Bas den Brok

Bas den Brok

PhD

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76
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Publications

Publications (76)
Book
Full-text available
Download the geological map of Elm as a vector file (tif-, twf-, ovr-format) here for free: https://cms.geo.admin.ch/ogd/geology/ga25-raster/products/173-Elm.zip *** study it online here (Geocover): https://s.geo.admin.ch/92d145e72e *** download it in PDF-format here: http://www.denbrok.ch/elm/01-GA25_1174_Elm_173_2021.pdf *** or buy a paper versio...
Book
Full-text available
Download explanatory notes in PDF format (full text) here: http://www.denbrok.ch/elm/01-GA25_1174_Elm_173_Erl.pdf *** Summary - The centerpiece of the UNESCO natural heritage Tectonic Arena Sardona is situated within the area of the map sheet Elm: the Glarus main thrust with the Tschingelhörner and the Martinsloch. In this region, the thrust is...
Data
Download full size tectonic overview map here: https://www.denbrok.ch/elm/03-GA25_1174_Elm_173_Tafel_II.pdf
Data
Download full size panoramas here: https://www.denbrok.ch/elm/04-GA25_1174_Elm_173_Tafel_III.pdf
Data
Full size PDF of cross sections can be downloaded here: https://www.denbrok.ch/elm/02-GA25_1174_Elm_173_Tafel_I.pdf
Article
Full-text available
Uplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test this hypothesis, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U–Th)/He (AHe), 15 fission-track (AFT), and four zircon (U–Th)/He (ZHe) sample ages. These data show differentia...
Article
Comment on: Ngako et al. 2008 (Pan-African tectonics in northwestern Cameroon: implication for the history of western Gondwana. Gondwana Research 14, 509–522.) --- The authors present a three-stage tectonic scenario highlighting N–S to NNE–SSW collision between the San Francisco Congo Craton (SFCC) and Eastern Saharan Block (ESB) tectonic indent fo...
Article
Full-text available
We present a geological map, profiles and the results of a detailed structural analysis of the Early Oligocene Engi Slates southwest of the village of Engi in the Sernft Valley in canton Glarus (Switzerland). In this area, the Engi Slates are folded on a deca- to hectometer scale into tight NW-vergent folds with sharp hinges. This took place during...
Article
Deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation (CPO) development. Cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol.% of added water and 20 mg of Mn2O3 powder in a Griggs solid medium deformation apparatus...
Article
Intracrystalline shear zones developed in a quartz single crystal that was experimentally deformed to 26% finite strain under conditions favourable for macroscopically ductile deformation. The crystal was uniaxially compressed parallel to 〈c〉, in the presence of 1 vol.% of added water, at a temperature of 800 °C, a confining pressure of 1200 MPa an...
Article
Full-text available
Den Brok & Jagoutz (2000) proposed that the Glarus Thrust would possibly have fol-lowed a pre-existing Miocene age erosional angular unconformity (Fig. 1; German: "Reliefüberschiebung"). Field work carried out in 2005 and 2006 in the area around the Landesplattenberg in Engi (Kanton Glarus) reveals that this angular unconformity may be found at the...
Article
Full-text available
Quartz single crystals were deformed by brittle processes in a diamond anvil cell at high confining pressure (800–1200 MPa) and room temperature in the presence of water: We investigated if quartz can be deformed in a diamond anvil cell (DAC) under an optical microscope with the aim of making in situ observations of quartz deformation processes at...
Conference Paper
Full-text available
Pressure solution is an important rock deformation mechanism, but a reliable pressure solution creep law does not exist. The major difficulty is that we do not understand the microstructure of grain boundaries in rocks that deform by pressure solution. It appears that the initially flat surface of a solid under stress is unstable and turns into a r...
Conference Paper
Full-text available
We aim at experimentally deforming quartz in the presence of water at high P-T conditions in an ETH-manufactured hydrothermal diamond-anvil cell (Miletich et al. 2000). Creep experiments (approximately constant load) will be carried out on disk-shaped quartz single crystals (~200 μm diameter; ~50 μm high) at a temperature of 800°C and a confining p...
Conference Paper
Full-text available
Experiments have been carried out to investigate lattice preferred orientation (LPO) development in quartz single crystals. Cylindrical samples of natural single crystals of Brazilian quartz were axially deformed in a Griggs solid medium deformation apparatus in the ductile deformation field. The experiments were performed at a temperature of 800°C...
Conference Paper
In the presence of water otherwise brittle materials may deform macroscopically ductile by water-assisted cataclastic creep. This is possible as long as (i) solubility is high enough, so that stress-corrosion can occur, and (ii) local stress is low enough, so that fracturing remains subcritical. Water-assisted cataclastic creep (WACC) may play an i...
Article
It is commonly assumed that the Yaounde nappe was deformed and emplaced onto the Congo craton during two compressional Pan-African deformation phases (D1 and D2). A detailed field and microstructural study of the mica schists, gneisses and pyriclasites of the Yaounde nappe in the Yaounde area was carried out and it appeared, however, that the D2 de...
Conference Paper
Full-text available
Minerals deformed ductile are often broken-up in numerous small 'subgrains', especially in case of quartz when deformed under Greenschist facies metamorphic conditions. Prior to about the nineteen sixtees, such subgrains were seen as a product of cataclastic deformation, of 'crushing' of the minerals. The subgrains were assumed to be slightly rotat...
Conference Paper
Full-text available
An experimental study has been carried out to investigate lattice preferred orientation (LPO) development in quartz single crystals. Cylindrical samples of natural single crystals of Brazilian quartz were axially deformed in a Griggs solid medium deformation apparatus in the ductile deformation field. The experiments were performed at a temperature...
Article
Theory and experiments have demonstrated that the initially flat surface of an elastically strained solid is morphologically unstable. The elastic strain energy of a rough, corrugated surface is lower than that of a flat one. Hence, stress forces the surface into a rough structure, but the associated increase in surface energy counteracts this roug...
Conference Paper
Full-text available
In tunnels cutting through anhydrite-rich rocks gypsum aggregates and fibrous gypsum veins are reported to develop soon after water gains access. The growth of the gypsum veins is associated with the build-up of differential stresses up to several MPa and may lead to large distortions. In the nature, fibrous gypsum veins are observed to develop und...
Conference Paper
Full-text available
It has been shown in theory (e.g., Srolovitz 1986, Leroy and Heidug 1994) that the nominally flat surface of an elastically stressed solid is instable with respect to the growth of perturbations with wavelengths (λ) greater than a critical wavelength (λc). For λ>λc the energy of the system may be lowered by the formation of a 'rough' surface of hig...
Conference Paper
Full-text available
The influence of gradients in bulk elastic strain energy on the dissolution and growth behaviour of minerals in rocks is commonly considered negligible. We experimentally observed, however, that regular arrays of macroscopically visible etch grooves may develop on the originally smooth free surfaces of soluble crystals held in an undersaturated aqu...
Article
The influence of gradients in bulk elastic strain energy on the dissolution and growth behavior of minerals in rocks is commonly considered negligible. We experimentally observed, however, that regular arrays of macroscopically visible etch grooves may develop on the originally smooth free surfaces of soluble crystals held in an undersaturated aque...
Conference Paper
Full-text available
Two types of in-situ deformation experiments were carried out at room P-T conditions on a very soluble elastic/brittle salt (sodium chlorate; NaClO3) used as rock analogue material under conditions where dissolution/precipitation processes and water-assisted subcritical microcracking are the dominant deformation processes. 1-Dead weight indentation...
Article
Recent experimental studies carried out on the kinetics of crystal growth in solution have revealed that mechanically induced elastic strain can have a strong influence on the crystal growth rate. We present experimental evidence to show that it can also strongly affect the dissolution rate. Sodium chlorate crystals held in an undersaturated sodium...
Article
Crystallographic preferred orientations (CPOs) in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms, where deformation is achieved by the movement of dislocations. In this paper we investigate the possibility of CPO-development by dissolution–precipitation creep or pressure solution. A numerical model...
Conference Paper
Full-text available
In Gesteinen, die bei schwach-bis mittelgradiger Metamorphose deformiert wurden, sind Mikrostrukturen, wie z.B. undulöse Auslöschung, Subkörner und Rekristallisate, typisch. Sie werden i.a. als das Resultat eines kristallplastischen Deformationsprozesses interpretiert. Das häufige Auftreten dieser Strukturen führte zu der Annahme, daß kristallplast...
Conference Paper
Full-text available
Solution transfer creep (STC) is an important natural compaction mechanism and may also play an important role during production-induced reservoir rock compaction. Commonly, STC is assumed to be driven by differences in grain boundary surface normal stress. However, P. Bjørkum has proposed that STC in quartz sandstone may also be driven by increase...
Conference Paper
Full-text available
Two types of experiments were carried out at room temperature on very soluble elastic/brittle salts used in our laboratory as a rock analogue material for in-situ microscopy study of dissolution/precipitation deformation processes ("pressure" solution, cataclastic solution creep): 1) The effect of elastic strain on the microstructure (micro topog...
Conference Paper
Full-text available
It is widely believed that the Glarus nappe complex in the Helvetic Alps in Switzerland is thrusted as an essentially rigid block over the-also essentially rigid-infra-Helvetic footwall block. At the thrust contact, more than 40 km displacement is thought to be localized in a 25-200 cm thin limestone layer (the famous Lochseiten limestone). Martin...
Article
Sodium chloride (NaCl) has been extensively used as a material to develop, test and improve pressure solution (PS) rock deformation models. However, unlike silicate and carbonate rocks, NaCl can deform plastically at very low stresses (∼0.5 MPa). This could mean that NaCl is less suitable for use as an analogue for rocks that do not deform plastica...
Conference Paper
Full-text available
Uniaxial dead-weight creep experiments were carried out on wet synthetic polycrystalline aggregates of sodium chlorate (NaClO 3) at room temperature and atmospheric pressure. Aim of the experiments was to study the deformation mechanism and resulting microstructures in wet rock analogues that are loaded below the plastic limit within the pressure s...
Conference Paper
Full-text available
Pressure solution indentation experiments were carried out on single crystals of an elastic-brittle salt (potassium-or K-alum) having a high solubility and fast dissolution/growth rate. The experiments were carried out to study why pressure solution indentation in this brittle material is several orders of magnitude faster than in rock salt, wherea...
Conference Paper
Full-text available
A series of in-situ deformation experiments were carried out to study (micro-scale) ductile shear zone development under conditions where dissolution-precipitation processes and water-assisted subcritical microcracking are the dominant deformation processes. Sodium chlorate (NaClO3) was used as a rock analogue material. This is a brittle salt, with...
Conference Paper
Elastic strains are commonly regarded to have a negligible effect on growth and dissolution rate, especially compared to crystal-plastic strain (crystal defects). Recent experimental work by Ristic et al. (1997) on K-alum and sodium chlorate has shown, however, that (tensile) elastic strain may have a very strong effect on growth and dissolution ra...
Article
Grain boundary migration (GBM) was studied in-situ at room temperature, atmospheric pressure and an applied diffmfwerential stress of ∼9.5 MPa under the optical microscope, in a wet aggregate of an elastic-brittle salt (sodium chlorate). The aggregate was previously deformed predominantly by a combination of grain boundary sliding, pressure solutio...
Article
Until about the late 1960s, macroscopically ductile deformation of quartz was seen as a microscopically cataclastic process by most geologists (cf. the origin of the name `mylonite'). Undulatory extinction, subgrains, recrystallised grains and even crystallographic preferred orientations were interpreted as due to water-assisted brittle deformation...
Article
Different, but reasonable and well-accepted assumptions made about grain-boundary structure during pressure-solution (PS) creep may easily have an effect of more than 10 orders of magnitude on the calculated PS deformation rate. Understanding of grain-boundary structure during PS creep is therefore extremely important. Experimental evidence is pres...
Article
Intense fibrosity develops in wet porous NaCl crystal aggregates (grain size 250-500 μm) held in a temperature (T) gradient field (0.5-4°C/mm) at temperatures between 20 and 50-60°C. In situ microscopic observation of the process shows that fibre growth is associated with T-gradient driven motion of tiny gas (air, water vapour) bubbles present in t...
Article
Many ductile shear zones are interpreted to operate by simple shear flow but some form under other flow regimes. Lineations and foliations in such shear zones can lie obliquely to those in simple shear zones, which can lead to erroneous tectonic interpretations on the assumption of simple shear flow. This paper describes a gently dipping shear zone...
Article
The structural analysis of a high-strain zone developed in medium- to high-grade metamorphic micaschists from the Cap de Creus area, Spain provides an example of the complex relationships between geometry, strain and kinematics to be found in deep crustal shear zones. This high-strain zone is composed of E-W trending structural domains characterize...
Article
Full-text available
In this article, 'we report a new set of procedures to fabricate synthetic analogues of granular rocks. These procedures permit accurate control of the most important structural parameters (i.e., grain size, porosity, cement content>. We were thus able to prepare two varieties of synthetic sandstones in which only the cement content significantly ....
Conference Paper
Full-text available
We want to experimentally investigate in situ the deformation of polycrystalline aggregates of brittle salts as rock analogues in the pressure solution deformation regime. We are specially interested in the deformation microstructures developed in an aggregate consisting of relatively large (porphyro-)clasts in a fine grained matrix. At present, we...
Article
Full-text available
In the Namurian strata at the Namur citadelle (Belgium) the macroscopic morphology of small-scale folds has been described as convolute bedding resulting from an Early Variscan "soft-sediment" deformation, i.e. by deformation of unconsolidated sediment. However, a microscopic study shows that these small-scale folds have an axial plane pressure sol...
Article
In a paper entitled “Ductility of garnet as an indicator of extremely high temperature deformation”, Ji & Martignole (1994) argue that garnets were flattened substantially by dislocation creep in the Morin shear zone (Grenville Province, Quebec, Canada) and that the temperature during deformation must therefore have exceeded 900°C. Ji & Martignole...
Article
By measuring the degree of flattening and the orientation of the c-axis of single quartz grains in a naturally deformed (“cleaved”) sandstone, Becker (1995) showed that: “quartz grains with a small angle between c-axis and the Z-axis of shortening exhibit the least amount of pressure solution, whereas grains with c-axes oriented at about 50” to Z m...
Conference Paper
Full-text available
The potential energy difference (∆U) across the rate controlling step during pressure solution (PS) is commonly assumed to be approximately equal to the mechanical work (W) done on the solid during deformation. This assumption is essential to all of the PS rate laws published in the literature to date. However, this assumption may be fine for rever...
Conference Paper
Full-text available
Little is known about micro physical and chemical processes during fluid assisted grain boundary migration (FAGBM) in rocks. Yet it is a very important process during ductile deformation in the middle and the lower continental crust. FAGBM probably significanty affects the rheological behaviour, has a large effect on the development of the microstr...
Conference Paper
Full-text available
Within the framework of irreversible thermodynamics, the driving force ( X) for stress induced dissolution at interfaces containing a free fluid phase can be expressed as: X = { ƒs + α (σn/ρs ) } − μ where ƒs and ρs are the Helmholtz free energy and the density of the differentially stressed solid phase respectively, σn is the interfacial normal...
Article
To test whether the weakening effect of added water in ductile deformation experiments on quartzite at high P-T conditions is associated with measurable changes in intragranular water content (c(sub w) we determined Fourier trnasform infrared (FTIR) spectra of single grains (grain size 150-250 micrometers; IR spot size approximately 70 um) in sampl...
Conference Paper
Full-text available
In deformation experiments performed with a Griggs-type solid-medium apparatus on natural quartz mono-and polycrystals (grain size 150-250 µm) at a temperature of 800°C, a confining pressure (Pc) of 1200 MPa, a strain rate of 10-7 s-1 , finite uniaxial compressive strains in the range 0.1-0.5, and in the presence of ~1 vol-% of added water (at a pr...
Technical Report
Full-text available
Why make a SYNTHETIC sandstone? To study the petrophysical properties of reservoir rocks, it is important to be able to vary ONE petrophysical property only, to find out the dependence on this specific property. In nature it is very difficult to find rocks in which only one property varies while other properties remain the same. Rocks are needed i...
Conference Paper
Full-text available
The question whether natural quartz can deform by dislocation creep (at appropriate chemical and physical conditions) might seem pretty much outdated to most structural geologists nowadays. Dislocation creep is widely believed to be one of the most important ductile deformation mechanisms in quartz rocks in the continental crust. This seems to be s...
Conference Paper
Full-text available
Small amounts of water have a large effect on the ductile flow behaviour of quartzite in experiments, and cause a significant mechanical weakening effect. The mechanism responsible for this "water-weakening" effect remains, however, poorly understood. It is commonly assumed, that added water dissolves in the crystal lattice as water-related point d...
Thesis
Full-text available
While it is well known from experimental work that small amounts of water (typically <1 vol%) greatly enhance the ductility of quartz rocks and cause a major mechanical weakening effect, the mechanism by which this occurs is poorly understood and the associated theological behaviour is poorly constrained. This presents a serious problem with regard...
Conference Paper
Full-text available
Deformation features frequently observed in deformed quartz rocks, such as undulatory extinction, deformation bands, subgrains and recrystallized grains, are commonly believed to be typical crystal-plastic deformation microstructures. However, quite similar deformation microstructures have now been produced experimentally in quartzite by stable ext...
Article
Natural quartzites have been experimentally deformed at a temperature of 1073 K, a confining pressure of 1.2 ±0.1 GPa, strain rates of 10-5, 10-6 and 10-7 s-1 and in the presence of 0.4 wt% of added water. Microstructures after deformation indicate that at strain rates of 10-5- 10-6 s-1 crystal plastic deformation is predominant. This is evident fr...
Technical Report
Full-text available
On the 23rd of March 1989 Martin Fleischmann and Stanley Pons of the University of Utah in Salt Lake City (U.S.A.) declared in a press conference that they had discovered a new source of energy for the future: cheap, easy, clean and unlimited. They went on to describe summarily their experiments involving electrolysis of heavy water (D 20) using a...
Conference Paper
Full-text available
The present paper reports an experimental investigation into the water-weakening of quartzite. The results indicate that the action of water is to enhance solution-precipitation creep (SPC) and microcracking (MC), rather than dislocation creep. This casts doubt on the widespread belief that water-weakening is an intracrystalline effect. Furthermore...
Conference Paper
Full-text available
A detailed experimental/microstructural study has been carried out to determine whether some of the water weakening effects seen in experimentally deformed natural quartzites might be explained by intercrystalline diffusional creep processes. Quartzite specimens (6mm diameter, grain size ~200 μm) were weld-sealed in Au-capsules with 0.4 wt-% of add...
Conference Paper
Full-text available
It is widely accepted in the literature that experimentally observed water weakening phenomena in quartz are due to the influence of a Hydrogen-Related Crystal Defect (HRCD) on intra-crystalline plasticity. However, the weakening effects observed in experiments on natural crystals deformed in the presence of water cannot be unambiguously related to...
Conference Paper
Full-text available
To determine if diffusional creep might explain some of the water weakening effects seen in expe-rimentally deformed polycrystals, we have conducted a detailed experimental/microstructural study. Quartzite specimens (6 mm in diameter, grain size ~200 μm) were weld-sealed in Au-capsules with 0.4wt-% water added and deformed in a Griggs rig (with NaC...
Article
Full-text available
Four deformation phases have been distinguished in the Cambro-Ordovician metasediments of the eastern Lys-Caillaouas massif. D1 only affected rocks of a stratigraphic unit below and Ordovician metaconglomerate horizon and is of pre-Variscan age. During D2 tight folds with steep E-W trending axial plane foliations S2 have formed, indicating N-S shor...
Article
Full-text available
The eastern Pyrenees are reported to show evidence of an angular unconformity between the Cambrian metasedimentary series of Canaveilles and Jujols and the Upper Ordovician Rabassa conglomerate (Llopis Lladó, 1965; Santanach Prat, 1972, 1974; Laumonier & Guitard, 1986; Laumonier, 1987, in press). Bedding within the Cambrian rocks trends obliquely t...

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Projects (3)
Project
The topography seen today throughout the Al Hajar Mountains is difficult to explain. Paleocene shallow marine sedimentary rocks are found at 2000 m above sea level on the flanks of the mountains. This suggests that the topography is younger than 40 Ma. Therefore, it is unlikely that the mountains were generated by the Late Cretaceous ophiolite obduction. The collision between the Arabian and Eurasian plates is often used to justify the uplift of the Al Hajar Mountains. This is demonstrated in the northwest region of the mountains at the Musandam Peninsula. Here, the mountains are uplifting due to continental collision. However, in the central mountains (where the highest peak is found), east of the Musandam Peninsula, the Arabia-Eurasia plate boundary transitions into a subduction zone (Arabia oceanic crust). The central mountains are 200 km outboard of this boundary. Mountainous topography forming on a subducting plate has not been documented before. It appears that the Arabia-Eurasia collision is not the driving force behind the uplift of the Al Hajar Mountains. The Oman area must have a much more complicated tectonic history than is currently described. Our goal is to constrain the uplift history of the Al Hajar Mountains.
Project
Geological mapping of Sheet 1174 Elm 1:25000 in the Glarus Alps for the Swiss Geological Survey (Swisstopo, Landesgeologie). The mapping area includes the UNESCO World Heritage site Swiss Tectonic Arena Sardona with the Glarus thrust.
Project
Orogenic processes, West-Gondwanaland reconstruction, and Metallogenesis