Under the blue waters of Indonesia's Banda Sea lies one of the deepest bodies of water on the planet – the Weber Deep. Measuring 7.2 km (4.47 mi) from top to bottom, it's the deepest point of the Earth's oceans not within a trench. How it got to be so deep is a question that has puzzled scientists, but now, thanks to a chance discovery of the biggest exposed fault plane on Earth, researchers finally have the answer to this question.

Oceanic trenches, which are the deepest chasms in the ocean, are formed during the subduction of two tectonic plates – basically, when they collide and one slides under the other. However, the Weber Deep is a forearc basin, which is basically a depression located in front of the Banda arc, a curved chain of volcanic islands. So how did this basin end up becoming like a trench?

An illustrated cross-section of the Banda arc(Credit: ANU)

"The abyss has been known for 90 years but until now no one has been able to explain how it got so deep," says Jonathan Pownall, a researcher in structure tectonics at the Australian National University (ANU).

Researchers at ANU hypothesized that it was due to two things; firstly, the final stages of the Banda slab rollback, which is the process by which a subducting plate pulls the overlying one down with it as it falls into the mantle, thus causing an extension in the latter and in certain cases, rifting when it is pulled apart; and secondly, extension along an as-yet undocumented low-angle fault, based on studies of the sea bed and what they knew about the region's geology.

Confirmation of the existence of the fault line, and therefore the second hypothesis, came earlier this year when Pownall chanced upon extensions of the fault line on the mountains of the Banda arc islands while on a boat journey.

"I was stunned to see the hypothesized fault plane, this time not on a computer screen, but poking above the waves," says Pownall.

Further analysis of high-resolution maps of the Banda Sea floor shows hundreds of straight parallel scars marring the rocks on the seabed. These wounds point to the likelihood that the abyss was formed when a piece of crust bigger than Belgium or Tasmania was ripped apart by 120 km (75 mi) of extension along a low-angle crack, or detachment fault.

According to the researchers, the fault, which they dubbed the Banda Detachment, represents a rip in the ocean floor exposed over 60,000 sq km (23,166 sq m). So extreme was the amount of extension that in some spots there was no longer any trace of oceanic crust.

Given that this fault lies within the Ring of Fire, this discovery will help researchers assess future tectonic hazards in the region. Indeed, earthquakes are a regular fixture in the Banda area, and their tremors can often be felt as far away as northern Australia.

The most catastrophic event ever recorded happened in 1629 when a "mega-thrust" earthquake with a magnitude reaching 9.2 resulted in a 15-m (49-ft) tsunami and nine years of seismic aftershocks – in comparison, the 2004 Boxing Day catastrophe in Sumatra measured 9.1. Since then, the quakes have been relatively tamer, though some researchers point out that enough strain has accumulated in the Banda subduction/collision zone to produce a catastrophe of similar size to the 1629 event.

In light of this, the discovery could not come at a better time.

"In a region of extreme tsunami risk," notes Pownall, "knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards."

A paper on the results of the algorithm was published in Geology.

Source: ANU

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