Rheological perspective using the new generation Griggs-type apparatus: New constraints from general shear experiments of Carrara marble

Quantifying rock rheology is fundamental to understanding and modelling the lithosphere’s dynamics. However, although most rocks of the lithosphere deform at high (> 0.5 GPa) – to very high (> 3 GPa) – pressure over geodynamic events, available mechanical laws have been produced at low pressure (0.3 GPa) using gas-medium deformation apparatuses. To explore rock rheology at higher pressure – typically above 1 GPa – a solid-medium apparatus is required, which involves substantial friction-related stress overestimations while the sample is deforming within the confining medium. Here we provide a series of deformation experiments that aim to quantify such a stress overestimation in the new generation Griggs-type apparatus. The main goal is to better estimate how the friction “baseline” evolves with pressure, alongside defining the starting point of the strain-stress curve more accurately. To do so, we performed general shear experiments of Carrara marble at a confining pressure ranging from 0.3 to 1.5 GPa, while systematically applying a temperature of 650 °C and a displacement rate of 10^-4 s^-1. Using relaxation steps to highlight the friction baseline in a ‘force-displacement’ plot, we document a slope that increases linearly with pressure, from 0.1° to 1.5°. Moreover, none of the highlighted baselines crosses the conventional hit-point, which is the commonly used reference to define the “zero” point of strain-stress curves in the Griggs-type apparatus. Such a mismatch involves additional stress overestimations that we propose to correct by using a new “hit-point” at the intersection between the baseline and mechanical curve. Thanks to the latter and applying a “baseline” correction, we document stress measurements equivalent to the ones documented for Carrara marble using the gas-medium Paterson press.