Figure F4. Schematic models to explain observed deep structural asymmetries between the Newfoundland (left) and Iberia (right) margins. A. Synrift extension of continental crust. In the central part of the rift, lower crust is thinned ductilely (dashes) but brittle upper crust has limited tectonic extension (e.g., Driscoll and Karner, 1998). B. At the time of Anomaly ~M3. The rift evolves asymmetrically, with a thin remnant of continental crust forming an upper Newfoundland plate and serpentinized peridotite and remnants of ductilely thinned lower crust forming a lower Iberia plate. Bending stresses may account for faulting in the cold brittle mantle footwall as it is exhumed. Basement depth differences on the two margins reflect buoyancy of thin continental crust vs. serpentinized mantle. The U reflection may be a synrift unconformity developed near sea level. C. Anomaly ~M3. Mantle is exposed on both sides of the rift at an early stage, but an asymmetric shear then develops. Melt extracted from the lower plate may permeate the Newfoundland upper plate and flood its surface to form the U-to-basement sequence in a submarine setting (dark gray). Basement depth differences reflect buoyancy differences caused by melt intrusion/extrusion on the Newfoundland side. D. Anomaly ~M3: ultra-slow seafloor spreading. Symmetrical spreading as depicted here is unlikely because it would not account for extensive exposure of serpentinized mantle on the Iberia side or the asymmetry in basement structure of the transition zones on the two margins. Rather, ocean crust may have formed in the western part of the rift by seafloor spreading after the initial exposure of mantle, with the ocean crust subsequently being isolated on the Newfoundland side by an eastward jump of the spreading axis. The U–basement sequence might be explained (e.g., by basalt flows capping the ocean crust or as a sedimentary phenomenon peculiar to the Grand Banks side of the rift). The Newfoundland ocean crust would be more buoyant, and thus shallower, than serpentinized mantle on the Iberia side; differences in basement roughness would reflect differing effects and degrees of tectonic extension in ultra-slow-spreading ocean crust (Newfoundland side) and serpentinized mantle (Iberia side). MOHO = Mohorovicic seismic discontinuity.