Preliminary geothermal model of the Sierra Nevada

Four heat-flow values from the central Sierra Nevada vary systematically from 1.3 μcal/cm2 sec near the crest to 0.45 μcal/cm2 sec at the eastern edge of the San Joaquin Valley. Corresponding heat-production values determined from drill cores by Harold Wollenberg and Alan Smith range from 0.9 μμcal/cm3 sec (higher than the average granite) to 0.07 μμcal/cm3 sec (lower than the average basalt) in these granitic and intermediate rocks. With Francis Birch and Robert Roy and their collaborators we find that heat flow q is related to surface heat production A0 by q = q0 + DA0, where q0 and D are empirically determined constants (0.4 μcal/cm2 sec and 10 km, respectively). This relation can be preserved under differential erosion (for which there is much evidence) only if the crustal heat sources are distributed vertically according to A(z) = A0 e−z/D. q0 is interpreted as the uniform contribution from the mantle. This exponential source-distribution law forms the basis for an idealized model by means of which it is possible to reconcile the observed distributon of heat flow, surface heat producton, seismic velocities, and crustal thickness, as well as the volume of sediments attributed to post-plutonic erosion of the Sierra and the generation of the Sierra Nevada batholith by anatexis in a great geosyncline as proposed by Paul Bateman and his associates. The model implies that in a period of remarkable activity ending in the early Tertiary the western Sierra was almost completely purged of heat-producing elements in a time interval of the order of 100 m.y.