From the continent seaward, Zone I represents thinned continental crust, Zone II the transitional crust, and Zone III the true oceanic crust. I on the Labrador margin while on the SW Greenland margin it is thinned sharply over a distance of only 20 km. Combined interpretation for this lower crustal block favors an origin of serpentinization rather than magmatic underplating. Comparison with the seafloor spreading magnetic anomaly data suggests that true oceanic crust started to form between Chrons 27 and 31, with the first clear magnetic anomaly occurring at Chron 27. The crustal cross-sections are used as a starting point in reconstruction of the rifting history.
Results suggest that the initial continental stretching started symmetrically.
At a later stage of stretching, more thinning occurred at the eastern side of the rift zone, leading to a lower crustal separation near the seaward end of the continental slope on western Greenland. A composite pure and simple shear model is favored in which all the lower crust in the rift zone moves with Labrador and most of the upper crust in the rift zone moves with Greenland. Serpentinization of upper mantle materials started after the crustal separation, forming undercrusting beneath the thinned upper crust on the southwestern Greenland margin and seaward of the lower crust on the Labrador margin. This proposed model explains all the new data presented in this thesis, as well as other observations that have been controversially addressed on similar margins.