A new data set of temperature-dependent clay mineral parameters and vitrinite reflectance of the Tuscan successions in the Northern Apennines (Italy) displays decreasing levels of thermal maturity from hinterland to foreland, and abrupt changes parallel to the strike of the chain which are structurally controlled by northeast-southwest–trending faults (e.g., Marecchia valley lineament). To the southeast of the Marecchia valley lineament, paleothermal indicators show deep diagenetic conditions in the hinterland and early diagenetic conditions in the foreland (Ro% ranges from 0.80% to 0.30%; illite content in mixed-layer illite-smectite [I-S] from 86% to 38%). To the northwest of the Marecchia valley lineament, in the hinterland, Ro% is up to 0.95% and mixed-layer I-S have an illite content of ∼87%–88%, both gradually decreasing toward the northeast (to Ro% of 0.33%, and illite in I-S of 50%). Thermal models allowed us to constrain the geometry of the Miocene thrust wedge with special regard to the original thickness and distribution of its allochthonous uppermost structural unit (Ligurian unit) across northeast-southwest–trending tectonic lineaments. The thickness of the Ligurian unit ranges from 1 to 1.5 km to the south of the Marecchia valley lineament, to 3 km to the north-northwest. This tectonic lineament affected wedge geometry, amounts of tectonic transport, and thickness of the uppermost structural unit, and, possibly, Neogene–Quaternary levels of exhumation.