The absence of a heat flow anomaly greater than ∼0.3 µcal/cm2/sec associated with the San Andreas fault is used to estimate the upper limit for the steady state or initial shear stress. Under the assumption that the long‐term rate of motion along the fault is 5 cm/yr and occurs primarily in the form of creep, this upper limit is about 100 bars. If the motion is primarily accomplished by faulting during large earthquakes and if the frictional stress is equal to the final stress as suggested by E. Orowan (1960), the upper limit is estimated to be about 200 bars. Without Orowan's assumption, the estimation of the upper limit is about 250 bars, based on earthquake energy‐magnitude‐moment relations. If the long‐term rate of motion along the San Andreas fault is only ∼2 cm/yr, these results are increased to 250, 350, and 400 bars, respectively.

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New measurements of heat flow are reported for one hundred thirty‐eight sites in the United States. The tabulation includes mean gradients, mean resistivities, and uncorrected and topographically corrected heat flow. Methods and calibrations are briefly described. In several areas, station density is adequte for preliminary contouring and for correlation with basement geology and radioactivity.

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We report 66 new heat flow and 24 new heat production measurements from the Sirt Basin, a late Jurassic‐Miocene sedimentary depression in north central Libya underlain by late Proterozoic basement. Heat flow determinations were made using bottom hole temperatures from oil wells and thermal conductivity measurements from drill core and cuttings; heat production measurements come from core samples of basement rock. Heat flow is fairly uniform throughout the basin, with a mean of 72 ± 9 (s. d.) mW m−2. It is not clear if heat flow from the Sirt Basin is elevated as a consequence of its origin as a late Mesozoic rift. The difference between the mean basin heat flow and the global mean heat flow from tectonically undisturbed late Proterozoic terrains (55 ± 17 mW m−2) is 17 mW m−2, but this difference lies within the uncertainties associated with these mean heat flow estimates. If heat flow from the Sirt Basin is elevated, it could be caused by enhanced crustal heat production and need not be attributed to thermal alteration of the lithosphere related to basin formation. Mean crustal heat production is 3.9 ± 2.1 μW m−3, 1/2 to 3 times greater than surface heat production in other Proterozoic terrains in Africa. From west to east, the pattern of heat flow across northern Africa is characterized by high (80–110 mW m−2) heat flow throughout most of northwestern Africa, normal to perhaps slightly elevated heat flow in the Sirt Basin, low to normal (35–55 mW m−2) heat flow in Egypt inboard of the Red Sea, and high heat (75–100 mW m−2) flow along the Red Sea. High heat flow near the Red Sea and in northwestern Africa along the Mediterranean coast can be readily attributed to Cenozoic tectonic activity, but high heat flow in the Paleozoic Sahara basins of southern Algeria is harder to understand within the tectonic framework of northern Africa. A possible explanation, advanced previously, is that elevated heat flow in the Sahara basins arises from a regional thermal anomaly within the north African lithosphere. If that explanation is correct, then the heat flow distribution in the Sirt Basin and in Egypt away from the Red Sea suggests that the postulated lithospheric thermal anomaly does not extend beyond the Sahara basins to the east.

Magnetic time‐variations between Tucson, Arizona and Sweetwater, Texas indicate that a zone of high electrical conductivity underlies the southwestern United States. The interpretation of this zone by Schmucker as a rise of the isotherms in the upper mantle is supported by six heat flow observations along the line of the geomagnetic profile. These and other observations indicate a high but variable heat flow in the Basin and Range Province which contrasts strongly with the uniform values of [Formula: see text] reported for the Texas Foreland. The width of this high heat flow anomaly, which may extend across the entire Basin and Range Province, suggests anomalously high temperatures in the upper mantle. This interpretation is further supported by magnetotelluric data between Phoenix, Arizona and Roswell, New Mexico and by the low seismic [Formula: see text] velocity and negative gravity anomaly. It is suggested that the “anomalous mantle” may be related to the tectonic evolution of the western United States and the late Cenozoic fault system.

NOTICE PORTIONS OF THIS REPORT ARE ILLEGIBLE. It has been reproduced from the best availab!e copy to permit the broadest posslble availa bllity,

Investigations of the distribution of U, Th, and K in deep profiles exposed by Precambrian doming have been presented for Vredefort, South Africa and have recently been completed for a similar structure near Sudbury, Ontario, Canada. An evaluation of the Vredefort data shows evidence of a major crustal repetition of the vertical radioelement distribution with depth. A simple explanation is that the South African basement is geochemically layered, with repetition occurring from layer to layer. This implies a sawtooth, or “serrated” distribution with depth. Preliminary results from the Sudbury investigation show a rise in heat production with distance from the Sudbury Structure, corresponding to decreasing depth in the basement rocks. The repetition of the radioelement distribution observed at Vredefort does not appear in the Sudbury data. Although Th and K make a jump at nearly the same depth, the U data show no corresponding increase. U loss is suspected in the project area, affecting the overall distribution of heat production in the Sudbury basement. Given its complex history and geographic location, the material in the sample area may not be representative of the Superior Province.