FLUID RELEASE AND DEFORMATION AT, AND BEYOND, THE BASE OF THE MEGATHRUST SEISMOGENIC ZONE (Invited Presentation)

Hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes, including the Namibian Damara Belt, where subduction-related deformation occurred at temperatures ≤ 550˚C. High intensity quartz vein networks are observed in mutually crosscutting relationship with amphibolite facies metamorphic fabrics. The veins reflect mixed-mode deformation and are kinematically consistent with plate interface shear and hydrofracture conditions involving low differential stress and fluid pressures in excess of the least principal stress. Vein quartz δ¹⁸O values range from 9.4 ‰ to 17.9 ‰ (n = 30). Excluding values obtained from long-lived high-strain zones involving basaltic slivers, δ¹⁸O values are within a much smaller range of 14.9 ± 1 ‰, consistent with quartz-H₂O equilibrium at 470 - 550 ˚C and a fluid source from progressive local breakdown of chlorite. This implies that a locally released fluid with δ¹⁸O of 12 ± 1 ‰ allowed extensive hydrofracture in low permeability shear zone rocks at temperatures typical of the plastic regime. The conditions and mechanics of vein system formation appear analogous to tectonic tremor observed at the downdip end of the seismogenic zone in active subduction zones. If the process of tremor is the same as for this vein system, then a corollary is that the near-periodic nature of tremor events relates to a regular nature in the build-up and release of fluid pressure. Also, an effect of tremor and vein formation would be a strengthening of the interface by quartz precipitation within a weaker, plastically deforming shear zone, potentially leading to migration of the megathrust and preservation of the vein system.