Microfabric evolution in pure and impure carbonate mylonites
and their role for strain localization in large-scale shear zones


Supervisor: PD Dr. Marco Herwegh and Prof. Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Switzerland

Summary

My Phd-thesis deals with the microfabric evolution of polyphase carbonate mylonites formed under physical conditions varying in time, and the role of microfabric evolution associated with large-scale thrust systems. For this purpose, samples of different stratigraphic units collected from profiles across large-scale shear zones were analyzed. The shear zones pertain to the Helvetic nappes in Switzerland (Morcles, Diablerets, Gelli-Wildhorn, Doldenhorn and Glarner nappe). The sample series contain metamophic gradients from anchizonal to lower greenschiest facies conditions going from frontal to rear parts. Quantitative geochemical (trace elements, isotopes), microstructural (e.g. grain size, shape preferred orientation (SPO)) and textural (crystallographic preferred orientation (CPO)) analyses were performed on sampled calcite mylonites consisting of variable contents of second-phase minerals.
In this study all calcite microfabrics are characterized by dynamic steady state fabrics that are controlled by (a) phase compositions and geochemistry of the mylonites like second-phase type and content, calcite chemistry, and synkinematic fluids, (b) physical conditions during deformation such as temperature, stress, and strain rate, and (c) their changes in space and time inducing strain localization and annealing. Since crustal rocks are rarely monomineralic, point (a) is particularly important. In case of carbonates, nano- and micro-scale second-phase particles (carbon flakes, sheet silicates, quartz, dolomite and others) as well as chemical impurities (e.g. Mg, Mn, Fe, Sr) are typical. At constant physical conditions, second-phase particles influence the mean grain size, grain size distribution, axial ratio, SPO and CPO of calcite. This thesis shows that only 2vol% of second-phase minerals are sufficient to completely change the calcite microfabric by pinning of grain boundaries. For example, mylonites with volume fractions (fp) of 40vol% second phases display calcite grain sizes (Dcc) half that of pure ones . The pinned average grain size of calcite (Dcc) can be expressed by the relationship Dcc=c*(dp/fp)m, where c is a temperature dependent constant, m=0.3, dp the grain size and fp the volume fraction of second phases . Moreover, second-phase controlled calcite aggregates are characterized by enhanced grain elongation, strong SPO, and CPO that is weaker and rotated in respect to the shear zone . Despite differences in synkinematic fluid flow and calcite chemistry along the thrust and between different nappes, calcite microfabrics are independent of differences in fluid flow and chemistry.
While the deformation microfabrics of calcite and second phases are controlled by a balance of nucleation, grain growth, pinning and grain size reducing mechanisms (dynamic recrystallization), physical parameters control and modify these microfabrics. With increasing metamorphic conditions from the diagenesis-anchizone transition to lower greenschist facies conditions, the grain size increases by a factor of 10 . Despite variations in strain rate between the nappes (10-11 to 10-12 s-1), microstructures display similar increases in grain sizes of both calcite and second phases towards higher temperatures for all nappes. This coupled grain growth of matrix and second phases can be expressed by grain coarsening maps for the microstructures investigated in this study. Such maps are based on the grain sizes of calcite and second phases, their volume fractions and estimates on the deformation conditions . Interestingly, the relationship between logDcc and 1/T in a typical Arrhenius-type diagram is non-linear and differs for second-phase (impure) and recrystallization (pure) controlled microstructures. This points to a competition between grain growth and grain size reduction that changes with temperature/stress. For future work, the new, calibrated grain coarsening maps provide an opportunity to predict microfabric changes in the Earth’s crust as function of increasing depth.
In light of large-scale thrust systems, it is important to keep in mind that extrinsic physical parameters change with time during shear zone evolution. Based on studies on the Glarus thrust, we are able to demonstrate that with ongoing deformation under retrograde conditions, strain localized progressively in the center of the shear zone. The decrease in shear zone width is manifest by a drastic decrease in grain size within the last meter from the thrust contact and . This fact supports the importance of temperature/stress to control the balance of grain growth and grain size reducing mechanisms, while the growth component decreases towards lower temperature conditions. In contrast to the recrystallized grain size, the densities of calcite twins increase . The CPO of calcite shows strongest textures at distances of 0.5-7m from the thrust and weaker ones in the direct vicinity to the thrust. This variability in microfabrics indicates that deformation mechanisms change with time and physical conditions from predominant dislocation creep to granular flow. The changes in microfabrics correlate with variations in the stable isotope compositions and suggest that fluids were present during deformation, a fact also manifest by different generations of recrystallized veins. We conclude that the enhanced isotopic exchange within the shear zone center is favored by deformation during localization involving processes like dynamic recrystallization (particularly grain boundary migration), grain size reduction and a change in deformation mechanisms leading to smaller grain sizes, higher permeabilities and higher finite strains. Hydrofracturing as well as permeability contrasts between hanging and footwall further intensify fluid flow. In all thrusts, tectonic breccias indicate that brittle conditions prevailed during the youngest deformation stage.
Based on microstructural criteria and variations of these structures along thrust systems, this thesis sheds new light on the understanding of the evolution of microfabrics in mono- and polymineralic carbonate mylonites in nature, as well as the evolution of structures in large-scale shear zones as function of changing physical and geochemical conditions with depth and time. This knowledge is relevant for one understanding of the deformation behavior within the Earth’s crust of a growing orogen.

Publications

Microstructural evolution of second-phase controlled carbonates in large-scale shear zones under prograde and retrograde conditions. (2006)

Entwicklung einer fluidbeeinflussten Scherzone am Beispiel der Glarner Hauptüberschiebung (Schweiz). (2006)

Evolution of polymineralic deformation microstructures in different large-scale shear zones under different influence of fluids. (2006)
download poster here

Fluid-assisted strain localization in the Glarus overthrust (Switzerland). (2006)

Microstructural variation in natural mylonites and their consequences for rheological extrapolations from laboratory to nature. (2006)

Strain Localization in Thrust Zones of Helvetic Nappes (Switzerland). (2005)

Grain coarsening maps: A new tool to predict microfabric evolution of ploymineralic rocks. (2005)

Second-phase particles in grains and at grain boundaries of calcite – how they affect the microfabric in natural carbonate mylonites. (2005)

Brittle-ductile transition and inferred seismicity in compressional orogens: the Helvetic Alps as an example. (2005)

Temperature-dependence of calcite microfabrics in polymineralic carbonate mylonites – A comparison between different Helvetic nappes. (2004)

Grain coarsening maps: A new tool to predict microstructural changes in ploymineralic rocks. (2004)

The growth kinetics in second phase affected systems: the dynamic case. (2004)

Parameter die das Mikrogefuege von unreinen Karbonatmyloniten beeinflussen: Ein Fallbeispiel aus dem Helvetikum (Schweiz). (2004)
download poster here (english version)

The Influence of Second Phases on Grain Boundaries of Mylonitic Microfabrics: Evidences From Natural Carbonate Mylonites. (2003)

Parameters affecting the microstructure of impure carbonate mylonites: A case study on samples from the Helvetic nappes (Switzerland). (2003)

Temperature and second-phase induced microstructural variations within impure carbonate mylonites. (2003)

 

Microstructural evolution of second-phase controlled carbonates in large-scale shear zones under prograde and retrograde conditions.

Andreas Ebert, Marco Herwegh and Alfons Berger
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

SGM4, Bern, 2006 (oral)

Steady state microfabrics of dynamically recrystallized rocks of large-scale shear zones are characterized particularly by (a) phase compositions, (b) physical conditions, and (c) changes of deformation conditions in space and time. Other parameters like matrix chemistry and synkinematic fluids are of minor importance, at least in case of carbonates deformed under natural conditions in the range between 200-400°C.
(a) Second phases in carbonate mylonites, in this study predominantly sheet silicates, have an important influence on shape and grain size of the matrix phase calcite. Due to the pinning effect of second phases, at constant temperature, the mean area-weighted grain size of calcite (Dcc) can be expressed by the relationship Dcc=c*(dp/fp)m (1), where c is a constant, m=0.3±0.05, dp the grain size and fp the volume fraction of second phases. Under constant physical conditions, dp depends on fp following the relation dp=k*fpn (2) with a constant k and n=0.15±0.05.
(b) The temperature dependency of equations 1 and 2 can be incorporated by changing both equations into an Arrhenius type form. The resulting equation of second-phase controlled grain growth of calcite is Dcc=c’*exp(-Qcc/RT)*(dp/fp)m (3). Likewise, temperature dependent grain growth of second-phases can be expressed as dp=k’*exp(-Qp/RT)*fpn (4). C’ and k’ are new constants, Qcc and Qp activations energies, R the gas constant and T the deformation temperature in K. Equations 1 and 2 characterize the steady state deformation microfabrics at constant physical conditions where dynamic steady state fabrics are controlled by a balance of nucleation, grain growth, pinning and grain size reducing mechanisms (dynamic recrystallization). In addition, equations 3 and 4 describe the competition between temperature induced grain growth and deformation induced grain size reduction. Interestingly, calcite microfabrics of different large-scale shear zones, which are corrected for a similar second-phase influence, fit all within the error on a linear trend characterized by a change in slope in an Arrhenius type diagram (logDcc vs. 1/T) at temperatures around 300-350°C. Therefore, resulting activation energies Qcc (for calculation see Geology data repository in Herwegh et al., 2005) are higher for T>300-350°C yielding values of 60±8 kJ/mol (second-phase controlled) and 80±8 kJ/mol (recrystallization controlled) than for T<300-350°C with values of 32±3 kJ/mol (second-phase rich) and 40±3 kJ/mol (pure). These differences in activation energies indicate that different sets of dominant processes are active during deformation at high and low temperatures and in pure and impure mylonites. In a similar way, activation energies Qp for second-phase grain growth yield an average of 40±7 kJ/mol. In contrast to calcite, the second-phase growth behaviour displays only one linear trend in an Arrhenius plot indicating the presence of one single growth mechanism for all samples investigated.
(c) During thrusting, i.e. deformation, physical conditions change along the thrusts but also with time. Deformation on the retrograde path leads to strain localization, which is characterized by a narrowing of the shear zone width and a distinct grain size reduction of matrix and second phases. In case of grain size reduction, resulting microstructures are similar to those developed at locations with equivalent peak metamorphic conditions at lower temperatures. Therefore, equations 1-4 hold also for deformation on the retrograde path.
In summary, both matrix grains and second phases change their microstructures with changing temperature/stress conditions and affect each other during simultaneous recrystallization cylces. The evolution of coupled grain growth under prograde conditions as well as of grain size reduction on the retrograde path can be displayed in grain coarsening/reduction maps, which can be calculated from equation 3 with substitution of dp by equation 4 (see Figure 1).
Figure 1. Grain coarsening/reduction map of calcite in second-phase and recrystallization controlled microstructures derived from equation 3 and 4 (Dcc = calcite grain size, Z = Zener parameter = dp/fp = grain size/volume fraction of second phases, Qcc = activation energies for second-phase (grey letters) and recrystallization (white letters) controlled microstructures for T>300-350°C (hT) and T<300-350°C (lT).
REFERENCES: Herwegh, M., Berger, A. & Ebert, A. (2005): Grain coarsening maps: A new tool to predict microfabric evolution of polymineralic rocks. Geology 33 (10): 801-804.

nach oben

Entwicklung einer fluidbeeinflussten Scherzone am Beispiel der Glarner Hauptüberschiebung (Schweiz).

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

TSK 11, Göttingen, 2006 (oral)

Lokalisierung unter retrograden Deformationsbedingungen kann häufig in groß-maßstäblichen Scherzonen beobachtet werden. Dabei nimmt die Scherzonenbreite kontinuierlich ab. Gleichzeitig passt sich das Gefüge (Korngröße, Kornform, Kornorientierung, Zwillingsdichte, kristallographische Orientierung, usw.) den neuen Umgebungsbedingungen (Temperatur, Spannung, Verformungsrate) an. Die Glarner Hauptüberschiebung in den Ostschweizer Alpen ist ein gutes Beispiel, um das Ausmaß und die Entwicklung einer Verformungslokalisierung zu bestimmen. In der Vergangenheit wurde sie detailliert in Hinblick auf ihre Isotopenverteilung und daraus resultierenden Fluidbewegungen und Überprägungen untersucht. Dies erlaubt das Zusammenspiel der Lokalisierung und der Fluidüberprägung zur Zeit der Platznahme der Glarnerdecke zu bestimmen. Im Fall der Glarner Hauptüberschiebung wurde permischer Verrucano über den sedimentären infrahelvetischen Komplex (Flysch und mesozoische Karbonate) geschoben. Dabei entstand zwischen dem Hangenden und Liegenden der bekannte Lochseiten Kalkmylonit. Die alpinen peak-metamorphen Bedingungen lagen im Bereich der Anchizone (230°C) im Norden und der Grünschieferfazies (350°C) im Süden.
Entlang der Überschiebungsbahn wurden im Abstand von wenigen Kilometern Probenserien vertikal zur Scherbahn genommen. Dabei wurde beginnend am Kontakt zum Verrucano bis zu 20m tief beprobt, wobei nahe am Kontakt im Dezimeter-Bereich beprobt wurde. Alle Proben zeigen stabilisierte Korngefüge, welche durch ein temperatur- und spannungskontrolliertes Wechselspiel von korngrößenreduzierenden Mechanismen und Kornwachstum charakterisiert sind. Als Konsequenz nimmt die mittlere Korngröße von Nord nach Süd zu, wobei sie aber gleichzeitig senkrecht zur Scherbahn abnimmt. Die Zwillingsdichte verhält sich entgegengesetzt zur Körngröße. Sie nimmt mit abnehmender Distanz zur Überschiebung zu. Änderungen der stabilen Isotope in vertikalen Profilen zeigen übereinstimmende Trends (Badertscher, 2001). d13C und d18O-Werte nehmen simultan mit der Körngröße zur Überschiebungsbahn hin ab. Zusammen mit synkinematischen Adern zeigen diese Isotopenänderungen, dass während der Deformation Fluide vorhanden gewesen sein müssen und das Gefüge beeinflusst haben.

nach oben

Evolution of polymineralic deformation microstructures in different large-scale shear zones under different influence of fluids.

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

EGU, Wien, 2006 (poster)

Geophysical Research Abstracts, Vol. 8, 02563, 2006
Calcite microstructures of polymineralic carbonate mylonites from different Helvetic nappes (Morcles, Diablerets, Doldenhorn, Glarner nappe) in Switzerland were analyzed in regard of variations due to temperature/stress and impurity content (i.e. second phase minerals). The aim was to determine the variability of microstructures (mean grain size of calcite Dcc and second-phases dp) between the different nappes, which were all deformed at similar conditions (peak temperature T /stress and volume fraction of second-phases fp). Grain sizes of both matrix calcite and second phases are dynamically stabilized resulting in steady state microstructures maintained by cycles of nucleation, growth and consumption. Dcc of every sample is affected by second-phase particles (predominantly sheet-silicates) in a similar way in all nappes. The second-phase influence is expressed by the Zener parameter (Z), a geometric factor defined by Z=dp/fp. Therefore, it is possible to compare different second-phase affected microstructures by taking Dcc for equal Z values. Hence, the relationship logDcc vs. 1/T and the resulting activation energies (Qcc) for each nappe can be determined. This relationship is identical for all nappes. However, Qcc varies within the nappes with T, showing an increase from around 20 to 80kJ/mol for the temperature range of 250 to 390°C, respectively. This suggests that for the aforementioned cycles the relative contribution of grain growth and grain size reduction change with temperature in an opposite manner. In contrast to Dcc, the second-phase grain size dp for constant fp differs between the nappes. The slopes of the relation logdp vs. 1/T are constant and so are the resulting activation energies for second phase growth (Qp 30kJ/mol). In nappes with higher synkinematic fluid flow the overall dp is smaller. This indicates that second-phase grain growth is transport and dissolution controlled. Fluids enhance rates of dissolution - mass transfer - precipitation for the second phases. Therefore, cycles of growth and consumption are faster and consequently dp smaller in mylonites affected by fluids. Hence, coupled grain coarsening of both matrix and second phases define the bulk microstructure while the deformation conditions control growth and grain size reducing processes of the associated phases.

nach oben

Fluid-assisted strain localization in the Glarus overthrust (Switzerland).

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

EGU, Wien, 2006 (oral)

Geophysical Research Abstracts, Vol. 8, 04971, 2006
In high-strain shear zones, deformation under retrograde conditions is a common feature, where the width of shear zone decreases continuously with ongoing deformation. The occurrence, degree, and geological significance of strain localization were studied for the Glarus nappe complex. Here, Permian Verrucano was thrusted over the sedimentary Infrahelvetic complex while the famous ‘Lochsiten calc-mylonite’ served as lubrificant, which was sandwiched between hanging and footwall. From front to rear, the peak metamorphic conditions increase from anchizone to lower greenschist facies, respectively.
Along the entire thrust, steady state microfabrics occur characterized by a temperature/stress controlled balance of grain size reducing mechanisms and grain growth. As a consequence, the mean grain size continuously increases along the thrust plane with increasing metamorphic conditions (i.e. from N to S). In a section perpendicular to the thrust, however, the grain size continuously decreases towards the center of the shear zone. In an opposite manner to the grain size reduction, calcite twin density increases. These changes in microfabrics go hand in hand with stable isotopes, where both d13C und d18O also decrease (Badertscher, 2001). This fact, in combination with the occurrence of synkinematic veins, indicates the presence of fluids during deformation.
The modifications in microfabric resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling. As a consequence strain localized, i.e. the width of the large-scale shear zone continuously decreased with time and reduced temperature. A texture weakening with continuous localization points to a simultaneous change in predominant deformation mechanisms from grain size insensitive to granular flow (dissolution-precipitation and grain boundary sliding processes). The enhanced isotopic fractionation towards the shear zone center is based on 3 major parameters directly attributed to strain localization: (1) ongoing dynamic recrystallization particularly grain boundary migration, (2) localization induced small grain sizes and therefore higher permeabilities, and (3) higher finite strains. The latest stage of deformation occurred under brittle conditions as manifest by sharp well defined brittle faults and local cataclasites.
References:
Badertscher, N., 2001. Deformation mechanisms and fluid flow along the Glarus overthrust, eastern Helvetic Alps, Switzerland. PhD Thesis, Université de Neuchâtel, Switzerland, 286 pp.

nach oben

Microstructural variation in natural mylonites and their consequences for rheological extrapolations from laboratory to nature.

M. Herwegh (1), A. Ebert (1), J.H.P. de Bresser (2)
(1) Institute of Geological Science, University of Bern, Switzerland,
(2) Department of Earth Sciences, Utrecht University, The Netherlands

EGU, Wien, 2006 (poster)

Geophysical Research Abstracts, Vol. 8, 04070, 2006
At first glance, the characteristics of natural microfabrics of calcite mylonites formed at mid-crustal levels almost invariably suggest grain size insensitive creep (GSI) to be the predominant deformation mechanism. This inference contradicts extrapolations of laboratory experiments, which predict grain size sensitive (GSS) flow to be dominant under a wide range of natural deformation conditions (e.g. de Bresser et al., 2001). This extrapolation problem between nature and experiment can be solved if quantified grain size distributions of natural microfabrics are used. These distributions act as input parameters for the rheological modeling of composite GSS+GSI flow (ter Heege et al., 2004) using constraints from nature on temperature and strain rate or stress. Results for selected natural calcite mylonites indicate predominance of grain size insensitive creep at low temperatures but with an increase of the contribution of grain size sensitive creep to the bulk flow with increasing T (Herwegh et al., 2005). This elegantly corroborates the inferences made from the natural microfabrics.
In nature, however, not only the deformation conditions (stress, temperature, strain rate), but also the composition of calcite mylonites (chemistry, second phase content, fluids) and the geodynamic evolution can vary substantially between individual largescale shear zones. This variability, of course, directly affects the resulting deformation microstructures, which again has implications for the extrapolation of rheological lab data to nature. Based on a series of different large-scale shear zones, we will demonstrate microstructural variations in calcite mylonites from the Alps and discuss what their consequences for rheological predictions are.
References:
ter Heege, J. H., de Bresser, J. H. P., Spiers, C. J., 2004. Composite flow laws for crystalline materials with log-normally distributed grain size: Theory and application to olivine. Journal of Structural Geology, 26, 1693-1705.
de Bresser, J. H. P., Evans, B., Renner, J., 2002. Predicting the strength of calcite rocks under natural conditions. In: de Meer, S., Drury, M. R., de Bresser, J. H. P., Pennock, G. M. (eds) Deformation mechanisms, rheology and tectonics: Current status and future perspectives. Geological Society, London, Special Publications 200, 309-329.
Herwegh, M., de Bresser, J.H.P., ter Heege, J., 2005. Combining natural microstructures with composite flow laws: An improved approach for the extrapolation of lab data to nature. J. Struct. Geol., 27, 503-521.

nach oben

Strain Localization in Thrust Zones of Helvetic Nappes (Switzerland).

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

SGM, Zürich, 2005 (poster)

Deformation under retrograde conditions is a common feature in high-strain shear zones, where the width of the shear zone decreases continuously with ongoing deformation. The occurrence, degree and geological significance of strain localization was studied for different Helvetic nappes (e.g., the Morcles, Diablerets, Doldenhorn and Glarus nappe). These nappes belong to three major nappe stacks: a western (Morcles, Diablerets, and Wildhorn nappe) a central (Doldenhorn, Gellihorn, and Wildhorn nappe) and an eastern (Glarus nappe complex) one, which all developed under similar tectono-metamorphic conditions. From frontal to rear parts and with increasing depth in the stack, the metamorphic conditions range from diagenesis to lower greenshist facies, respectively. Strain rates are similar and the deformation was mainly concentrated in im-/pure carbonate rocks with varying contents of second-phase minerals like sheet silicates, quartz, dolomite and ores.
The observed steady-state calcite microstructures in these carbonate mylonites are characterized pre-dominantly by a temperature/stress controlled balance of grain size reducing mechanisms and grain growth as well as second-phase pinning. Particularly high second-phase contents and/or retrograde strain localization, with an enhanced grain size reduction component compared to syndeformational grain growth, result in mylonites characterized by small steady state grain sizes. In order to detect strain localization, the field geologist has to discriminate this phenomenon from a simple second-phase effect. This can either be obtained by selecting pure rocks, where a second-phase influence can be excluded or one has to be able to correct for the second-phase effect. Besides abrupt grain size reduction, the occurrence of localized horizons with bimodal grain size distributions, intense twinning even of small sized calcite grains and discrete shear zones cross-cutting older deformation structures represent additional criteria to identify localized shear zones. Partly, the zones characterized by grain size reduction are accompanied by cataclasites.
In the samples analyzed, the ‘retrograde microfabrics’ occur in thin layers within the former peak metamorphic mylonites. Towards the center of these strain localization zones the grain size continuously decreases indicating a continuous reduction of the shear zone width with decreasing metamorphic grade. Such strain localization phenomena were found in all nappes, where the mylonitic grain sizes were reduced to sizes smaller than 5 µm for both, pure and second-phase affected microstructures. The latter point indicates that rock strength hardly differs between pure and second-phase controlled carbonate mylonites during retrograde deformation.
Despite the common appearance of strain localization phenomena in all nappes investigated, there exist remarkable differences in regard of the width of the localized shear zones. In the Morcles and Doldenhorn nappe, strain localization is evident only within a few mm to cm, while in the Diablerets and Glarus nappe the calcite grain size decrease occurs over distances of several dm to m. This could be caused by a variety of different parameters like (a) differences in steady state grain size developed under peak metamorphic and retrograde conditions, (b) differences in fluid content (high fluid content = fast recrystallization cylcles), or (c) differences in the cooling histories depending on the position of the nappe in the nappe stack.

nach oben

Grain coarsening maps: A new tool to predict microfabric evolution of ploymineralic rocks.

Marco Herwegh, Alfons Berger and Andreas Ebert
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

in Geology, 2005, v.33, no.10, p.801-804

Polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments, as long as no mineral reactions occur. The grain coarsening in such rocks is complex because the different phases influence each other, and it is this interaction that controls the rate of grain coarsening of the entire aggregate. We present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters, including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information. Based on samples from polymineralic carbonate mylonites that were deformed at different temperatures, we demonstrate how the mathematical relation can be calibrated for this natural system. Using such datasets for other lithologies, grain coarsening maps can be generated, which allow the prediction of microstructural evolution in polymineralic rocks. Such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths, such as structural geology, geophysics, geodynamics, and metamorphic petrology.

nach oben

Second-phase particles in grains and at grain boundaries of calcite – how they affect the microfabric in natural carbonate mylonites.

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

DRT, Zürich, 2005 (oral)

In the past, many investigations were performed on deformed pure carbonate rocks in nature and experiment. But so far, little is known about deformation in impure carbonate rocks, although most limestones contain considerable amounts of second-phases such as sheet silicates, quartz, dolomite, anhydrite and other minor phases. For this reason we analyzed carbonate mylonites, with 0-40 vol% second-phase particles, from basal thrust planes of three Helvetic nappes in Western and Central Switzerland.
The results show that the mean calcite grain size Dcc is influenced by two major parameters: the temperature T and the second-phases. While Dcc increases with T, second-phase particles pin the calcite grain boundary after the relationship: Dcc=c*(dp/fp)m with a temperature dependent variable c, a constant exponent m, the grain size dp and the volume fraction fp of the second-phases (modified after Zener in Smith, 1948). In case of pure mylonites, the calcite grain size is stabilized by a balance of grain growth and grain size reduction (dynamic recrystallization).
In contrast, in impure samples Dcc is limited by pinning depending on dp and fp. Furthermore, two types of second-phases can be distinguished, second-phases (a) included in calcite grains (ISP) and (b) at calcite grain boundaries (BSP). These second-phases display the following relationship: (1) dp coarsens with fp and T for ISP and BSP, (2) at constant T, ISP particles are smaller than BSP, and (3) the ratio of ISP vs. BSP increases with T and decreasing fp. These relationships are controlled by interacting processes: (i) driving forces for calcite grain boundary migration, (ii) pinning of calcite grain boundaries and (iii) intergranular diffusion.
In terms of Dcc-T dependencies, the microstructures of the three nappes are similar, if corrected for the same impurity content, i.e. without second-phase influence. Interestingly the relationship logDcc ­ 1/T is non-linear and the CPO weakens with increasing second-phase content. These observations can be explained by simultaneously active GSS and GSI creep, while the relative contributions of the two end member mechanisms change as a function of T and second-phase content.
In contrast, the relation logdp - 1/T is linear with constant slope for all nappes. But the positions of each trend shift between the different nappes, a fact which might be explained by differences in fluid activity. The shifts correlate with variations in the intensity of synkinematic veining between the different nappes. A higher fluid activity may cause faster cycles of deformation and grain growth resulting in smaller grain sizes dp.

nach oben

Brittle-Ductile Transition and Inferred Seismicity in Compressional Orogens: The Helvetic Alps as an Example

Marco Herwegh (1), Andreas Ebert (1) , Hans de Bresser (2), Adrian Pfiffner (1)
(1) Institute of Geological Sciences, Universtiy of Berne, Baltzerstrasse 1-3, CH-3012 Berne, Switzerland
(2) Faculty of Earth Sciences, Utrecht University (the Netherlands)

DRT, Zürich, 2005 (oral)

In compressional orogens, the brittle-ductile transition represents an important rheological marker. Seismicity generally diminishes below this transition zone, which is typically located at depths of about 10-15km. Geophysical investigations have resulted in a wealth of data on this phenomenon, but many aspects remain ill-understood. In particular, this concerns (a) the spatial evolution of the zone of seismicity as a function of the overall evolution of the orogen, (b) the character of the related micro- to mesoscale structures, (c) the processes controlling the development of structures, and (d) the resulting overall rheology of the deforming rock. In order to unravel these points, we studied carbonate fault rocks from large-scale thrusts, along three N-S sections through the Western, Central and Eastern Helvetic Alps of Switzerland. These sections are characterized by temperatures ranging from 180-200°C in the north to 360-400°C in the south.
With increasing temperature, the dominant micro- and mesoscale structure in the carbonate fault rocks changes from a system of calcite veins in irregularly oriented directions with intensely twinned calcite grains and prominent occurrence of stylolites (at T <230°C), to a structure of more parallel but undulating veins in which the degree of deformational overprint by dynamic recrystallization increases as a function of the relative age of the vein (at T <230-330°C). At temperatures >330°C the early formed veins are parallel to the thrust plane and are completely recrystallized. At all temperatures the structures are found to be overprinted by late brittle faults and/or localized cataclasites, oriented slightly oblique to the shear plane and showing only minor occurrence of calcite precipitation.
The evolution of these structures can be correlated with (a) increasing pore fluid pressure towards higher temperature, (b) prograde changes in dehydration reactions as a function of temperature and time, and (c) a reduction of rock strength with increasing temperature. Our data suggest that the carbonate rocks were affected by cycles of vein formation that resulted from enhanced fluid activity and low pore fluid pressures at depths <7-8km and low temperatures. This probably represents prograde seismically active domains induced by hydrofracturing. With increasing depth under peak metamorphic conditions, vein formation reduced owing to two counteracting parameters: (a) reduced dehydration, and (b) enhanced pore fluid pressures at higher lithostatic pressures. At the same time an overall increase in ductile deformation resulted in a strength decrease of the carbonate rocks. Here, composite flow of simultaneously active grain size insensitive and grain size sensitive creep mechanisms occurred, with an increasing contribution of the latter to overall deformation with depth. An exhumation induced temperature reduction promoted drastic strain localization during a final episode of deformation, which occurred first under still ductile conditions, but then changed to brittle fracturing and cataclasis. This domain might have become seismically active again (retrograde seismicity), but with less hydrofracturing under ‘drier conditions’.
Summarizing, vein systems in the Helvetic Alps form a demonstration of brittle and ductile deformation in a compressional orogen that change as a function of crustal level (temperature, fluid activity, rheology), space and time, probably affecting locations and intensity of seismicity.

nach oben

Temperature-dependence of calcite microfabrics in polymineralic carbonate mylonites – A comparison between different Helvetic nappes.

Andreas Ebert, Marco Herwegh and Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

SGM, Lausanne, 2004 (oral)

Microfabrics from carbonate mylonites with variable second-phase content and types from different Helvetic nappes (Morcles, Diablerets, Wildhorn, Doldenhorn and Glarus nappe) were analysed. With this respect, size (dp) and volume fraction (fp) of the second-phases as well as the temperature, which increases from Diagenesis in frontal parts to epizonal metamorphic conditions in the rear of each nappe, influence the stabilization of the size of the calcite grains (Dcc).
In impure samples the grain size Dcc of the matrix calcite is affected by pinning of the calcite grain boundaries by second-phases, following the relation Dcc=c*(dp/fp)m (modified after Zener in Smith, 1948), where c is a constant and m the exponent of the Zener parameter Z=dp/fp. If a closed system is assumed, the temperature-induced second-phase coarsening leads to a smaller number of second-phases per unit area but simultaneously to larger inter-particle distances. Hence, calcite grains can grow until their moving grain boundaries are pinned again by the second-phases. Thus coarsening of the polymineralic aggregate is delimited by the kinetics of the coupled growth of both phases. The analyses show that different types of second-phases influence Dcc in a distinct manner. Platy minerals like sheet silicates induce higher m values than is the case for blocky dolomite or spherical quartz grains.
Dcc of pure samples is controlled by dynamic recrystallization, i.e. by the balance of grain growth and grain size reducing mechanisms.
Similar to calcite in pure samples, the occurrence of nucleation, growth and dissolution phenomena of the second-phases indicates a cyclical behaviour. In case of the Doldenhorn nappe, dp is smaller than in the Morcles nappe at same temperatures, although Dcc in the pure samples is identical. The latter indicates similar deformation conditions, i.e. no difference in stress and strain rate. The smaller dp might therefore be attributed to faster recrystallization cycles of the second-phases due to a higher fluid activity. This is in fact evident by a more frequent occurrence of syndeformational veins in the Doldenhorn nappe. These variations in recrystallization cycles affect the calcite matrix because differences in c and Z occur, while m values are similar in all nappes.
To summarize, analysis of the relation between second-phases and the matrix grains is a useful tool to estimate the processes, their kinetics, microstructural evolution and related temperatures during deformation of polymineralic rocks.

nach oben

Grain coarsening maps: A new tool to predict microstructural changes in ploymineralic rocks.

Marco Herwegh, Alfons Berger and Andreas Ebert
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

SGM, Lausanne, 2004 (oral)

Grain coarsening in polymineralic aggregates is one of the most fundamental processes in metamorphic rocks and mylonites. As function of time, temperature, grain growth kinetics and chemistry of the mineral assemblage, a complex interaction between coarsening of the matrix and second-phases is required. Due to limitations of experimental investigations (e.g. time, mineral stability etc.), however, the kinetic parameters required to predict grain coarsening are difficult to obtain and even more problematic to apply to nature. This study presents an approach, where natural samples can be used to calibrate the grain coarsening of polymineralic aggregates in grain-size stabilized systems. In form of grain coarsening maps this calibration can then be applied to predict microstructural changes in the earth’s interior. Generally, the average grain size (dg) changes continuously with time (where Ðdg depends on Ðt and T) but different grain size stabilizing processes can lead to a steady state grain size. In this contribution we will restrict on mylonites, where deformation stabilizes an average steady state grain size.
For polymineralic aggregates with less than 50vol% second-phase the grain size of the matrix mineral (dg) is related to the volume fraction (fp) and size (dp) of the second-phase in the following manner (eq. 1): dg = c’*exp(-Qg/RT)*(dp/fp)m, where c’ is a constant and m an exponent. The term exp(-Qg/RT) expresses the temperature-dependence, with Qg as the activation energy, R as the gas constant and T the as temperature. The coarsening behaviour of the second-phases can be defined as (eq. 2): dp = k*exp(-Qp/RT)*fpn. Where again the activation energy (Qp), a constant k and an exponent n are required. Combining equations 1 & 2 links the coarsening of the second-phase and the matrix mineral. Based on a series of natural samples with variable second-phase content and different temperatures all parameters can be calibrated. Based on these data and knowledge about the stability field of the mineral assemblage, a grain coarsening map can be generated for the polymineralic aggregate (Figures a & b).
The grain coarsening map in Figures (a & b) is based on samples from calcite mylonites from the Doldenhorn nappe deformed in the temperature range 340-380°C with sheet silicates as predominant second-phase. Here, second-phase controlled and recrystallization controlled microstructures (in pure samples) have to be distinguished. The map predicts that second-phase controlled microstructures are only important up to 550°C because at higher temperatures the dynamically recrystallised grain size of the matrix calcite becomes smaller than the inter-particle distance of the second-phases. In the enlarged second-phase controlled field (Figure b), a net can be constructed consisting of two types of grid lines (1) calcite grain size (dg) trends as function of the ratio dp/fp and T (eq. 1) and (2) second-phase coarsening trends at constant volume fraction but different T (eq. 2). Based on the grain coarsening map predictions about the microfabric changes can be made. Note that the presented approach might also be applicable for regional metamorphic rocks because after about 1 million years, grain growth kinetics predicts only limited changes with time anymore. In case of regional metamorphic rocks, the field of recrystallization controlled microstructures would become much smaller or even diminish in the grain coarsening map because of the reduced efficiency of dynamic recrystallization. Alternatively to the example used, other rock types like quartz, feldspar, sheet silicate, olivine or spinell rich polymineralic rocks would be promising candidates for this approach.
To summarize, grain coarsening maps are a promising tool to predict microstructural changes in polymineralic rocks. Linking these predictions with the associated geophysical properties would help to investigate the changes of rheology, seismic anisotropy, electric conductivity and permeability during dynamic processes occurring in the earth’s interior.
Figure captions Figure a: Grain coarsening map for the system calcite-muscovite. Figure b: Enlarged field for second-phase controlled microstructures. Arrow indicates increasing size (dp) of the second-phase at constant volume fraction (fp). dg: calcite grain size; sp cont: second-phase controlled; recr: recrystallization.

nach oben

The growth kinetics in second phase affected systems: the dynamic case.

Marco Herwegh, Alfons Berger, Andreas Ebert
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

GSA, Denver, 2004 (oral)

GSA Abstracts with Programs, Vol. 36, No. 5, Abstracts No. 76146

Deformation of a polymineralic rock, consisting of a matrix mineral and a subordinate quantity of second phases, requires interaction of different deformation and metamorphic processes, where the degree of interaction depends on a variety of extrinsic and intrinsic parameters. Of special interest in this context are processes related to interactions between matrix mineral and second phases responsible for the evolution of the microstructures.
Studies on naturally deformed mylonites show that syndeformational grain growth of the second phases represents the major rate controlling process. At constant temperature, the grain size of the second phases (dp) increases with increasing volume fraction (fp). This tendency persists with increasing temperature but at constant fp the second phase size increases indicating thermally-activated and transport-controlled growth of the second phases. The second phase coarsening can be modeled by (1) dp = k*exp(-Q/RT)*fpn , where k is a constant, Q the activation energy, T the temperature and n the growth exponent of the second phases. Since in polymineralic rocks the matrix grains are mostly affected by Zener pinning, syndeformational growth of the second phases controls directly the grain size of the matrix grain size (D): (2) D = c*Zm, where c is a constant and m the exponent of the Zener parameter (Z= dp/fp). The microstructural evolution of a mylonite during deformation can now be predicted by combining equations (1) and (2).
Based on natural mylonites formed at different temperatures in a large-scale crustal shear zone we will demonstrate that second phase coarsening is enhanced in the shear zone and that Q, n, m, k and c can be directly obtained from the samples allowing the prediction of the microstructural evolution in polymineralic mylonites.

nach oben

Parameter die das Mikrogefuege von unreinen Karbonatmyloniten beeinflussen:
Ein Fallbeispiel aus dem Helvetikum (Schweiz).

Andreas Ebert, Marco Herwegh & Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1-3, CH-3012 Bern

TSK 10, Aachen, 2004 (poster)

Verschieden reine Karbonatmylonite mit unterschiedlichen Gehalten an Zweitphasen, €berwiegend Schichtsilikate, aus den basalen Ueberschiebungsbahnen der Morcles- und Doldenhorn Decke in der Westschweiz wurden hinsichtlich moeglicher Einflussfaktoren auf das Kalzit-Mikrogefuege untersucht. Diese sind die Temperatur, der Volumenanteil und die Korngroesse der Zweitphasen und deren Mineralogie.
Die Kalzitkorngroesse ist in allen untersuchten Proben stabilisiert und zeigt eine Abhaengigkeit vom Zener Parameter Z=dp/fp (dp=Korngroesse und fp=Volumenanteil der Zweitphasen), wobei zwei Typen der Korngroessenstabilisierung unterschieden werden koennen: (a) dynamische Rekristallisation in reinen Karbonatlagen und (b) Korngroessenstabilisierung durch Zweitphasen in unreinen Lagen, wobei Aenderungen der Kalzitkorngroesse um bis zu 50% durch unterschiedliche Groessen (1-11mm) und variierende Volumenanteile (0-30%) der Zweitphasen resultieren koennen.
Bei konstanter Temperatur nimmt in zweitphasenstabilisierten Mikrostrukturen die Kalzitkorngroesse mit Z markant zu, waehrend in rekristallisationsdominierten Bereichen die Kalzitkorngroesse nahezu konstant ist. Gleichzeitig nimmt mit abnehmendem Z die Elongation der Kalzitkoerner zu, kristallographisch bevorzugte Orientierungen werden abgeschwaecht und die Vorzugsorientierungen werden zunehmend asymmetrisch. Gegen hoehere Temperaturen bleiben die beobachteten Trends bestehen, wobei die stabilisierten Kalzitkorngroessen zunehmen.
All diese Punkte deuten auf einen Uebergang von Dislokations- zu Diffusionskriechen als dominanten Deformationsmechanismus hin. Im Vergleich zwischen Morcles- und Doldenhorn-Decke findet bei letzterer der Uebergang von zweitphasen- zu rekristallisationsdominierten Mikrogefuegen bei niedrigeren Z Werten bzw. kleineren Zweitphasenkorngroessen statt. Dieser Unterschied kann durch unterschiedliches Zweitphasen-Wachstum oder Fluid-Aktivitaet verursacht sein.

nach oben

The Influence of Second Phases on Grain Boundaries of Mylonitic Microfabrics: Evidences From Natural Carbonate Mylonites.

Andreas Ebert, Marco Herwegh & Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1, CH-3012 Bern

AGU, San Franzisco, 2003 (poster)

Impure carbonate mylonites were sampled along basal thrusts of two Helvetic nappes in western Switzerland (Morcles and Doldenhorn nappe). The two nappes built up large-scale recumbent folds with intensively deformed and elongated inverted limbs. In both nappes the temperature rises towards the rear of each tectonic unit. Both type, mineralogy (sheet silicates, dolomite, quartz) and content of second phases vary within the samples.
In this study we investigated the influence of volume fraction, grain size, shape, mineralogy of second phase minerals, and the temperature on the mylonitic microfabric. In all samples, calcite shows a steady state grain size, but the grain size varies as a function of (a) size and volume fraction of the second phase minerals and (b) temperature.
(a) At constant temperature, two major microstructure controlling parameters can be distinguished: second phases in impure and dynamic recrystallization in pure mylonites. In impure calcite mylonites pinning and dragging of calcite grain boundaries by second phase minerals stabilize the calcite grain size (Dcc) after: Dcc=c*Zm with Z= dp/fp (size dp and volume fraction fp of second phase minerals, modified after Zener in Smith, 1948). In this way the calcite grain size is inversely proportional to the second phase content. In contrast, the average calcite grain size is maintained constant by dynamic recrystallization via a balance of grain growth and grain size reducing mechanisms. Different lithologies with different second phase mineralogy from the same locality (constant temperature) show the same Dcc-Z trends, indicating that the shape of the second phases (elongated sheet silicates, blocky dolomite, spherical quartz) has no major influence on the calcite grain size. However, the elongation of calcite grains increases with decreasing Z, while in pure samples the elongation is constant.
(b) With increasing temperature the steady state grain size Dcc of calcite increases, while the aforementioned Dcc-Z trends persist. Thus for both types of microstructure c is temperature dependent. In light of crystallographic preferred orientation, first investigations show a shift from random distribution to point maxima with increasing Z and temperature.
This study shows that already small amounts of second phases can drastically influence the microstructure of mylonites and the associated deformation mechanisms. The aforementioned relation allows to take the influence of second phases on calcite grain size into account, which is an important requirement for rheological interpretations.

nach oben

Parameters affecting the microstructure of impure carbonate mylonites: A case study on samples from the Helvetic nappes (Switzerland).

Andreas Ebert, Marco Herwegh & Adrian Pfiffner
Institute of Geological Sciences, University of Bern, Baltzerstr. 1, CH-3012 Bern

SGM, Basel, 2003 (oral)

Carbonate mylonites with different amounts of second phase minerals, in particular sheet silicates, were sampled along the basal thrusts of the Morcles and Doldenhorn nappe (western Switzerland).
Several parameters affecting the microstructure of calcite in carbonate mylonites were analysed: temperature, volume fraction and grain size of second phase minerals, and the mineralogy of second phases. In all samples investigated, calcite shows steady state grain sizes, which vary as function of (a) size and fraction of second phases and (b) temperature.
(a) Under isothermal conditions, recrystallization and second phase controlled grain sizes can be distinguished. While a balance of grain growth and grain size reducing mechanisms maintains a recrystallization grain size in pure samples, the calcite grain size in impure mylonites varies in the following manner: Dcc = c*Zm where Z = (dp/fp) (grain size dp and volume fraction fp of second phase minerals, modified after Zener in Smith, 1948). In this way, pinning and dragging of calcite grain boundaries by second phases stabilizes the calcite grain size in impure carbonate mylonites. Surprisingly, different lithologies from the same locality (constant temperature), i.e., different second phase mineralogy (sheet silicates, dolomite, quartz and nannoparticles) and distribution, seem to have no influence to the Dcc-Z trends. In light of calcite grain shapes, impure samples show a decreasing grain aspect ratio (minor/major axis) with decreasing Z, while the elongation is nearly constant in pure samples.
(b) The trends observed under isothermal conditions persist with increasing temperature but the steady state grain size of calcite increases. In other words, a temperature dependence occurs, which mainly influences c.
Quantification of the aforementioned relation allows the correction of the calcite grain size for different Z, allowing the comparison of samples with variable second phase content from different thrust planes. In case of the Doldenhorn and Morcles nappes, the m values are very similar and for pure samples the steady state grain sizes show and identical variation with temperature. With increasing impurity content, however, the stabilized grain size between the two nappes depart, yielding calcite grain sizes smaller in the Morcles than in the Doldenhorn nappe. The reason for this discrepancy, might be related to differences in the growth kinetics of second phases, which might be different in the two nappes due to differences in the fluid activity during thrusting.

nach oben

Temperature and second-phase induced microstructural variations within impure carbonate mylonites.

A. Ebert, M. Herwegh & A. Pfiffner

DRT, St.Malo, 2003 (poster)

Carbonate mylonites were sampled along the basal thrusts of the Morcles and Doldenhorn nappes representing the lowest exposed tectonic units of the Helvetic nappe stack in western Switzerland. Both nappes form large scale recumbent folds with intensively deformed inverted limbs right above the thrust plane. Temperature are higher towards the rear of the nappes.
From a microstructural point of view, the mylonites all contain small amounts of second phase minerals, in particular sheet silicates. Depending on size (dp) and volume fraction (fp) (Zener parameter Z=dp/fp), they affect the grain size (Dcc) of the matrix calcite. Two general trends can be distinguished: (a) grain size variations within the individual sample (i.e., constant temperature) and (b) grain size changes with temperature. (a) Grain sizes at the mm scale are controlled by differences in the Zener parameter. In this context it is important to note that the impurity content varies in the range of 0-10 vol% inducing grain size variations as much as 50%. Despite this variability, the relationship Dcc = c(dp/fpm) allows to correct for the impurity content. M and c are factors describing the second phase distribution and the material constant. Using this relationship, calcite grain sizes at constant second phase content can be estimated. This is a prerequisite to compare grain size variations between different locations. (b) In both tectonic sections the grain sizes increase rearward in the nappe along the thrust, i.e., grain size increase with temperature.
In case of the Doldenhorn nappe, this grain size increase is more pronounced than in the Morcles nappe. The reason for this difference cannot solely be attributed to different temperature gradients. Additional differences as strain rate, fluid content, chemical compositions of calcite or annealing are required. Jumps in grain size, as seen for example in the rear of the Morcles nappe could indicate late faulting.

nach oben