Reflection profiles characterize the structure and the upper Mesozoic to Cenozoic deposits of the Gulf of Ciidiz region. Two long ENE-WSW multichannel seismic lines (ca. 400-500 km long) are analyzed to study the evolution of the area from the continental shelf to the Horseshoe and Seine abyssal plains. The huge allochthonous deposits emplaced in this region (the so called "Olistostrome" of the Gulf of Cadiz) are described in terms of three different domains on the basis of the seismic architecture, the main tectonic features and the nature of the basement, oceanic or continental. The eastern domain extends along the continental shelf and upper and middle slope and corresponds to the offshore extension of the Betic -Rifean external front. It is characterized by salt and shale nappes later affected by extensional collapses. The central domain develops along the lower slope between the Betic-Rifean front and the abyssal plains and is characterized by a change in dip of the allochthonous basal surface and the basement. The allochthonous masses were emplaced by a combined gravitational and tectonic mechanism.The nOlihern boundary of this domain is marked by the occurrence of an outstanding WNW -ESE-trending thrust fault with a strike-slip component, termed here as the Gorringe-Horseshoe fault. The westernmost domain corresponds to the abyssal plains, where the distal emplacement of the allochthonous body takes place; it is characterized by thrust faults affecting both the sedimentary cover and the oceanic basement. The allochthonous masses show a less chaotic character and the thickness decreases notably. These domains represent different evolutionary steps in the mechanisms of emplacement of the allochthonous units. The eastern domain of the allochthonous units was emplaced as part of the pre-Messinian orogenic wedge related to the collision that gave rise to the Betic-Rifean Belt, whereas the allochthonous wedge of the central and western domains were emplaced later as a consequence of the NE-SW late Miocene compression that continues in present times.

Hydrothermal vents jetting out water at 380 degrees +/- 30 degrees C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Galápagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

The Gulf of Cadiz, located at the southwestern Iberian margin, is characterized by widespread seismicity, compressional and strike‐slip fault plane solutions and by a large, elongated positive free‐air gravity anomaly, the Gulf of Cadiz Gravity High (GCGH). Multichannel seismic profiles across and along GCGH, together with bathymetric and gravity data, allow us to study in detail the tectonic architecture and crustal structure of the Gulf of Cadiz. The upper shelf and slope of the Gulf of Cadiz includes the main structural domains of the Betic fold and thrust belt. In the middle part of the Gulf, the Paleozoic basement crops out on the shallow Guadalquivir Bank and is associated with the largest signature of the GCGH, whereas toward the outer part of the Gulf, the basement deepens progressively. A large NW‐SE normal fault and conjugate NE‐SW faults define a prominent basement high associated with the GCGH. Modeling of the GCGH suggests localized crustal thinning of 10 km along the central part of the Gulf of Cadiz, probably generated during the Mesozoic rifting episode between the Iberian and African plates. Concentric wedges of fold and thrust belts and large allochthonous masses were emplaced in the Gulf of Cadiz during the Neogene compressional phase. The final emplacement of these units becomes progressively young from the SE (pre‐early Langhian) toward the foreland in the NW (late Tortonian). Seafloor surface ruptures, pockmarks, and submarine landslides provide evidence of active faulting in the Gulf of Cadiz. To accommodate the present‐day convergence between the African and Eurasian plates, previously extensional faults have probably been reactivated and inverted at depth, as suggested by the intermediate depth seismicity.

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[1] Controversial evolutionary models have been proposed for the Gibraltar Arc system, a complex interaction zone between the Eurasia and African plates. Here we derive new mantle anisotropic constraints from SKS splitting measurements on a dense network of about 90 broad-band stations deployed over South Iberia and North Morocco. The inferred fast polarization directions (FPD) clearly show a spectacular rotation along the arc following the curvature of the Rif-Betic chain, while stations located at the South and South-East edges show distinct patterns. These results support geodynamical processes invoking a fast retreating slab rather than convective-removal and delamination models. The FPD variations along the Gibraltar arc can be explained by fossil anisotropy acquired during the Western Mediterranean Eocene subduction, while changes to the South and South-East of the Rif-Betic chain could be the imprint of a flow episode around an Alboran high velocity slab during its Miocene fragmentation from the Algerian slab. Citation:

[1] The crustal structure of the northern Gulf of California transtensional margin has been investigated by a 280-km-long NW-SE profile, including deep multichannel seismic reflection and densely sampled refraction/wide-angle reflection seismic information combined with gravity modeling. The seismic and gravity modeling constrains two thinned crustal areas, corresponding to the upper Delfín and the upper Tiburón basins. On both sides of the profile, toward the Baja California Peninsula and the Mexico mainland, a progressive thickening of the continental crust is observed. Our results indicate that the crustal thickness is 19 km below the coastline, and it decreases to 14 and 17 km below the upper Delfín and upper Tiburón basins, respectively. In the area between both basins, the crust thickens to 19.5 km. There are significant lateral thickness variations for the different levels of the crust. The interpreted structure is consistent with the existence of an aborted rift below the upper Tiburón basin. Prominent dipping reflections in the multichannel data under upper Tiburón basin and the ridge between upper Tiburón and upper Delfín basins can be explained as a mylonite like zone related to a detachment fault. This interpretation suggests that the structural evolution of upper Tiburón basin could be controlled by a major fault that cuts through the upper crust and merges into a zone of subhorizontal reflections in the lower crust. The mode and locus of extension have evolved from a core complex in upper Tiburón to a narrow rift mode in upper Delfín basin.

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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