Magmatism and extension rates at rifted margins
Magmatism and extension rates at rifted margins
Petroleum Geoscience (May 2018) 24 (4): 379-392
- Arctic Ocean
- Arctic region
- Atlantic Ocean
- Central Atlantic magmatic province
- continental margin
- extension
- Jan Mayen
- Kolbeinsey Ridge
- magmatism
- magnetic anomalies
- North Atlantic
- Northeast Atlantic
- Norwegian Sea
- numerical models
- plate tectonics
- rates
- Reykjanes Ridge
- rifting
- sea-floor spreading
- South Atlantic
- spreading centers
- conjugate margins
Rifted margins are commonly classified as either magma-rich or magma-poor. Magma-poor margins are often implicitly related to ultraslow-slow extension. Conversely, therefore, magma-rich margins should represent more rapid extension. Although supported by numerical modelling, these relationships are based on limited data and depend on a perhaps spurious comparison between continental margins and spreading ridges. Three case studies from the Atlantic margins are therefore presented here as a local, by no means complete, examination of this concept. Extension rates for magma-poor margins are mainly derived from offshore Iberia, while the best documented rates on magma-rich margins are probably those in the NE Atlantic. Particularly for the NE Atlantic, there is a dependence on the initial oceanic spreading rate as pre-break-up rates are very difficult to quantify. Our two southerly examples, the Central Atlantic and southern South Atlantic, are both magma-rich in parts and have been described as opening during ultraslow-slow plate separation. Both would therefore seem to contradict the positive 'rate-magmatism' correlation. However, on closer examination, a wide range of initial extension rates are actually possible. This is largely due to poor constraints on break-up ages. The assumption that break-up is synchronous with flood basalt extrusion is flawed, and may have caused initial extension rates to have been significantly underestimated. Additionally, averaging between widely spaced oceanic magnetic anomalies allows for a wide range of extension rates. New, well-constrained ages and event chronologies from critical areas of conjugate margins are needed to determine whether this relationship has global validity.