Opening of the North Atlantic Basin - Lessons from the South

Chris C. Parry
New Exploration Ventures, ConocoPhillips, Stavanger, Norway.

The Atlantic Mid Ocean Ridge can be traced from the Bouvet triple junction at latitude 54 degrees south, some 10000 kilometers northwards via Iceland into the Norwegian Sea before joining with the Gakkel Ridge in the Arctic Ocean, via the Fram Strait.

Along the length of the divergent boundary of the Atlantic Mid Ocean Ridge, the spreading center is offset by regularly spaced transform boundaries. These can be traced shoreward as deep seated continental fracture zones beneath the sediment cover.

Lister et al., 1986, described upper plate and lower plate passive margins, separated by a detachment fault, which give rise to asymmetric conjugate margins after final continental break up. The upper plate is characterized by a narrow continental shelf, with relatively little sedimentary accommodation space. It is relatively unstructured and has experienced uplift related to underplating. While on the opposite side of the mid ocean ridge, the conjugate lower plate is characterized by a wide continental shelf, which has abundant sedimentary accommodation space. It is complexly structured and exhibits bowed up detachment faults. Transfer faults offset marginal features and can cause the upper/lower plate polarity to change along the strike of the margin.

The Fram Strait is a transform margin, which was initiated in the Eocene as a result of the onset of spreading in the North Atlantic. The sliding of the North American Plate past the Eurasian Plate during the opening of the North Atlantic created an upthrust zone that formed due to space constraints associated with low angle convergent strike slip or transform motion. The easiest direction for space relief for the squeezed sediments is vertical and a zone of downward tapering wedges and upthrust margins is created.

The Atlantic Mid Ocean Ridge transform boundaries can be traced across the oceanic crust towards the coast line forming basement structural highs. These are related to volcanic activity along strike of these “leaky” fracture zones in the oceanic crust. These structures set up the initial structural framework of the continental margin basins. Syn-rift and post-rift deepwater sedimentation on-lap these basement highs and the influence of the transfer zones continues to propagate into younger strata by differential compaction. These differential compaction faults both act as a hydrocarbon migration pathway from deep seated source rocks to shallower reservoirs, as well as influencing deepwater sediment delivery systems.

These zones of long-lived crustal weakness can be subsequently reactivated during later tectonic episodes, giving rise to inversion structures and complex compressive and transpressive/transtensional features. In offshore Equatorial Guinea, reactivation of the Ascension Fracture Zone during Senonian times created a series of transpressional anticlines, one of which contains the Ceiba Field.

Using the South Atlantic as an analogue, the integration of gravity, magnetic and seismic data has been used to construct a simple symmetrical spreading model for the opening of the Norwegian Sea between Iceland and the island of Jan Mayen.

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