--> --> Crustal architecture and polarity shift throughout Atlantic rifted margins: An overview

AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Crustal architecture and polarity shift throughout Atlantic rifted margins: An overview

Abstract

Rifted continental margins are typically described as magma-poor or magma-rich and through a process of extension of the continental crust, breakup of its conjugate segments results in the formation of new oceanic crust. During this process, thinning of the continental crust results in an asymmetric geometry of the conjugate margins where deep crustal detachments on its distal domain accommodate final pulses of extension. Ultimately, the asymmetry of the two end-members of the rifted margins results in the formation of opposing lower-plate and an upper-plate (sensu Lister et al., 1986), where the crustal detachments respectively dip outward or inward the thick proximal domain. Examples of alternating plate architecture are found in several domains of the Atlantic, namely on the Norway-Greenland, Iberia-Newfoundland, Morocco-Nova Scotia or Angola-Brazil. However, the transition of the upper vs. lower-plate along the margin strike is yet unclear in other regions of the world. In the case of the Iberia-Newfoundland conjugate margins, used here as a type-example, syn-rift to post-rift evolution is characterised by the presence of the two distinct architectures, of which the Southwest Iberian Margin is an example of an upper-plate whereas the Northwest Iberia reveals a lower-plate (e.g Afilhado et al., 2008). Transition along each segment is accommodated through long-lived and wide transfer zones bounded by first-order strike-slip corridors broadly defining a promontory on the proximal margin (the Tagus Fault and the Nazaré Fault Zones). Transition towards South Iberia and the Gulf of Cadiz is accommodated by the Messejana-Plasencia Fault Zone and the Sagres Plateau. Similar examples of alternating plate architecture and polarity change along-strike, with uplifted zones bounded by first-order strike-slip are also found offshore Morocco (Tefelney Plateau), Norway-Greenland (Helland Hansen Arch) or the Demerara Plateau (French Guiana). In the case of the Congo-Angola margin this transition occurs either as a diffuse or a sharp boundary. These transcurrent regional features are likely inherited from the fragmentation of older terrains and ultimately control the overall evolution of the margins. Given the distinct geometry of each segment of the asymmetric margin the potential to form large syn-rift basins is dissimilar and ultimately controls the potential to form widespread source rocks. Moreover, thermal fluxes that control maturation may also be distinct and could explain distinct maturities along a margin.