Large scale mass wasting as a possible mechanism of formation of highly thinned continental crust and the S reflector on the Galicia rifted margin

The generally accepted model for the formation of the Atlantic rifted margins of the Galicia Bank (GB) and Newfoundland calls for an extended period of rifting, probably in several discrete stages, during which listric faults cut progressively deeper into continental crust and eventually reach the upper mantle. In these models, the margin developed a major low-angle west-dipping detachment fault (S) that controlled the last stage of the breakup, forming the deep Galicia basin, and stretching the continental crust to less than 3 km thickness. An oft cited puzzle has been the relative scarcity of syn-rifting sediment, as evidenced by reflector fanning, in the basins formed by fault block rotation above the detachment fault. This has been explained by 1) very low sediment supply during the syn-rift period, 2) reworking of syn-rift sediment in the post-rift stage, 3) non-rotational block displacement over a flat detachment, and/or 4) rapid fault movement during progressive rifting across the basin. While one or more of these explanations might apply to individual basins, it seems unlikely that they can explain the apparent paucity of synrift sediment throughout the whole margin. Therefore, we have chosen to examine an alternative hypothesis for the last stages of the formation of the rift between Galicia and Newfoundland that can explain this lack. Specifically, we suggest that comparatively rapid mass wasting on the western and southern flanks of the GB played an important, and heretofore unrecognized, role in the history of the margin . The mass wasting in question would have taken the form of rotational slumping and could have occurred some millions of years after the initiation of seafloor spreading, possibly masking aspects of the past tectonic history. We suggest that the low-angle detachment fault (S) in the conventional model may be the surface of separation of a very large landslide, moving to the west off the western flank of the GB. The landslide material (above S) included blocks of continental crust and earlier syn-rift sediment. The hypothesized landslide rupture occurred along crustal listric normal faults that were the first to penetrate to the upper mantle. Below S was upper mantle, exposed at the seafloor prior to slumping. In this concept, the Peridotite Ridge becomes a feature of the ocean crust formation prior the landslide event, rather than a feature exposed by detachment tectonics. Taking this possible very large slump into account suggests reevaluating much of the ongoing discussion of non-volcanic rifting processes, and the symmetry and asymmetry of conjugate rifted margins.