Geologists Find Largest Exposed Fault on Earth

An international team of geologists from the Australian National University and Royal Holloway University of London has for the first time documented the Banda Detachment fault in eastern Indonesia and worked out how it formed. The research is published in the journal Geology in 2016.
“The find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire – an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions,” said lead author Dr. Jonathan Pownall, from the Australian National University.
“The abyss has been known for 90 years but until now no one has been able to explain how it got so deep.”
“Our research found that a 4.3-mile (7 km) deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth’s largest-identified exposed fault plane.”
By analyzing high-resolution maps of the Banda Sea floor, Dr. Pownall and co-authors found the rocks flooring the seas are cut by hundreds of straight parallel scars.
These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 74.5 miles (120 km) of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.
“This fault, the Banda Detachment, represents a rip in the ocean floor exposed over 14.8 million acres (60,000 sq. km),” Dr. Pownall said.
“The discovery will help explain how one of the Earth’s deepest sea areas became so deep.”
“This was the first time the fault has been seen and documented by researchers,” said co-author Prof. Gordon Lister, also from the Australian National University.
“We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology.”
“I was stunned to see the hypothesized fault plane, this time not on a computer screen, but poking above the waves,” Dr. Pownall said.
“Rocks immediately below the fault include those brought up from the mantle. This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero.”
According to the team, the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.
“In a region of extreme tsunami risk, 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,” Dr. Pownall said.


http://geology.geoscienceworld.org/content/44/11/947.full