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The ocean-continent boundary off the western continental margin of Iberia—II. Crustal structure in the Tagus Abyssal Plain
Pinheiro, L.M.; Whitmarsh, R.B.; Miles, P.R. (1992). The ocean-continent boundary off the western continental margin of Iberia—II. Crustal structure in the Tagus Abyssal Plain. Geophys. J. Int. 109(1): 106-124
In: Geophysical Journal International. Wiley: Oxford. ISSN 0956-540X, more
Peer reviewed article  

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  • Pinheiro, L.M.
  • Whitmarsh, R.B.
  • Miles, P.R.

Abstract
    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.

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