Metamorphic Controls on Brittle Fracturing in Ductile Shear Zones along the Subduction Interface - Perspectives from the Rock-Record and Phase Equilibrium Models

Abstract:

Episodic tremor and slow slip (ETS) phenomena are observed along numerous active subduction zones and appear to require conditions of low but variable viscosity and low effective stress. However, there is a scarcity of exhumed rocks that preserve insight into how and where these conditions arise. Three exposures of exhumed shear zones on Kyushu, SW Japan, represent a transect along the plate interface at temperatures between 300 and 500 ^oC, spanning conditions where ETS phenomena are commonly observed.

At all our localities, ocean plate stratigraphy is intermingled in a broad, ductile shear zone with an intense phyllosilicate-rich foliation. Evidence for local dissolution and precipitation implies a pressure solution origin for the foliation. Tensile quartz veins cut and/or are deformed along the foliation, indicating local, episodic brittle fracturing during bulk ductile shearing. The quartz veins suggest that pore fluid pressure was locally and temporally greater than the least principal stress, and that differential stresses were low (10s of MPa) to allow for tensile failure. Oxygen isotopes in quartz veins combined with quartz–water fractionation factors suggest d¹⁸O 10.9 to 13.9‰ for vein-forming fluids, consistent with a metamorphic fluid source. Calculated mineral assemblages predict that during subduction, for a sample of metabasalt between 250 to 600 ^oC and 2 to 17 kbar, fluid is released in three discrete pulses relating to the breakdown of prehnite and pumpellyite at ~275 ^oC and < 7 kbar, lawsonite at 300 - 500 ^oC and > 7 kbar, and chlorite at 450 - 500 ^oC over the entire pressure range. This suggests that fluid production along the plate interface exposed on Kyushu, exhumed from relatively warm slab conditions, occurred first due to prehnite-pumpellyite breakdown and again due to chlorite breakdown.

High anisotropy and potentially low permeability within the thrust likely restrict fluid flow to along or within the plate interface. Together with this permeability structure, we suspect that fluids supplied by localised dehydration reactions lead to local attainment of the fluid overpressures required for generating the quartz veins in shear zone exposures. The conditions of dehydration reactions correlate with the inferred conditions of tremor sources along active margins.