AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Major normal faulting on distal parts of South Atlantic margins: geometry, timing & processes

Abstract

Recent frontier exploration of very distal margins enabled to image seaward-verging normal faults with throws reaching several thousands of meters in various areas of both African & Brazilian margins. Deep seismic profile interpretations, including 3D seismic, suggest a sudden rise of the Mohorovic discontinuity below the footwalls of these giant faults, together with fault rooting within the subcontinental mantle, thus offsetting the Moho itself. These faults localized at the limit of the continental crust, from which the subcontinental mantle is exhumed. Fault activity is generally accompanied by salt deposition and sealed below the top of the salt, enabling to date it Late Aptian (115-113 My.) in age. This period corresponds to the phase when the South Atlantic margin experienced the lithospheric break-up process, just preceding the generalized marine flooding synchronous to the steady-state activity of the mid-oceanic ridge. Observations replaced within the evolution of the rifted margins, the faults are thought to accommodate the thermal cooling, which followed the exhumation of the mantle. Lost crustal and thermal supports resulted in a rapid subsidence, mainly localized on the line of lost crustal support, synchronously with the focusing of main tectonic and thermal activity more distally at the nascent mid-oceanic ridge. The process can be seen as “border-free” effect due to the lithospheric break-up. The sinking footwall side of the giant faults, compared to the crustal-supported footwall, created major topography at the base of the Aptian salt, which may constitute exploration targets, where risking evaluation differs from a classical approach.