Magnetic Anomalies in the Wharton Basin (Part 2 of 2)

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Magnetic Anomalies in the Wharton Basin (Part 2 of 2)

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One of the root causes of all this activity could be the age of the lithosphere, that ever-spreading, always-moving seafloor crust. “South of eastern Java,” Dr Dyment said, pointing to a brightly coloured map on his computer, “the lithosphere is about 120 million years old. South of western Java, it’s younger, maybe 80 million years old. But alongside Sumatra, the crust is much younger, as young as 45 million years old. And then, of course, on the other side of that, things start aging again.

"This not only means that the lithosphere plunging under Sumatra and Java is not of the same age, it is also not of the same buoyancy. Old crust is cold, so it subducts relatively smoothly, but close to central Sumatra, the crust is much younger, warmer, and buoyant, so it resists subduction, which may be why we have been getting so many of these big earthquakes.” 

In fact, if you look at Dr Dyment’s map, you can see this resistance expressed in the curve of the subduction-zone line just west of the Mentawai Islands. One can imagine the buoyant warmer lithosphere have more trouble plunging into the earth than the areas to the north and south. In this context, earthquakes are not just a result of fault lines and subduction zones — they are also a product of the descending lithosphere’s age.

This screen shot from Dr Jerome Dyment’s computer shows how younger, more buoyant lithosphere is resisting subduction west of Sumatra. This resistance may be one explanation for the recent strong earthquakes this area has been experiencing over the past several decades. Image courtesy of Dr Jerome Dyment, IPGP.

In the Wharton Basin, one other condition may be coming into play, and that is the diffused plate boundary that lies within the Indo-Australian plate, a zone of perhaps 1,000 kilometres (km) in width. To the north is the Indian Plate, which has been halted in its collision with Eurasia, creating the Himalayas. To the southeast is the Australian plate, which continues to subduct under Sumatra. But the Australian plate’s proximity to the Indian plate is also forcing the Australian plate to turn slowly clockwise on an axis point, or pole, located roughly 1,000 km south of the southern tip of India. 

“The recent large earthquakes may be evidence that the Indo-Australian plate is breaking apart,” Dr Dyment said, “and that may mean that the diffused plate boundary we were speaking about earlier may not be so diffused after all.”

Dr Jerome Dyment of IPGP explains why we may be getting so many big earthquakes. 

I asked Dr Dyment if perhaps the Wharton Basin’s history as a former spreading centre is part of the geological equation. “I don’t think the former spreading centre in the Wharton Basin is playing a particular role,” he replied, “but I could be wrong. It is certainly something to be investigated. Rather, I think the activity is simply getting more focused in the Wharton Basin because we are far away from the pole of the plate’s rotation.

"Because this area of the plate is experiencing more motion, that results in more deformation. And where you have more deformation, there is a greater chance that we could be seeing the start of a new, undiffused, plate boundary. That’s my best guess,” he added, “but it is just a guess. That’s why we are taking this cruise.”

Click here for Part 1.

To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise.

All photographs are taken by Ben Marks, unless otherwise stated.

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