Comment on: 'The 3D shear experiment over the Natih field in Oman: the effects of fracture-filling fluids on shear propagation' by C.M. van der Kolk, W.S. Guest and J.H.H.M. Potters

We find errors in theory and application in the paper by van der Kolk et al. (Geophysical Prospecting 49 , 179–197 (2001)) that invalidate their arguments for the cause of the reduction in the velocity of the vertically propagating slower split shear-wave in the gas cap over the Natih field in Oman. We suggest that existing theories are adequate and can explain the anomaly.     

Discrete-element modelling of detachment folding

A two‐dimensional, discrete‐element modelling technique is used to investigate the initiation and growth of detachment folds in sedimentary rocks above a weak décollement level. The model depicts the sedimentary rocks as an assemblage of spheres that obey Newton's equations of motion and that interact with elastic forces under the influence of gravity. Faulting or fracturing between neighbouring elements is represented by a transition from repulsive–attractive forces to solely repulsive forces. The sedimentary sequence is mechanically heterogeneous, consisting of intercalated layers of markedly different strengths and thicknesses. The interlayering of weak and strong layers within the sedimentary rocks promotes the localization of flexural flow deformation within the weak layers. Even with simple displacement boundary conditions, and straightforward interlayering of weak and strong layers, the structural geometries that develop are complex, with a combination of box, lift‐off and disharmonic detachment fold styles forming above the décollement. In detail, it is found that the modelled folds grow by both limb rotation and limb lengthening. The combination of these two mechanisms results in uplift patterns above the folds that are difficult, or misleading, to interpret in terms of simple kinematic models. Comparison of modelling results with natural examples and with kinematic models highlights the complexities of structural interpretation in such settings.     

Pumpellyite in Low-grade Metamorphism

The regional metamorphism of the Kant? Mountains in Japan producedrocks of the following facies, with rising temperature: chloritefacies, pumpellyite-chlorite facies, glauco-phane-schist facies,and greenschist facies. This relationship is compared with theprogressive metamorphic zones in other regions where pumpellyitehas been found. Almost similar relations appear to hold in manymetamorphic terrains. Pumpellyites in glaucophanitic metamorphicterrains have, generally, low Fe'/R'R ratios. Physical andchemical conditions responsible for the formation of pumpellyiteare also discussed.     

Fluid inclusion studies of the Abawso gold prospect,near the Ashanti Belt,Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en échelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H₂O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H₂O and H₂O-CO₂ inclusions over CO₂ inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO₂ inclusions from a H₂O-CO₂ parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO₂±N₂ inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.Editorial handling: E. Frimmel     

Kinetics and Equilibrium Sorption Studies of 4-Nitrophenol on pyrolyzed and activated oil shale residue

Sorption of the organic pollutant 4-nitrophenol (4-NP) by pyrolyzed and activated Jordanian oil-shale was studied. Pyrolyzed oil shale was prepared using a fluidized bed reactor at 520 °C in the presence of nitrogen. Physical activation was carried out by treating the pyrolyzed oil shale with CO₂ at 830 °C, while chemical activation was achieved by using KOH and ZnCl₂ as impregnating agents. Batch kinetics and isotherm studies were conducted to evaluate the sorption process. Effects of contact time, initial sorbate concentration, sorbent concentration, temperature, pH and inorganic salts (NaCl and KCl) on the sorption process by the different sorbents were considered. Chemically activated oil shale, pretreated with ZnCl₂, gave the highest uptake of 4-NP. The isotherm experimental data fit reasonably well to Langmuir, Freundlich and Redlich-Paterson models. Three kinetic models, namely the Morris-Weber, Lagergren, and Pseudo-Second-Order model (PSOM), were applied to represent the experimental results for both pyrolyzed and ZnCl₂-oil shale sorbents.     

Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation

Samples of quartz-bearing rocks were heated above the α (trigonal)–β (hexagonal) phase transformation of quartz (625–950°C) to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron X-ray and electron backscatter diffraction. The trigonal–hexagonal phase transformation does not change the orientation of c- and a-axes, but positive and negative rhombs become equal in the hexagonal β-phase. In naturally deformed quartzites measured by neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable twinning and memory loss after the phase transformation. In experimentally deformed quartz rocks, where twinning was induced mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it does not account for the persistence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause of texture memory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal stresses are released during the phase transformation and the material returns to its original state prior to deformation. Similarly, on surfaces there are no tractions and thus texture memory is partially lost.