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Leg 81 drilling results on the west margin of Rockall Plateau, combined with available geophysical data, provide the first transect of the "dipping reflector" type of passive margin. Unlike passive margins characterized by large tilted fault blocks, this type is characterized by an oceanward dipping suite of reflectors and may be the predominant type of rifted margin. The "dipping reflector" margin can be divided into four structural zones: the ocean crust (Zone I), an outer high (Zone II), the area of dipping reflectors (Zone III), and a "landward" zone (Zone IV) of subhorizontal reflectors. The Leg 81 transect sampled Zones II, III, and IV at four sites. The dipping reflectors were penetrated at two sites, 553 and 555. At these sites, the dipping reflectors consist mainly of subaerial tholeiitic flow basalts and minor interbedded sediments; high acoustic impedance contrasts between flows/ packets of flows and sediments may cause the reflections. The flow basalts making up much of the sequence were very probably erupted entirely within the Anomaly 24B-25 reversed polarity interval and possibly a good deal less. Effusive basalt eruption was succeeded by a major phase of pyroclastic volcanism that ceased just prior to Anomaly -24B and is recorded as a widespread ash-fall deposit throughout the NE Atlantic. Two hypotheses have been proposed to explain the dipping reflectors. Mutter et al. (1982) consider that they are formed by some kind of subaerial seafloor spreading. By contrast, Roberts et al. (1979) and Hinz (1981) propose that they have formed by voluminous eruption on stretching continental crust. The Leg 81 results neigher prove nor disprove that the dipping reflectors are underlain by oceanic or continental crust, although preliminary gravity interpretation favors the latter. However, the Leg 81 results provide data and in turn constraints on reasonable models for the formation of this type of margin.

Evolution of Volcanic Rifted Margins: Synthesis of Leg 81 Results on the West Margin of Rockall Plateau