Rheology and geodynamic modelling: the next step forward

The application of continuum mechanics and microstructural analysis to geological studies over the past 30 years has spurred earth scientists to reassess fundamental tectonic processes such as subduction, collision and rifting in terms of dynamics. Armed with new analytical methods, geologists have returned to the field to look at rock structures with more mechanistic eyes. The advent of sophisticated computers, programs, and laboratory deformation equipment has facilitated the simulation of geodynamic processes that range in scale from the grain to the lithosphere. The result has been specialization, with the concomitant opening of communication gaps between geodynamicists, field geologists and rock mechanicists. Partly, these gaps reflect differences of perception and approach. In order to bridge these gaps, a workshop was organized after the DRM conference to debate how field and laboratory studies of deformed rocks can improve our understanding of lithospheric rheology, and in turn, how this understanding can be used to refine dynamic models of orogenesis. The workshop hosted participants with backgrounds in structural geology, experimental rock mechanics, metamorphic petrology and both numerical and analogue modelling. This paper summarizes the main controversies and conclusions reached during the workshop. For the sake of brevity, referencing in this summary is restricted to literature referred to during the oral presentations and to comments made by speakers themselves (names italicized).     

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted ⁴⁰Ar-³⁹Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a ⁴⁰Ar-³⁹Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma.     

Investigations on the Quaternary Glaciation in the Khumbu Himal (Nepal, East-Himalaya)

Based on geomorphological field work the mapping of former lateral/terminal moraines in the middle segment of the Bhotekoshi Nadi is quite similar to the geomorphological findings of Heuberger (1956: Fig. 2). The deductive calculation of the equilibrium line altitudes is oriented on the reconstructed types of former glaciers. Based on the geomorphological findings of this valley segment the relation of Neoglacial and Late Glacial ice margins of the glaciers of the tributary valleys of the Bhotekoshi Nadi is not conclusive. A post-late glacial maximum snow-line depression of at least 950 m has not obligatory caused an ice fill of the Bhotekoshi Nadi below 3200 m. This revised version was published online in July 2006 with corrections to the Cover Date.     

The natural enrichment of ferruginous weathering products and its implication for water quality in the Hunchun Basin, China

In order to evaluate the extent of a natural enrichment of Fe and Mn over the whole of the Hunchun Basin, the stability of Fe-Mn mineral phases, their status, the weathering rate of source materials, and soil characteristics were determined. A variety of samples were collected from sediments, soil profiles, and surface soils. In the solutions after a sequentially selective dissolution using sodium pyrophosphate (p), acid oxalate (o), and dithionite-citrate-bicarbonate (d), Si, Al and Fe were dominant in the d and p fractions, and Mn in the o and p fractions. The results showed that the existing phases and status of Fe and Mn were distinctly different, and that the abundance of amorphous Mn phases made thermodynamic calculation difficult. An application of these ferruginous weathering products was therefore made to understand basin development. The first terrace of the Hunchun River is more active than the second terrace, which can be inferred from it having a higher value in Fe_d/Fe_t. Amorphous Fe phases such as Fe(OH)₃ and Fe₃(OH)₈ were controlling factors of Fe in the thermodynamic calculations. The transformation of the amorphous Fe phases to more stable phases was hindered by high contents of smectite and gibbsite, amorphous opaline produced from rice roots, and organic phosphate pesticides or fertilizers in soils. Source-rock deduction using water samples suggested granitic weathering. Precipitation around the Hunchun Basin is gradually becoming more acidic. Therefore, in the future it is inevitable that ecological and environmental problems related to the Fe phases mentioned above will occur in the drinking water supply for this area, especially in that of the first terrace.     

Characterisation of tobelite (NH4)Al2[AlSi3O10](OH)2 and ND4-tobelite (ND4)Al2[AlSi3O10](OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

Tobelite (NH₄) Al₂ [AlSi₃O₁₀] (OH)₂, the ammonium analogue of muscovite, and its deuterated form ND₄-tobelite (ND₄) Al₂ [AlSi₃O₁₀] (OD)₂ have been synthesised at 600?°C and 200 and 500 Mpa using a well homogenised, stoichiometric SiO₂-Al₂O₃ oxide mix with Al₂O₃ in excess of 5 mol% and a 25% NH₃ solution whose relative abundance was such that the amount of NH₄ ⁺ stoichiometrically available was in excess of 50%. Characterisation of both tobelite and ND₄-tobelite using IR-spectroscopy, Rietveld refinement of X-ray powder diffraction data, and electron microprobe analysis indicate that, similar to K⁺ in muscovite, the NH₄ ⁺ or ND₄ ⁺ molecule occupies the interlayer site. IR absorption bands caused by NH₄ ⁺ and ND₄ ⁺ can be explained, to a very good approximation, on the basis of T_d symmetry. Nevertheless, substantial line broadening and the occurrence of shoulders indicate a deviation from ideal T_d symmetry. However, even at 77?K, no discrete splitting of the degenerate states could be confirmed. The OH stretching frequencies observed for synthetic tobelite are quite similar to those for muscovite, indicating that the replacement of K⁺ by NH₄ ⁺ has no effect. The low FWHH of the OH bands indicate that the hydroxyl groups are well ordered within the structure. Rietveld refinement of tobelite and ND₄-tobelite indicates that all samples synthesised consist of the 3 different mica polytypes which are typical of muscovite – namely 1M (C2/m), 2M ₁ (C2/c) and 2M ₂ (C2/c). Tobelite and ND₄-tobelite synthesised at 500 Mpa principally contain the 1M polytype, whereas the principle polytype for ND₄-tobelite synthesised at 200 Mpa, is 2M ₂. Rietveld refinement of X-ray diffraction spectra for tobelite synthesised at 200 Mpa was problematic due to the very broad FWHH of the X-ray peaks indicating poor crystallinity. In comparision to synthetic muscovite, the cell dimensions observed for tobelite and its deuterated analogue are quite similar except for the lattice constant c. Due to the larger radius of NH₄ ⁺ or ND₄ ⁺ compared to K⁺ cation, the c-direction is expanded form 10.275 Å in muscovite to approximately 10.540 Å in tobelite and ND₄-tobelite.     

Comparisons of xylem sap flow and water vapour flux at the stand level and derivation of canopy conductance for Scots pine

Summary Simultaneous measurements of xylem sap flow and water vapour flux over a Scots pine (Pinus sylvestris) forest (Hartheim, Germany), were carried out during the Hartheim Experiment (HartX), an intensive observation campaign of the international programme REKLIP. Sap flow was measured every 30 min using both radial constant heating (Granier, 1985) and two types of Cermak sap flowmeters installed on 24 trees selected to cover a wide range of the diameter classes of the stand (min 8 cm; max 17.5 cm). Available energy was high during the observation period (5.5 to 6.9 mm.day^–1), and daily cumulated sap flow on a ground area basis varied between 2.0 and 2.7 mm day^–1 depending on climate conditions. Maximum hourly values of sap flow reached 0.33 mm h^–1, i.e., 230 W m^–2.Comparisons of sap flow with water vapour flux as measured with two OPEC (One Propeller Eddy Correlation, University of Arizona) systems showed a time lag between the two methods, sap flow lagging about 90 min behind vapour flux. After taking into account this time lag in the sap flow data set, a good agreement was found between both methods: sap flow = 0.745^* vapour flux,r ² = 0.86. The difference between the two estimates was due to understory transpiration.Canopy conductance (g _c ) was calculated from sap flow measurements using the reverse form of Penman-Monteith equation and climatic data measured 4 m above the canopy. Variations ofg _c were well correlated (r ² = 0.85) with global radiation (R) and vapour pressure deficit (vpd). The quantitative expression forg _c =f (R, vpd) was very similar to that previously found with maritime pine (Pinus pinaster) in the forest of Les Landes, South Western France.With 6 Figures     

Disturbed vertical E×B plasma drifts in the equatorial F2 region at solar minimum deduced from observed NmF2 and hmF2 variations

Ground-based ionosonde and magnetic-field observations on the equatorial station Huancayo, ESRO4 neutral-composition measurements, and theoretical model calculations were used to analyze disturbed E×B vertical plasma drift during the phase of solar minimum in 1973. Vertical drifts calculated for disturbed days do not show the systematic decrease often mentioned in publications, and demonstrate strong dependence on IMF-B_z changes. It is confirmed with the help of our drift calculations that B_z turnings to a northward direction result in a decrease (up to reversal) of normal Sq (eastward during daytime and westward at nighttime) in the zonal component of electric field. Southward B_z excursions enhance normal E_y both in daytime and nighttime hours. Model predictions of E_y’s reaction to IMF-B_z changes are discussed.     

Calibration of regional pollen data to construct maps of former forest types in southern Sweden

A method was developed to construct maps of former forest types based on regional pollen data in southern Sweden. The considered species were Alnus, Betula, Carpinus, Corylus, Fagus, Fraxinus, Juniperus, Picea, Pinus, Populus, Quercus, Salix, Tilia and Ulmus. A network of 37 regional pollen sites with high data quality from lakes and peat deposits were selected from Sweden south of 60 ° N. Pollen percentage values were calculated and converted into estimates of tree composition. For controlling the reliability of the reconstruction, the estimates from the core-tops were compared with present day forest inventory data, and local pollen diagrams were compared with the regional pollen diagrams. An inverse distance weighted interpolation algorithm was used to generate maps for each tree species distribution at 2000 BP, 1500 BP, 1000 BP, 500 BP and 0 BP. A supervised classification routine was implemented to generate nine different forest types common to the five studied time intervals. The maps show that the amounts and patterns of distribution of the species and the forest types have varied in a significant but systematic manner through time. The changes are due to human activities, migrational patterns and changes in climate. These maps will be of value as a basis for future landscape planning, forestry and conservation of biodiversity.This is the 20th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.