The ocean-continent boundary off the western continental margin of Iberia-II. Crustal structure in the Tagus Abyssal Plain

Abstract

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.

Diachronous evolution of Late Jurassic–Cretaceous continental rifting in the northeast Atlantic (west Iberian margin)

“…Late Oxfordian -early Kimmeridgian extension in the latter basin has been considered the precursor of ocean spreading in the Tagus Abyssal Plain [Wilson et al, 1989] with recent models indicating a Valanginian age for this latter event [Pinheiro et al, 1992]. Alternatively, the oldest magnetic anomaly in the Tagus Abyssal Plain may be M20 [Srivastava et al, 2000], which may indicate a latest Jurassic (late Tithonian, 147 Ma) age for continental breakup in southwest Iberia (Figure 3).…”

“…The data in this paper confirm that outer proximal basins were structurally independent from the inner proximal margin during the last extension episode in southwest Iberia (rift 3). Therefore, latest Jurassic -Early Cretaceous continental breakup west of the study area [Pinheiro et al, 1992;Srivastava et al, 2000] must have been immediately preceded by significant extension in the outer proximal margin during the advanced rifting stage (Figures 15 and 16). Comparatively, after a peak in subsidence around the Oxfordian-Kimmeridgian boundary [Wilson et al, 1989;Hiscott et al, 1990a], the Lusitanian Basin and Santiago do Cacém areas record a widespread sea level fall during the late Kimmeridgian [Inverno et al, 1993].…”

Constraints on Moho Depth and Crustal Thickness in the Liguro-Provençal Basin from a 3d Gravity Inversion : Geodynamic Implications

“…Thin crust overlying a high velocity layer is found on the western margin of Iberia (3-4 km thick above a 7.3 to 7.6 km/s layer, Whitmarsh et al, 1990;Whitmarsh et al, 1993), in the Tagus abyssal Plain (2 km thick above 7.6 km/s layer increasing to 7.9 km/s towards Moho, Pinheiro et al, 1992) and its conjugate Newfoundland margin off Grand Banks (2-3 km thick above 7.2 to 7.7 km/s layer, Reid, 1994), accross Southwest Greenland margin (2.5 km thick above 7.0 to 7.6 km/s layer, Chian and Louden, 1994) and its conjugate Labrador margin (1-2 km thick above a 6.4 to 7.7 km/s layer, Chian et al, 1995) On the southern Newfoundland margin the high velocity body is limited by one or two landward dipping reflectors rising to basement surface seaward and connecting to Moho landward (Keen and de Voogd, 1988;Reid, 1994). The similarity with the T reflector of the Gulf of Lion is striking.…”

Ocean-Continent Transition in the Iberia Abyssal Plain

“…The second rifting phase consisted of extension in the Late Jurassic. The last phase of extension occurred in the Early Cretaceous (from Valanginian to early Aptian time), coincided with the south-to-north breakup of Iberia from the Grand Banks and has been well documented based on geological and geophysical data at sea Whitmarsh et al, 1990;Pinheiro et al, 1992).…”

“…A recent geophysical study of the Tagus Abyssal Plain (Pinheiro et al, 1992), using seismic refraction, seismic reflection and magnetic profiles, showed that the oceanic crust adjacent to the OCT is unusually thin (2 km) and that there is a transitional region between thinned continental crust and the thin oceanic crust, which, although not truly oceanic (for example, it has no seafloor-spreading magnetic anomalies), has a magnetization far stronger than is usually associated with continental crust. The thin oceanic crust is underlain by a 7.6 km/s layer, which is probably serpentinized peridotite.…”