Indicators of organic metamorphism provide valuable tools for analyzing the thermal history of tectonostratigraphic terranes. Paleotemperature estimates derived from vitrinite reflectance, for example, are more precise than values based upon inorganic mineral assemblages in low‐grade rocks. Isothermal geometries must be interpreted within the context of structural and stratigraphic data, but, by doing so, direct comparisons of thermal history can be made between adjacent terranes. Data from the Franciscan Complex of California show that three basic types of thermal anomalies are related to terrane amalgamation: (1) maturity inversion across terrane boundaries, (2) shear heating along terrane boundaries, and (3) resetting of maturity gradients via thrust loading. In addition, thermal overprints result if there is a substantial rise in local or regional geothermal gradient following terrane amalgamation; under these circumstances, homogenization of thermal maturity may be evident across terrane boundaries. If associated magmatic rocks or hydrothermal veins can be dated, an upper limit is established on the suturing event. Thermal data are also useful for resolving structural ambiguities on a local scale; clear distinctions can be made, for example, between faulted contacts, diapirs, and mildly tectonized depositional contacts or unconformities.