The maximum depth of meteoric fluid ingress into Earth's lithosphere is not well constrained. A principal barrier to surface fluid penetration is thought to be the transition from brittle to ductile stress-strain regimes in the crust because of the implicit shift from hydrostatic to lithostatic fluid pressures. Studies of Tertiary metamorphic core complexes exposed in the Cordilleran orogen of the southwestern United States addressed the question of whether or not surface fluids breached the ductile-brittle transition.
The lower plates of core complexes are exhumed along crustal-scale shear zones (detachment faults) that span the ductile-brittle transition. Lower-plate rocks have thus undergone ductile strain followed by brittle deformation during their unroofing. There is abundant evidence for reaction with fluids both above and below core complex detachment faults. Indications are that some of these fossil fluids were surface-derived. The timing of fluid-rock interaction below detachment faults therefore has important implications for the depth of meteoric fluid penetration.
Fricke et al. [1992] and Morrison [1994]
examined indicators of fluid-rock reaction in the lower plates of the Ruby
Mountains-East Humboldt Range core complex of northeastern Nevada and
the Whipple Mountains core complex of southeastern California,
respectively. Both studies found that quartz-feldspar 
O
O
systematics were indicative of exchange with meteoric fluid, but
Fricke et al. concluded that 
O
O
exchange
occurred during the waning stages of ductile deformation of the lower
plate while Morrison asserted that exchange occurred after passage of
the lower plate rocks into the brittle regime.
The critical question of whether or not meteoric fluids passed
through the ductile-brittle transition remains unresolved.
Outstanding difficulties include estimating depths from temperatures
in tectonically active terranes, and accounting for the complexity of
variable strain rates.
Acknowledgments. The author has benefitted from valuable discussions with D. Rumble, R. Ilchik, D. Bell, J.D. Frantz, J.M. Brenan, J.M. Palin, and J. Morrison during compilation of this report. Reviews by two anonymous referees, J.M. Brenan, K. Verosub, and R. Pielke improved the clarity of presentation.