Up-Scale Cascade of Fluid Pathways in an Exhumed Accretionary Wedge - the Link Between Slow Slip, Tremor and Megathrust Earthquakes

In plate-convergent settings, episodic tremor and slow-slip (ETS) with reoccurrence times of several months to years occurs mostly downdip from the seismogenic zone. Such events are well detected with geophysical measurements. It is speculated that fluids play a major role, and several geological studies on exhumed accretionary prisms indeed demonstrate the presence of fracture meshes including shear, crack-seal and extension veins indicative for ETS. What remains poorly understood is how fluids release and migrate over a multitude of scales and what their effect on the deformation is. On the base of the exhumed accretionary wedge of the European Alps, we use veins as a proxy to study fluid pathways across different scales in slate sequences. We apply field information and synchrotron X-ray fluorescence microscopy (SXFM), the latter yielding high-resolution trace-element maps. At the μm-cm-scale the slate matrix within vein arrays shows pervasive dynamic dissolution-precipitation processes and ultrathin veinlets (10 μm) with spatial densities up to 10⁵ m^-1. The ductile microphysical deformation processes in slate seams are mutually overprinted by brittle fracturing of the veinlets. This suggests alternating cycles of slow slip deformation and dehydration in slate seams and fast hydrofracturing and precipitation in the case of veinlet formation. This sequence is typical for slow slip and related tremor activity. The large number of μm-scale brittle structures enables to collect fluids pervasively with a small energy release only. In this way, fluids are collected and fed into foliation sub-parallel veins (m-scale) making up individual deca-meter veins, which themselves are concentrated in vein arrays (width of 5 m and spacing of 100 m). The sub-parallel veins with average thicknesses of 1 cm have smaller spatial frequencies compared to the veinlets and indicate a localized fluid flux at the 100 m to km scale with a stronger but less frequent energy release. The vein arrays are eventually channelling fluids into mega-arrays (width of 100 m and spacing of 350 m), which themselves feed into deca-km-scale thrusts. Fluid flux along these major thrusts is extremely localized and could ultimately generate devastating megathrust earthquakes with very high energy.