Slip partitioning and fluid pressure in a heterogeneous plate interface at seismogenic depths: an example from the Gwna Complex, Anglesey, UK

Abstract:

Plate interface deformation commonly involves varied lithologies (sandstone, mudstone, basalt, carbonates, chert) within a tabular shear zone. Minerals within each lithology are subject to variable stress-dependent deformation mechanisms, meaning the rheology of a plate interface shear zone depends strongly on geometry, composition and applied shear and normal stress. We compare steady state slip speed modelled based on observations in the Gwna subgreenschist subduction Complex (Wales, UK) exhumed from seismogenic depths, with geodetically-observed velocities for aseismic creep (10-100 mm/yr) and slow slip (300-1500 mm/yr).

Localised (<10 m thickness) mélange shear zones derived from illitic siliciclastic sediments and chloritic altered volcanics anastomose throughout the Gwna Complex. Illitic mélange has a clast-in-matrix structure cut by variably-deformed quartz-dominated veins. In contrast, chloritic mélange is cut by foliation-parallel deformed calcite veins. We use the mineralogy of these mélange units to construct stress-strain rate curves at 260°C (derived from chlorite geothermometry) and ~17 km depth (based on a 15 °C/km geotherm). With a differential stress of 39±12 MPa, from calcite piezometry and vein geometry, we then use the shear zone geometry to estimate likely shear velocities.

At hydrostatic pore fluid pressures (λ=0.4), viscous deformation mechanisms (crystal plasticity, pressure solution) dominate and modelled slip rates within metre-thick siliciclastic quartz and mm-thick calcite-dominated shear zones are lower than required for plate boundary deformation. At near-lithostatic pore fluid pressures (λ=0.95), frictional slip distributed throughout matrix-forming chlorite and illite causes modelled slip rates to exceed 1 m/s. No clear evidence of seismic slip is present within the subduction complex. For stress constraints from the subduction complex and velocity constraints from modern margins to agree, sub-lithostatic pore fluid pressures or strain-rate dependent deforming thicknesses are required. The absolute values of this modelling are likely imprecise, but mixed frictional-viscous mechanisms and intermediate pore fluid pressures are consistent with observations from both active and exhumed plate interface shear zones at seismogenic depths.