The ocean‐continent transition (OCT) on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading. The nature of the OCT off western Iberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by Leg 149 of the Ocean Drilling Program. Here we concentrate on the interpretation of a new digital magnetic anomaly chart for the Iberia Abyssal Plain between 39.5° and 42.2°N. The most striking anomaly is a trough which exists immediately east of, and locally parallel to, anomaly J and is closely coincident with a basement peridotite ridge. We conclude from modeling surface magnetic anomalies and a deep‐tow magnetometer profile that seafloor spreading began about 129.9 m.y. ago (Barremian) at a rate of 10.0 mm/yr, consistent with drilling and seismostratigraphic results from the southern Iberia Abyssal Plain. A second feature is the existence of linear low‐amplitude isochron‐parallel magnetic anomalies east of the peridotite‐ridge trough and a clear change in the trend of these anomalies east of about 11°15′W. It is possible to model these anomalies by a series of 7‐ to 30‐km‐wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust. The west‐to‐east anticlockwise change in trend at 11°15′W suggests either a major change in stress regime and/or in the type of magnetic anomaly source. We speculate that the region between the peridotite ridge and 11°15′W represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading. We explain the change in trend at 11°15′W by a model featuring a rift which propagated through continental crust from south to north. We consider three scenarios for the formation of the transitional zone crust; first, continental crust is surrounded and impregnated by intrusive and extrusive material, formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate, second, such crust is surrounded by unroofed upper mantle material, or third, the zone is the product of ultraslow seafloor spreading. Present evidence tends to support the first scenario. In this scenario, the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust, fill the space caused by additional extension, and form the transitional crust, already described, over a broad zone at least several tens of kilometers wide. At breakup, upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization. In a final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading, initially producing thin oceanic crust because of a restricted melt supply. The width of continuous technically extended continental crust in the southern Iberia Abyssal Plain is...

S U M M A R Y An 80 km long reversed seismic refraction line (Line 5) was shot over the Tagus Abyssal Plain off Portugal. The main P-wave reflected and refracted phases were modelled both for traveltime and amplitude. The resulting P-wave velocity/depth model has the following features: (a) an extremely thin crust of about 2 km; (b) the absence of oceanic layer 3; and (c) very low upper mantle velocities between 7.6 and 7.9 km s-'. This very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at Atlantic fracture zones, and in particular to the structures found in profiles shot along the transform valley and near ridge-transform intersections. A magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of Line 5.As an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the Tagus Abyssal Plain is located at about 11"30'W, in a symmetric position with respect to the ocean-continent transition in the conjugate South Newfoundland Basin, then magnetic anomalies can be modelled simply by assuming sea-floor spreading west of 11'45'W at 10 mm yr-' beginning at M11 time (133 Myr BP), and blocks of rifted continental crust to the east. The location of the proposed ocean-continent transition in the Tagus Abyssal Plain is marked by a well-defined N-S linear magnetic anomaly which is adjacent to the oldest sea-floor spreading block. East of the proposed ocean-continent transition there is an increase in the depth to basement similar to that found east of the ocean-continent transition in the Iberia Abyssal Plain and elsewhere. This model also allows us to explain why Purdy's (1975) seismic refraction line A-AR in the Tagus Abyssal Plain cannot be interpreted as a conventional reversed pair because most of Line A was shot over the ocean-continent transition zone and most of Line A R over thinned continental crust.Remarkably similar velocity/depth structures to that under Line 5 are found close to the ocean-continent transition zone off the whole of western Iberia, in areas which show no clear evidence of fracture zones. Therefore it appears more likely that the seismic structure of Line 5 is due to its proximity to the ocean-continent transition than to a local association with a fracture zone and further, that its structure is typical of this transition off the western margin of Iberia. We also suspect that the low upper mantle velocities associated with the ocean-continent transition indicate the widespread occurrence of serpentinized peridotite.

S U M M A R YThe western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean-continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east-west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 41"15'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12'10' and 12'30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11"W of the OCB in the Tagus Abyssal Plain.

The NE dipping slab of the Hellenic subduction is imaged in unprecedented detail using teleseismic receiver function analysis on a dense 2‐D seismic array. Mapping of slab geometry for over 300 km along strike and down to 100 km depth reveals a segmentation into dipping panels by along‐dip faults. Resolved intermediate‐depth seismicity commonly attributed to dehydration embrittlement is shown to be clustered along these faults. Large earthquakes occurrence within the upper and lower plate and at the interplate megathrust boundary show a striking correlation with the slab faults suggesting high mechanical coupling between the two plates. Our results imply that the general slab rollback occurs here in a differential piecewise manner imposing its specific stress and deformation pattern onto the overriding Aegean plate.

Western • is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean-continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2-4 kin) oceanic crust underlain by 7.6 inn s -1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s -1 layer is serpentinized peridofite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental brea•p. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.