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

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Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth’s subduction zones1,2. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel—the shear zone marking the plate boundary3—has not been recognized in the field, 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 500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching 150 °C. The fossil zone records several surprises. Two décollements 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 3 km 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.

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Figure 1: Schematic models of tectonic erosion.
Figure 2: Geological setting of the Northern Apennines.
Figure 3: Photographs of mesoscopic structures characterizing the Apennine fossil subduction channel.


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We thank J. P. Morgan for discussions and G. Ruggeri for sharing preliminary results on fluid inclusion analysis. This work is a contribution to PRIN ‘Dynamics in subduction complexes: mass transfer in fossil systems and comparison with modern examples’.

Author Contributions All authors participated in collecting the data, interpretation of results and developing the model. P.V. wrote the paper. G.B. conceived the project.

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Correspondence to Paola Vannucchi.

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