Tagus Abyssal Plain Research Articles

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

SUMMARY The 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 4115'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 11W of the OCB in the Tagus Abyssal Plain.

Cenozoic deformation and Mesozoic abandoned spreading centre in the Tagus Abyssal Plain (west of Portugal): results of a multichannel seismic survey

The seismic stratigraphy of the sedimentary section that fills in the Tagus Abyssal Plain is specified by means of new multichannel seismic data. Six sedimentary sequence units are identified. These, ranging from Late Jurassic to Recent, have been dated by extrapolation of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilling results and from sampling by dredging and submersible surveys. Based on this stratigraphic calibration and other geophysical data, a geodynamic evolution can be established for the Tagus Abyssal Plain. Faulting and folding in these sedimentary units clearly indicate that both the Portuguese Margin and the adjacent oceanic crust were deformed by a Miocene compressive event. An Oxfordian–Kimmeridgian rifting in the Lusitanian Basin and on the Portuguese Margin may herald a sea-floor spreading in the Tagus Abyssal Plain. The continent–ocean boundary is marked by landward-dipping reflectors similar to those observed along the Canadian margin. The oceanic structure of the Tagus Abyssal Plain is compared with those of the small oceans and (or) slow-spreading-rate centres. The eastern Tagus Abyssal Plain could be underlain by a Latest Jurassic (M-21)–Earliest Cretaceous (M16) spreading centre that was abandoned in Earliest Cretaceous (142 Ma), whereas the western Tagus Abyssal Plain could be underlain by a younger crust (from M10 to J) formed after a ridge jump. This younger oceanic domain may have a western complement in the Newfoundland Basin.

Coherence of bottom‐reflected sound in the Tagus Abyssal Plain

Coherence of bottom‐reflected sound in the Tagus Abyssal Plain was determined as a function of bearing at frequencies 50–500 Hz for a fixed grazing angle of 15°. SUS charges were dropped at 244 m depth in a circular pattern of 28 km radius around a receiver at 2500 m depth in water 5100 m deep. The bottom reflected signals were isolated and processed to remove the decorrelating effects of varying bubble pulse periods. Computed was a coherence function ρ(ω,φ) = |Smn(jω)|4/Sm(ω)Sn(ω), where S is a sample estimate of the power spectral density, j is −1, ω is the radian frequency, and m, n are the indices of adjacent shots at an average bearing φ. For a given frequency, the measured coherence function vs bearing exhibits a degree of symmetry in which features occur in pairs 180° apart. This observation suggests a directionally dependent, coherence‐degrading effect operating at the bottom, in the water column, or both, and significant over an aperture of 5 km. [Work supported by NORDA Ocean Programs Office.]

The Eastern End of the Azores-Gibraltar Plate Boundary

Summary The known relative motion of the African and Eurasian plates at the eastern end of the Azores-Gibraltar plate boundary implies consumption of oceanic lithosphere at the low rate of 1–1.5 cm/yr. This section of the plate boundary, which crosses the area 34–38deg;N, 6–14° W, has none of the characteristic features of zones of oceanic lithosphere consumption (e. g. arcuate trench, island are volcanics, deep seismicity). A block of oceanic crustal and upper mantle material has been upthrust to form a 120 km long, 5 km high ridge, called Gorringe Ridge. Basalts, gabbros and ultramafics have been dredged from the northern face of this ridge. A mechanism for slow consumption is proposed which accounts for the anomalous features of this part of the plate boundary, and for the formation of Gorringe Ridge. Four deep seismic refraction lines show the major characteristics of the crustal structure to be 3–4 km thick sediment sequences, low upper mantle velocities of 7.3-7.6 km s−1 and typical oceanic moho depths of about 11 km below sea level. Continuous seismic reflection profiles show 1–2 km of basement relief in the Western Horseshoe and Tagus Abyssal Plains and define a zone of acoustically opaque sediment in the Eastern Horseshoe Abyssal Plain. The free-air gravity anomaly contour map shows 480 mgal. of relief. Closely-controlled models for the structure of Gorringe Ridge devised to fit north-south gravity profiles across the plate boundary require a large body with a density of at least 3.0 g cm−3 beneath the ridge to explain its associated 390 mgal. free-air anomaly. The plate boundary is not well defined by the relocated epicentres but crustal structures suggest a 100 km southward offset immediately west of Gorringe Ridge. The calculation of finite difference poles of rotation for the African plate relative to Eurasia shows there have been two major periods of relative motion across the plate boundary in this region during the past 72 My. The slow compressive phase of the last 10 My was preceded 60–72 My before present by right lateral strike-slip motion at a rate of 5.5 cm/yr. The direction of these relative motions was respectively perpendicular and parallel to 060°, a trend which is predominant in the bathymetry of this area and in the geology of southern Spain. This suggests that for the past 72 My a single line of weakness has existed in the lithosphere along which all motion between the African and Eurasian plates has been accommodated.

Minéralogie comparée des sédiments du Tage (Portugal) et de quelques sables profonds de la plaine abyssale correspondante

A mineralogical analysis was made of sand sampleso btained seaward of the River Tagus. These samples came from the continental shelf near the Tagus mouth and from sandy horizons in turbidites of the Tagus Abyssal Plain. All samples are extraordinarily similar in mineralogy. These observations support the hypothesis that the sediments of the Tagus Abyssal Plain were derived from the alluvium of the Tagus River.