Metamorphic pressure-temperature (P-T) paths from the upper and lower plates of the Fall Mountain nappe, southwest New Hampshire, reveal different thermal histories in the two structural levels. Upper-plate rocks experienced early low-P, high-T (contact?) metamorphism to peak P-T conditions of 700-750 °C, 3-4.5 kbar. These P-T conditions were followed by loading to approximately 5-6 kbar, and then by nearly isobaric cooling to approximately 500 °C, 4.5 kbar. Upper-plate rocks then experienced minor heating with unloading and finally cooling. The lower-plate rocks also experienced early low-P, high-T (contact?) metamorphism but only to P-T conditions of 480-510 °C, 2-3.5 kbar. Nappe emplacement resulted in nearly isothermal loading to 500 °C, 5-6 kbar, which was followed by either heating or cooling, depending on the proximity of the sample to the upper plate.
The early pressures recorded in both the upper- and lower-plate rocks suggest a depth of approximately 10 km for origination of the nappe; the post-nappe pressures (5-6 kbar) imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness. Diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m.y., and U/Pb chronology on zircon suggests that it occurred between 400 and 410 Ma. 4OAr/39Ar cooling ages on micas indicate that the terrane was cooled to approximately 300 °C by 340 Ma.