AN ASSESSMENT OF CLAST MACROFABRICS IN GLACIGENIC SEDIMENTS BASED ON A/B PLANE DATA

Previous studies of clast macrofabrics in glacigenic deposits have concentrated on A‐axis orientations and dips, and a variety of control samples are available based upon such measurements. Like clast A‐axes, A/B planes will also tend to rotate to parallelism with the direction of shear and therefore should also provide meaningful data on the direction and cumulative impact of shear by the depositing/deforming medium (i.e. glacier ice). The measurement of A/B plane dip and orientation avoids the potential problem of the transverse orientations observed for clast A‐axes and provides poles‐to‐plane data, thereby strengthening the modality of samples and providing clear visual impressions of stress directions. Such data also enable more significant inter‐sample comparisons of fabric strength and clast dip angles, which are significant when assessing the impact of shearing in sediment genesis. We present data on clast A/B plane dip directions and angles from subglacial tills, glacitectonite continuums, subglacially lodged clasts and glacimarine/glacilacustrine deposits using traditional methods of statistical and graphical macrofabric analysis. These sample sets will serve as control data for future macrofabric analyses that utilize A/B planes. The separation of the unequivocally lodged clast component from subglacial till samples allows us to demonstrate the influence of deformation and ploughing in the relative weakening of till fabrics as proposed by some researchers. High angles of A/B plane dip in glacigenic subaqueous deposits appear to be well developed in the glacilacustrine setting investigated here, confirming previous studies based on A‐axis dips, but less convincing in the glacimarine sediments of the Canadian Arctic, thereby widening the range of fabric strengths in subaqueous glacigenic deposits. Significant overlaps of A/B plane fabric shape envelopes reflect the strain history of subglacial and subaqueous depositional environments, which is unsurprising given the hybrid nature of glacigenic deposits, but the statistical isolation of the lodgement component from subglacial traction tills strongly suggests that the continuum of sample plots on modality/isotropy graphs reflects the range of strain histories in glacitectonites and subglacial traction till.     

Seismic and volcanic hazards in Peru: changing attitudes to disaster mitigation

Over the last 15 years there have been dramatic shifts in the consensus over how best to cope with natural hazards in economically developing regions such as South America. One very positive outcome of the United Nations sponsored International Decade for Natural Disaster Reduction (IDNDR 1990–2000) has been that there is now greater interchange between the work of earth scientists examining the processes and mechanics of hazard occurrence and impact, and social scientists exploring the causes of human vulnerability to hazard – and thereby disaster. This paper traces the development of this new understanding with reference to earthquakes and volcanic eruptions in Peru, one of the most hazardous countries in South America. Particular focus is placed on the excellent progress currently being made by scientists in better understanding the physical dimensions of natural hazard exposure, and the ground-breaking work by social scientists in promoting new approaches to understanding and mitigating human vulnerability to disaster. The paper concludes by emphasizing the need to build on this research to produce more inclusive, incultured and unified strategies of disaster mitigation at the local, national and international levels.     

Kilauea East Rift Zone Magmatism: an Episode 54 Perspective

On January 29–30, 1997, prolonged steady-state effusionof lava from Pu'u'O'o was briefly disrupted by shallow extensionbeneath Napau Crater, 1–4 km uprift of the active Kilaueavent. A 23-h-long eruption (episode 54) ensued from fissuresthat were overlapping or en echelon with eruptive fissures formedduring episode 1 in 1983 and those of earlier rift zone eruptionsin 1963 and 1968. Combined geophysical and petrologic data forthe 1994–1999 eruptive interval, including episode 54,reveal a variety of shallow magmatic conditions that persistin association with prolonged rift zone eruption. Near-ventlava samples document a significant range in composition, temperatureand crystallinity of pre-eruptive magma. As supported by phenocryst–liquidrelations and Kilauea mineral thermometers established herein,the rift zone extension that led to episode 54 resulted in mixtureof near-cotectic magma with discrete magma bodies cooled to     

Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia

The northern margin of the Inland Branch of the Pan-AfricanDamara Orogen in Namibia shows dramatic along-strike variationin metamorphic character during convergence between the Congoand Kalahari Cratons (M₃ metamorphic cycle). Low-P contact metamorphismwith anticlockwise P–T paths dominates in the westerndomains (Ugab Zone and western Northern Zone), and high-P Barrovianmetamorphism with a clockwise P–T path is documented fromthe easternmost domain (eastern Northern Zone). The sequenceof M₃ mineral growth in contact aureoles shows early growthof cordierite porphyroblasts that were pseudomorphed to biotite–chlorite–muscoviteat the same time as an andalusite–biotite–muscovitetransposed foliation was developed in the matrix. The peak-Tmetamorphic assemblages and fabrics were overprinted by crenulationsand retrograde chlorite–muscovite. The KFMASH P–Tpseudosection for metapelites in the Ugab Zone and western NorthernZone contact aureoles indicates tight anticlockwise P–Tloops through peak metamorphic conditions of 540–570°Cand 2·5–3·2 kbar. These semi-quantitativeP–T loops are consistent with average P–T calculationsusing THERMOCALC, which give a pooled mean of 556 ± 26°Cand 3·2 ± 0·6 kbar, indicating a high averagethermal gradient of 50°C/km. In contrast, the eastern NorthernZone experienced deep burial, high-P/moderate-T Barrovian M₃metamorphism with an average thermal gradient of 21°C/kmand peak metamorphic conditions of c. 635°C and 8·7kbar. The calculated P–T pseudosection and garnet compositionalisopleths in KFMASH, appropriate for the metapelite sample fromthis region, document a clockwise P–T path. Early plagioclase–kyanite–biotiteparageneses evolved by plagioclase consumption and the growthof garnet to increasing X_Fe, X_Mg and X_Ca and decreasing X_Mncompositions, indicating steep burial with heating. The developedkyanite–garnet–biotite peak metamorphic parageneseswere followed by the resorption of garnet and formation of plagioclasemoats, indicating decompression, which was followed by retrogressivecooling and chlorite–muscovite growth. The clockwise P–Tloop is consistent with the foreland vergent fold–thrustbelt geometry in this part of the northern margin. Earlier formed(580–570 Ma) pervasive matrix foliations (M₂) were overprintedby contact metamorphic parageneses (M₃) in the aureoles of 530± 3 Ma granites in the Ugab Zone and 553–514 Magranites in the western Northern Zone. Available geochronologicaldata suggest that convergence between the Congo and KalahariCratons was essentially coeval in all parts of the northernmargin, with similar ages of 535–530 Ma for the main phaseof deformation in the eastern Northern Zone and Northern Platformand 538–505 Ma high-grade metamorphism of the CentralZone immediately to the south. Consequently, NNE–SSW-directedconvergent deformation and associated M₃ metamorphism of contrastingstyles are interpreted to be broadly contemporaneous along thelength of the northern margin of the Inland Branch. In the westheat transfer was dominated by conduction and externally drivenby granites, whereas in the east heat transfer was dominatedby advection and internally driven radiogenic heat production.The ultimate cause was along-orogen variation in crustal architecture,including thickness of the passive margin lithosphere and thicknessof the overlying sedimentary succession. KEY WORDS: Pan-African Orogeny; P–T paths; pseudosections; low-P metamorphism; contact metamorphism; Barrovian metamorphism     

Abundance and Partitioning of OH in a High-pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa

Concentrations of OH, and major and trace elements were determinedin a suite of mantle-derived megacrysts that represent the crystallizationproducts of a kimberlite-like magma at     

The Late Devensian and Flandrian history of the Teith valley, Scotland

Two episodes of glaciation are identified in the Teith valley, central Scotland. During the earlier episode, the valley was wholly occupied by an ice sheet, but during the later episode, correlated with the Loch Lomond Readvance, ice only occupied the upper part of the valley. The deglaciation which followed each episode was marked in particular by sequences of kame and outwash terraces. A terrace related to the second episode grades into a large fan buried beneath a sequence of marine deposits in the nearby Forth valley. The latest of these, the carse clays, are related to Flandrian terraces in the Teith valley.     

FORTRAN PROGRAMME FOR THE ANALYSIS AND GRAPHICAL PRESENTATION OF GEOLOGICAL DATA

Computers are being used increasingly for the analysis of quantitative geological data. An outline is given of the advantages of contemporaneously using both computer and a connected automatic plotter. It is suggested that the quality of the automatic plotter output is such as to preclude the services of a draftsman in the preparation of many graphics for publication.