We present a new approach to identifying the source and age of paleofluids associated with low-temperature deformation in the brittle crust, using hydrogen isotopic compositions (dD) and 40 Ar/ 39 Ar geochronology of authigenic illite in clay gouge-bearing fault zones. The procedure involves grain-size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to dD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). dD values of the authigenic illite population, obtained by extrapolation, are 289 6 3&, 290 6 2&, and 297 6 2& (VSMOW) for samples KSL, RES4-1, and G1G2, respectively. These correspond to dD fluid values of 262& to 285& for the temperature range of 1258C 6 258, indistinguishable from present-day precipitation values. dD values of the detrital illite population are 245 6 13&, 260 6 6&, and 264 6 6& for samples KSL, G1G2, and RES4-1, respectively. Corresponding dD fluid values at 3008C are 226& to 245& and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 6 3.4 Ma (authigenic) and 95.8 6 7.7 Ma (detrital) for sample G2 and 24.6 6 1.6 Ma (authigenic) and 96.5 6 3.8 Ma (detrital) for sample RES4-1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated preexisting, weak clay-rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time.