ALBERT

Notes: 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-1. 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 ocean–continent 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-1 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 AR 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.

Notes: Two separate sets of experiments with digital ocean-bottom seismographs (DOBS) and airguns, on continental rise areas off Madeira and west of Portugal, produced en echelon second arrivals from the sediment layer on record sections. Traveltime and synthetic seismograrn modelling indicate that the arrivals represent multiply-reflected refracted phases which have undergone reflection within the sediment layer itself. Further, although the P-wave contrast at the intrasediment reflecting horizon is relatively small, the modelling indicates a large downward increase in S-wave velocity from 100–250 m s−1 (Poisson's ratio of at least 0.42) to about 1200 m s−1 (Poisson's ratio of about 0.25). A reflection event can usually be found on reflection profiles along the refraction lines at almost exactly the same ‘depth’ as the intrasediment reflector. In one case such an event can be traced to a nearby Deep Sea Drilling Project (DSDP) borehole where it is associated with the transition from ooze to chalk. This, and other circumstantial evidence, suggests that the intrasediment reflector marks an important increase in lithification within the sediment layer. If so it means that, in future, straightforward OBS experiments may be used to measure the depth of this increase without resorting to the drill.