Stress measurements in the earth's crust - recrystallized grain size piezometry revisited

EGU2012-13016 The dynamically recrystallized grain size is the most reliable paleo-piezometer to determine the differential stress in the Earth’s crust and mantle. Knowledge on the stress magnitude is enigmatic to quantify tectonic processes in orogens and plate tectonic forces in general. Owing to this significance, a considerable number of research groups has proposed different theoretical concepts of piezometers in the last couple of years. The recrystallized grain size has been suggested to be not only a function of stress, but also of temperature, strain rate, strain and other parameters. In the meantime, data of experimental studies and from natural shear zones have been collected. Hence, empirical piezometer models and theoretical concepts can be confronted with these data sets. A recrystallized grain size compilation of quartz mylonites from shear zones worldwide indicates that specific grain sizes are less frequent corresponding to transitions in the recrystallization mechanisms. This indicates that the recrystallized grain size development is significantly controlled by the different recrystallization mechanisms in natural mylonites. This relationship should be constrained by any valid piezometer or dynamic recrystallization model which is, however, not the case. Most of the piezometer models assume a temperature dependence via an activation energy term. While the majority of these models predicts a decrease in recrystallized grain size with increasing temperature one implies an increase with increasing temperature. However, neither a decrease nor an increase has reliably been shown by deformation experiments on different minerals. In fact, experimental data on dislocation creep of quartz do not show any temperature-dependence within the error of the given stress-recrystallized grain size measurements. A strain rate dependence is – if at all – less important and also not constrained by experimental data. Also a water-dependence of the piezometer does not exist for quartz and there is contradicting experimental evidence for olivine. Experimentally deformed quartz samples display 2d- recrystallized grain size distributions close to a normal distribution with a slight tendency to a positive skewness. The dispersion of the distribution does not change over the experimental range of strain (7 – 46 %) and also not with the volume proportion of recrystallized grains (2 – 60 %). Hence, increasing strain does not change the recrystallized grain size distribution. In summary, no dependence on temperature, strain rate, strain, grain size distribution and water content can be observed for the quartz piezometer. There is only evidence for the dependence on the recrystallization mechanism. Therefore, it is recommended to refer to the original empirical piezometer relationship in which the recrystallized grain size is only a function of the differential stress when measuring stress in the Earth’s crust

Elastic anisotropies of deformed crustal rocks in the Alps

The crust within collisional orogens is very heterogeneous, in composition as well as in type and intensity of deformation, leading to highly variable physical properties at small scales. This causes difficulties for seismic investigations of tectonic structures at depth since the diverse and partially strong upper crustal anisotropy might overprint the signal of deeper anisotropic structures in the mantle. We characterized the range of elastic anisotropies of deformed crustal rocks in the Alps according to the crystallographic preferred orientation (CPO) of their constituent mineral phases. In the Lago di Cignana area of the Italian Western Alps, we sampled eclogites, blueschists and greenschists of oceanic origin (Zermatt-Saas-Zone). From the lower crustal and mantle rocks of the Ivrea Zone near Finero metagabbros and marbles were collected. The Adula Nappe in the central Alps, which was intensely deformed during the Alpine orogeny, was sampled for rocks from a typical deformed upper continental crust within the Alps. The two major rock types, orthogneisses and paragneisses and small lenses of eclogites, amphibolites and marbles were sampled. CPOs of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. Similarities can be found within the mafic samples. The eclogite and amphibolite lenses within the continental crust, the eclogites and blueschists of oceanic origin, as well as the metagabbros of the lower crust exhibit highest P-wave velocity (VP) in lineation direction. An exception within the mafic samples is the greenschist, which shows a distribution of high VP within the foliation plane. P-wave anisotropy (AVP) of the metagabbros and eclogites is generally low with about 1.5%. The greenschist, blueschist and amphibolite samples show higher AVP of 2-4.5%. Marble also yields highest VP in lineation direction and high AVP of 8%. It can be distinguished from all other samples in the set by its high VP/VS ratio of 1.8. The felsic rocks of the continental crust show high variability. Those with high mica contents also show VP maxima in the foliation plane. They can be distinguished from greenschists, which show an average VP of 7.30 km/s, by their generally lower average VP of 6.18 - 6.81 km/s. To approximate an average for upper crustal rocks units, we picked common CPO types of rock forming minerals within gneiss samples of the Adula Nappe representing the most common lithology. These data were used to determine an average elastic anisotropy of a typical crustal rock within the Alps yielding 4 %. This value is an approximation, which can be used for seismic models at a lithospheric scale. At a crustal or smaller scale, however, local variations in lithology and deformation as displayed by the range of elastic anisotropies within the sample set need to be considered. In addition, larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal or smaller scale seismic model

Reading stress from crystals

Tectonic stresses lead to the deformation of the Earth’s crust and upper mantle. They are the driving force behind plate motion and mountain building, and they control earthquake distribution and intensity. These stresses can be measured from deformation microstructures of previously deeply buried rocks using paleo-piezometers. A new assessment of microstructural data from naturally deformed quartz-bearing rocks indicates that the mechanism of recrystallization fundamentally affects the piezometers. Our findings point out major inaccuracies of stress estimates published in the last 40 years, and it prepares the field for a new piezometer generation that will provide a significantly improved assessment of the stress states in lithospheric plates and plate boundaries

Causes and consequences of the great strength variability among soft Nankai accretionary prism sediments from offshore SW-Japan

Geophysical Research AbstractsVol. 16, EGU2014-10666, 2014EGU General Assembly 2014© Author(s) 2014. CC Attribution 3.0 License.Causes and consequences of the great strength variability among softNankai accretionary prism sediments from offshore SW-JapanMichael Stipp (1), Kai Schumann (1), Bernd Leiss (2), and Klaus Ullemeyer (3)(1) GEOMAR, Marine Geodynamics, Kiel, Germany ([email protected], [email protected]), (2)Geowissenschaftliches Zentrum, Universität Göttingen, Germany ([email protected]), (3) Institut für Geowissenschaften,Universität Kiel, Germany ([email protected])The Nankai Trough Seismogenic Zone Experiment of the International Ocean Discovery Program (IODP) isthe very first attempt to drill into the seismogenic part of a subduction zone. Offshore SW-Japan the oceanicPhilippine sea plate is subducted beneath the continental Eurasian plate causing earthquakes of magnitude 8.0 to8.5 and related tsunamis with a recurrence rate of 80-100 years. For the tsunamigenic potential of the forearc slopeand accreted sediments their mechanical strength, composition and fabrics have been investigated. 19 drill coresamples of IODP Expeditions 315, 316 and 333 were experimentally deformed in a triaxial cell under consolidatedand undrained conditions at confining pressures of 400-1000 kPa, room temperature, axial shortening rates of0.01-9.0 mm/min, and up to an axial strain of64% (Stipp et al., 2013). With respect to the mechanical behavior,two distinct sample groups could be distinguished. Weak samples from the upper and middle forearc slope ofthe accretionary prism show a deviatoric peak stress after only a few percent strain (< 10%) and a continuousstress decrease after a maximum combined with a continuous increase in pore pressure. Strong samples fromthe accretionary prism toe display a constant residual stress at maximum level or even a continuous stressincrease together with a decrease in pore pressure towards high strain (Stipp et al., 2013). Synchrotron textureand composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinementprogram MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth downto 523 m below sea floor corresponding to a preferred mineral alignment due to compaction. Experimentallydeformed samples have generally stronger textures than related undeformed core samples and they show alsoincreasing strength of the illite and kaolinite textures with increasing axial strain. Mechanically weak samples havea bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite [001]-polefigures have maxima >1.5 mrd. Strong samples which were deformed to approximately the same amount of strain(up to 40%) have no calcite and a bulk clay content of 24-36 vol.-%. Illite, kaolinite and smectite [001]-pole figuremaxima are mostly <1.5 mrd, except for one sample which was deformed to a considerably higher strain (64%).The higher clay and calcite content and the stronger textures of the mechanically weak samples can be related toa collapsing pore space of the originally flocculated clay aggregates. This process is insignificant in the strongsamples from the prism toe, for which deformation would tend to involve large rock volumes and lead to straindissipation. The weak samples from the forearc slope which become even weaker with increasing strain mayprovoke mechanical runaway situations allowing for earthquake rupture, surface breakage and tsunami generation.Stipp, M., Rolfs, M., Kitamura, Y., Behrmann, J.H., Schumann, K., Schulte-Kortnack, D. and Feeser, V.2013. G-Cubed 14/11, doi: 10.1002/ggge.20290

POWTEX Neutron Diffractometer at FRM II - new perspectives for in-situ rock deformation analysis

EGU2012-13521 In Geoscience quantitative texture analysis here defined as the quantitative analysis of the crystallographic preferred orientation (CPO), is a common tool for the investigation of fabric development in mono- and polyphase rocks, their deformation histories and kinematics. Bulk texture measurements also allow the quantitative characterisation of the anisotropic physical properties of rock materials. A routine tool to measure bulk sample volumes is neutron texture diffraction, as neutrons have large penetration capabilities of several cm in geological sample materials. The new POWTEX (POWder and TEXture) Diffractometer at the neutron research reactor FRM II in Garching, Germany is designed as a high-intensity diffractometer by groups from the RWTH Aachen, Forschungszentrum Jülich and the University of Göttingen. Complementary to existing neutron diffractometers (SKAT at Dubna, Russia; GEM at ISIS, UK; HIPPO at Los Alamos, USA; D20 at ILL, France; and the local STRESS-SPEC and SPODI at FRM II) the layout of POWTEX is focused on fast time-resolved experiments and the measurement of larger sample series as necessary for the study of large scale geological structures. POWTEX is a dedicated beam line for geoscientific research. Effective texture measurements without sample tilting and rotation are possible firstly by utilizing a range of neutron wavelengths simultaneously (Time-of-Flight technique) and secondly by the high detector coverage (9.8 sr) and a high flux (�~1x10 7 n/cm2s) at the sample. Furthermore the instrument and the angular detector resolution is designed also for strong recrystallisation textures as well as for weak textures of polyphase rocks. These instrument characteristics allow in-situ time-resolved texture measurements during deformation experiments on rocksalt, ice and other materials as large sample environments will be implemented at POWTEX. The in-situ deformation apparatus is operated by a uniaxial spindle drive with a maximum axial load of 250 kN, which will be redesigned to minimize shadowing effects inside the cylindrical detector. The HT deformatione experiments will be carried out in uniaxial compression or extension and an upgrade to triaxial deformation conditions is envisaged. The load frame can alternatively be used for ice deformation by inserting a cryostat cell for temperatures down to 77 K with a triaxial apparatus allowing also simple shear experiments on ice. Strain rates range between 10-8 and 10-3 s-1 reaching to at least 50% axial strain. The deformation apparatus is designed for continuous long-term deformation experiments and can be exchanged between in-situ and ex-situ placements during continuous operation inside and outside the neutron detector