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Fifty‐one new and 309 published thermochronometric ages (nine systems with closure temperatures ranging from ~450 to 70°C) from the Graubünden region of the Central Alps demonstrate that a pronounced thermal mismatch between the Austroalpine allochthon (Alpine “orogenic lid”) and the Pennine zone persisted until at least 29 Ma and, allowably, until circa 18 Ma. The observed mismatch supports previous suggestions that the famous “overthrust” between the Austroalpine allochthon and the Pennine zone, historically regarded as primarily an Eocene top‐north thrust fault, is in fact primarily an Oligocene‐Miocene normal fault that has a minimum of 60 km of displacement with top‐south or top‐southeast sense of shear. Two hallmarks of Alpine geology, deposition of the foredeep Molasse and emplacement of the Helvetic nappes, appear to be coeval, peripheral manifestations of crustal thickening via the interposition of the Pennine zone as a northward intruding wedge between the Austroalpine “lid” and the European cratonic margin, with the Helvetic system (European margin) acting as the “floor” of the wedge. We presume the Penninic wedge is driven by the buoyant rise of subducted crust no longer able to remain attached to the descending slab. If so, emplacement of the Pennine wedge could have occurred mainly after Adria was juxtaposed against cratonic Europe. What causes mountains to rise? The most common setting for their development is a place where the Earth's tectonic plates collide. Like the jaws of a closing vice, the plates squeeze crust between them, greatly deforming it by faulting and folding. One of the best known examples of this complex process is the European Alps, which formed between colliding continental plates of Europe and “Adria” (Italy). The traditional view is that large faults within the Alps are a direct expression of the sliding of the southern margin of the European plate beneath Adria (a process called subduction) to great depths in the Earth (100 km or more). Here we analyze the timing of motion of the largest fault in the Alps of eastern Switzerland (the Austroalpine “overthrust”) using a variety of thermochronometers (heat‐sensitive “clocks” that provide time of cooling). Our results show that the timing and sense of motion on the fault do not reflect relative motion between the two plates as previously believed. Instead, displacement on the faults we now see within the mountain belt mainly occurred late in the collision process, when the mechanical attachment of the subducted European crust to the downgoing plate was lost due to heating, which caused it to buoyantly rise relative to its dense surroundings in the Earth's mantle. Returning this wedge of crust to near the surface greatly thickened the overall crust along the collisional “suture” of the two continents, causing the Alps to rise. Thermal histories of the Austroalpine hanging wall and Pennine footwall, constrained by nine thermochronometric systems, show that they did not thermally equilibrate until after 29 Ma, and possibly not until 18 MaThe thermal data indicate that the Austroalpine‐Pennine contact zone is a top‐south to top‐southeast normal fault with at least 60 km of displacement, active during the main phase of Alpine mountain buildingDevelopment of topography and an orogenic crustal root, as reflected in deposition of the perialpine molasse and deformation of Europe (Helvetics) circa 33‐15 Ma, may result primarily by buoyancy‐driven interposition of the Pennine zone between the Austroalpine “lid” and the European cratonal margin and not from the convergence between Apulia and Europe