White Mountain is centrally located in the bedding-plane portion of the Eocene Heart Mountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126–340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where Kmax is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a significant amount of CO2 by flash heating. This report is the first to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.