Figure F11. Schematic models to explain observed deep structural asymmetries between the Newfoundland (left) and Iberia (right) transition zones. 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. 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 on the Iberia side. Differences of basement depth on the two margins reflect buoyancy of thin continental crust (shallow) vs. serpentinized mantle (deep). The U reflection could be compared to a synrift unconformity developed near sea level, basalt flows, or high-velocity sediments (see C and D). C. Alternate model at Anomaly ~M3; mantle is exposed on both sides of the rift at an early stage, followed by development of an asymmetric shear. 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. Basement depth differences between the two margins reflect buoyancy differences caused by melt intrusion/extrusion on the Newfoundland side. D. Second alternate model at Anomaly ~M3; ultra-slow seafloor spreading. Symmetrical spreading is unlikely because it would not account for extensive exposure of serpentinized mantle on the Iberia side or 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 initial exposure of mantle, with the ocean crust subsequently being isolated on the Newfoundland side by a jump of the spreading axis (from a failed rift [FR] eastward to a more central location). The U–basement sequence might be explained by basalt flows capping the ocean crust or as a high-velocity sedimentary sequence. More buoyant Newfoundland ocean crust would be shallower than serpentinized mantle on the Iberia side, and differences in basement roughness would reflect dissimilar tectonic extension in the two kinds of lithosphere. MOHO = Mohorovicic discontinuity.