“…The directions of azS Hmax , derived above for the plate boundary channel flow model, are virtually identical to the average directions of strain rate from high-resolution decadal GPS measurements of velocity [Beavan et al, 2016], which are~100°for the Southern Lakes region and~105°for the central Southern Alps (Figure 5). This would make sense if the GPS-derived strain rates reflect uniform elastic straining of the brittle crust, which in dynamical terms implies that it is either coupled to an underlying uniform ductile flow [Lamb, 1994, Bourne et al, 1998 or the result of edge forces imposed by the bounding plates on an isotropic uniform elastic layer [Solomon et al, 1975;Forsyth and Uyeda, 1975;Richardson et al, 1976;Schubert et al, 1978, Wallace et al, 2004, Wallace et al, 2007, Lamb and Smith, 2013, so long as gravitationally induced stresses are a negligible component of the stress field [Jeffreys and Singer, 1959;Artyushkov, 1973;Wortel et al, 1991;Flesch et al, 2000Flesch et al, , 2007. However, the topography of the Southern Alps in central South Island suggests that gravity is likely to be a contributing part of the stress field.…”