Earth Observatory Blog
The Collision Zone
Subduction zones are the most violent collisions on Earth. They are so powerful that even scientists who are familiar with the dynamics of plate tectonics are humbled by images of the aftermath.
The massive seamount was no match for the force of subduction. A clear, deep scar runs through the undersea mountain, a testament to the unrelenting motion of tectonic plates.
Dr Satish Singh, Chief Scientist on the MIRAGE II expedition, stares in awe at the imposing image. For a moment it seems like he is going to dive into an explanation of the mechanics of the fault, but its magnificence renders him speechless. “It is beautiful,” he marvels and pauses before examining the image more closely.
Rocks are born, and they die. When their time comes, rocks are consumed by subduction zones, where they sink into the mantle of the Earth. During their lifetime, they collect many stories. Scientists onboard R/V Marion Dufresne are determined to reconstruct the scene of the collision in the Wharton Basin to discover those stories still buried amidst the chaos.
A Tiny Piece of the Puzzle
The main focus of MIRAGE II is to gather seismic reflection profiles of the seafloor in the Wharton Basin. But to decide exactly where to focus their study, scientists have relied on previous bathymetric surveys. A seismic reflection profile is a fairly complex image, which is why the relative simplicity of bathymetry is much appreciated.
Ms Marie-Laure Fournasson works at the Institut de Physique du Globe de Paris and is currently a participant in the Floating Summer School programme onboard R/V Marion Dufresne. Marie-Laure spends a great deal of her time on the ship processing bathymetric data.
Today, she is working on a bathymetric image that shows a mix of geological features. While this image represents just a tiny piece of the puzzle, it is enough for the scientists on-board to begin speculating.
“I am learning a lot,” shares Marie-Laure. “I had to familiarise myself with this software at first and now, I enjoy seeing the difference between the raw data and the rendered image. I feel like a magician! Processing bathymetric data is a skill that will be very useful for my career in the future.”
Many of the faults in this image were probably caused by the bending of the plate as it approached the subduction zone, located 80 kilometres (km) away from where the image was acquired. The image also shows a ridge that runs from north to south—a fracture zone. This ridge is a result of the spreading of the ocean floor, where the crust breaks and allows new material to come up from the depths of the mantle, creating what looks like a mountain range.
The smaller mountains in the image are volcanoes. Volcanoes typically occur beneath spreading ridges or along volcanic arcs. The volcanoes in this image do not fit either description; their origin is mysterious. When it comes to studying plate tectonics through geophysics and geology, each piece of evidence is a signature of a certain moment in time, and must be made sense of.
Scientists onboard R/V Marion Dufresne work to model future outcomes, but the screens of the Science Control Room display neither future nor past events. The images are simply from measurements of the subduction zone in the present.
The team has to discern which forces are playing an active role in the present, which ones were active in the past, and which are likely to activate in the future. It might sound simple, but it is dizzying to watch the researchers jump with ease from one time-scale to another, just by looking at an image that may seem unintelligible to the untrained eye.
Finally, in the image, a channel comes into view. Upon closer examination, this channel could become the key – to decode time, to replay the collision of the tectonic plates. “We are very lucky to have found this,” says geologist Dr Kyle Bradley.“This image could give us a chance to bridge the gap between the 2012 earthquake and the long-term geology of this region.”
The Wharton Basin likes to push the limits of these scientists’ knowledge, and they love to push back.
Follow the progress of MIRAGE II between 25th September and 20th October 2017 on the EOS blog, and spread the word using #MIRAGEcruise.