SUMMARY AND CONCLUSIONS

The west Iberia Margin is a nonvolcanic rifted margin which, following rifting that began in the Late Triassic, broke away from Newfoundland in the Early Cretaceous by a process of rifting which propagated from south-to-north.

The following summarizes the principal conclusions of this chapter.

1. Off west Iberia the ocean/continent transition is defined by an 80-130-km-wide region between the most oceanward tilted basement fault blocks (continental crust?) and a 300-km-long narrow margin-parallel peridotite ridge. Leg 149 drilled a west-to-east transect of holes across the ocean/continent transition, three of which reached acoustic basement, beginning at the peridotite ridge.
2. Sites 897 and 899 sampled serpentinized peridotites with similar, but not identical, petrologies that experienced a similar history of exhumation from the deep mantle to the surface. They crystallized at 1170°-1230°C, and this was followed by ductile shear deformation at 880°-1000°C. After limited partial melting, secondary minerals crystallized at around 30 km depth.
   Mylonitization, then low-temperature deformation and serpentinization, followed as the rocks were exhumed at the seabed. One important difference between the two sites is the fivefold greater strength of remanent magnetization of the Site 899 cores, which is reflected in the amplitudes of magnetic anomalies observed over the sites. The peridotites are neither clearly subcontinental nor suboceanic.
3. Site 900 sampled a flasered cumulate gabbro basement. The rare-earth element patterns are ambiguous and have been matched by different authors to both island arc and N-MORB basalts. The light rare-earth element pattern fits a transitional MORB parent magma. Nd and Pb isotope ratios also strongly suggest a MORB parent. The gabbro therefore likely formed in a magma chamber from a melt that was little contaminated, if at all, by continental crust. Traces of the primary mineralogy indicate dynamic crystallization at depths of at least 13 km and temperatures typical of granulite facies conditions. We think this happened either in the upper mantle beneath ultraslow spreading oceanic crust or by underplating under thinned continental crust. During exhumation to the seabed the rocks underwent retrograde metamorphism at 280 crystallization of some plagioclases at 136.4
4. Upper Barremian to upper Aptian debris-flow and mass-flow deposits were encountered above acoustic basement in holes at Sites 897 and 899, both of which are now situated on substantial basement elevations hundreds of meters above the adjacent basins. They are overlain by poorly fossiliferous latest Maastrichtian to Eocene thin claystones and conglomerates (lag deposits) of similar age to the sediments at the same level in the flanking basins. We explain these deposits by rapid Aptian uplift (or at least less subsidence than in the flanking basins), ~10 m.y. after the onset of 10 mm/yr seafloor spreading west of the peridotite ridge., followed in Eocene time by eventual blanketing by the sediments of the Iberia Abyssal Plain. This also explains the long history of seawater alteration of serpentinized peridotite at both sites.
5. The above results are explained here by two possible hypotheses for the development of the west Iberia ocean/continent transition, neither of which fully explains all the observations. The first hypothesis is that the ocean/continent transition is underlain by crust formed by ultraslow (~5 mm/yr) seafloor spreading which, by analogy with the slow-spreading Mid-Atlantic Ridge, allows the seabed exposure of peridotite and gabbro by extensive faulting. This hypothesis explains the drilling results but has difficulty in explaining the magnetic anomalies or the minute volume of basalt in the cores. The second hypothesis envisages an ocean/continent transition of tectonically and magmatically disrupted continental crust. It explains the Site 900 MORB gabbro as material underplated under thinned continental crust or as an aborted point-of-initiation of sea- floor spreading and the alkaline-to-transitional characters of igneous clasts in Site 897 and 899 deposits. Its main problem is the lack of unequivocal continental basement samples, although there is evidence of reworked continental sediments in the cores. Both hypotheses tend to expect a highly heterogeneous crust in the ocean/continent transition that may be hard to characterize other than by further basement sampling.