Calcite Twin Research Articles

Comparison of Paleostress magnitudes from calcite twins with contemporary stress magnitudes and frictional sliding criteria in the continental crust : Mechanical implications

Comparison of Paleostress magnitudes from calcite twins with contemporary stress magnitudes and frictional sliding criteria in the continental crust : Mechanical implications

Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt

Research Article| March 01, 2007 Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt O. Lacombe; O. Lacombe 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar K. Amrouch; K. Amrouch 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar F. Mouthereau; F. Mouthereau 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar L. Dissez L. Dissez 27/99 Wellington Street, Bondi, NSW 2026, Australia Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (3): 263–266. https://doi.org/10.1130/G23173A.1 Article history received: 14 Aug 2006 rev-recd: 13 Nov 2006 accepted: 20 Nov 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation O. Lacombe, K. Amrouch, F. Mouthereau, L. Dissez; Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt. Geology 2007;; 35 (3): 263–266. doi: https://doi.org/10.1130/G23173A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract Mechanically induced calcite twins in veins and host rocks of Late Cretaceous to Miocene age in Iran have been used to determine regional Arabia-Eurasia collisional stresses. A late folding stress regime with a compression oriented 025° (±15°) has been identified across the Zagros belt and the southern Iranian Plateau. This late Neogene stress pattern agrees with the current stress field determined from the focal mechanisms of basement earthquakes and suggests that the Hormuz salt décollement poorly decouples the basement and cover stress fields. Our data show that the collisional state of stress has been relatively constant since ca. 5 Ma. The magnitudes of the stresses obtained from the twinning analysis are unexpectedly low, and, to a first approximation, they are constant across the Zagros simply folded belt. This result supports an overall mechanism of buckling of the detached Zagros cover. Internal viscous-plastic processes help to relieve stress within this cover, thus lowering its seismogenic potential. Beyond these regional implications, this study underlines the potential of paleostress analyses in constraining both the tectonics and the mechanics of ancient and active foreland fold belts. You do not currently have access to this article.

Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt

Research Article| March 01, 2007 Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt O. Lacombe; O. Lacombe 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar K. Amrouch; K. Amrouch 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar F. Mouthereau; F. Mouthereau 1Université P. et M. Curie-Paris 6, Laboratoire de Tectonique, UMR 7072 CNRS, Paris, France Search for other works by this author on: GSW Google Scholar L. Dissez L. Dissez 27/99 Wellington Street, Bondi, NSW 2026, Australia Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (3): 263–266. https://doi.org/10.1130/G23173A.1 Article history received: 14 Aug 2006 rev-recd: 13 Nov 2006 accepted: 20 Nov 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation O. Lacombe, K. Amrouch, F. Mouthereau, L. Dissez; Calcite twinning constraints on late Neogene stress patterns and deformation mechanisms in the active Zagros collision belt. Geology 2007;; 35 (3): 263–266. doi: https://doi.org/10.1130/G23173A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mechanically induced calcite twins in veins and host rocks of Late Cretaceous to Miocene age in Iran have been used to determine regional Arabia-Eurasia collisional stresses. A late folding stress regime with a compression oriented 025° (±15°) has been identified across the Zagros belt and the southern Iranian Plateau. This late Neogene stress pattern agrees with the current stress field determined from the focal mechanisms of basement earthquakes and suggests that the Hormuz salt décollement poorly decouples the basement and cover stress fields. Our data show that the collisional state of stress has been relatively constant since ca. 5 Ma. The magnitudes of the stresses obtained from the twinning analysis are unexpectedly low, and, to a first approximation, they are constant across the Zagros simply folded belt. This result supports an overall mechanism of buckling of the detached Zagros cover. Internal viscous-plastic processes help to relieve stress within this cover, thus lowering its seismogenic potential. Beyond these regional implications, this study underlines the potential of paleostress analyses in constraining both the tectonics and the mechanics of ancient and active foreland fold belts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleo-deviatoric stress magnitudes from calcite twins and related structural permeability evolution in minor faults: Example from the toarcian shale of the French Causses Basin, Aveyron, France

The relationship between deformation and so-called fluid paleotransfers in minor faults has been analysed in an argillaceous formation located in the Causses Basin in France. The fluid paleotransfers are related to the fault activity to a large extent. We attempt to estimate the intensity of paleo-deviatoric stress magnitudes under which the fault activity may have occurred and consequently, the change in the structural fault permeability. The paleo-deviatoric stress magnitudes were calculated with the inverse method of Etchecopar applied to calcite twinning. The measured crystals are contained within the core zone of minor faults and this study is based on a previous complete microtectonic and microstructural analysis of the faults. In this paper, analysis of calcite twinning has been applied for the first time to vein fillings associated small faults in a context of relatively weak deformation, a condition ensured by the tectonic and structural analysis. Calculation and discussion of the paleo-deviatoric stress tensors in relation to the evolution of the structural fault permeability and to the hydraulic behaviour of the faults are the aim of this paper. The analysed faults, created and active during the same tectonic event, were permeable under a ( σ 1– σ 3) mean value of 40–50 MPa. On the other hand, the reactivation of faults during a second tectonic event implies mean ( σ 1– σ 3) value higher than 40–50 MPa, especially for the faults that are poorly oriented with respect to the principal tectonic stress directions. The core zone of these faults remained sealed and impermeable or became permeable by development of microcracks inside the pre-existing fillings.

Comparison of paleostress magnitudes from calcite twins with contemporary stress magnitudes and frictional sliding criteria in the continental crust: Mechanical implications

The evolution with depth of paleo-differential stress magnitudes (mostly derived from calcite twinning paleopiezometry) in various tectonic settings is compared to the modern differential stress/depth gradients deduced from in situ measurements. Despite dispersion, both independent sets of stress data support to a first-order that the strength of the continental crust down to the brittle-ductile transition is generally controlled by frictional sliding on well-oriented pre-existing faults with frictional coefficients of 0.6–0.9 under hydrostatic fluid pressure. Some ductile mechanisms may, however, relieve stress and keep stress level beyond the frictional yield, as for instance in the detached cover of forelands. The main conclusion is that despite inherent differences, contemporary stress and paleostress data can be combined to bring useful information on the strength and mechanical behaviour of the upper continental crust over times scales of several tens of Ma.

Exhumation of high-pressure rocks under continuous compression: a working hypothesis for the southern Hellenides (central Crete, Greece)

Combined kinematic, structural and palaeostress (calcite twinning, fault-slip data) analyses are used to study the exhumation mechanism of the high-pressure rocks exposed on the island of Crete (southern Aegean, Greece). Our study shows that the evolution of windows in central Crete was controlled by two main contractional phases of deformation. The first phase (D1) was related to the ductile-stage of exhumation. NNW–SSE compression during D1 caused layer- and transport-parallel shortening in the upper thrust sheets, resulting in nappe stacking via low-angle thrusting. Synchronously, intracontinental subduction led to high-pressure metamorphism which, however, did not affect the most external parts of the southern Hellenides. Subsequent upward ductile extrusion of high-pressure rocks was characterized by both down-section increase of strain and up-section increase of the pure shear component. The second phase (D2) was associated with the brittle-stage of exhumation. D2 was governed by NNE–SSW compression and involved conspicuous thrust-related folding, considerable tectonic imbrication and formation of a Middle Miocene basin. The major D2-related Psiloritis Thrust cross-cuts the entire nappe pile, and its trajectory partially follows and reworks the D1-related contact between upper and lower (high-pressure) tectonic units. Eduction and doming of the Talea Window was accompanied by gravity sliding of the upper thrust sheets and by out-of-the-syncline thrusting. Late-orogenic collapse also contributed to the exhumation process. Therefore, it seems that the high-pressure rocks of central Crete were exhumed under continuous compression and that the role of extension was previously overestimated.

Calcite e-twins as markers of recent tectonics: insights from Quaternary karstic deposits from SE Spain

Superficial Pleistocene karstic coatings found in the boundary region between the Iberian and Betic Chains (Spain) are often offset by normal faults. These travertines were sampled to investigate whether mechanical twins in calcite can develop at low paleodepths. Surprisingly, twinned grains are very abundant in sparry grains, which are found in the proximity of the fault planes (mean twin densities 28 twins/mm). Paleostress determinations based on the analysis of calcite e-twinning yielded the same principal stress orientations as the fault population analysis, i.e. a NW–SE extension tectonic regime. Therefore, calcite e-twin analysis is a useful method with which to establish the orientation of recent stress tensors. Calcite e-twins develop under conditions of minimum overburden (∼300 m) during fault slip. The low differential stresses during the fault slip event suggest that the critical resolved shear stress on the twin plane must be lower than the expected mean value of 10 MPa or that local stresses experienced by rock in and near the fault zone exceed ambient stress conditions during fault slip.

Paleostress fields from calcite twins in the Pyeongan Supergroup, South Korea

Abstract Paleostress fields of the Late Paleozoic to Early Mesozoic Pyeongan Supergroup that is distributed along the northeastern part of the Ogcheon Belt in South Korea were investigated using the calcite strain gauge (CSG) technique. Combining the results of this study with those of other studies investigating the relationship between twin strain, twin density and width, which are used as indicators of deformation conditions in the natural low‐temperature deformation of limestone, it was estimated that calcite twins in the study area were probably formed at temperatures lower than 170°C. From two samples, two different principal paleostress directions were inferred from calcite twins, while only one direction was inferred from two other samples. This result suggests that deformation occurred during two or more different tectonic events in the Pyeongan Supergroup during the Mesozoic era. The maximum shortening axis was oriented in two directions, northeast–southwest and northwest–southeast, respectively, which coincide well with the paleostress directions inferred from the stress inversion for many fault sets. Combining the results of the paleostress analysis from this and other studies, we hypothesize that the directions of the maximum shortening axis in the Pyeongan Supergroup changed from northeast–southwest during the pre‐Daebo orogeny period (Late Triassic period) to northwest–southeast during the syn‐Daebo orogeny period (Early Jurassic to Early Cretaceous period) in the Mesozoic era.

Intraplate paleostresses reconstructed with calcite twinning and faulting: improved method and application to the eastern Paris Basin (Lorraine, France)

This study deals with the reconstruction of intraplate paleostresses. Calcite twinning inversion provides paleostress directions and differential values, even in poorly outcropping areas. However, existing methods are not well adapted to multi-phase deformation cases, and the calculation of stress values provides unreliable estimates. Modifications were brought to a calcite twinning inverse method and to the processing of results. In a new twin data separation process, the whole set of data is used to calculate each tensor. The variability of yield stress value for twinning with strain hardening is taken into consideration. A first order yield stress-calcite grain size dependence function has been established with a sample showing various grain sizes. This modified method of calcite twinning inversion was applied in the eastern Paris Basin (Lorraine, France). In this platform area, measured fractures are preferentially trending NE–SW and NW–SE at all scales. The Meso-Cenozoic paleostress succession reconstructed with fault slips is complex. Jurassic sedimentation occurred during the Tethys rifting. A mainly strike–slip phase with σ 1 successively striking NNW and NNE is attributed to the Tertiary Pyrenean orogenesis, and is followed by WNW extension in the Oligocene. After local ENE compression, WNW compression was probably due to the Alpine Late Miocene collision. The calcite twinning inversion has confirmed these tectonic events and their succession order, and the directions of stress axes were specified. Using grain size- and strain rate-sensitive yield stress values, the differential stresses can be estimated with an uncertainty of few MPa. They range between 20 and 30 MPa, depending on the tectonic event. The Oligocene paleodepths of the sampled calcite, estimated using differential stress values, are structurally sensible. Thus, the original calcite twinning inverse method has been improved. Complex paleostress successions such as in platform areas may be deciphered by combination with fault slip inversion. Even if additional improvement is possible, this calcite twinning inverse method opens new ways to determine stress distributions in tectonic plates.

Calcite twin morphology: a low-temperature deformation geothermometer

Twinning of the e-plane is the dominant crystal–plastic deformation mechanism in calcite deformed below about 400 °C. Calcite in a twin domain has a different crystallographic orientation from the host calcite grain. So-called thin twins appear as thin black lines when viewed parallel to the twin plane at 200–320× magnification under a petrographic microscope. Thick twins viewed in the same way have a microscopically visible width of twinned material between black lines. Calcite e-twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse-grained calcite. In this paper, we present a compilation and analysis of data from limestones of the frontal Alps (France and Switzerland) and the Appalachian Valley and Ridge and Plateau provinces (eastern United States) to document this temperature dependence. Mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 °C and thick twins dominate above 200 °C. Above 250 °C dynamic recrystallization is an important deformation mechanism in calcite. Mean twin intensity (twin planes/mm) correlates negatively with temperature, and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation. These relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data.

Tectonics of the Northern Bresse region (France) during the Alpine cycle

Combining fieldwork and surface data, we have reconstructed the Cenozoic structural and tectonic evolution of the Northern Bresse. Analysis of drainage network geometry allowed to detect three major fault zones trending NE–SW, E–W and NW–SE, and smooth folds with NNE trending axes, all corroborated with shallow well data in the graben and fieldwork on edges. Cenozoic paleostress succession was determined through fault slip and calcite twin inversions, taking into account data of relative chronology. A N–S major compression, attributed to the Pyrenean orogenesis, has activated strike-slip faults trending NNE along the western edge and NE–SW in the graben. After a transitional minor E–W trending extension, the Oligocene WNW extension has structured the graben by a collapse along NNE to NE–SW normal faults. A local NNW extension closes this phase. The Alpine collision has led to an ENE compression at Early Miocene. The following WNW trending major compression has generated shallow deformation in Bresse, but no deformation along the western edge. The calculation of potential reactivation of pre-existing faults enables to propose a structural sketch map for this event, with a NE–SW trending transfer fault zone, inactivity of the NNE edge faults, and possibly large wavelength folding, which could explain the deposit agency and repartition of Miocene to Quaternary deformation.

Strain partitioning of deformation mechanisms in limestones: examining the relationship of strain and anisotropy of magnetic susceptibility (AMS)

In order to investigate the relationship between rock strain and anisotropy of magnetic susceptibility (AMS), strain partitioning and AMS analysis was conducted at 35 sites from two stratigraphically adjacent Paleozoic limestone units in the Patterson Creek and Wills Mountain anticlines in the central Appalachian orogen of West Virginia. In addition, anisotropy of anhysteretic remanent magnetization (AARM) was conducted on selected samples to examine the role of preferentially oriented magnetite on the AMS fabric. Strain is partitioned into bed-normal shortening due to compaction solution strain (≤35.0% shortening), bed-parallel shortening due to tectonic solution strain (≤13.3% shortening), calcite twinning strain (≤5.8% shortening), and grain-boundary-sliding (≤26.7% shortening). The AMS fabrics in the rocks were found to be a result of a complex interaction between rock lithology, deformation mechanisms, and strain magnitude. Although all the rocks have experienced the same deformation conditions, six different AMS fabrics are exhibited. Each of the different AMS fabrics is a composite fabric resulting from the overprinting of three components: (1) an inherent primary depositional AMS fabric that is attributed to preferentially oriented phyllosilicates in the rock matrix; (2) a diagenetic and/or compaction AMS fabric formed during burial that is due to preferentially oriented phyllosilicates in solution structures and in the rock matrix; and (3) a tectonic AMS fabric that was imparted on the rocks by layer-parallel-shortening deformation prior to folding, and is also attributed to preferentially oriented phyllosilicates in solution structures and in the rock matrix, as well as twinning of ferroan calcite.

Synfolding remagnetization and deformation: results from Palaeozoic sedimentary rocks in West Virginia

SUMMARY Palaeomagnetic analysis was conducted at 27 sites from two stratigraphically adjacent Palaeozoic units along two transects (Meldey and Rig) of the Patterson Creek anticline in the Appalachian orogen of West Virginia to better understand the relationship between deformation and remagnetization. While no primary directions were observed, we isolated identical, well-defined, secondary Late Palaeozoic magnetizations in carbonates from the Late Silurian Tonoloway Formation and the Early Devonian Helderberg Group. Standard application of conventional palaeomagnetic fold tests suggests that the magnetizations are synfolding in the Helderberg Group and pre-folding to early synfolding in the Tonoloway Formation. A fold test on all data combined indicates a synfolding magnetization (best grouping at 60–65 per cent unfolding), but the resulting distributions are not circular. An alternative interpretation, based on optimal differential unfolding at the site mean level yields one group with a nearly pre-folding, or early synfolding, magnetization (>70 per cent unfolding) that is both circular and concordant with North America's apparent polar wander path. The second group has a ‘variable synfolding’ magnetization that is elliptical at the optimum unfolding level and falls off the path. Differences in either the age of remagnetization or viscous partial thermoremanent magnetization contamination cannot account for this result. The sites falling into the pre-folding group are biased towards the Tonoloway Formation, the Medley Transect, the northwest limb, away from small folds and lower dip angle relative to the sites in the synfolding group. The two groups defined by the fold test results also have a relationship with both total strain and strain partitioning. The rocks containing the early synfolding remanence exhibit less total strain and, in particular, less pressure solution strain relative to grain boundary sliding and calcite twinning. It is likely that the remagnetization in the entire area was coincident and occurred prior to folding even though some sites reveal synfolding directions. The variation in the behaviour of the remagnetization relative to folding is unclear at present but it indicates that pressure solution and stylolite formation are important processes in remagnetization. In any event, the relationship between deformation and magnetization is not simple. Apparently identical magnetizations from adjacent sites may have very different histories and/or responses to deformation. While the exact relationship between remagnetization and deformation remains elusive this study demonstrates that significant variation in behaviour can exist at a very local scale.

Strain analysis from calcite e-twins in the Cameros basin, NW Iberian Chain, Spain

Calcite twins and c-axes from sparry grains that fill micro-veins were used to unravel the deformational history of the Cameros Massif, an inverted Mesozoic sedimentary basin located in the northwestern part of the Iberian Chain (Spain). Calcite c-axis and microvein orientations indicate the existence of two extension directions (NW–SE and NE–SW) during the basin formation probably related to the slip along two syn-sedimentary faults. Calcite e-twins were developed only in micro-veins from the frontal ramp area, i.e. in the northeast side of the basin, where the stresses were higher during the basin inversion. Twin analysis yields three different orientations (NW–SE, E–W and NE–SW) for the minimum horizontal extension direction (i.e. Shmax under coaxial conditions), which suggests that the basin inversion was developed through several thrusting episodes, during which the frontal and lateral ramps of each one of these episodes are interchanged. Moreover, the perpendicularity between c-axes and the minimum extension directions suggests that the basin inversion was produced along the same faults that controlled the basin formation. The minimum extension directions deduced from e-twins are also consistent with the shortening directions established previously for the Tertiary evolution of the Iberian Chain, which suggest that the basin inversion involved several tectonic events during the Neogene. These results show the advantage of the combined c-axis and e-twins analysis to unravel deformational and kinematic histories.

Paleostress magnitudes associated with development of mountain belts: Insights from tectonic analyses of calcite twins in the Taiwan Foothills

Determinations of stress magnitudes based on inversion of calcite twin data from fault‐related folds in the Taiwan Foothills show a decrease of differential stresses during synfolding erosion and exhumation, suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks. Regional frictional conditions linked to shallow and/or deep décollement tectonics probably also influenced stress magnitudes. Combination of the twin inversion technique with fracture analysis and rock mechanics data provides first‐order estimates of principal stress values related to the major Pleistocene shortening event in Taiwan. These stress estimates are compared to previous stress estimates in fold‐thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens. At the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland. This decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation. Differential stresses estimated from natural deformation consequently provide an independent support to the depth‐dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes.

Towards pyrrhotite/magnetite geothermometry in low-grade metamorphic carbonates of the Tethyan Himalayas (Shiar Khola, Central Nepal)

In Mesozoic metacarbonates of the Tethyan Himalayas (Shiar Khola area, Central Nepal) two characteristic remanent magnetisations (ChRM1 and ChRM2) were identified by their unblocking temperature spectra. The ChRM1 is carried by pyrrhotite (unblocking temperature: 270–360°C) and the ChRM2 by magnetite (unblocking temperature spectra: 430–580°C). The temperature-related formation of pyrrhotite at the expense of primary magnetite during low-grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite/magnetite ratio. This new method can be used as a geothermometer for T≤300°C in low-grade metamorphic carbonates, where other methods are not available. This method is applied for the first time in the Tethyan Himalayas of Central Nepal.In the Shiar Khola valley, systematic variations in the ferrimagnetic content of the metacarbonates along an E–W profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite, derived from natural remanent magnetisation (RPYR/MAG) and saturation magnetisation (SPYR/MAG). Over a stretch of 10km the RPYR/MAG and SPYR/MAG increase from W to E from ~0.42 to ~0.91 and ~0.48 to ~1.0, respectively. Based on temperature estimates, the eastern part experienced upper anchizone–epizone (~250–300°C) conditions, while the western part underwent only diagenesis (~200°C). The temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non-magnetic method.

Transverse fold evolution in the External Sierra, southern Pyrenees, Spain

Fault-slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern margins of the Jaca basin, southern Pyrenees, Spain. The Spanish Pyrenean foreland consists of thrust sheets and leading-edge décollement folds which developed within piggyback basins. Guara Formation limestones on the margins of the Jaca basin were deposited synchronously with deformation and are exposed in the External Sierra. Within the transverse folds, principal shortening axes determined from P and T dihedra plots of fault-slip data show a shift from steep shortening in stratigraphically older beds to NNE–SSW horizontal shortening in younger beds. Older strata are characterized by extensional faults interpreted to result from halotectonic (salt tectonics) deformation, whereas younger strata are characterized by contraction and strike-slip faults interpreted to result from thrust sheet emplacement. The interpretation of the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes determined from finite strain analysis, calcite twins, and regional thrusting directions determined from fault-related folds and slickenlines. This study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny.

Determination of deviatoric stress tensors based on inversion of calcite twin data from experimentally deformed monophase samples. Part II. Axial and triaxial stress experiments

Three samples of a bioclastic bajocian limestone from Burgundy (France) were experimentally deformed under axial and triaxial conditions at 200 and 300°C. In each sample, a series of three mutually orthogonal thin sections were studied using a U stage. The intracrystalline deformation of the calcite grains involved mainly twinning on the e planes. Precise measurement of the e twin lamellae combined with the Etchecopar inverse method (1984) allowed the calculation of the four parameters of the reduced stress tensor responsible for the twinning. As the principal differential stress values of the experimental deviatoric stress tensors were known, the critical resolved shear stress (CRSS) values τs for twinning on the e planes were then determined in each case: at 200°C and 4.8% strain, τs is close to 15MPa; at 300°C and at 8.7 and 9.4% strain, the calculated τs values are close to 10.1 and 13.4MPa, respectively. When compared with the data published by Turner et al. (1954), Turner and Heard (1965) and Lacombe and Laurent (1996), our results confirm that the CRSS values are more or less dependent on both the temperature and the bulk amount of strain. Consequences for the estimation of the actual principal differential stress values (σ1−σ3) are described, particularly for very weakly deformed cover rocks in the forelands of mountain chains.

Cenozoic folding and faulting in the south Aquitaine Basin (France): insights from combined structural and paleostress analyses

New fieldwork, surface data (e.g. drainage network anomalies) and SPOT satellite imagery are combined with sub-surface data (seismic profiles and drill-cores) to analyse the structural setting of the south Aquitaine Basin. Cenozoic paleostresses are determined through inversion of fault slip and calcite twin data (quarries and drill cores), allowing reconstruction of the Cenozoic structural and tectonic evolution. The main tectonic event, the NNE ‘Pyrenean compression’, from the Late Cretaceous to the Oligocene, is responsible for thrusting and folding along N110° axes and strike-slip reactivation of major NNW and NE–SW faults. Some fold axes turn along NNW major wrench faults, and compression locally undergoes deviation to ENE trends. NNE extension locally occurred at anticline hinges. After a minor WNW extension, a Miocene NNW compression occurred and changed into a perpendicular ENE extension, responsible for nearly N–S normal faulting. These multiple states of stress reflect two major compressional events (NNE and NNW); their variety mainly reveals local accommodation due to numerous inherited structures, in the general context of Eurasia–Africa convergence.

Mechanical twinning in calcite considered with the concept of ferroelasticity

The mechanical twinning of calcite is compared with ferroelastic behavior. For calcite a paraelastic prototype phase does not exist, therefore a virtual prototype is defined with cubic m3m symmetry. Using this condition the strain tensors of all domain states of mechanical twinning are calculated. With the use of the strain tensors, application of Sapriel's strain compatibility law gives the crystallographic orientations of all possible twin planes between different domain states. The findings indicate that the twin walls are {1 0 0} and {1 1 0} planes with respect to the morphological rhombohedral lattice. These twin plane orientations from the equated strain tensors are in accordance with the r-and e-twin systems commonly observed in calcite.