Heat-flow measurements made in Lake Tahoe, California-Nevada, demonstrate that the transition from subnormal heat flow in the Sierra Nevada to above-normal heat flow in the Basin and Range province occurs west of the assumed physiographic boundary between these two areas, contrary to earlier belief. In addition, these data, together with data of other workers, clearly reveal the sharpness of this transition, which suggests that the causative thermal sources and (or) sinks must be restricted to depths not greater than the uppermost mantle. The way in which heat-flow data constrain the current hypotheses of crustal structure and evolution of the Sierra Nevada–Basin and Range provinces is illustrated with a tectonic model that employs a post-Cretaceous shallow-dipping subduction zone beneath the Sierra Nevada and an active upper-mantle diapir under the Basin and Range province during late Cenozoic time.

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