Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan Plateau

Thermochronological data from the Songpan-Ganze˛Fold Belt and Longmen Mountains Thrust-Nappe Belt, on the eastern margin of the Tibetan Plateau in central China, reveal several phases of differential cooling across major listric thrust faults since Early Cretaceous times. Differential cooling, indicated by distinct breaks in age data across discrete compressional structures, was superimposed upon a regional cooling pattern following the Late Triassic Indosinian Orogeny. ⁴⁰Ar/³⁹Ar data from muscovite from the central and southern Longmen Mountains Thrust-Nappe Belt suggest a phase of differential cooling across the Wenchuan-Maouwen Shear Zone during the Early Cretaceous. The zircon fission track data also indicate differential cooling across a zone of brittle re-activation on the eastern margin of the Wenchuan-Maouwen Shear Zone during the mid-Tertiary, between 38 and 10 Ma. Apatite fission track data from the central and southern Longmen Mountains Thrust-Nappe Belt reveal differential cooling across the Yingxiu-Beichuan and Erwangmiao faults during the Miocene. Forward modelling of apatite fission track data from the northern Longmen Mountains Thrust-Nappe Belt suggests relatively slow regional cooling through the Mesozoic and early Tertiary, followed by accelerated cooling during the Miocene, beginning at ca. 20 Ma, to present day.

Regional cooling is attributed to erosion during exhumation of the evolving Longmen Mountains Thrust-Nappe Belt (LMTNB) following the Indosinian Orogeny. Differential cooling across the Wenchuan-Maouwen Shear Zone and the Yingxiu-Beichuan and Erwangmiao faults is attributed to exhumation of the hanging walls of active listric thrust faults. Thermochronological data from the Longmen Mountains Thrust-Nappe Belt reveal a greater amount of differential exhumation across thrust faults from north to south. This observation is in accord with the prevalence of Proterozoic and Sinian basement in the hanging walls of thrust faults in the central and southern Longmen Mountains. The two most recent phases of reactivation occurred following the initial collision of India with Eurasia, suggesting that lateral extrusion of crustal material in response to this collision was focused along discrete structures in the LMTNB.     

Hydrochemical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK

In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ¹⁸O and δ²H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l⁻¹, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ¹⁸O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ¹⁸O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.     

Phytoplankton and particulate matter in Carlingford Lough, Ireland: An assessment of food availability and the impact of bivalve culture

In an attempt to assess the impact of bivalve culture in Carlingford Lough, Ireland, the seasonal cycles of nutrients, particulate matter, chlorophylla, and phytoplankton in the lough was investigated in 1992. Chlorophyll levels showed an increase in April, corresponding to the annual spring bloom, and levels remained relatively high (2–12 mg m^?3) throughout the summer before dropping to a winter minimum by December. Throughout the summer the phytoplankton community was dominated by diatoms, with microflagellates becoming an increasingly larger fraction of the biomass in autumn and winter. Dinoflagellates were only present on occasion in low numbers during the summer months. Seasonal variations in nitrate, phosphate, and silicate concentrations at all stations showed characteristic winter maxima and summer minima. Nitrate concentrations had reached a minimum undetectable level by June, at a time when the main freshwater input from the Clanrye River had dropped to <0.3 m³ s^?1. Particulate organic carbon (POC) composed approximately 5% of the suspended matter, with highest values in winter due to resuspension. Levels of biologically available POC, as determined by a modified BOD technique, were greatest in summer, and an inverse relationship was observed between total POC and its fraction that was biologically available. Most of the labile fraction was considered to be phytoplankton, and remineralization during the summer is suggested as a mechanism for maintaining high productivity during the summer months. Although the phytoplankton biology was uncoupled with that outside the lough, it is concluded that there is scope for expansion of the local bivalve mariculture industry without altering the ecosystem of the lough. The upper limit on such expansion would be set by practical considerations such as availability of space and site suitability due to water quality.     

Reviews

THE PERMAFROST ENVIRONMENT by S. A. Harris. 23 × 14 cm, viii and 275 pages. Croom Helm: London 1986 (ISBN 0 7099 3713 X) $A57.50 (hard).

ENVIRONMENTAL POLICIES: An International Review edited by C. C. Park. 14 × 22 cm, 315 pages. Croom Helm: London 1986 (ISBN 0 7099 2062 8) $A56.25 (hard).

A MODERN DICTIONARY OF GEOGRAPHY by J. Small and M. Witherick. 15 × 23 cm, vi and 233 pages. Edward Arnold: London 1986 (ISBN 0 7131 6434 4) $A28.95 (soft).

PROGRESS IN POLITICAL GEOGRAPHY edited by M. Pacione. 14 × 22 cm, 275 pages. Croom Helm: London 1985 (ISBN 0 7099 2087 3) $A56.25 (hard).

THE PRICE OF WAR: Urbanization in Vietnam 1954–1985, by N. Thrift and D. Forbes. 14 × 22 cm, xiv and 188 pages. Allen & Unwin: London 1986 (ISBN 0 04 3012108) $A59.95 (hard).

INDUSTRIALISATION AND LABOUR FORCE PROCESSES: A Case Study of Peninsular Malaysia (Research Papers on Development in East Java and West Malaysia No. 1) by T. G. McGee and seven others. 17 × 24 cm, xviii and 244 pages. Research School of Pacific Studies, Australian National University: Canberra 1986 (ISBN 0 86784 991 6) $A15.00 plus postage and packing (soft).

CITIES OF THE WORLD: A Compilation of Current Information on Cultural, Geographical, and Political Conditions in the Countries and Cities of Six Continents. Based on the Department of State's ‘Post Reports’ (second edition) edited by M. W. Young and S. L. Stetler. 16 × 23 cm, 3,056 pages in 4 volumes. Gale Research Company: Detroit 1985 (ISBN 0 8103 2059 2) $US235.00 (hard). Supplement 758 pages. 1986 (ISBN 0 8103 1110 0) $US84.00 (hard).     

Evidence for frost on Rhea's surface

We have observed Rhea (S5) at 1.6 μm and 2.2 μm at Mt. Wilson using the Caltech photometer on the 1.52m and 2.54m telescopes. The infrared spectral reflectances relative to 0.55μm are 0.8 (±0.1 p.e.) at 1.65μm and 0.6 (±0.1 p.e.) at 2.2μm. Such absorption bands in the near infrared are not consistent with spectra of most rocks or minerals; even carbonaceous chondritic materials have nearly flat reflectances over this spectral region. Frosts, however, have strong absorption bands in the 1–3μm region. In particular, the broadband infrared reflectances of Rhea are similar to those of the Galilean satellites Europa (J2) and Ganymede (J3) and also the rings of Saturn (all of which are known from high resolution scans to have water frosts on their surfaces). The broadband photometry does not have sufficient resolution to identify the frost species: but Rhea's low density, high albedo and relatively flat reflectance from 0.3μm to 1.1μm as well as the low infrared reflectances reported here are consistent with the presence of water ice on Rhea's surface.