Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary

Tectonic erosion of the overriding plate by the downgoing slab isbelieved to occur at half the Earth’s subduction zones. In situinvestigation of the geological processes at active erosive marginsis extremely difficult owing to the deep marine environment andthe net loss of forearc crust to deeper levels in the subduction zone.Until now, a fossil erosive subduction channel—the shear zonemarking the plate boundary—has not been recognized in thefield, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is 500m thick. It is representative of the first 5km of depth, with its deeper portions reaching 150 C. The fossil zone records several surprises. Two decollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at 150 C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.