Rift history and volcanism at continental margins: Research is published in 'Nature'

The surface of the Earth is shaped by powerful forces. Among the greatest are those associated with plate tectonics – the large-scale movements of the rocky outer shell of the planet called the lithosphere, which is composed of the crust and upper mantle.

Where the lithosphere is being pulled apart it can fracture, causing geological faults and the formation of deep rifts. Ultimately, this fracturing can result in continental breakup.

“Rifting is often associated with volcanism and magmatism – the formation of new igneous rocks from cooling magma. The breakup of Europe and North America, for example, was marked by massive outbursts of volcanic activity,” said Professor Tim Minshull, Head of the School of Ocean and Earth Science.

Geologists disagree about the exact relationship between rifting and magmatism.Based on studies in the North Atlantic, it was previously thought that magmatism at rifted margins was largely controlled by mantle temperature at the time of breakup. But detailed observations from other parts of the world have complicated the picture.

For example, despite evidence for high mantle temperatures around the time of continental breakup, the continental margins of the Northwest Indian Ocean display little evidence for extensive magmatism.

Using a new computer model, researchers have now reconciled these observations and identified factors controlling breakup magmatism at different rift margins. The model accounts for extensional episodes that often precede continental breakup, and was previously tested against observations from south-west Greenland. According to the model, the low extension rate observed at this margin extension prior tobreak-up thinned the lithosphere, aiding magma upwelling.

In the new study,the researchers focused on rifting between India and the Seychellesin the northwest Indian Ocean, where the extension rate is higher. Here, continental breakup is associated with the Deccan Traps, a vast onshore flood basalt province in west-central India. Formed between 60 and 68 million years ago, around the time of the disappearance of the dinosaurs, the Deccan Traps is one of the largest volcanic features on Earth and famous for its fossil beds.

The onshore flood basalt province of the Deccan traps is similar in size to the North Atlantic Volcanic Province and it has been assumed that the associated continental margins would also display volcanic characteristics. However, the researchers found little evidence for magmatism or thick underlying crust at the Seychelles-Laxmi Ridge, which was approximately 1000 km from the Deccan Traps at the time of breakup.

However, they did find evidence for voluminous magmatism in an area known as the Gop Rift, which is underlain by thick oceanic crust and is known to have opened up before the main breakup between the Seychelles and India. Based on their computer model, the researchers find that observed characteristics of the Gop Rift can be explained if it opened up around six million years before the main Deccan eruption and tapped anomalously hot mantle.

“What we believe happened is that the opening of the Gop Rift partially exhausted the thermal anomaly and limited the potential for the mantle to generate further magmatism” said Tim: “Once extension had migrated southward to the Seychelles-Laxmi Ridge margin, what remained of the thermal anomaly had cooled down. These factors explain why the oceanic crust seaward of the Seychelles and Laxmi Ridge is thinner than expected.”

It is clear then that the history of rifting, including prior extension events, can exert considerable control over the formation of volcanic margins.

“Our computer simulations showed that mantle temperature alone is unable to explain the observations in either the northwest Indian Ocean or in the North Atlantic,” said Tim: “Instead we found that rift history can either suppress or enhance the production of magma and is just as important as mantle temperature in controlling magmatism at rift margins.”