The ocean-continent boundary off the western continental margin of Iberia-I. Crustal structure at 40°30'N

Abstract

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.

P-wave velocity structure in the northern part of the central Japan Basin, Japan Sea with ocean bottom seismometers and airguns

“…In a case that a spreading center exists in a region of anomalously hot asthenospheric mantle near plumes, the crust is thicker than normal (White et al, 1992). Also, slightly thicker crust than that of a typical oceanic crust was found in the transition zone between the continental crust and the oceanic crust as seen in passive margins (e.g., Whitmarsh et al, 1990). The continental crust may have been thinned to less than 1/5 of its original thickness at non-volcanic margins, which means that the thickness of a thinned continental crust is comparable to that of the oceanic crust.…”

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).…”

“…The thin oceanic crust is underlain by a 7.6 km/s layer, which is probably serpentinized peridotite. Whitmarsh et al (1990;in press, 1992) The western margin of Galicia Bank, the third of the three segments, has been studied with seismic refraction and reflection profiles, and has also been sampled extensively with dredges, submersibles and by drilling (Horsefleld, 1992;Mauffret and Montadert, 1987;. A seismic refraction model across the margin shows a thinned continental crust at the OCT adjacent to a moderately thinned (5-km) oceanic crust, which thickens rapidly to the west.…”