Continents have re-shaped and seas have parted, but one fragment of the ocean floor has remained locked in place for more than 200 million years. The Ionian basin – a patch of seafloor under the Mediterranean – is the oldest-known section of the seabed to have remained static, held by irregular-shaped continental joints that prevent its motion.

The Ionian Sea carries its years well – scientists have debated its true age and origins for nearly 40 years. Some studies consider the basin’s unusually thin floor to have formed from “thinned” continental plates, while others suggest it arose from oceanic crust. Estimates of the basin’s age vary, with some saying it originated 250 to 290 million years ago, in Permian times, and others putting its origins approximately 195 million years ago in the Jurassic period.

Apennine mountains flank the Ionian Sea. Credit: Fabio Speranza, National Institute of Geophysics and Volcanology, Rome.

“What we can say for sure is that this is a very old basin lying on top of a very thin crust,” said geophysicist Fabio Speranza of the National Institute of Geophysics and Volcanology in Rome, Italy.

Speranza on Monday presented 3-D models that predict the age of the Ionian Sea floor based on magnetic variations in the thinnest parts of its crust.

The models, presented during the American Geophysical Union’s Fall Meeting in San Francisco, suggest the Ionian seafloor may not be as old as previously thought and that it is more likely to have originated in the ocean.

Magnetic anomalies – peaks or troughs of magnetic activity – are etched into the ocean floor when chemicals in rocks respond to shifts in the Earth’s magnetic fields. Matching the patterns to the Earth’s magnetic history can reveal when the anomalies were formed.

Speranza and his colleagues focused on available data from a 9 kilometer-long stretch of the Ionian seafloor that was relatively free of sediment.

“In the deepest part of the thinnest crust, we found two positive anomalies,” he said.

Constructing 3-D models of this region, the group first tested the possibility that the Ionian basin had been formed by thinned-out continental plates. When they assumed that the deepest layers of the seafloor had been formed by serpentinite, typical of continental crust, the models failed to recreate the magnetic variations revealed by the data.

“Our target was to get the real anomaly replicated,” said Speranza.

Modeling the Ionian seafloor using an oceanic crust origin worked better. Simply by modeling a reverse polarity for the oceanic crust – a flip-flop of magnetic fields – the researchers could recreate the magnetic anomaly. Matching their 3-D models to various models of magnetic variations over time, the Ionian seafloor most resembled the patterns seen 220-230 million years ago.

And as Speranza said, “We confirm that the Ionian Sea is still the oldest ocean fragment in place known in the world.”

– Jyoti Madhusoodanan is a science communication graduate student at UC Santa Cruz