Horseshoe Abyssal Plain Research Articles

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.

Late Quaternary emplacement on the Horseshoe Abyssal Plain: ERRATUM

ABSTRACT The following address is the correct address for S.M. Lebreiro and I.N. McCave, the first two authors of the article, Late Quaternary emplacement on the Horseshoe Abyssal Plain, published in Journal of Sedimentary Research, vol. 67, No. 5: University of Cambridge Earth Sciences Department Cambridge CB2 3EQ, U.K. End_of_Record - Last_Page 236-------

Late Quaternary Turbidite Emplacement on the Horseshoe Abyssal Plain (Iberian Margin)

ABSTRACT The Horseshoe abyssal plain (HAP) is located on the Iberian margin at approximately 4800 m water depth, and is confined within topographic elevations with a relief of about 3000 m, with the exception of Gorringe Bank, which rises to only 20 m below sealevel. The plain is built up by an alternation of turbidites of the order of meter-thick beds, numbered H2, H3, H8, and H13, contrasting with sets of thinner beds of the order of decimeters thick (H4,5,6,7 and H9,10,11,12), and pelagites centimeters in thickness. An intensive study of eight piston cores, using visual observation (color and thickness), relative stratigraphic position of units, magnetic susceptibility logs, calcium carbonate content, and mineralogy of turbid te bases, concludes in a bed-by-bed correlation of all individual turbidites. The major source of terrigenous material feeding the HAP is the Sao Vicente canyon, which incises the Portuguese shelf, while minor sources are the surrounding seamounts. The elongate geometry of the abyssal plain with its single dominant source of sediments produced laterally continuous deposits from both large and small flows. These covered the entire 356 km length of the plain. Grain-size analysis of the four thickest turbidites (H2, H3, H8, and H9) demonstrates only slight downcurrent fining and vertical grading, expressed best in the ratio of coarse silt to fine silt plus clay. A small amount of sand is carried the length of the plain. The thicker units all show the same pattern of thickness, with a maximum in the middle of the plain around a topographic constriction and bend. This is most plausibly explained as due to reduction in flow speed of an initially supercritical flow causing enhanced deposition Some of the beds appear to have a double coarse layer in the base, which may indicate partial reflection of flows from the side of the basin. It is suggested by application of equations for flow behavior that both thick ( 3 m) and thin ( 0.3 m) beds are due to supercritical flows a few tens of meters high. However, the thick beds resulted from high-concentration flows (Cv 4% by volume) whereas thinner beds require low concentration (Cv < 1%) to run out over the full length of the basin. The stratigraphy is tied into the dated oceanic pelagic record by analysis of foraminifera in the pelagic layers above the turbidites and through recognition of two Heinrich layers (H-1 and H-2, ages 14.3 and 21 ka). The resulting age framework shows higher turbidite frequency in the glacial (2.7/kyr) than interglacial (Holocene) (1.0/kyr). This also gives higher mass flux during the glacial. Emplacement of turbidites cannot be clearly related to sea-level changes but may well be due to seismic activity. However, one of the largest earthquakes in human experience (Lisbon in 1755) triggered only a thin turbidite, invalidating the term seismite for thick turbidites.

Crustal thinning in the Southwestern Iberia Margin

The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide‐angle seismic reflection data, whereas offshore we have used coincident steep and wide‐angle reflection data along a NE‐SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

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 &gt; ∼ 13.25°C, salinity &gt; ∼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.

Sampling and observation of oceanic mantle and crust on Gorringe Bank

THE structure and nature of the uppermost oceanic mantle and crust has been, and still is, the subject of a major international research effort1. Yet the direct field evidence known to us on this subject, whether obtained by drilling in the I.P.O.D. oceanic crust programme2 or by submersible surveying in the Famous programme3, has been either very incomplete or limited to the upper part of layer 2. As a result, the problem of the comparison between ophiolites and oceanic crust is still debated1. We report here the preliminary results of an in situ geological survey in which we have sampled and observed what we believe to be an almost complete geological section starting a few kilometres below the Moho discontinuity and ending in the upper part of layer 2 (ref. 4). This section outcrops on the flanks of Gorringe Bank, which is a ridge 200 km long and 60 km wide between Tagus and Horseshoe abyssal plains in the Atlantic ocean, South West of Cape Saint Vincent, Portugal (Fig. 1).

Long-Distance Turbidite Correlations in Horseshoe Abyssal Plain: ABSTRACT

Long-Distance Turbidite Correlations in Horseshoe Abyssal Plain: ABSTRACT

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.

Heat-flow near a North Atlantic fracture zone

Fifteen new heat-flow measurements are reported, fourteen of which are located in or near a fracture zone near 43°N on the mid-Atlantic Ridge. These data, combined with earlier nearby measurements, show the fracture zone to have an observable, positive effect on the regional heat-flow field. A good correlation is observed between large near-bottom water potential temperature gradients and the occurrence of significant variations with depth in the measured rate of heat-flow. One measurement was taken in the Horseshoe abyssal plain west of Gibraltar and gave a value of 1.38 μcal/cm 2s.

Deep Sea Photographs of an Active Seismic Fault Zone near Gibraltar Straits

IN October 1969, the RV Jean Charcot made a geophysical survey of the Horseshoe seamount and abyssal plain west of Gibraltar Straits and south-west of Cape Saint-Vincent, Portugal; this is the eastern end of the Azores–Gibraltar seismic zone1 which is a compressive plate boundary2,3. During this survey a series of deep-sea photographs were obtained near the location of the last major earthquake of February 28, 1969 (magnitude 7.3 to 8.0, location 36.0° N 10.6° W according to the USCGS preliminary report).