Heat flow and radioactive heat generation have been measured and the data compiled across southern British Columbia in the region of the Lithoprobe Southern Canadian Cordillera Transect. Heat flow in the trench-arc zone between the continental margin and the Garibaldi volcanic belt is very low, but in the volcanic belt it is high and very irregular. Farther inland, to the east, the heat flow is moderately high, with the highest values in southeastern British Columbia, associated with high surface radioactive heat production. The thermal data from the central and eastern interior of southern British Columbia define a single heat-flow province with a reduced heat flow of 63 mW/m2 flowing into the upper crust. This indicates a warm, thin lithosphere similar to that of the Basin and Range of the United States to the south. Occurrences of seismic reflective bands in the lower crust of the Cordillera were compared with temperatures calculated from surface heat flow and heat generation using a simple one-dimensional conductive model. The 450 °C isotherm corresponds approximately to the brittle– ductile transition, and deeper crust may be rheologically detached from the upper crust. Where the thermal data approximately coincide with the transect seismic reflection lines, the 450 °C isotherm often corresponds to the top of characteristic sub-horizontal reflector bands, as found in Phanerozoic areas elsewhere around the world. The lower limit of the reflective band in a number of Cordilleran reflection sections is near the 730 °C isotherm, which corresponds to the transition from present "wet" amphibolite- to "dry" granulite-facies conditions. This control of the depth to the deep crustal reflective bands by present temperature provides support for the model of the reflectors being produced by fluids trapped at lithostatic pressure (layered porosity), a model that can also explain the high electrical conductivity in the deep crust of the area. The probable rheological detachment of the lower crust and a possible nonstructural origin of the deep reflectors require that interpreted lower crustal structural boundaries such as the top of the basement of the North American craton under the Lithoprobe Southern Canadian Cordillera Transect be treated with caution. However, there is no doubt that many seismic reflectors are related to crustal structures, and the model is presented as an explanation for bands of seismic reflectors in the lower Phanerozoic crust, not as a model for all seismic reflectors.