetric spreading, creating a sequence of "zed pattern" reorganizations (Fig. 4a, Menard & Atwater, 1968). This is also the type of reorganization described qualitatively in the White (1997) thermal model.
White (1997) proposed the now accepted conceptual thermal mechanism, in which radial flow of warmer plume asthenosphere expanding away from Iceland progressively changes the lithospheric rheology from brittle to ductile. This evolution is described in words but not with a figure, but this progressive thermally-induced rheological change in behavior is similar to the thermally-induced lithospheric weakening model of Vogt & Johnson (1975) in two important ways. In both models the causative mechanism is lithospheric thermal weakening, & propagating rifts are not involved in either model, so presumably the detailed plan form evolution would be essentially as predicted by Fig. 3. Certainly no subsequent authors have thought that any revision to Vogt & Johnson's (1975) Fig. 3 schematic evolution was necessary, & ridge rotation, along with ridge propagation, are the two basic alternative seafloor spreading reorientation mechanisms known (Fig. 4, from Kearey & Vine, 2003). (A third possible mechanism would be an instantaneous “ridge jump” or “synchronous” reorientation as proposed for the Woodlark Basin (Goodliffe et al., 1997), but this mechanism does not apply here because a clear diachronous pattern is evident along the Reykjanes Ridge flanks).
Fig. 4: (a) Ridge rotation mechanism of plate boundary reorganization (after Menard & Atwater, 1968) vs. (b) reorganization by rift propagation. Only the propagating rift mechanism creates a failed rift & zone of transferred lithosphere (oblique isochrons between failed rift & inner pseudofault), & thus there is a definitive test between this & the Fig. 4a ridge rotation mechanism. From Kearey & Vine (2003).
Although there had been very brief suggestions that propagating rifts might have been involved (Johansen et al., 1984; Mercuriev et al., 1994), this was never accepted as the reorganization mechanism in this area. The reason the propagating rift alternative was ignored was that the younger seafloor record (Fig. 1) of the part of the Reykjanes Ridge flanked by V-shaped ridges (Vogt, 1971) seemed to prove that there had been no rift propagation (Johansen et al., 1984), while a great deal of data & modeling were interpreted as being consistent with a purely thermal origin of these structures. Thus the thermal model has always been assumed to be true. However, results from our prior work (Hey et al., 2010) show the strong probability of rift propagation farther north associated with the V-shaped ridges on the younger Reykjanes Ridge lithosphere, suggesting it is a possible explanation for this earlier reorganization as well.
