ALBERT

Description: The application of plagioclase geothermometry to plagioclase-bearing volcanic ash layers and to the glassy margins of pillow basalts from the fast-spreading East Pacific rise, the moderately spreading Gorda and Juan de Fuca ridges, and the slow-spreading mid-Atlantic ridge has shown that magma temperatures, as well as average An contents of plagioclases, are negatively correlated with spreading rates. A detailed investigation of the major element chemistry of volcanic glasses from each of these areas suggests that the observed consistent element-element covariances among individual populations of samples have been caused by fractional crystallization of the magmas. The regularity of chemical variation and the similarity of magma temperatures within each population of samples suggest that magmas ascending from beneath each ridge have had similar evolutionary histories. Vector analysis of the chemical data of all samples of volcanic glasses indicate that each population of samples from each of the spreading centers is chemically distinct, even though all samples have been subjected to similar amounts of fractional crystallization. The compositional distinctiveness of each population of oceanic tholeiites probably reflects differences in the depths at which the magmas were generated. Calculated magma temperatures and geothermal gradients calculated from published heat flow measurements can be used to estimate depths of magma generation of about 16 km beneath the East Pacific rise and about 23 km beneath the mid-Atlantic ridge.

Description: The asymmetry of the continental margin around southern Africa can be related to Mesozoic sediment thicknesses, which were in turn controlled by the local structural setting. On the west coast, the Orange Basin sediments were built out as a thick wedge over the margin of the continent by discharge from the Orange River, whereas on the Agulhas Bank, sedimentation was confined to continental areas. Off the east coast the extremely narrow margin of the continent did not form an effective trap for sediments, which were readily carried beyond it. Cainozoic sediments are thin, and modify the Mesozoic sediment pile only locally on the outer shelf and slope.

Description: Geodetic data along the San Andreas fault between Parkfield and San Francisco, California (latitudes 36°N and 38°N, respectively), have been re-examined to estimate the current relative movement between the American and Pacific plates across the San Andreas fault system. The average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907-1971. Between 36°N and 37°N it appears that most, if not all, of the plate motion is accommodated by fault creep. Although strain is presumably accumulating north of 37°N (San Francisco Bay area), the geodetic evidence for accumulation is not conclusive.

Description: An analysis of the reproducibility of Geodolite measurements at distances ranging from 1 to 35 km indicates a standard deviation for each length measurement of about σ = (a2 + b2L2)1/2, where a = 3 mm, b = 2 × 10−7, and L is the line length. Thus σ ranges from 3 to 8 mm for line lengths of 1 and 37 km, respectively. Corrections for atmospheric refractivity must be determined from temperature and humidity readings made with an aircraft flying along the line of sight at the time of the range measurements in order to attain this precision. Even at this level of precision, determination of the strain accumulation at sites along the San Andreas fault system will require annual observation of many line lengths over a period of at least 5 years.