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One of the most compelling evidences that megathrust subduction interfaces are weak (e.g. Duarte et al., 2015), is the devastating 2011 Tohoku-Oki megaquake, whereby the megathrust accomplished a nearly complete stress drop (Hasegawa et al., 2011; Brodsky et al., 2017) and a co-seismic switching of the maximum and minimum stress orientations (Hasegawa et al., 2012).We studied the calcite veins of an exhumed thrust fault coming from the base of the Sestola Vidiciatico Tectonic Unit shear zone (SVU, Northern Apennines of Italy), interpreted as a field analogue of the shallow portion of subduction megathrusts (Tmax ̴ 100°-150° C) (Vannucchi et al., 2008). Observed crosscutting relationships suggest a cyclical formation of incompatible deformation features: extensional veins exploiting pre-existent cleavage planes at low-angle to the thrust, the main low-angle thrust faults associated with mixed hybrid-shear and implosion-breccia type veins and high-angle extension veins coupled with thrust-parallel pressure solution cleavage (Mittempergher et al., 2018).To characterize the type of fluids involved, we measured trace elements and Rare Earth Elements (REEs) concentrations by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) in calcite veins and their wall rock, finding marked difference between REE patterns of extension and shear veins, suggesting different sources of fluid for each type of vein. In particular, a strongly positive Eu2+ anomaly recorded exclusively in the shear veins can derive from an exotic fluid, different in redox conditions and likely higher in temperature. To the contrary, extension veins REE patterns point out for a very local fluid circuit and source.All these microstructural and geochemical features suggest a cyclical shifting of the principal σ1 and σ3 stresses through time. This was controlled by interrelated changes in permeability, fluid pressure and composition and easily achieved thanks to the low differential stress, which represented a necessary condition for extension and hybrid veins formation.References:Duarte J. C., Schellart W. P. and Cruden A. R., 2015. How weak is the subduction zone interface? Geophysical Research Letters, 42, 2664-2673.Hasegawa A., Yoshida K. and Okada T., 2011. Nearly complete stress drop in the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake. Earth Planets Space, 63, 703-70.Hasegawa A., Yoshida K., Asano Y., Okada T., Iinuma T. and Ito Y., 2012. Change in stress field after the 2011 great Tohoku-Oki earthquake. Earth and Planetary Science Letters 355-356, 231-243.Brodsky E., Saffer D., Fulton P., Chester F., Conin M., Huffman K., Moore J. C. and Wu H.-Y., 2017. The postearthquake stress state on the Tohoku megathrust as constrained by reanalysis of the JFAST breakout data. Geophysical Research Letters, 44, 8294-8302.Vannucchi, P., Remitti, F. and Bettelli, G., 2008. Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary. Nature, 451, 699-03.Mittempergher S., Cerchiari A., Remitti F. and Festa A., 2018. From soft sediment to fluid assisted faulting at the base of the Sestola-Vidiciatico tectonic Unit in the Northern Apennines (Italy). Geological Magazine, 155(2), 438-450. [ABSTRACT FROM AUTHOR]