Flat‐slab subduction is an atypical form of subduction where the downgoing plate assumes a low‐angle or subhorizontal geometry as it descends beneath the overriding plate. These systems have profound impacts on upper‐plate deformation and volcanism, yet there are outstanding questions regarding the causes, initiation, and termination of flat‐slab subduction. The Pampean flat‐slab region, located in the South American Cordillera, is an ideal locale to study the evolution of these systems due to its well‐constrained geologic history and the continuity of subduction along the western margin of the continent. In this work, we utilize S wave receiver functions (SRFs) to measure the thicknesses and geometry of the subducting Nazca plate lithosphere and the overriding South American plate within the region. Results from this study show overthickened Nazca Plate crust (~15–20 km thick) in the flat‐slab region. Prior to the slab steepening, SRFs indicate a velocity increase rather than a decrease at the top of slab crust, which is consistent with a transition from basalt to eclogite within the crust as it dehydrates. Margin parallel cross sections are consistent with differing transitions between flat‐subduction and more‐typical subduction occurring to the north and south. In the north, the transition appears to be gradual, consistent with slab bending. To the south, SRFs indicate an abrupt change, consistent with slab tearing. These differences are consistent with significant variations in slab rheology along the margin, which may reflect differing thermal regimes. This work has global implications for the three‐dimensional effects of flat‐slab subduction.