Morphological features of the Portimao Bank Seamount (NW Gulf of Cadiz Margin) related to Pliocene-Quaternary activity

Abstract

The main aim of this work is the knowledge of the detailed morphology of the Portimao Bank which is an E-W structural seamount located in the distal sector of the South Portuguese margin, northeastern area of the Gulf of Cadiz, and it is bordered to the north by the Don Carlos Valley and to the south by the Cadiz Valley. This seamount has been described like a morphostructural high related to a Variscan basement high, strongly deformed by the NW convergence of Africa respect to Eurasia during the Pliocene-Quaternary as a pop-up structure (Terrinha et al., 2009). Current uplift has been related to thrust faults characterized by successive reactivations (Medialdea et al., 2004) during the Pliocene and Quaternary (Vazquez et al., 2003). This Bank exhibit a complex morphology result of tectonics, halokinetic and oceanographic processes mainly since Upper Miocene times to now. This seamount has an elongated and asymmetric W-E shape of 93 km long and 15-28 km wide, and it summit is located at 1500 m water depth. The total surface is around 1800 km2. The top is flat (approximately 0.2º) in the eastern sector, while in the central and western part higher gradients are found (2º-4º). The slope gradient increases on the flanks (4-15º); the southern flank is the steepest (12º-15º) and presents escarpment morphology, while the north-eastern flank is less steep (4-5º). Along the surface of this high, in terms of morphological features and deposits, ridges, depressions, domes, mass movement deposits and escarpments have been identified on the Portimao Bank. A central circular depression of radius 6 km bounded between the western and eastern structural styles of the bank. The highlight in halokinetic processes is the presence of two domes in the central sector of the Bank that exhibit conical shapes. The diapirs uplift the recent sedimentary cover but they do not outcrop. The main dome is related to the Don Carlos Diapir uplift that are not outcrop in the seafloor. This is a circular dome-shaped area (47 km2) rising around 320 m above the high and surrounded by a circular depression. The other bulge, located 4 km towards the northwest of the Don Carlos diapir, is cone-shaped, occupies a smaller area (2.5 km2) and is 100 m high. Related to diapir activity, a 32 km-long ridge feature oriented W-E has been mapped on the top towards the north of the bank, which has been related to recent diapiric activity. Escarpments are found in the north-eastern part of the Portimao Bank which are 28 km long and rising 170 m above the seafloor. Along the northern and southern flanks of the Portimao Bank it is possible identify several slides scars with different features. In the southwestwern one , it has been possible identified five scar slides developed in a quasi radial manner from the D. Carlos Diapir in SW direction between 18 to 12 km long and 3 to 5 km wide. Theese slide scars seem to originate from the Don Carlos diaper uplift, and they extend downslope on the southern flank. Along the southeastern flank, it has been identified a slide scar 8 km long and 9 km wide with smaller slide scars inside. In the north western sector flank, the dimension of slide scars increase from the D. Carlos Diapir towards east, from 3 to 5 km long, and 2 km wide. In the northeastern sector, three slide scars with sizes that decrease from west to east have been identified. The largest is 8,6 km long and 11 km wide and the smallest measure is 2 km long and 2,5 km wide. In the northern sector has been observed the most recent and active deformation feature, which correspond to a ridge delimited at the seafloor surface by an anticline fold related to a blind thrust or diapirism, which causes marked deformation of the current sedimentary units