Basalts of unusual composition from the Chyulu Hills,Kenya

Five Ne-normative basalts are unusually rich in Cr, to a lesser degree in Co, and have very high La/Lu ratios and high contents of several excluded trace elements resembling kimberlites in these and other aspects. They seem to represent mantle-derived magmas, only slightly modified by shallow level crystal/liquid fractionation. Cr was apparently excluded from the crystalline phases in equilibrium with these magmas in the upper mantle.The Chyulu rocks differ in several respects from other Gregory Rift basalts, but closely resemble those from Jan Mayen Island near the Mid-Atlantic ridge. The generation of these unusual basalts is therefore not related simply to mantle composition or tectonic setting, but may reflect unusually high-T (1450°C), low-P (15–29 kbars) environments in the zones of last effective equilibration of these magmas with mantle rocks.     

On the stability of close binaries in hierarchical three-body systems

The Hill-type stability (cf. closure of the zero-velocity curves in the circular restricted three-body problem) of general hierarchical three-body systems is examined analytically in the case where the total mass of the binary is small in comparison to the mass of the external body (e.g. systems of the type Planet-Satellite-Sun, Planet-Planet-Star, etc.). This is compared with results derived by Szebehely, Markellos and Roy in the Planet-Satellite-Sun case of the circular restricted three-body problem. It is demonstrated how the Hill-type stability is affected by the sense of revolution of the binary, i.e. corotational or contrarotational, and the mass ratio within the binary. The effect of the difference in longitudes of the bodies in their orbits is also examined.     

Simulation of pebble abrasion on coastal benches by transgressive waves

Tumbler simulations of abrasion processes affecting rocks on basaltic marine benches have clarified the effects of mass and volume of rock fragments on their rate of wear. Several experiments indicate that rate of basalt wear increases with mass in the interval 1 g to 8 g, but is not related to volume of fragments abraded in seawater. Relatively small quantities of rock flour suspended in the immersant seawater substantially reduced the rate of basalt wear. Basalt and calcareous beachrock responded differently in systems with both rock types tumbling together. Examination of the cuttings from these low-energy encounters suggests that abrasion on elevated benches along exposed seacoasts reduces virtually all clastic material to particles of silt size or smaller, producing essentially no sand.     

ENVIRONMENTAL CONSEQUENCES OF HUMAN ACTIVITY ON THE YELLOW RIVER AND ITS DELTA,CHINA

The Yellow River is unique among the major rivers of the world because of its extremely heavy sediment load in relation to its small discharge. The Yellow River has changed courses numerous times during the past 5000 to 6000 years. With its large sediment load, most of which comes from the Loess Plateau of central China, the river has created a number of deltas along the coast of northern China. The abandoned deltas are eroding, whereas the present delta being formed in Bohai Bay is growing rapidly. Human activity, including extensive agriculture, heavy water consumption, the use of river-control structures, and war, has been responsible for many of the changes occurring in the Yellow River and its delta. [Key words: hydrology, loess, floods, Yellow River, Bohai Bay, China.]     

Geomorphic constraints on the active tectonics of southern Taiwan

Taiwan is a region of rapid active tectonics, yet the study of the tectonic processes that shape the interior of the island is difficult due to the high rates of erosion and dense vegetation. We use digital topography to look for indications of active deformation preserved in the local geomorphology. In particular, anomalies in the regional pattern of drainage are used to infer zones of enhanced tectonic activity. The apparent anticlockwise rotation of major river systems in plan view indicates the presence of a diffuse zone of left-lateral shear running down the southeastern side of Taiwan. Asymmetries in the catchments of individual drainage basins show the influence of varying rates of uplift across southern Taiwan, with the most rapid uplift close to Taitung at the indentation point of the Luzon arc with the Chinese continental margin. Our interpretations, though based predominantly on remote-sensing observations, are consistent with the available field evidence. This study demonstrates the usefulness of drainage basins as tectonic markers in the quantification of regional strain and uplift, which may have wider applicability in other deforming parts of the world.