In the Dauphinoise zone of the French Western Alps, a posttectonic thermoremanent magnetization, carried by pyrrhotite, has been isolated. The paleomagnetic signature is interpreted as the record of successive partial thermoremanent magnetizations of opposite directions acquired during the early Miocene slow cooling of the epimetamorphic marly limestones of the studied area. A local vertical thermal gradient of 22 ± 2 °C/km was derived from the unblocking temperature of a reversal recorded at different altitudes. Using this vertical gradient, we have produced a paleotemperature map at the time of the reversal (about 20 Ma). The map shows a horizontal regional thermal gradient of 7 °C/km; the higher temperatures were toward the crystalline basement. Paleomagnetic directions demonstrate that this horizontal gradient was present during cooling. Its origin can be explained by large-volume fluid circulation along the major fault that forms the boundary between the crystalline basement and its Liassic cover.