Hydrofracturing of Sediment and Hydrated MORB’s during Subduction of Ocean Crust

Abstract:

Dehydration accompanying metamorphism of sediment and hydrated mafic ocean crust during ocean plate subduction can potentially lead to fracturing and intraplate tremors. As rocks are buried, they lose porosity and permeability and eventually pass into the lithostatic pressure regime where the porosity and permeability are thought to be very small. Pore fluid pressure generation stemming from metamorphic reactions was modeled using the program SUTRAMET (Nabelek et al., 2014), which is a modified version of the program SUTRA (Voss & Provost, 2002). The program allows modeling of metamorphic reactions at high pressures and temperatures and incorporates transient porosity and permeability changes due to overpressure and changing volumes of metamorphic assemblages. Permeabilities (k) of 10^-18 and 10^-20 m² within the brittle regime were modeled. The model assumes continuous subduction with fluids generated at horizons where specific P-T conditions for reactions exist. Temperature gradient was assumed to be 10°C/km and the vertical component of subduction velocity to be at 0.85 cm/y. The model reactions in sediment (Campito Formation of western California as a proxy) and average MORB were generated by the program Theriak-Domino (de Capitani and Petrakakis, 2010). The initial sediment mineralogy includes albite, K-feldspar, phengite, chlorite, and lawsonite. The final mineralogy for the 10°C/km gradient is quartz, phengite, garnet, and pyroxene with jadeite and acmite components. The initial mineralogy of MORB includes chlorite, amphibole, pumpelyite, albite and quartz and the final eclogite mineralogy includes garnet, cpx, kyanite and quartz.

Calculations show that at k = 10^-18 m², pore fluid pressure can reach 450 MPa. Pore pressure is reduced by negative ∆V of mineral assemblages stemming from reactions; nevertheless, for the garnet-forming reaction, pore pressure is about 110 MPa. Assuming a tensile strength of 15 MPa and fracture aperture given by rock compressibility, fracturing allows reduction of pore pressure to this level. That is not the case, however, for k = 10^-20 m². Even with fracturing and accounting for negative ∆V, pore pressure can reach 300 MPa for garnet-forming reactions. This pressure may exceed the sheer strength within the brittle regime of a subducting slab.