A Teleseismic Delay Time Study Across the Central African Shear Zone In the Adamawa Region of Cameroon, West Africa

Summary. P -wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the Adamawa Plateau and the Central African Shear Zone of central Cameroon, to determine the variation in crust and upper mantle velocity associated with these structures. A plot of the mean relative residuals for the stations shows a long wavelength (> 300 km) variation of amplitude 0.45 s. the slowest arrivals are located over and just to the north, of the faulted northern margin of the Adamawa Plateau. the residuals do not correlate with topography, surface geology or the previously determined crustal structure, in any simple way.
The Aki inversion technique has been used to invert the relative residuals into a 3-D model of velocity perturbations from a mean earth model. the results show the region is divided roughly into three blocks by two subvertical boundaries, striking ENE and traversing both the crust and upper mantle down to depths greater than 190km. the central block, which is 2 per cent slower than the adjacent blocks, roughly corresponds to the Central African Shear Zone. the Adamawa Plateau, as an individual uplifted area, is explained by the interaction of a regional anomalous upper mantle associated with the West African Rift System, and the Central African Shear Zone, which provided a conduit for heat flow to the surface.     

Last interglacial sea levels on the south coast of New South Wales

Remnants of the Last Interglacial shoreline occur at Middle Lagoon on the far south coast of New South Wales. Relict beach sediments can be traced to a height of at least +4.8 m and are indicative of a former mean sea level of about +3 m. Thermoluminescence (TL) ages of 126 ± 13 ka and 114 ± 15 ka were determined for beach and aeolian facies respectively. Sands in the lower part of an exposure on the adjacent Gillards Beach gave TL ages of 108 ± 13 ka, but sands in the upper part of that exposure gave an age of 19.9 ± 3.5 ka. This chronological evidence of a stratigraphic unconformity in what was initially taken as pedogenic differentiation at Gillards Beach is supported by contrasting electron traps and colour centres in crystal lattices of quartz grains in these two samples. No tectonic displacement is apparent. This site provides the first evidence of the Last Interglacial sea level for 1000 km along the coast between Gippsland and Newcastle.     

SUMMER MEASUREMENTS OF DISSOLVED ION CONCENTRATIONS IN ALPINE STREAMS,BLANCA PEAK REGION,COLORADO*

Chemical weathering is commonly assumed to be negligible in the denudation of alpine landscapes. Spot samples of dissolved ions from alpine streams on the Blanca Massif had minimum average concentrations in 1978 of 35.53 ppm (range: 14.9–75.6 ppm) and in 1979, 20.23 ppm (range: 7.35–35.0 ppm). Correlations between the variation in the concentration of dissolved ions (silica, pH, and minimum ion concentration) and the elevations and drainage-basin areas of sample sites were significant at the 0.05 level.     

Wind velocity and sand transport on a barchan dune

We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations.     

Geomorphology, Stratigraphy, and Radiocarbon Chronology of LlanquihueDrift in the Area of the Southern Lake District, Seno Reloncaví, and Isla Grande de Chiloé, Chile

Glacial geomorphologic features composed of (or cut into) Llanquihue drift delineate former Andean piedmont glaciers in the region of the southern Chilean Lake District, Seno Reloncav', Golfo de Ancud, and northern Golfo Corcovado during the last glaciation. These landforms include extensive moraine belts, main and subsidiary outwash plains, kame terraces, and meltwater spillways. Numerous radiocarbon dates document Andean ice advances into the moraine belts during the last glacial maximum (LGM) at 29,363–29,385 ¹⁴C yr BP , 26,797 ¹⁴C yr BP , 22,295–22,570 ¹⁴C yr BP , and 14,805–14,869 ¹⁴C yr BP . Advances may also have culminated at close to 21,000 ¹⁴C yr BP , shortly before 17,800 ¹⁴C yr BP , and shortly before 15,730 ¹⁴C yr BP . The maximum at 22,295–22,567 ¹⁴C yr BP was probably the most extensive of the LGM in the northern part of the field area, whereas that at 14,805–14,869 ¹⁴C yr BP was the most extensive in the southern part. Snowline depression during these maxima was about 1000 m. Andean piedmont glaciers did not advance into the outer Llanquihue moraine belts during the portion of middle Llanquihue time between 29,385 ¹⁴C yr BP and more than 39,660 ¹⁴C yr BP . In the southern part of the field area, the Golfo de Ancud lobe, as well as the Golfo Corcovado lobe, achieved a maximum at the outermost Llanquihue moraine prior to 49,892 ¹⁴C yr BP . Pollen analysis of the Taiquemmire, which is located on this moraine, suggests that the old Llanquihue advance probably corresponds to the time of marine isotope stage 4. The implication is that the Andean snowline was then depressed as much as during the LGM. A Llanquihue-age glacier expansion into the outer moraine belts also occurred more than about 40,000 ¹⁴C yr BP for the Lago Llanquihue piedmont glacier.