Values of the velocity dependence parameter ( a - b ) as a function of temperature. Colours indicate the magnitude of the post-step velocity imposed in performing each upward or downward velocity step, as shown in (a). Solid tie-lines designate a step down in velocity, while dashed lines designate a step up. Errors in (a-b) are estimated to lie within ±0.0024 (see error bar in the upper plot). (a) Results for wet illite/quartz gouges, previously reported by Den Hartog et al. (2012a). Open symbols are calculated using peak friction values of stick-slip events and differ from the (a-b) values for stick-slip events originally reported by Den Hartog et al. (2012a; Den Hartog and Spiers 2013), which were based on mean friction values. (b) Results for wet (near-)pure illite gouge. (c) Results for dry illite/quartz (see also Den Hartog et al. 2012a).