AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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An atlas of character: a model for the control of passive margin development

Abstract

As the amount of modern, long offset and long record length 2D seismic acquired over the Atlantics passive increases, so the key physical characteristics of the margins are being revealed. Three of these characteristics are; 1) Continental – Oceanic transition: abrupt continental margins to hyper-extended margins, 2) Magmatism: magma rich - both clastic rich and clastic poor Seaward Dipping Reflector (SDR) rich to magma poor margins, and 3) Asymmetry: margins with wide syn-rift remnants (salt rich or salt free) to margins apparently devoid or with narrow syn-rift remnants. Each of these endmember series display localized variation on the basin and margin scale and indeed some are observed as part of continuums others are binary distributions. This presentation will take the form of a review of a number of 2D seismic lines from both the Atlantic and other passive margins. These data extend from the continental section out over oceanic crust, offering a chance to compare and contrast the influence of these key characteristics on each margin, and observe the degree of variation between them. For each characteristic a model will be presented that offers an explanation for why that characteristic endmember, and continuum set if appropriate, has developed. Abrupt Continental margins have narrow zones of transition separating oceanic crust from thick (>30km) continental crust, whilst hyper-extended margins display fragments of extension and shear thinned continental crust drawn out far from the shelf. These may display a zone of exhumed mantle between the final continental crust fragments and oceanic crust. We propose a model where variation in abruptness of a margin along-rift relates to variation in heat from mantle during rifting. Magma rich margins display wide zones of SDR’s comprising largely flood basalts but varying from clastic-rich (often inboard representing final syn-rift sedimentation whilst sub-areal flood basalts of pre-early drift are erupting) to clastic-poor outboard produced during drift and early oceanic crust formation. Magma poor margins apparently display no flood basalts and outboard of the final continental crust fragments either transit into oceanic crust directly or exhumed mantle then oceanic crust. Whilst the amount of subaerial and submarine magmatism is clearly a function of relative sea level, we propose a model for undulations along-rift in a syn-rift setting. Conjugate margins are often somewhat dissimilar, or asymmetric. The amount of syn-rift section above thinned continental crust remaining post drift varies significantly. Some margins display very narrow syn-rift sections post drift, whilst others have syn-rift systems >100km broad. Where the preserved syn-rift is wide post rift salt is often observed yet this is not ubiquitous and may have required a common but specific environmental control to form post rift. Margins where syn-rift, thinned crust is broad often have narrower syn-rift preserved on their conjugate margins, demonstrating rift asymmetry, although this is very variable. A model explaining observed asymmetry and variations in this characteristic is proposed. To conclude, a simple model is introduced that seeks to provide a mechanism for both the formation of the key structural characteristics on passive margins and the observed variation in these characteristics.