A complete Alpine cross section integrates numerous seismic reflection and refraction profiles, across and along strike, with published and new field data. The deepest parts of the profile are constrained by geophysical data only, while structural features at intermediate levels are largely depicted according to the results of three-dimensional models making use of seismic and field geological data. The geometry of the highest structural levels is constrained by classical along-strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units. Because the transect is placed close to the western erosional margin of the Austroalpine nappes of the Eastern Alps, it contains all the major tectonic units of the Alps. A model for the tectonic evolution along the transect is proposed in the form of scaled and area-balanced profile sketches. Shortening within the Austroalpine nappes is testimony of a separate Cretaceous-age orogenic event. West directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the Meliata-Hallstatt Ocean further to the east. Considerable amounts of oceanic and continental crustal material were subducted during Tertiary orogeny, which involved some 500 km of N-S convergence between Europe and Apulia. Consequently, only a very small percentage of this crustal material is preserved within the nappes depicted in the transect. Postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments, both detachments nucleating at the interface between lower and upper crust. Large scale wedging of the Adriatic (or Apulian) lower crust into a gap opening between the subduced European lower crust and the pile of thin upper crustal flakes (Alpine nappes) indicates a relatively strong lower crust and detachment between upper and lower crust.

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Geophysical‐geological transect and tectonic evolution of the Swiss‐Italian Alps

https://epic.awi.de/2393/1/petschick-etal_1996.pdf

Clay mineral distribution in surface sediments of the South Atlantic: sources, transport, and relation to oceanography

Clay mineral crystallinity and crystallite (domain) size data determined by X-ray diffraction (XRD) are methods extensively used in the characterization of very low-grade metamorphic conditions. However, the lack of sufficient interlaboratory standardization has made comparisons between different research groups unreliable due to significant variations in numerical results obtained, a consequence of the different machine conditions, measurement methods and sample preparations used during analysis. A calibration approach to the standardization of data using rock chip standards is presented, which allows data sets produced by different research groups to be directly and quantitatively compared. A standardized scale, the crystallinity index standard (CIS), is proposed, with illite crystallinity anchizonal boundary limits of 0.25dΔ2θ and 0.42dΔ2θ, and equivalent illite crystallite sizes of 52 and 23 nm, respectively, determined by the Warren-Averbach method. Calibrating both old and new data will enable more reliable comparisons between similar and contrasting geological environments, and should improve the accuracy and reliability of correlations made between XRD data and other indicators of very low-grade metamorphism, hence increasing the value of such clay mineral studies.

Interlaboratory standardization and calibration of day mineral crystallinity and crystallite size data

Terra Nova, 22, 354–360, 2010



Abstract

The Raman spectrum of carbonaceous material (CM) from advanced diagenesis (∼200 °C) to low-grade metamorphism (∼320 °C) is documented in the Helvetic flysch of the Glarus Alps (Switzerland). The spectrum is complex, with several defect bands at ∼1200 (D4), ∼1350 (D1), ∼1500 (D3) and ∼1620 (D2) cm−1. We document the evolution of these bands relative to the ‘graphite’ G band with increasing metamorphic grade, and we show that this qualitative evolution may be used as a proxy for temperature in the Glarus Alps. We develop a robust peak-fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region. Further work in other geological contexts is needed to assess whether the spectral evolution observed in the Glarus Alps may be generalized and to critically assess the potential for calibrating a general, empirical and quantitative thermometer based on the Raman spectrum of CM in low-grade rocks.

Evolution of the Raman spectrum of carbonaceous material in low‐grade metasediments of the Glarus Alps (Switzerland)

Foreland basin stratigraphy can be considered as the result of three interacting processes: thrust deformation, which builds the tectonic load, sedimentary and erosional processes which redistribute that load, and the flexural response of the lithosphere. The resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities. To understand the development of these unconformities, we present a simple model of these three processes, coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography. Our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere. The model describes the thrust wedge-foreland basin system in terms of four parameters: (1) the effective elastic thickness of the foreland plate (Te), (2) the sediment transport coefficient (K), (3) the thrust wedge advance rate, (4) the surface slope of the thrust wedge. The model is applied to the Oligocene-Miocene North Alpine Foreland Basin (NAFB) of eastern Switzerland. The stratigraphy of the NAFB can be simplified into two large-scale shallowing upward cycles separated by an unconformity at the base of the Burdigalian (22 Ma). Geological information is taken from the NAFB to estimate suitable values for the parameters listed above. Assuming a linear elastic lithospheric rheology, the Te value is estimated at 10±5 km from decompacted sediment columns. Data to constrain the sequential development of the thrust wedge come from structural geology. The early stages (40–24 Ma) of compression involved a relatively low-angle thrust wedge with an advance rate of approximately 2–4 mm/yr. At about 24 Ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate. The value for the transport coefficient has been estimated from previous studies. Prior to attempting to simulate the broad-scale geometry of the NAFB the role of each parameter was assessed individually. The values for Te and K are held constant throughout the simulation of the NAFB at 7.5 km and 500 m²/yr, respectively. The geometry of the base Burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge. The surface slope angle of the wedge is increased from 2.5° to 4° over 0.2 m.y., and the rate of thrust advance is decreased from 2 mm/yr to 0.2 mm/yr, during the thickening event between 24 and 23.8 Ma. The rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and, after a time lag, erosion of the distal stratigraphy over the forebulge, so simulating the base Burdigalian unconformity.

Simulation of Foreland Basin Stratigraphy using a diffusion model of mountain belt uplift and erosion: An example from the central Alps, Switzerland

Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mossbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods. With increasing metamorphic grade based on illite “crystallinity” data (XRD and IR) the following continuous changes are observed: (i) the 1Md→2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 μm decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age. These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra. Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.

P. Roggwiler

Papers

The evolution of illite to muscovite; mineralogical and isotopic data from the Glarus Alps, Switzerland