Assessment of the efficiency of thermal pressurisation using natural pseudotachylyte-bearing rocks

Monday, 15 December 2014
Nicolas Brantut and Thomas M Mitchell, University College London, London, United Kingdom
The efficiency of thermal pressurisation as a dynamic weakening mechanism during earthquake slip relies on the thermal and hydraulic properties of the rocks surrounding the slip zone. A common approach to test the effect of thermal pressurisation is to make theoretical predictions using fault rock properties (permeability, porosity, compressibility) measured on exhumed fault rock samples. However, it is generally not possible to test whether those predictions are valid. Here, we attempt to assess the effectiveness of thermal pressurisation by comparing theoretical predictions of temperature rise to field estimates based on pseudotachylytes-bearing rocks. We measure hydraulic and transport properties of a suite of fault rocks (a healed fracture damage zone, an unhealed fracture damage zone and the intact parent rock) from the pseudotachylyte-bearing Gole Larghe fault in the Adamello batholith (Italy), and use them as inputs in realistic numerical simulations of thermal pressurisation. We find that the melting temperature can be reached only if damaged, unhealed rock properties are used, and not with the intact or the healed fault rock properties. A 10 fold increase in permeability, or a 4 fold increase in pore compressiblity of the intact rock is required to achieve melting. Our results emphasise the existence of dynamic damage processes which strongly modify fault rocks properties during earthquake propagation, and indirectly affect earthquake propagation itself by decreasing the efficiency of thermal pressurisation as a weakening mechanism.