The southern cape conductive belt (South Africa): Its composition, origin and tectonic significance

Magnetometer array studies have led to the discovery and mapping of the Southern Cape Conductive Belt (SCCB) crossing the southern tip of Africa from west to southeast coasts. The SCCB lies just south of the Namaqua-Natal Belt of cratonic rocks remobilized about 1000 m.y. B.P. It is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m.y. Relationships between the SCCB and the basement geochronology, geology and tectonics are considered in detail. These relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a Proterozoic subduction which stopped about 1000 to 800 m.y. B.P. Associated with this subduction we propose a Proterozoic range of Andean mountains, whose roots are now exposed in the Namaqua-Natal Belt. Later subduction further south, near the present south coast, is proposed to account for the intrusion, between the south coast and the SCCB, of the Cape Granites in the time interval 600-500 m.y. B.P. There is some evidence for a third, yet more distant, subduction episode off Permian Gondwanaland. After outlining this tectonic history, the paper turns to a closer examination of the hypothesis that the Southern Cape Conductive Belt consists of partly serpentinized basalt accumulated at the top of a Proterozoic subduction. A large static magnetic anomaly, which correlates with the SCCB over most of its length, is well fitted by a model which strongly supports this hypothesis. Bouguer gravity anomalies along western and central profiles likewise support the hypothesis. A discussion follows of the process of formation of the proposed block of serpentinized marine rocks, beginning with serpentinization of the crust near oceanic ridges by reaction of warm, porous, newly-extruded basalt with seawater convecting through it. The serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction, where it accumulates because of its low density. Examples of such accumulations are cited. Finally, it is shown that any iron in the olivine and pyroxene in the original basalt precipitates, when these silicates are hydrated to serpentine, as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites. In particular the Beattie static magnetic anomaly requires, in our model calculation, a very high susceptibility readily attainable in basalt at 15–20% serpentinization. The authors know of no other rock able to provide this high susceptibility. A similar percentage of serpentinization provides the density required to model the gravity anomalies.     

Freeze thaw cycles in Toronto, Canada in a changing climate

Summary Freeze thaw cycles are examined in Toronto Canada. Using data from 1960 to 1989 for three Toronto area weather stations, trends in freeze thaw activity, the relationship to mean monthly temperature and projections of freeze thaw activity are examined. For downtown Toronto the annual frequency of freeze thaw cycles is decreasing significantly, most notably in the shoulder months of October and April. At the Pearson International Airport and the Toronto Island Airport similar annual trends were not found, however there was evidence of decreased freeze thaw activity in April and October. Polynomial curve fitting provided functional relationships between mean monthly temperature and freeze thaw activity. These relationships enabled the assessment of freeze thaw activity under synthetic warming conditions. The results of this analysis show that the warming of the magnitude typically projected for the rest of this century will not likely generate a significant change in the freeze thaw activity although there are indications that the freeze thaw season will contract.     

Southern Quebec (Canada) summer-season heat spells over the 1941–2000 period: an assessment of observed changes

Summary Summer-season (May–September) daily maximum temperature (T _max) and daily minimum temperature (T _min) observations and three types of heat spells obtained from these temperature observations at seven weather stations located in southern Quebec (Canada) for the 60-year period from 1941 to 2000 are studied to assess temporal changes in their characteristics (i.e. frequency of occurrence, seasonal hot days and extremal durations of heat spells). Type-A and Type-B heat spells are obtained respectively from T _max and T _min observations and Type-C heat spells from simultaneous joint observations of T _max and T _min using suitable thresholds and spells of duration ≥1-day and ≥3-day. The results of this investigation show that the majority of the selected percentiles (i.e. 5P, 10P, 25P, 50P, 75P, 80P, 90P, 92P, 95P, and 98P) of T _max observations show a negative time-trend with statistically significant decreases (at 10% level) in some of the higher percentiles and in the maximal values at four out of seven stations. Almost all of the selected percentiles (same as for the T _max) and the maximal and minimal values of T _min observations show a positive trend, with statistically significant increases for all seven stations. Examination of frequencies of occurrence of heat spells, seasonal hot days and annual extremes of heat spell durations indicate that many of these characteristics of heat spells have undergone statistically significant changes over time at some of the stations for Type-A and Type-B heat spells as compared to Type-C heat spells. The Type-C heat spells are generally small in number and are found to be relatively temporally stable. More severe Type-C heat spells, i.e. the ones having T _max and T _min values simultaneously above very high thresholds and with duration ≥3-day have been rarely observed in southern Quebec.     

On the Power Distribution of Solar p MODES

We present a simple idealized model of stochastically forced solar oscillations, and compare the distribution function of energy, averaged over 12-hour intervals, with that of low-degree solar modes. We find that the simulated energy distribution is similar to the observations, except at extremely high energy.     

The geoid and single-cell mantle convection

The geoid shows an antisymmetric departure from the spheroid of best fit. A single zero-elevation contour divides its surface into nearly equal strips in one of which the elevation is everywhere positive and in the other everywhere negative. These two areas are interleaved roughly like the strips covering a tennis ball. This pattern may indicate global single-cell convection in the mantle. It is argued that on this convection hypothesis, the upcurrents underlie the low-geoid strip, although the opposite view could be supported. No simple relation is to be expected between the proposed whole-mantle convection and plate motions, because other constraints act on plates and because the asthenosphere will partially decouple the whole-mantle motions from the lithosphere. However, the proposed whole-mantle convective system is consistent with rapid northwestward motion of the Pacific plate, with fast spreading of the East Pacific Rise and with slow spreading of the North Atlantic Ridge. Seismological velocity anomalies in the mantle, while highly relevant to whole-mantle convection, do not at present decide for or against the hypothesis here advanced.     

Subglacial drainage system structure and morphology of Brewster Glacier,New Zealand

A global positioning system and ground penetrating radar surveys is used to produce digital elevation models of the surface and bed of Brewster Glacier. These are used to derive maps of subglacial hydraulic potential and drainage system structure using three different assumptions about the subglacial water pressure (P_w): (i) P_w = ice overburden; (ii) P_w = half ice overburden; (iii) P_w = atmospheric. Additionally, 16 dye‐tracing experiments at 12 locations were performed through a summer melt season. Dye return curve shape, together with calculations of transit velocity, dispersivity and storage, are used to infer the likely morphology of the subglacial drainage system. Taken together, the data indicate that the glacier is underlain by a channelised but hydraulically inefficient drainage system in the early summer in which water pressures are close to ice overburden. By mid‐summer, water pressures are closer to half‐ice overburden and the channelised drainage system is more hydraulically efficient. Surface streams that enter the glacier close to the location of major subglacial drainage pathways are routed quickly to the channels and then to the glacier snout. Streams that enter the glacier further away from the drainage pathways are routed slowly to the channels and then to the snout because they first flow through a distributed drainage system. Copyright © 2008 John Wiley & Sons, Ltd.