Judith Bunbury

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. Along the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

THE SIGNIFICANCE OF HIMALAYAN RIVERS FOR SILICATE WEATHERING RATES : EVIDENCE FROM THE BHOTE KOSI TRIBUTARY

The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/86Sr ratios of their dissolved load results from the mixing of a small component ( 90%) of carbonate-derived material some of which is characterised by a high 87Sr/86Sr ratio (up to 0.8). Elevated 87Sr/86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.

Fluxes of Sr into the headwaters of the Ganges

Himalayan weathering is recognized as an important agent in modifying sea water chemistry, but there are significant uncertainties in our understanding of Himalayan riverine fluxes. This paper examines causes of the variability, including that of the seasons, by analysis of downstream variations in Sr, 87Sr, and major ions in the mainstream, in relation to the composition of tributary streams from subcatchments with differing geologic substrates. Water samples were collected over four periods spanning the premonsoon, monsoon, and postmonsoon seasons. Uncertainties in the relative fluxes have been estimated, using Monte Carlo techniques, from the short-term variability of mainstream chemistry and the scatter of tributary compositions. The results show marked seasonal variations in the relative inputs related to high monsoon rainfall in the High and Lesser Himalaya, contrasting with the major contribution from glacial melt waters from the Tibetan Sedimentary Series (TSS) at times of low rainfall. Much of the spread in previously published estimates of the sources of Sr in Himalayan rivers may result from these seasonal variations in Sr fluxes. The annual fluxes of Sr into the headwaters of the Ganges are derived from the three main tectonic units in the proportions 35 ± 1% from the TSS, 27 ± 3% from the High Himalayan Crystalline Series (HHCS), and 38 ± 8% from the Lesser Himalaya. The particularly elevated 87Sr/86Sr ratios characteristic of the HHCS and the Lesser Himalaya enhance their influence on seawater Sr-isotope composition. The TSS contributes 13 ± 1%, the HHCS 30 ± 3%, and the Lesser Himalaya 57 ± 11% of the 87Sr flux in excess of the seawater 87Sr/86Sr ratio of 0.709.

Controls on the 87Sr/86Sr Ratio of Carbonates in the Garhwal Himalaya, Headwaters of the Ganges

The episodic variation of the seawater 87Sr/86Sr ratio has been attributed to either variations in the Sr flux or the Sr‐isotopic composition of the riverine‐dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentrations of dissolved Sr concentrations with high 87Sr/86Sr ratios has raised the possibility that collisional orogens play a critical role in moderating the variations in seawater 87Sr/86Sr ratios. Here we describe Himalayan carbonates and calc‐silicates from Garhwal, the headwaters of the Ganges, with extreme 87Sr/86Sr ratios (>1.0). Elevated Sr‐isotope ratios result from exchange with Rb‐rich silicate material during both Himalayan and pre‐Himalayan metamorphic episodes, and the carbonates contribute a significant fraction to the Ganges 87Sr flux. Particularly elevated 87Sr/86Sr ratios are found in calc‐silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc‐silicates from this unit confirms that carbonate horizons have increased 87Sr/86Sr ratios as a result of isotopic exchange over length scales of 10–30 cm. We conclude that metamorphism of carbonates may cause elevation of their 87Sr/86Sr ratios and that uplift of metamorphosed carbonates may be a consequence of collisional orogens, which contributes to the elevation of seawater 87Sr/86Sr ratios.

Interstitial magmatic scapolite in glass-bearing crystalline nodules from the Kula Volcanic Province, Western Turkey

Abstract Scapolite oikocrysts, up to 3 cm in diameter, occur in cognate, cumulate nodules from the 2 Ma riftrelated, alkali-basaltic Kula Volcanic Province in Western Turkey. Three scapolite-bearing nodules were found within deposits originating from two adjacent cones. They contain primocrysts of clinopyroxene, biotite, apatite and titanite, together with interstitial kaersutite, biotite, plagioclase and scapolite. Kula scapolites fall mid-way between the meionite (Ca4[Al6Si6O24]CO3) and marialite (Na4[Al3Si9O24]Cl) end-members (~Me45-71). We believe this to be the first report of interstitial magmatic scapolite.

Real and Literary Landscapes in Ancient Egypt

During the past thirty years the Survey of Memphis and others have acquired more than two hundred borehole logs from the Capital Zone of Egypt. Combining these boreholes with maps and satellite images, we show that, during the past five thousand years, the geography of the Nile has been in constant flux with mean rates of migration around 2 m/y and one of its channels becoming extinct, by nature or through human intervention. Re-visiting ancient texts in the light of this changing environment, we show that the literary settings of both fictional and historical texts were real landscapes known to the authors. Hence we infer that ancient descriptions of landscape can be interpreted in a more literal way than before and that the authors were not as prone to writing of a metaphorical realm as was previously thought.

Survey and excavation at the Gebel El-Asr gneiss and quartz quarries in Lower Nubia (1997-2000)

Presentation des fouilles effectuees dans les carrieres de gneiss et de quartz a Gebel el-Asr (basse Nubie) de 1997 a 2000. Les AA commentent les decouvertes (steles, jarres de stockage...).

Geomorphological Development of the Memphite Floodplain Over the Past 6,000 Years

Abstract The Memphite ruin mounds around the modern town of Mit Rahina in northern Egypt form a part of a region around which the capital of Egypt mi grated through time. Some of these migrations were the responses to landscape changes and the area is one that is subject to a number of types of landscape change. The delta and river systems as well as the deserts that surround Memphis changed profoundly as global temperatures rose at theend of the last ice age. This paper summarises the main landscape processes that affected the area and pro poses a model for river migration and delta-head change in the Memphite floodplain.