GeoScienceWorld
Volume

Advances in Interpretation of Geological Processes:

Refinement of Multi-scale Data and Integration in Numerical Modelling

Edited by M. I. Spalla, A. M. Marotta and G. Gosso

Abstract

Iterative comparison of analytical results and natural observations with predictions of numerical models improves interpretation of geological processes. Further refinements derive from wide-angle comparison of results from various scales of study. In this volume, advances from field, laboratory and modelling approaches to tectonic evolution – from the lithosphere to the rock scale – are compared. Constructive use is made of apparently discrepant or non-consistent results from analytical or methodological approaches in processing field or laboratory data, P–T estimates, absolute or relative age determinations of tectonic events, tectonic unit size in crustal scale deformation, grain-scale deformation processes, various modelling approaches, and numerical techniques. Advances in geodynamic modelling critically depend on new insights into grain- and subgrain-scale deformation processes. Conversely, quantitative models help to identify which rheological laws and parameters exert the strongest control on multi-scale deformation up to lithosphere and upper mantle scale.

  1. Page 1
    Abstract

    Submicroscopic microchemical disequilibrium in minerals is extremely widespread. Disequilibrium recrystallization is promoted by water in metamorphic terranes and near granites, contact aureoles, and faults. Recrystallization is energetically less costly at almost any temperature than diffusive re-equilibration.

    Radiogenic isotopes (except 4He) never diffusively re-equilibrate faster than major elements forming the mineral structure. Isotopic inheritance tied to relicts was demonstrated for zircon, monazite, amphibole, K-feldspar, biotite and muscovite.

    The mechanism for resetting the isotope record in nature depends more on the availability of recrystallization-enhancing water than on reaching a preset temperature. Laboratory diffusion experiments on hydrous minerals were plagued, to a variable but always large extent, by dissolution–reprecipitation. Mineral geochronometers should be viewed as ‘geohygrometers’ that essentially date the fluid circulation episodes.

    Thanks to submicroscopic petrology, isotopic disequilibria can be put into context with petrogenetic disequilibria. Analytical advances allow the successful dating of each mineral generation. This has opened up a much richer wealth of data on the PTAXd history of rocks, which in the long run will also improve our ability to develop credible numeric models.

  2. Page 17
    Abstract

    40Ar/39Ar apparent age spectra have been measured for unusually retentive potassium feldspars (K-feldspar) from the South Cyclades Shear Zone, Ios, Greece. Our results imply that the Argon Partial Retention Zone (Ar PRZ) for the most retentive domains in potassium K-feldspar can expand into the ductile regime, even when temperatures in excess of about 400–450 °C apply. In such cases K-feldspar could be used as a geochronometer to estimate the timing and duration of deformation and metamorphism events. Therefore, we have reassessed traditional methods used to analyse Arrhenius plots by simulating the effect of step-heating experiments on argon loss. Fractal multidomain diffusion models were used, with theoretical distributions of diffusion domain size and volume. We discovered a Fundamental Asymmetry Principle that offers objective constraints on slope fitting to allow an analysis to be consistent with the multidomain diffusion hypothesis, and which consistently leads to the estimation of higher activation energies. Reanalysis of existing datasets is encouraged to allow reassessment of the significance of the average values reported. Retentive diffusion parameters for K-feldspar might prove to be commonplace.

  3. Page 35
    Abstract

    In the past fractal (ruler) dimension (Dr) of quartz grain-boundary sutures and area–perimeter fractal dimension (Da) of quartz grains, respectively, have been shown to depend on temperature (T) and strain rate. However, the application of these methods to gauge temperature and strain rate in naturally deformed intrusive rocks has not yet been tested. In the present study Dr and Da are calculated in 12 thin sections from different parts of a syntectonic granite (Godhra Granite, India). Of these, six belong to the northern part, two to the central part and four to the southern part of the granite. Earlier work on the Godhra Granite showed both a strain and a temperature gradient, with high temperature in the north and high strain in the south. Microstructural studies reveal that the quartz grain-boundary sutures are less serrated in the northern samples compared to those from the remaining part of the granite. The northern samples contain abundant high-temperature solid-state deformation fabrics that formed between 675 and 725 °C (quartz chessboard pattern thermobarometry). Using a Dr v. T plot given by earlier workers, a Dr value of 1.05–1.14 is expected for the above T range. Dr calculations of quartz sutures from the northern samples give a median of 1.11 and most of the sutures have Dr <1.14. These data fit well with the expected temperature range in which the quartz chessboard pattern formed in the Godhra Granite. The central and southern parts of the granite are dominated by myrmekites (500–670 °C), recrystallized feldspars (450–600 °C), deformation twins in feldspar (400–500 °C) and kinked biotite (<300 °C). The expected Dr of quartz sutures under the above medium–low temperature ranges are 1.07–1.23, 1.11–1.25, 1.16–1.28 and <1.27, respectively. Dr calculations reveal that most of the quartz sutures from the central+southern part have Dr >1.14, and the median values are 1.18 (centre) and 1.17 (south). Using the Dr v. T plot, these Dr values indicate that most of the textures in the central+southern part of the Godhra Granite formed in the temperature range of 450–600 °C, which fits well with the temperature range required for the development of medium–low temperature fabrics that dominate this part of the granite. Thus, it is concluded that Dr of quartz sutures can be used as a geothermometer in syntectonic granites. Da for northern and southern samples is 1.10 and 1.14, respectively. Strain rates of the order of approximately 10−7 and 10−11 s−1, respectively, are obtained for high (675 °C) and low temperature (300 °C) using area-perimeter fractal dimension (Da) values. Although these are higher than geological strain rates that are known in nature (10−12–10−15 s−1), the calculated values for the lower-temperature range are similar to strain rates estimated for intrusions (10−10–10−12 s−1). The calculations indicate that the method to calculate strain rate using Da of quartz grains fails to give geologically reasonable strain rates for high temperature in a syntectonic granite. However, the method maybe useful in obtaining reasonable strain rate estimates for lower temperatures.

  4. Page 49
    Abstract

    A strain gradient was mesoscopically recognized in sheared leucogneisses cropping out near Mount Montalto (Calabria, southern Italy) in the Aspromonte–Peloritani Unit on the basis of field observations. In order to investigate the relationship between textural and physical anisotropy, a microstructural and petrophysical study was carried out on selected mylonites exhibiting different stages of deformation. The main mineral assemblage is Qtz+Pl+Kfs+Wm, displaying SC and shear-band textures; mica-fish and ribbon-like quartz are widespread.

    As strain increases K-feldspar, biotite and premylonitic low phengite white mica transformed to synmylonitic high phengite white mica and quartz, accompanied by an increasing albitization. Different quartz c-axis patterns are ascribable to non-coaxial progressive deformation; we suggest that deformation proceeded under greenschist- up to amphibolite-facies conditions owing to a local increase in shearing temperature.

    Laboratory seismic measurements were carried out on sample cubes (43 mm edged) cut according to the structural frame (foliation, lineation) of the rock. At 400 MPa and room temperature the averages of compressional (Vp) and shear-wave velocities (Vs) are very similar: 5.70–5.91 and 3.36–3.55 km s−1, respectively. Seismic anisotropy and shear-wave splitting are related to the modal amounts of constituent minerals (in particular mica) and their crystallographic preferred orientation. Importantly, anisotropy is lowest in the most strained rock.

  5. Page 69
    Abstract

    Electrical impedance measurements were performed on deforming fine-grained (c. 300 µm) synthetic halite rocks containing small quantities of water in order to study the distribution of intercrystalline brine. The experimental conditions were 125 °C and 50 MPa confining pressure. The resistivity at the predeformational, heated and hydrostatically pressurized state suggests that brine is interconnected in halite. The resistivity progressively increases with deformation, reflecting the change in distribution. In this paper we applied a simple tube model to the resistivity change, and found that the change must be caused by deformation of a thin fluid path with an initial aspect ratio of less than 2×10−4. Brine must, therefore, exist on grain boundaries as a thin fluid film. Previous studies on dihedral angles, however, showed that brine cannot be interconnected under our experimental conditions. The variation in grain-boundary energy cannot explain the coexistence of grain-boundary brine with a positive dihedral angle. The observed resistivity change requires grain-boundary brine to be very thin (<100 nm). Such a thin fluid film might have properties distinct from the bulk fluid, and coexist with brine pores at grain corners and grain faces.

  6. Page 79
    Abstract

    This contribution presents a quantitative microstructural analysis of a polycrystalline aggregate of gypsum, deformed in torsion (T=70–90 °C) at γ (shear strain) ranging from 0 to 4.82. Quantitative microstructural analysis is used to compare the evolution of microstructures observed by optical microscope with those obtained from analysis of X-ray and neutron diffraction data. This analysis shows that during experimental deformation, gypsum accommodated strain by brittle and plastic deformation mechanisms, developing Riedel-like microfaults with plastic foliations and crystallographic preferred orientation (CPO). The relations of microstructures show that with increasing strain, the Riedel systems start from R planes with an angle of ≈30° to the Imposed Shear Plane. This angle decreases (5°–15°) when strain increases, and Y planes develop. Quantitative texture analysis (QTA) shows that S-foliations start developing at low γ and maintain their orientation up to high γ, and that the most active slip system is the (010) along normal to (100) and the [001]-axis. Shape preferred orientation (SPO) of gypsum does not coincide with the theoretical orientation as it does not decrease with increasing strain. This discrepancy is explained by the role of the brittle shear planes that impose a back rotation to gypsum. No brittle to plastic transition occurs. But both plastic and brittle structures contemporaneously accommodate and localize strain.

  7. Page 99
    Abstract

    Slab detachment or breakoff is directly associated with phenomena like morphological orogenesis, occurrence of earthquakes and magmatism. At depth the detachment process is slow and characterized by viscous rheolgy, whereas closer to the surface the process is relatively fast and plastic. Using a 2D mantle model 1500 km deep and 4000 km wide we investigated, with finite-difference and marker-in-cell numerical techniques, the impact of slab age, convergence rate and phase transitions on the viscous mode of slab detachment. In contrast to previous studies exploring simplified breakoff models in which the blockage responsible for inducing breakoff is kinematically prescribed, we constructed a fully dynamic coupled petrological–thermomechanical model of viscous slab breakoff. In this model, forced subduction of a 700 km-long oceanic plate was followed by collision of two continental plates and spontaneous slab blocking resulting from the buoyancy of the continental crust once it had been subducted to a depth of 100–124 km. Typically, five phases of model development can be distinguished: (a) oceanic slab subduction and bending; (b) continental collision initiation followed by the spontaneous slab blocking, thermal relaxation and unbending – in experiments with old oceanic plates in this phase slab roll-back occurs; (c) slab stretching and necking; (d) slab breakoff and accelerated sinking; and (e) post-breakoff relaxation.

    Our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models. The results also demonstrate a non-linear dependence of the duration of the breakoff event on slab age: a positive correlation being characteristic of young (<50 Ma) slabs while for older slabs the correlation is negative. The increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness, which increases both the slab thermal relaxation time and duration of the necking process. In older slabs this tendency is counteracted by negative slab buoyancy, which generate higher stresses that facilitate slab necking and breakoff. A prediction from our breakoff models is that the olivine–wadsleyite transition plays an important role in localizing viscous slab breakoff at depths of 410–510 km due to the buoyancy effects of the transition.

  8. Page 115
    Abstract

    The active tectonics of the Western Alps reveals contrasting regimes: ongoing extension at the heart of the chain and transpression–compression at its external sectors. The active processes currently affecting this region are still a matter of debate. The classical models proposed in the literature invoke: Eurasia–Adria plate collision, counterclockwise motion of the Adria microplate, slab retreat of the subducted continental lithosphere and slab-detachment. More recently, several authors prefer the hypothesis of tectonics driven by isostasy–buoyancy forces. To better understand the influence of these processes on the velocity, strain and stress fields at the surface and in the crust, we developed 2D viscoelastic numerical models along a vertical cross-section perpendicular to the Western Alps. We run our models with different driving forces in order to investigate, one by one, the geodynamic processes proposed in the literature. Results are compared with available geodetic, geological and seismotectonic data. In order to bring into coincidence model predictions and observations, an important vertical isostatic readjustment must be included in the modelling, together with a slight horizontal compression (0.5 mm year−1), probably due to Africa–Eurasia convergence. We show that the subduction process in this Alpine region is likely to be dead and that buoyancy forces may be dominating the present-day tectonics.

  9. Page 129
    Abstract

    A finite-element thermomechanical model is used to analyse present-day crustal deformation in the surroundings of the Calabrian Arc. The major structural complexities of the Tyrrhenian area are taken into account, along with the rheological properties of the rocks resulting from a thermal analysis. A comparison between the results obtained from a model composed of three wide rheologically uniform blocks and those obtained from the thermomechanical model allows us to better constrain the geophysical assumptions and shed light on the roles of the different active mechanisms acting in the Tyrrhenian. Our comparative analysis enlightens the crucial role played by lateral rheological heterogeneities when deformation is analysed at short wavelengths of a few hundred kilometres of the Tyrrhenian, driving the observed diffuse SW–NE extension within the regional context of active Africa–Eurasia convergence. Furthermore, a χ2 analysis based on comparisons with GPS data confirms the hypothesis that a significant part of the Africa–Eurasia convergence is absorbed through the Calabrian subduction.

  10. Page 149
    Abstract

    In orogenic belts high pressure–low temperature (HP–LT) metamorphism can widely affect units derived from both the oceanic and the continental lithosphere. In order to verify whether high P/T (pressure/temperature) ratios recorded in the continental lithosphere can result from tectonic erosion, ablative subduction and recycling in the mantle wedge, we implemented a 2D numerical model to simulate oceanic subduction beneath a continent. Particular attention is paid to the role played by mantle hydration within the continental crust recycled in the wedge region. A comparison between hydrated and non-hydrated models highlights that hydration is fundamental in allowing the recycling of crustal material at shallow depths (≤150 km for a convergence rate of 1 cm year−1), making the uprising and exhumation of buried crustal material during active subduction possible. The recycled crustal material can originate from any crustal level. The Tmax and Pmax distributions within the final marker configuration show that crustal recycling induces the coupling of volumes that reached different depths during their paths in the corner flow. To verify the reliability of this model we compare predictions with natural geological data from the Austroalpine Sesia–Lanzo Zone (SLZ), the largest eclogite-facies crustal fragment of early Alpine age and whose Alpine tectonic evolution has been interpreted as compatible with a cycle of burial at depth and exhumation during active subduction of the oceanic lithosphere. The relationships between natural PT estimates and predicted PT values show that the simulated geodynamic scenario generates a thermal regime coherent with that affecting the subducted continental crust of the SLZ, which may have been stable for a long time during Alpine subduction, allowing the SLZ rocks to accomplish their burial and exhumation path under an active subduction regime.

  11. Page 173
    Abstract

    The 3D reconstruction of geological bodies is an excellent tool for the representation of crustal structures and is applied here to understand related heterogeneities in the grain-scale fabrics; the western portion of the Languard–Tonale Alpine tectono-metamorphic unit (Austroalpine domain, Central Alps) allows evaluation of the per cent volume of textural reworking during polyphase pre-Alpine and Alpine deformations. The structural and metamorphic overprinting during the last deformation imprint involved less than 50% of rock volume; this estimate is obtained by discriminating domains that homogeneously recorded structural and metamorphic re-equilibration during crenulation–decrenulation cycles. These domains are reconstructed using a geograhpical information system (GIS) to manipulate field data and interpretative cross-sections as a means to constrain their 3D volumes. The degree of fabric evolution is integrated at the microscale with the estimate of the reactants/products ratio to infer the progress of metamorphic transformation related to advancing degree of mechanical reactivation. The correlation between degree of fabric evolution and progress of synkinematic metamorphic reactions shows that differences between pristine mineral assemblages v. pre-existing fabrics influence the rate of reaction accomplishment. Fabric evolution and degree of metamorphic transformation increase proportionally once above the threshold value of 60% of volume affected by fabric rejuvenation; metamorphic degree also influences the progress of metamorphic reactions.

  12. Page 189
    Abstract

    Feedback relations between deformation and metamorphic mineral reactions, derived using the principles of non-equilibrium thermodynamics, indicate that mineral reactions progress to completion in high-strain areas, driven by energy dissipated from inelastic deformation. These processes, in common with other time-dependent geological processes, lead to both strain, and strain-rate, hardening/softening in rate-dependent materials. In particular, strain-rate softening leads to the formation of shear zones, folds and boudins by non-Biot mechanisms. Strain-softening alone does not produce folding or boudinage and results in low-strain shear zones; strain-rate softening is necessary to produce realistic strains and structures. Reaction–mechanical feedback relations operating at the scale of 10–100 m produce structures similar to those that arise from thermal–mechanical feedback relations at coarser (kilometre) scales and reaction–diffusion–mechanical feedback relations at finer (millimetre) scales. The dominance of specific processes at various length scales but the development of similar structures by all coupled processes leads to scale invariance. The concept of non-equilibrium mineral stability diagrams is introduced. In principle, deformation influences the position of mineral stability fields relative to equilibrium stability fields; the effect is negligible for the quartzcoesite reaction but may be important for others. Application of these results to the development of structures and mineral reactions in the Italian Alps is discussed.

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