AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Evidence and significance of buried magmatic features offshore the West Iberian Margin

Abstract

West Iberia is a type-example of a magma-poor rifted margin that underwent multiphased extension of the continental crust ultimately leading to breakup by the Aptian-Albian. Located on the southernmost segment of the North Atlantic it records the complete Neo-Tethys to Atlantic evolution with a complex syn-rift to inversion history. Nonetheless, throughout the proximal margin three major magmatic events can be recognised: 1) the tholeiitic cycle from the Central Atlantic Magmatic Province associated with the onset of Syn-rift II; 2) the transitional cycle from the Late Jurassic syn-rift III; and 3) the post-rift Late Cretaceous alkaline cycle. The post-rift section of the West Iberian Margin shows evidence of some enigmatic features buried under the sediments of the Estremadura Spur. Using high resolution 2D and 3D seismic data covering the area, the presence of numerous and widespread igneous features is revealed and presented in detail for the first time. These are characterised by high amplitude signal, some with saucer-shape and others either crosscutting the strata or parallel to the main reflectors. Based on their geometry, seismic response and attributes, these can be unambiguously assigned to dykes and sills. Additionally, a buried volcano (the Fontanelas seamount) can be recognised with an estimated height of 3000 m and covering an area of about 400 km2. Along the volcano, associated lava flows can be imaged. Given the seismic-stratigraphic position of these features post-dating the syn-rift strata and underlying the Tertiary sediments the igneous features can be tentatively assigned to the alkaline Late Cretaceous magmatic cycle that is described on outcrops from the proximal margin of central and south Portugal. Locally, some features correlate with distinct magnetic and gravimetric geophysical anomalies. Confirming the nature of the rocks, the extent and magnitude of the intrusive/extrusive event, which is likely more significant than anticipated, will help clarifying the impact on possible petroleum systems, not only on the influence on maturation of hydrocarbons sourced from underlying units, but also on the speculative reservoir and sealing potential of these igneous rocks.