Abstract — Although nonlinear fracture flow and non-Fickian transport may be common in natural settings, the mechanisms driving and controlling these intertwined phenomena are seldom scrutinized. Here we investigated the critical role of recirculation zones (RZs) in controlling nonlinear flow and non-Fickian transport through numerical simulation experiments within three-dimensional real fractures. RZs were quantitatively mapped based on flow fields modeled by solving the Navier-Stokes equations across increasing Reynolds number (Re). The development and growth of RZs, which are related to aperture expansion and contraction, determine the degree of flow nonlinearity. Moreover, expanding RZs have more capacity to capture and later release solutes back to the main flow. This always results in non-Fickian transport with bi- and sometimes multi-modal residence time distributions (RTDs). The time interval between the RTD modes are related via a power law. The Re and RZ volume are sufficient for characterizing the bimodal RTD.