Potassium enrichment within the upper plate of the Whipple detachment fault, southeastern California, occurred as a discrete pulse between 18.5 and 14 Ma, following the most intense extension and prior to the last episode of movement on the detachment fault. This timing is not consistent with a hydrothermal model for K enrichment in which the K is derived from propylitic alteration of lower-plate rocks. Instead, the temporal and spatial distribution of K enrichment supports a diagenetic model in which the K was derived from leaching of Tertiary alkalic volcanic rocks and dissolution of rhyolitic ashes by alkaline-saline brines in a lake and circulating ground water. The most intense alteration is commonly associated with mapped upper-plate faults, although not all mapped faults display evidence of K metasomatism. Fracture porosity present along active faults may have provided a conduit for K-bearing fluids, thus marking a set of faults as coeval and cogenetic.