The ECORS deep seismic profile and additional geological and geophysical data are used to constrain the balancing of a structural section crossing the Pyrenees. To minimize the effects of the mid-Cretaceous strike-slip motion along the North Pyrenean fault, we have chosen to restore the geometry to the period after the Albian-Cenomanian strike-slip faulting and before the Late Cretaceous compressional tectonics. At least 100 km of shortening must be accounted for in the deep crust in order to balance the cross section. The estimated length of the top of the Iberian Paleozoic basement is 40 km shorter than the length of the layered Iberian lower crust as measured on the ECORS seismic line.A variety of restorations are thus discussed to accommodate this discrepancy. The first solution considers that the discrepancy is due to an initial absence of lower crust underneath part of the Iberian

The South-Pyrenean Tertiary Basin was an east-west trending foreland trough which developed from Early Eocene to Miocene times in a north-south trending compressive regime.

The Devonian basins of western Norway represent shallow to deep exposures of a synthetic extensional sedimentary basin and provide field evidence for ductile extensional deformation within the basin fill and for the evolution of a brittle low-angle fault and ductile shear zone along the basal contact. The motion along this low-angle (5-25 ~ detachment is synchronous with both deposition and tilting (25 ~ of the huge (up to 25-km thick) overlapping coarse detrital Middle Devonian series. Such a geometry requires a minimum dip-s!ip offset of 50 km.The structural data are consistent with fault-rock associations along the basal contact and with the prograde greenschist metamorphism observed in the southern basin: deeper and deeper levels are observed from N to S. Except along the highly sheared and retrogressed basal shear zone, the footwall remained unaffected by deformation during basin development.We discuss three crustal models for basin development and propose that the displacement along the basal contact of the basins is transformed into pervasive ductile flow within the lower crust both at some distance to the side of the basin and beneath the basin.A key problem for large-scale extensional tectonics is to determine the relationships between brittle faulting near to the surface and ductile stretching at depth, which controls the structure and evolution of extensional sedimentary basins.Using a theoretical approach, McKenzie (1978) proposed a model for lithospheric stretching.

Massive ore-grade zinc, copper and iron sulphide deposits have been found at the axis of the East Pacific Rise. Although their presence on the deep ocean-floor had been predicted rhere was no supporting observational euidence. The East Pacific Rise deposits represent a modern analogue of Cyprus-type sulphide ores associated with ophiolitic rocks on land. They contain at least 29% zinc meral and 6 % merallic copper. Their discovery will prouide a new focus for deep-sea exploration, leading to new assessmenrs of the concentration of metals in the upper layers of the oceanic crust.-THE area of the deposits of ore-grade zinc, copper and iron sulphide was explored and sampled in February-March 1978 by the manned diving saucer CYANA during the expedition CYAMEXi. The expedition, the only submersible diving programme that has so far been conducted on the East Pacific Rise (EPR), is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-yr study devoted to detailed geological and geophysical investigations of the EPR crest. The ore deposits were sampled in water depths of close to 2,620 m a t two neighbouring sites near 20" 54' N 109" 03'W. (refs 2-5) about 9 0 km north of the Rivera transform fault and 240 km south of the Tamayo transform fault (Fig. 1). Three dives of Cyana (CY 78-06, 08 and 12) crossed the two sampling sites, and we collected samples during two of these dives (CY 78-08 and 12). However, during al1 dives in the EPR axial zone, signs of hydrothermal activity were seen, including colonies of dead giant clams, fields of pillow lavas with pronounced colourstaining at the base of pillows, and coloured deposits on exposed scarp surfaces of normal faults and open fissures'. Coral-like growths, possibly of native sulphur, occur in other locations, including a sedimented fault-scarp about 1.0 km to the West of where the sulphide ores were sampled. Sampling sites The two sites where the sulphides were sampled lie on the lightly sedimented flanks of steep-sided structural depressions, about 20-30 m deep, 20-30 m wide. and about 600-700 m west of the axis of the 'extrusion zone' where the youngest lavas occur. Whereas the extrusion zone is marked by a 50 m-high sedimentfree discontinuous ridge with n o fissures or faults, the structural *The authors are al1 members of the CYAMEX Scientific Team.

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