The Eastern End of the Azores-Gibraltar Plate Boundary

Abstract

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.