Tagus Abyssal Plain Research Articles

The Rift and Continent‐Ocean Transition Structure Under the Tagus Abyssal Plain West of the Iberia

AbstractThe West Iberia margin is the focus of intense research since the 1980s, with some of the most exemplary geophysical cross‐sections and drilling expeditions. Those data sets have been used to create conceptual models of rifting used as a template to interpret margins worldwide. We present two collocated ∼350 km long lines of multi‐channel seismic (MCS) streamer data and wide‐angle seismic (WAS) data collected across the Tagus Abyssal Plain (TAP). We use travel‐times of first arrivals identified at WAS and reflected seismic phases identified at both WAS and MCS records to jointly invert for the P wave velocity (Vp) distribution and the geometry of a sediment unconformity, the top of the basement, and the Moho boundary. The Vp model shows that the TAP basement is more complex than previously inferred, presenting abrupt boundaries between five domains. Domain I under the foot of the slope and Domain III under the abyssal plain display Vp values and gradients of thin continental crust. In between, Domain II displays a steep Vp gradient and high Vp values at shallow depth that support that basement is made of exhumed partly serpentinized mantle. Domain IV and Domain V, further oceanward, have oceanic crust Vp structure. The new results support an unanticipated complex rift history during the initial separation of Iberia and America. We propose a geodynamic scenario characterized by two phases of extension separated by a jump of the locus of extension, caused by the northward propagation of the oceanic spreading center during the J‐anomaly formation, which terminated continental rifting.

Seismic evidence of exhumed mantle rock basement at the Gorringe Bank and the adjacent Horseshoe and Tagus abyssal plains (SW Iberia)

The Gorringe Bank is a gigantic seamount that separates the Horseshoe and Tagus abyssal plains offshore SW Iberia, in a zone that hosts the convergent boundary between the Africa and Eurasia plates. Although the region has been the focus of numerous investigations since the early 1970s, the lack of appropriate geophysical data makes the nature of the basement, and thus the origin of the structures, still debated. In this work, we present combined P-wave seismic velocity and gravity models along a transect that crosses the Gorringe Bank from the Tagus to the Horseshoe abyssal plains. The P-wave velocity structure of the basement is similar in the Tagus and Horseshoe plains. It shows a 2.5–3.0km-thick top layer with a velocity gradient twice stronger than oceanic Layer 2 and an abrupt change to an underlying layer with a five-fold weaker gradient. Velocity and density is lower beneath the Gorringe Bank probably due to enhanced fracturing, that have led to rock disaggregation in the sediment-starved northern flank. In contrast to previous velocity models of this region, there is no evidence of a sharp crust–mantle boundary in any of the record sections. The modelling results indicate that the sediment overlays directly serpentinite rock, exhumed from the mantle with a degree of serpentinization decreasing from a maximum of 70–80% under the top of Gorringe Bank to less than 5% at a depth of ∼20km. We propose that the three domains were originally part of a single serpentine rock band, of nature and possibly origin similar to the Iberia Abyssal Plain ocean–continent transition, which was probably generated during the earliest phase of the North Atlantic opening that followed continental crust breakup (Early Cretaceous). During the Miocene, the NW–SE trending Eurasia–Africa convergence resulted in thrusting of the southeastern segment of the exhumed serpentinite band over the northwestern one, forming the Gorringe Bank. The local deformation associated to plate convergence and uplift could have promoted pervasive rock fracturing of the overriding plate, leading eventually to rock disaggregation in the northern flank of the GB, which could be now a potential source of rock avalanches and tsunamis.

Seismic triggering of landslides and turbidity currents offshore Portugal

[1] Sediments in deep water basins often include turbidites that record sediment input from adjacent continental margins. In seismically active areas, where turbidity currents are triggered by earthquakes, the basinal turbidite sequence may thus contain a record of palaeoseismicity, which can be used to infer the frequency of earthquakes affecting the margins of the basin. This is particularly useful where large earthquakes have a recurrence interval than is greater than the historical record. However, turbidity currents can be triggered by several processes, and it is often difficult to trace individual turbidites to their precise source areas and to assign a definite trigger to a particular turbidite. Here, we demonstrate that turbidites emplaced at ∼6600 and ∼8300 Cal yr BP in the Tagus Abyssal Plain, off Portugal, correlate with erosional hiatuses in two submarine canyons on the continental margin. The turbidites are sourced from simultaneous landsliding in both canyons, requiring regional triggers interpreted as earthquakes. An earthquake recurrence interval for the continental margin of ∼4000 years is estimated by extrapolation to deeper turbidites in the basin sequence. However, the example of the 1755 earthquake, which caused widespread devastation in southwest Iberia, shows that palaeoseismic interpretations must be made with caution. The 1755 earthquake had a magnitude >8.5 and yet the associated turbidite in the abyssal plain is typically ∼5 cm thick, while older turbidites can be >1 m thick. Given the large 1755 earthquake magnitude, the difference in turbidite thickness is unlikely to be related to the relative size of triggering earthquakes. Instead, we suggest that the offshore location of the 1755 earthquake, coupled with low sedimentation rates during the Holocene, may have limited the size of the associated turbidite.

Decoupled crust-mantle accommodation of Africa-Eurasia convergence in the NW Moroccan margin

[1] The extent of the area accommodating convergence between the African and Iberian plates, how this convergence is partitioned between crust and mantle, and the role of the plate boundary in accommodating deformation are not well-understood subjects. We calculate the structure of the lithosphere derived from its density distribution along a profile running from the Tagus Abyssal Plain to the Sahara Platform and crossing the Gorringe Bank, the NW Moroccan margin, and the Atlas Mountains. The model is based on the integration of gravity, geoid, elevation, and heat flow data and on the crustal structure across the NW Moroccan margin derived from reflection and wide-angle seismic data. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thickening beneath the margin (LAB > 200 km depth) followed by thinning beneath the Atlas Mountains (LAB ∼90 km depth). At crustal levels the Iberia-Africa convergence is sparsely accommodated in a ∼950 km wide area and localized in the Atlas and Gorringe regions, with an inferred shortening of ∼50 km. In contrast, mantle thickening accommodates a 400 km wide region, thus advocating for a decoupled crustal-mantle mechanical response. A combination of mantle underthrusting due to oblique convergence, together with a viscous dripping fed by lateral mantle dragging, can explain the imaged lithospheric structure. The model is consistent with crustal shortening estimates and with the accommodation of part of the Iberia-Africa convergence farther NW of the Gorringe Bank and/or off the strike of the profile.

Meddy, spiral arms, and mixing mechanisms viewed by seismic imaging in the Tagus Abyssal Plain (SW Iberia)

Meddy, spiral arms, and mixing mechanisms viewed by seismic imaging in the Tagus Abyssal Plain (SW Iberia)

Lithospheric structure of the Gorringe Bank: Insights into its origin and tectonic evolution

[1] The Gorringe Bank is a 5000 m high seamount near the Atlantic coast of Iberia characterized by a 9 m high geoid anomaly and a ∼120 mGal Bouguer anomaly relative to the surrounding abyssal plains. It has been linked to a NW directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed-down Eurasian oceanic crust along the Tagus Abyssal Plain. However, estimations of crustal shortening have yielded dissimilar results, and the deep structure of the ridge remains highly unknown. We present a restored cross section and a new model of the lithospheric structure based on gravity, geoid, elevation, and the presence of serpentinized peridotites. At least 20 km of shortening took place along a flat-ramp-flat thrust fault, and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70% at the surface to 20% at 14 km depth and 0% at 40 km. The topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness Te of ∼30 km. The evolution of the Gorringe Bank since the Late Jurassic is interpreted in relation to Eurasia-Africa-North America plate motion in four stages: (1) transtension between Newfoundland-Iberia and Africa, which generated small oceanic basins and mantle exhumation; (2) opening of the North Atlantic and seafloor spreading at the NW side of the exhumed Gorringe, which produced gabbro intrusions and serpentinization; (3) a quiescent tectonic period dominated by subsidence and sediment accumulation; and (4) a transpressional plate boundary between Eurasia and Africa with NW directed subcrustal thrusting and generation of the present Gorringe relief.

Detailed 2-D imaging of the Mediterranean outflow and meddies off W Iberia from multichannel seismic data

Reprocessing of a 326-km long multichannel seismic line acquired in the Tagus Abyssal Plain off W Iberia in 1991 allowed detailed imaging of the thermohaline structure of several mesoscale features within the water column. The interpretation was supported by subsurface float measurements, Sea Level Anomaly (SLA) maps and Sea Surface Temperature (SST) images contemporaneous with the acquisition of the seismic data. Clear images were obtained of the reflective patterns associated with one previously known and one newly discovered meddy, one cyclone, the upper and lower cores of the Mediterranean Undercurrent, and the interface of the high-salinity tongue of the Mediterranean Water with the North Atlantic Central Water. These reveal a complexity and a detail of the lateral variations of the thermohaline structure not easily observed by conventional physical oceanography tools. The mesoscale structures were imaged via reflections from oceanic fine structures of scale 30 m or less. We compare the characteristics of observed reflections with known mechanisms of fine-structure production. Most of the observed reflections are consistent with internal waves and thermohaline intrusions as previously hypothesized. We postulate a new mechanism to explain the formation of the steeply sloping reflections that outline the meddy and other features, involving frontogenetic isopycnal advection, formation of thermohaline intrusions, and tilting of the intrusive layers by mesoscale shear flows. The imaging technique therefore shows the relationship between mesoscale features and the fine-scale oceanographic phenomena associated with mixing, including steeply-sloped structures that would otherwise not be tracked using CTD profiles alone.

Response of a multi-domain continental margin to compression: Study from seismic reflection–refraction and numerical modelling in the Tagus Abyssal Plain

The effects of the Miocene through Present compression in the Tagus Abyssal Plain are mapped using the most up to date available to scientific community multi-channel seismic reflection and refraction data. Correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line IAM-5. Four structural domains were recognized. In the oceanic realm mild deformation concentrates in Domain 1 adjacent to the Tore–Madeira Rise. Domain 2 is characterized by the absence of shortening structures, except near the ocean–continent transition (OCT), implying that Miocene deformation did not propagate into the Abyssal Plain. In Domain 3 we distinguish three sub-domains: Sub-domain 3A which coincides with the OCT, Sub-domain 3B which is a highly deformed adjacent continental segment, and Sub-domain 3C. The Miocene tectonic inversion is mainly accommodated in Domain 3 by oceanwards directed thrusting at the ocean–continent transition and continentwards on the continental slope. Domain 4 corresponds to the non-rifted continental margin where only minor extensional and shortening deformation structures are observed. Finite element numerical models address the response of the various domains to the Miocene compression, emphasizing the long-wavelength differential vertical movements and the role of possible rheologic contrasts. The concentration of the Miocene deformation in the transitional zone (TC), which is the addition of Sub-domain 3A and part of 3B, is a result of two main factors: (1) focusing of compression in an already stressed region due to plate curvature and sediment loading; and (2) rheological weakening. We estimate that the frictional strength in the TC is reduced in 30% relative to the surrounding regions. A model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented. This model is consistent with both the geological interpretation of seismic data and the results of numerical modelling.

Neogene Through Quaternary Tectonic Reactivation of SW Iberian Passive Margin

Southwest Portugal, the Gulf of Cadiz and Morocco are under the potential threat of natural hazards linked to seismicity and tsunami generation. We report the results of two multi-channel seismic (MCS) surveys carried out in 1992 and 1998 along the continental margin and oceanic crust of SW Iberia. This MCS data set shows the evidence of the compressional deformation which involves both the continental and the oceanic crust of the study area. The area of deformation extends from the southern border of the Tagus Abyssal Plain to the Seine Abyssal Plain, encompassing the continental margin of SW Portugal. Most of the structures observed are probably related to a Mid-Miocene phase of Africa-Europe plate convergence. In this paper we discuss the recent advances on the identification of the tectonic structures that are still active and that may generate great earthquakes and tsunamis. The tectonic structures identified are located respectively at the Guadalquivir Bank, along the eastern border of the Horseshoe Abyssal Plain and along the southern continental slope of SW Portugal.

Structure of the Mediterranean Undercurrent and Mediterranean Water spreading around the southwestern Iberian Peninsula

[1] RAFOS float and hydrographic data are analyzed to investigate the pathways, rates, and mechanisms of Mediterranean Water (MW) spreading into the North Atlantic from the Gulf of Cadiz, with an emphasis on processes other than meddies. We find that the transition of the Mediterranean outflow from a density current to an intermediate-depth jet occurs as it enters Portimao Canyon south of Portugal due to rapidly steepening topography and associated turbulent entrainment. This feature was not seen in recent plume model simulations due to their use of smoothed topography. In addition to the traditional lower salinity maximum (or core), a denser, deeper continuous vein of MW was also traceable from Portimao Canyon to Cape St. Vincent. The mean velocity structure of the Mediterranean Undercurrent changes from a ∼30-km-wide jet with peak mean westward speed of ∼0.2 m s−1 along the southern Iberian slope, to a broader, much weaker northward flow with maximum mean speed of only ∼0.06 m s−1 along the western Iberian slope, due in part to changes in the bottom slope. Lower core MW spread mainly (1) southward into the interior Gulf of Cadiz, (2) northward along the western Iberian slope, and (3) westward into the deep Tagus Abyssal Plain. The average spreading rate was 0.01–0.02 m s−1 toward the west–northwest direction. Meddies forming near the slope were found to have an indirect but profound impact on MW spreading, often diverting MW approaching from upstream into the interior. The results are compared to recent hydrography-based studies of MW spreading pathways.

Direct land/sea correlation of the Eemian, and its comparison with the Holocene: a high-resolution palynological record off the Iberian margin

Abstract High-resolution pollen, dinocyst and isotopie profiles covering the marine isotope stage 5 (MIS 5) are presented from core MD952042 (Tagus abyssal plain, 37°47'N, 10°09'W). Both marine and terrestrial proxies indicate the occurrence of a Bølling-Allerød-Younger Dryas-like event at the beginning of MI substage 5e. The terrestrial Eemian stage coincides with both the lightest oxygen isotope values of substage 5e and the heavier ones approaching the 5e/5d transition. Accordingly, the Eemian is not equivalent to MI substage 5e, as the Holocene is not equivalent to MIS 1. Remarkably, both pollen and dinocyst data reflect the same climatic pattern on land and ocean, and they evidence a succession of climatic events that the isotope signal does not identify. The Eemian began with a Mediterranean vegetation that was gradually replaced by Eurosiberian formations indicating a change from Mediterranean to oceanic climates. In the middle of the Eemian, warming conditions were interrupted by an event corresponding to a slight cooling resulting from an increase in precipitation over land and ocean. Finally, a warming trend characterised the last phase of the Eemian. The occurrence of small climatic changes during this interglacial is inconsistent with the dramatic variability suggested by the GRIP ice-core record.

Dinoflagellate cyst evidence of ‘Heinrich-like events’ off Portugal during the Marine Isotopic Stage 5

Dinoflagellate cysts were analysed from IMAGES core MD952042 (37°48′N; 10°01′W) retrieved from the Tagus Abyssal Plain. Previous results of stable isotope and magnetic susceptibility measurements as well as of planktonic foraminiferal temperature reconstruction from this core, suggest the occurrence of “Heinrich-like events” (i.e. large ice-sheet decay) during Marine Isotopic Stage 5 (MIS 5). Dinoflagellate assemblages of this time period have revealed six dinocyst events that are characterised by peaks in Bitectatodinium tepikiense percentages. These events occur synchronously with “Heinrich-like events” previously identified. They are coeval with major retreats of the forest on land, indicating, therefore, drastic changes in the regional climate. However, results from the Ice-Rafted Detritus (IRD) analysis of the >150 μm lithic fraction shows that MIS 5 of MD952042 has only recorded one significant input of iceberg discharge, located at the MIS 6/MIS 5 transition. It seems therefore that it is the only event that could be called a “true Heinrich event”.

Recognition and stratigraphical significance of the Aptian unconformity in the Lusitanian Basin, Portugal

Recent sedimentologic and palynologic studies in the Belasian Sandstone (Lusitanian Basin) have greatly improved the knowledge of the palaeogeographic significance and chronostratigraphical position of the unit. This paper focuses particularly on the lower boundary, demonstrating its unconformable nature and proposing a relationship with the beginning of oceanic opening in the Atlantic sector near the Lusitanian Basin, north of the Tagus Abyssal Plain, during the early Aptian. This extensional event may have caused thermal and isostatic basement uplift that triggered the synchronous and widespread onset of a coarse-grained depositional system that overlies the breakup unconformity. This can be considered as a type 1 sequence boundary with the transgressive surface juxtaposed, and the Belasin sandstone as a transgressive and immediate post-rift systems tract. Comparison with similar unconformities in other basins points to the hypothesis that the breakup between Iberia (Lusitanian Basin) and Newfoundland affected sea level throughout the North Atlantic region, reinforcing the recognition of the lower Aptian unconformity as one of the most important sequence boundaries of the Atlantic and Tethyan Cretaceous.

The Droguing of Meddy Pinball and Seeding with Alace Floats

This paper reports the results of a hydrographic survey and the successful deliberate deep droguing of a meddy (Pinball) and the seeding of its core with two ALACE floats. The drogued buoy results give important kinematic properties of the eddy core in real timesemicolon the ALACE have allowed the position of the meddy to be tracked for seven months. Pinball was found against the continental slope near Lisbon canyon. The maximum core salinity was 36·564 psu, at a depth of 1260 m, but the maximum rotation rate with period ~2·5 d was in the upper core near 700 m, where temperatures reached 13·2°C. The azimuthal transport to a radius of 50 km was ~13 Sv. Pinball moved from the continental slope near Lisbon to the central Tagus Abyssal Plain, returned towards the continental slope and then moved westwards crossing the central Tagus Abyssal Plain a second time. At times it had a marked remote sensing infra-red sea-surface signature. It moved ~550 km over 204 d and the near real-time data meant that, in principle, this eddy could have been re-surveyed, redrogued or reseeded with floats during this period.

Deep‐Sea Macrobenthic Communities at Contrasting Sites off Portugal, Preliminary Results: II Spatial Dispersion

AbstractSmall‐scale, biogenic processes are thought to be important in creating a mosaic of microhabitats that allow co‐existence of the large numbers of rare species found in deep‐sea sediments. However, the large‐scale effects of hydrodynamic disturbance seem likely to be important in high‐energy settings on the deep‐sea bed. This is investigated by examining spatial dispersion patterns in the macrobenthic populations at two deep‐sea sites off Portugal. The widely differing conditions of sediment disturbance at the two sites are related to differences in macrobenthic species diversity and dominance.Closely grouped, transponder‐mapped, replicate box core samples, each divided into 25 “vegematic” subcores permitted analysis of dispersion patterns at two spatial scales at each site. One site was located on the Tagus Abyssal Plain (TAP), and the other in the nearby Setubal Canyon (SC) where ripple bedforms indicate vigorous bed flow, probably with a tidal periodicity. Greater aggregation at the scale of tens of metres is indicated from the observed numbers of SC species per box core being wide underestimates of the value predicted by rarefaction of the pooled total compared to those from TAP. Variance/mean ratios for individual species of bivalves and tanaids indicated more aggregation in spatial distribution between box cores from SC than from TAP. Dispersion chi‐square analysis also indicated more non‐randomness at SC between box cores than for the TAP cores. However, low faunal abundances in the TAP samples render the results of analyses for this site somewhat inconclusive and therefore the trend indicating less aggregation at TAP compared to SC must be regarded as weak for the groups examined. Analyses of the distribution of tanaids, and bivalves at a 100 × 100 mm subcore scale showed no convincing significant departure from random expectation, but low abundances in these groups reduces the sensitivity of the tests applied. Between‐core aggregation detected at SC may reflect habitat variation resulting from the varying depth within the area of sampling, while within‐core patchiness could be influenced by the ripple bed forms on the sediment.

Deep‐Sea Macrobenthic Communities at Contrasting Sites off Portugal, Preliminary Results: I Introduction and Diversity Comparisons

AbstractSpecies diversity, faunal composition and abundance in the deep‐sea macrobenthos are investigated at two, adjoining deep‐sea areas off Portugal experiencing contrasting hydrodynamic conditions in order to test for the importance of disturbance in structuring the community. The first was located at 5,035 m depth on a presumably low current‐energy site on the Tagus Abyssal Plain (TAP) and the second was centered in the lower end of the Setubal Canyon (SC) at 3,400 m depth where high levels of hydrodynamic activity are inferred from ripple bedforms visible in sea bed photographs. Closely grouped, transponder‐mapped, “vegematic” box core samples were taken at each site. Agglutinated foraminifers numerically dominate the macrofauna (retained by 300 μm sieve) at both sites. Seagrass content in the SC cores indicates downslope detrital input from shallow water which is absent in the TAP samples. This must contribute, possibly with high levels of resuspended particles in near‐bed flow, to the almost tenfold difference in abundance of metazoan macrofauna at the two sites. Expected species diversity measured from rarefaction is highest at TAP, both for pooled data and when foraminifers. tanaids and bivalve are considered separately. Evenness (equitability) in species' abundances is also higher at TAP than at SC. These differences are compared with benthic community data from the hydrodynamically disturbed High Energy Benthic Boundary Layer (HEBBLE) site on the Nova Scotia continental rise, and from the Rockall Trough (N.E. Atlantic). In common with these sites, strong bed flow at SC is thought to result in relatively high abundances, lower species richness and greater unevenness by acting to periodically disturb and reduce populations. Differences in the degree to which different faunal groups seem to be affected might be related to differences in lifestyle, particularly burrowing depth.

An AMS radiocarbon method to determine the emplacement time of recent deep-sea turbidites

This paper reports the evaluation of a novel method based on accelerator mass spectrometer (AMS) radiocarbon dating of planktonic foraminifera to evaluate the times of emplacement of recent deep-sea turbidites. It assumes that the standing stock of relatively large ( > 150 μm) planktonic foraminifera in the bioturbated surface mixed layer of box cores represents an integration of the flux from the water column since the turbidite was emplaced, and exploits the potential of the AMS method to analyse the resultant small samples which can be readily separated. The method is evaluated by application to three Atlantic turbidites which form the present-day sediment surfaces on the Horseshoe, Madeira and Tagus Abyssal Plains. The time of emplacement of Madeira Abyssal Plain turbidite α is estimated at 1120, 730 and 870 years before present (each ±120–140 years) in three different cores. The resultant weighted mean emplacement time, 930 ± 76 years before present, is somewhat older than previous estimates by geochemical methods. The times of emplacement for the Horseshoe (240 and 140 years before present, each ± 120 years and Tagus (300 ± 120 years before present) Abyssal Plains are both consistent with initiation by the Lisbon earthquake of 1755.

A shallow meddy (a smeddy) from the secondary Mediterranean salinity maximum

A smeddy, or shallow meddy with temperature and salinity core characteristics of the Secondary Mediterranean Salinity Maximum (SMSM), was found near 36.75°N, 12.5°W in March 1992, west of Cape St. Vincent and some 600 km from the Strait of Gibraltar. A detailed survey defined the temperature, salinity, nitrate, silicate, oxygen, and light transmission structure of the smeddy. The smeddy core had a maximum salinity of 36.483 psu at a depth of 775 m, and a maximum temperature of 13.55°C at a depth of 725 m. The salinity anomaly was 9 standard deviations from the mean at a depth of 600 m. The property distributions suggest that about 0.5 km of a central water column (∼500–1000 m) was traveling with the smeddy, though the dynamic influence extended from the surface to a depth of about ∼1.5 km. The velocity field was derived from dynamic height differences, the gradient equation, a drogued Argos buoy, and acoustic Doppler current profiler (ADCP) measurements. At the core depth of ∼700 m a number of well‐defined structures were found that characterized the horizontal influence of the smeddy. Surrounding an inner core (temperature > ∼ 13.25°C, salinity > ∼36.4 psu) of radius about 7 km, in near solid body rotation (with center period ∼3.7 days), there was a region of maximum azimuthal currents (∼20 cm/s) at a radius of ∼12 km. At a radius of ∼17 km, horizontal gradients of properties reached maximum values. This water mass boundary was thought to define the horizontal extent of the water actually traveling with the smeddy, giving it an aspect ratio of 1.5%. The smeddy was observed to move about 150 km southward over a period of ∼30 days (∼6 cm/s), passing from the Tagus Abyssal Plain to the Horseshoe Abyssal Plain. Remote sensing and other hydrographic data are used to suggest one route whereby water found in the core of the isolated eddy reached the Horseshoe Abyssal Plain.

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

SUMMARY 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 1130'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 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.

Iberian plate motions during the Mesozoic

As more data are collected on the deep parts of the conjugate margins of Canada and Eurasia, models of the early kinematics of Iberia become better constrained. In order to specify the motions during phases of continental rifting to the west and to the north of Iberia, we used recent data on the early oceanic spreading in the Tagus abyssal plain and added simple assumptions on the geometry. The Tagus abyssal plain was formed after the end of continental rifting on the southwest Portuguese margin in latest Jurassic time, before chron M-0 (118 Ma). As a first step we close the space adjacent to the M-0 anomaly to keep open only the oldest oceanic part of the Tagus plain which is assumed to be latest Jurassic to earliest Cretaceous in age. In a second step we close this space. The main results are as follows: 1. (1) The Galicia Bank slid along the southern flank of Flemish Cap; this motion is interpreted as a strike-slip motion or as a very oblique rifting. 2. (2) A Late Jurassic reconstruction of Iberia and North America is obtained without considerable overlap and without large motion of microplates (such as Galicia Bank). This is possible because a large amount of continental extension occurred over a long time to the north of the Grand Banks and permitted early motion between North America and Eurasia. 3. (3) A tight initial reconstruction can only be reached if strike-slip motion is located between southern Eurasia and northern Eurasia along a shear zone following the English Channel and the transform area between Galicia Bank and Flemish Cap.