Finding Fault: Trace of old subduction zone found in Italy

A 200-kilometer-long, 500-meter-thick layer of rocks now lying high in the mountains of Italy is recognized as the remains of an erosive subduction zone that was active under the sea millions of years ago, scientists say. The first-of-its-kind discovery provides new clues about ancient seismic activity along this interface between tectonic plates, and also insight into what may be happening along many quake-prone subduction zones today.

ANCIENT SCRAPE. Now preserved high in the mountains of Italy, the Adriatic tectonic plate was subducting beneath the European plate millions of years ago. The dotted orange line depicts the boundary between the Adriatic plate and the material trapped in the subduction zone; arrows indicate ancient plate motions. The European plate lies above the area pictured. Remitti

When a tectonic plate made of ocean crust and one formed of relatively light continental rocks collide, the continental plate typically overrides the oceanic plate, forcing it back down into Earth’s mantle. When friction between the moving plates locks them in place, immense stresses build up along these subduction zones—stresses that can, when released, trigger great earthquakes and devastating tsunamis (SN: 1/8/05, p. 19).

About half of the world’s subduction zones, which typically lie in deep water far offshore, are considered erosive margins, where the ocean-crust plate is scouring the front edge of the overriding continental plate, says Francesca Remitti, a geologist at the University of Modena and Reggio Emilia in Italy. That action destroys evidence of the processes that occur at such tectonic interfaces, so the team’s geologic find, in the Apennines of north central Italy, is especially rare.

Why and how portions of a subduction zone could end up being preserved in a mountainous region “are wonderful questions that jump out of this paper,” says Stephen T. Johnston, a geologist at the University of Victoria in Canada.

Today, those rocks sit between 1 and 1.5 km above sea level. About 12 million years ago, however, the rocks lay about 3 or 4 km beneath the seafloor, where the edge of the Adriatic tectonic plate was subducting beneath the European plate. Eventually, this mountain-building collision lifted the subduction zone out of the sea and preserved it, Remitti and her colleagues suggest in the Feb. 7 Nature.

In many places, the rocks are crisscrossed by thin veins of calcite. Those veins, along with other characteristics of the rocks, indicate that seismic activity along the ancient subduction zone occurred at depths of 4 km and greater, says Remitti. That’s surprising, she notes, because modern subduction zones don’t exhibit seismic activity above depths of 5 km or so.

Although many research teams have used seismic waves to construct CT-scan-like images of these zones, those provide only inferences about the processes taking place along such tectonic interfaces, says Remitti. Several groups of geologists, including Harold J. Tobin of the University of Wisconsin–Madison, are now mounting ocean-drilling expeditions to obtain rock samples from active subduction zones.

Remitti and her team “have a big advantage in having samples of an actual subduction zone to study in real detail,” says Tobin. The big challenge, he notes, will be discriminating processes that occurred while the rocks were forming from those that modified the rocks after they were lifted from the sea millions of years ago.

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