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Plate Tectonic Evolution of the Nova Scotian Margin Changing a Paradigm

David Roberts1, Matt Luheshi2, Keith Nunn2, Janis Makris3, Bernard Colletta4, and Hamish Wilson2
1RPS Energy and Roberts Geoscienc1es, Henley-on-Thames, United Kingdom.
2RPS Energy, Henley-on-Thames, United Kingdom.
3Geopro GmbH, Hamburg, Germany.
4BEICIP Fran-lap, Rueil-Malmaison, France.

The main drive behind the understanding the plate tectonic evolution of the Nova Scotian Margin is to investigate the possibility of demonstrating an Early Jurassic anoxic shallow marine environment. This has the potential to transform perceptions about prospectivity offshore Nova Scotia by introducing a regional, large scale oil prone source bed.

Evaluation of this possibility necessarily requires taking a view on the rifting and subsidence history of the Nova Scotia/Morocco conjugate margin pair. The end member hypotheses are volcanic versus non-volcanic rifting. The former allows a simple robust case to be made for a syn to early post rift shallow marine environment. The current paradigm calls for a transition along strike from volcanic in the south-west to non volcanic to the north-east.

The Nova Scotia Play Fairway Analysis project has undertaken an integrated analysis of all the available data and relevant analogues. The team concludes that the rifting history is much more likely to have followed a basic volcanic process. This position is based on the view that the major change in age and the nature of rifting ,from volcanic to non volcanic, between the North and Central Atlantic occurs at the Newfoundland fracture Zone (NFZ). This indicates that this is the most likely position of a discontinuity in lithospheric fabric (thickness and/or material properties). Hence there is a prima facie case to expect the NFZ to be the location of a first order change in rifting style (between volcanic and non- volcanic).

Starting from this position the project team find that the geophysical evidence is consistent with a primarily volcanic rifting process that continues along the whole Scotian Margin. There is no need to invoke an along strike transition from magmatic to non-magmatic behaviour.

The classical indicators of volcanic passive margins are observations of Seaward Dipping Deflectors (SDRs) and associated magnetic anomalies. The revised magnetic compilation produced as part of this project shows credible evidence for a weakened ECMA (East Coast Magnetic Anomaly) extending to the NFZ. Modelling of this anomaly indicates that it is consistent with the presence of basalts at positions that would be expected relative to the continent ocean transition (COT). The reduction in amplitude of the ECMA is interpreted merely as the expression of a propagating volcanic rift that is weakening towards the NE of the margin.

The lack of SDRs has been reported at length as being a prime reason for invoking a transition in rifting style from magmatic to non volcanic half way along the Scotian margin. Our view has been that absence of evidence for SDRs NE of Novaspan line 1400 is quite likely due to seismic imaging quality. This is a plausible geophysical view given the complexity of the salt units. This view is emphatically demonstrated by a comparison of 2D/3D seismic in analogous conjugate position offshore Morocco. There is now persuasive evidence that SDR’s are clearly visible offshore Morocco both on proprietary data and on published data (Roeser et. al. 2002). The examples of SDRs offshore Morocco are at locations that are conjugate with locations to the North East of SMART 2 and SMART 1 (i.e. beyond the locations where SDRs are not observed on the Nova Scotia margin).

Based on these key observations, we conclude that the Scotian margin should be classified as volcanic in origin throughout, with the possible nuance implied by reduction in the volume of basalts extruded moving along strike towards the NFZ.

 

AAPG Search and Discovery Article #90130©2011 3P Arctic, The Polar Petroleum Potential Conference & Exhibition, Halifax, Nova Scotia, Canada, 30 August-2 September, 2011.

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