Pebble Deformation Research Articles

High-strain deformation of conglomerates: Numerical modelling, strain analysis, and an example from the Wutai Mountains, North China Craton

Conglomerates have been widely used to investigate deformation history and rheology, strain, vorticity and viscosity. Previous studies reveal that several factors, such as pebble shapes and concentrations, as well as material properties, affect conglomerate deformation. However, how pebble concentration and interaction between pebbles affect deformation is not understood very well. We use the 2D numerical modelling platform ELLE coupled to the full field crystal visco-plasticity code (VPFFT) to simulate the deformation of conglomerates with various viscosity contrasts between pebbles and matrix and different pebble concentrations, with both linear (stress exponent n = 1) and power-law (n = 3) viscous rheologies, under simple shear conditions up to a shear strain of ten. Pebbles can behave as effectively passive, deformable or effectively rigid. An increase in pebble concentrations/viscosity contrasts enhances pebble deformation, but reduces their rotation. A mean aspect ratio (Rf) - orientation (ϕ) plot is proposed to gain an estimate of pebble deformation behaviour and the amount of bulk strain. Closely spaced rigid or deformable pebbles can form clusters that mechanically act as single inclusions. Rigid clusters rotate and survive for only short strain increments, whereas the more stable deformable ones keep on elongating with minor rotation. We provide a natural example of deformed conglomerates from the Wutai Mountains, North China Craton. These consist of banded-iron-formation (BIF) pebbles embedded in a schistose matrix. Using the mean Rf-ϕ plot, a finite strain of ∼6 under simple shear could be determined. The viscosity of the pebbles is estimated at about 5–8 times that of the matrix for a linear rheology (n = 1), or 2 to 5 times if a power-law rheology with n = 3 is assumed.

Study on the Thermally-Induced Stress and Relaxation of Ceramic Breeder Pebble Beds

Ceramic breeder pebble beds undergo complex thermally-induced stress build-up and relaxation processes during reactor operations due to the pebble bed thermal expansion and creep deformation. Understanding such processes can facilitate the evaluation of a solid breeder performance, including bed stress/strain equilibrium status, which will guide the design of stable blanket operation and assessment of lifetime. The efforts of this study cover both experimental testing and numerical modeling for this purpose. Measured stresses in pebble beds show a decreasing trend with thermal cycles, until ultimately reaching a saturated state. This stress relaxation is mainly caused by the combined effect of bed plastic rearrangement and accumulation of creep deformation under compressive stresses and high temperatures. As bed stress is reduced, the creep deformation becomes less significant and further cyclic operation would not alter the pebble bed mechanical state. To validate the thermally-induced stress and its variation with cycles, experiments of thermal stress measurement have been designed and conducted for pebble beds heated by both continuous and pulsed power sources. Moreover, the effects of mechanical pre-compaction were investigated with emphasis on understanding the relationship between the bed stress-state evolution and maintaining adequate levels of thermal contact between the pebbles and the coolant structure. The results of this study presents valuable data to serve as a basis for validation of the most recent pebble bed numerical models.

内蒙古大青山水磨沟发现末次冰期冰川堆积物

The current Quaternary glaciers in China only distribute on the high mountains surrounding the Tibetan Plateau in the west of 105°E. In the east of 105°E, glaciers only exist in Taibai Mountain, Changbai Mountain, and Snow Mountain and Yushan Mountain in Taiwan. Whether the Quaternary glaciation reached the lower elevation foothills and uplands in the east of 105°E is usually causing intense disputations among the researchers. This discovery of the glacier deposits in the Shuimogou areas of the Daqing Shan Mountain has provided confident evidence that the glaciers did cover the lower elevation hill ranges during the late Quaternary glaciation. Daqing Shan Mountain is located in the middle part of the Yinshan Mountain, Inner Mongolia. The highest peak is 2200 m above see-level. Remote sensing images indicate that there are two glacier eroded depressions within the Shuimogou valley. In the northern depression, the glacier deposits distribute around 500 m in NS direction and 400 m in EW direction and overlap the Proterozoic marble. The glacial deposits are mainly brownish red tillites composed with 60% sandy mud and 40% pebbles. There is no clear bedding within these morainic sediments. Pebbles have poor sorting, well rounding, and are cemented by mud and muddy sand. The major components of the pebbles are granite, gneiss, diabase, plagio-amphibolite. No underlying Proterozoic marble pebble has been found in the tillites, so the tillite components should experience long-distance transportation from their source regions. The pebbles have following characters:(1) the smooth pebble surface is coated with brownish red iron jacket;(2) the glacial striae on the pebble surface show random directions;(3) on all of outcrops, almost all pebbles with the size larger than 10 cm show the fractures of the strong extensional twisted shear deformation. The presence of intensive twisted pebbles distributing in the loose sediments indicates that the pebbles must be deformed during their transportation—glacier movement. During the outcrop creep of the glacier, the shearing force from the squirming of the ice tore the pebbles and created the pebble deformation. The tillites are covered by the landslides from nearby hill slopes and the flood deposits. A 0.5 cm thick of dark gray mud bed is interlayering within the middle of the flood deposits. The clays in the mud have been sampled for 14C-AMS age dating(DQS04-1). The age dating was performed in Beta laboratory. The 14 C age was corrected by 2? and the results are 13565 to 13440 cal a BP. This result indicates that the morainic debris deposited before 13.5 ka. These Daqing Shan tillites should form from the latest Quaternary glaciation. The identification of these glacier deposits proved the existence of Quaternary glacier in the foothill and highland area, east of the 105°E longitude around the 41°N latitude region. This discovery has significant importance for the study of geological history in this region and shines a light on the research of the climate evolution in East Asia and the Quaternary environment change in China.

A Measurement Platform Scheme and Data Post-processing Method for Thermal Conductivity of Li4SiO4 Pebble Bed

The effective thermal conductivity of tritium breeder pebble bed is an important thermal parameter and must be known for the thermo-mechanical design of solid tritium breeder blankets. In order to obtain the parameter, experimental measurement is an effective method. A measurement platform was designed by University of Science and Technology of China for CFETR solid blanket scheme to measure the immediate thermal conductivity data and study the effect of pebble bed temperature, the purge gas pressure and pebble deformation on the thermal conductivity of pebble bed. Measurements were performed based on about 1 mm diameter Li4SiO4 pebbles in the temperature range between 100 and 800 °C, with purge gas pressure ranging from 0.1 to 0.3 MPa. This paper described a measurement platform scheme by thermal probe method. On the other hand, for the sake of increasing the precision of thermal conductivity data transformed from temperature data, some improvements for the data post-processing using Monte Carlo inversion method were made in this paper too.

Reappraisal of strain estimations and measurement methods in the Hammamat Group sediments: Comparison of primary and secondary grain fabrics with new data from Wadi Zeidun and Wadi Arak basins, CED, Egypt

In this study, pebble and sand grain shape and orientation data from undeformed and deformed conglomerates and sandstones in the Egyptian Central Eastern Desert (CED) Wadi Zeidun and Wadi Arak basins were collected, with the aim of providing more accurate tectonic strain estimations for deformed Hammamat sediments by comparing their primary and secondary sedimentary fabrics. Common assumptions about primary pebble fabrics and pebble deformation behaviour in previous strain studies of the region are shown to be invalid. This raises doubts about the accuracy of earlier reported strain estimates for the Hammamat that were based on such assumptions. For low strains in the Hammamat sediments, the best strain measurement method is normalized Fry (NFry) analysis of sand grain centre-to-centre distance data. An alternative is conventional Fry analysis of pebble ellipse centre-to-centre distance data, provided that large numbers of pebbles (∼300) are used, and the pebbles form closely packed aggregates with high degrees of anticlustering. This study also provides systematic guidelines for pebble axial length and orientation measurements in the field. The progressive changes in the mechanical behaviour of pebbles with increasing bulk strain are discussed.

Pliocene to Quaternary deformation in the Var Basin (Nice, SE France) and its interpretation in terms of “slow-active” faulting

Seismic hazard assessment of active faults in slow orogenic domains is a challenging issue. In this paper we present a multi-disciplinary approach based on a Digital Elevation Model (DEM), 3D-geological modelling, fracture analysis, and strain analysis of pebbles in a Pliocene molasse basin. The basin is cross-cut by "slow-active" faults of the Donaréo and St Blaise-Aspremont fault system. The DEM shows a topographic disturbance emphasized by slope gradients and the drainage system, which is ascribed to the Plio-Quaternary fault trace. Fracturation analysis evidences two fault corridors oriented approximately N150°E and N20°E. Paleo-stress analysis provides orientations similar to those derived from the focal mechanisms of current regional seismicity, with the main stress σ1 oriented N20°E and a (σ2 − σ3)/(σ1 − σ3) ratio of 0.31. The σ2 versus σ3 permutations are in agreement with ongoing strike-slip deformation at least since the early Pliocene. Discontinuous fracturation and comparison with seismic monitoring on regional active fault zones suggest that shallow seismicity may be expressed by low-magnitude (Mw < 4) seismic swarms. Deformation of pebbles occurs mainly by pressure-dissolution processes. Pebble striation orientations show a bimodal distribution, parallel to the two fault strands. Pebble deformation and the paucity of striated surfaces along the main faults suggests rare seismic deformation and long-lasting aseismic creep processes. Geometrical 3D analysis shows the formation and migration of a Plio-Quaternary basin about 500 metres east of the main fault system, together with folding and tilting of the post-Messinian Pliocene molasse. These observations indicate that the fault remained active from the Pliocene to the Quaternary, and possibly up to the present time. However, the estimates of the minimum slip rate on the faults of about 0.02 mm a−1 vertical and 0.03 mm a−1 horizontal are unlikely to produce any significant high-magnitude earthquakes, but rather swarm-like low-magnitude seismicity with long temporal recurrence.

Characterization of High Temperature Mechanical Properties for Lithium Metatitanate Pebble Beds

Lithium metatitanate (Li2TiO3) is a candidate tritium breeding material for fusion reactor blankets. In an actual blanket the pebbles are geometrically confined in a containing structure subjected to displacement and stress constraints under operating conditions. For an accurate description of pebble behavior during operations, existing data of pebble deformation under compressive loads is inadequate. In this study a uniaxial compression experimental apparatus was built to investigate time dependent creep on a single pebble as a function of applied force and temperature relevant to blanket conditions. Experimental results shows that at a temperature range of 700-800 °C, the crush load of Li2TiO3 pebbles is reduced by about half of its room temperature crush load. The experimental results allow for the establishment of a pebble creep failure map as a function of force and temperature, which provides guidance to the blanket designs. The study also introduces an FEA model based on experimental deformation data to obtain the constitutive equation needed for Discrete Element Model simulation of the pebble bed.

Effects of Thermal Expansion Mismatch on Solid Breeder Blanket Pebble Bed and Structural Clad Thermomechanics Interactions

In this paper, a thermomechanical interaction of a ceramic breeder pebble bed and structural plate is studied based on a recently developed discrete numerical simulation code. The calculations take into account the coupling effect between structural wall deformation and pebble bed deformation, which greatly increases the computing time and complicates the computational procedures in determining the particle-wall contact characteristics. Specifically, the model is applied to a lithium orthosilicate packed bed for the evaluation of the deformation of a circular plate due to bed thermal expansion, while the results are compared with SCATOLA's experimental data. Numerical results using a fixed boundary condition show reasonable agreement with the experimental data. In addition, numerical simulations confirm an irreversible plate deformation after a thermal cycle run as observed in the experiments, although these experimentally observed deformations are larger than that of the numerical estimations. Moreover, numerical results of contact forces at contact points provide information concerning the mechanical integrity of the bed at a moderately high temperature.

Neoproterozoic Deformation at a Boundary Zone Between the Nellore-Khammam Schist Belt and Pakhal Basin, SE India: Strain Analysis of Deformed Pebbles

In the Kinnerasani area in southeastern India, the terrain boundary between the Archean Nellore-Khammam Schist Belt and the Proterozoic Pakhal Supergroup overlying the Dharwar-Bastar cratons can be observed. We analyzed the mesoscopic and microscopic structural features of the highly deformed pebbles in the basal conglomerate bed of the Pakhal Supergroup that occurs at the terrain boundary. The results of the analysis of the pebbles suggest that: 1) deformation of pebbles resulted from ductile deformation during peak metamorphism 2) the mode of strain is plane strain to constrictive and maximum elongation located to be vertical and 3) the apparent stretch of the pebbles is up to ∼ 300%. In the Nellore-Khammam Schist Belt, quartz grains constituting the quartz layer of the feldspathized gneiss folded by the last-phase deformation also show vertical maximum stretching in constrictive strain. This observation suggests that the deformational features, at least the mode of strain, during the last-phase deformation is comparable to the deformation forming elongated pebbles of the Pakhal conglomerate. The last-phase deformation structures of the Nellore-Khammam Schist Belt are well observed near the terrain boundary. This indicates that the Pakhal deformation overprinted the rocks of the Nellore-Khammam Schist Belt near the boundary, and that their tectonic juxtaposition occurred during or before this deformation period. Because the Pakhal deformation took place during or soon after the peak metamorphism of the Pakhal Supergroup, which is known to be ∼1000 Ma, and the last metamorphism of the Nellore-Khammam Schist Belt in the Khammam area were reported to be ∼1100 Ma. The tectonic juxtaposition between the Pakhal Supergroup and Nellore-Khammam Schist Belt was around 1000∼1100 Ma.

Strain Analysis in the Basal Unit of Delhi Supergroup in Kishangarh Area (Rajasthan): As Deduced from Deformed Pebbles

Abstract The arkosic quartzites overlying the rocks of Aravalli Supergroup along an unconformity cum thrust contact, exhibit strong deformation resulting in preferred dimensional orientation of the pebbles. The three dimensional strain is of constriction type where the mean value of k varies from 1.33 to 6.52 from locality to locality. The shapes of the pebbles prior to deformation were mainly of constriction type. The path of pebble deformation has been traced. The strain in these pebbles is related to folding and later thrusting (gliding) during the first phase of deformation.

Quartz deformation and the recognition of recrystallization regimes in the Flinton Group conglomerates, Ontario

The clastic metasedimentary rocks that form the Flinton Group occur as arcuate belts within the Grenville Structural Province, southeastern Ontario. Metamorphic isotherms transect these rocks and allow comparison of deformation over an approximate temperature range of 550–650 °C. Two geometrically discrete but temporally continuous deformation phases contribute significantly to pebble deformation within the conglomeratic units. There is a general increase in both finite strain, over which there is a strong compositional control, and the frequency of D2 minor structures with increasing metamorphic temperature. D1 is characterized by the creation of a subgrain structure and progressive misorientation of the subgrains as strain accumulates. Ultimately this leads to formation of high-angle boundaries defining recrystallized grains of a size similar to that of the precursor subgrains. D2 produces a new, finer subgrain structure, elongated grains, grain boundary migration (bulging), and small recrystallized grains. Empirical relationships between stress and subgrain diameter suggest that D1 is a low stress (5–6 MPa) deformation and D2 is a higher stress (12–80 MPa) deformation. This emphasizes that rotation recrystallization defines a low-temperature, low-stress regime, whereas migration recrystallization is typical of higher temperatures and stresses. Although dislocation creep predominates in most pebble types, evaluation of the deformation in terms of non-uniform flow laws can explain the compositional control of strain in the quartzite pebbles and suggests that significant grain boundary sliding occurred.

Deformation of pebbles in relation to associated structures in parts of Singhbhum Shear Zone, Eastern India

The Singhbhum Shear Zone separates the rocks of a highly metamorphosed northern group from an unmetamorphosed southern group. It had been recognised by earlier workers as a thrust zone in which the thrust movements were achieved by slip along a pre-existing schistosity. Deformed conglomerates crop out in discontinuous bands along the Shear Zone. Strain determinations from these conglomerates have been made to ascertain the strain pattern within the Shear Zone. None of the existing methods of determining strain from stretched pebbles can be applied to these conglomerates. An approximate value for shortening perpendicular to the schistosity can however be obtained from cross-cutting buckled veins. Detailed study of various small-scale structures clearly indicates that the principal structures of this region cannot be explained simply by a slipping on the schistosity.

A Discussion on natural strain and geological structure - The control of ductility on the deformation of pebbles and conglomerates

Pebbles are commonly used parameters for the determination of finite strain in deformed rocks. In high grade metamorphic environments, rocks probably behave as viscous fluids and a theory exists which relates the deformation experienced by a pebble to that of the host rocks. However, some deformed conglomerates are found in low grade metamorphic rocks where the assumption of viscous behaviour is unrealistic The deformation of artificial conglomerates made of geological materials, at room temperature and varying confining pressure is described. In these experiments, pebbles deform by cataclasis at surprisingly low applied loads and large finite strains are achieved. The amount of deformation experienced by pebbles of different rock types depends mainly on their yield strengths and ductility contrasts with respect to the matrix. A theoretical analysis assuming that pebble and matrix behave as workhardening Bingham materials during deformation relates the strain experienced by a pebble to that of the host rock. The results suggest that significant pebble deformation can occur during gravitational loading of sediments. An attempt is made to verify this idea by analysing the shape of pebbles in conglomerates of the Upper Witwatersrand System. At some sites the pebbles appear to have deformed during gravitational compaction while at others a tectonic deformation has been superimposed upon the pre-tectomic strain.

Granite diapirism in the Rum Jungle area, Northern Australia

Early studies in the Rum Jungle area suggested an intrusive relationship between the Rum Jungle and Waterhouse “Granites”, and the overlying sediments. It was later shown that the granitic “intrusions” were Archaean basement complexes onto which Lower Proterozoic sediments had been deposited. Polyphase folding was postulated as being responsible for doming of the basement and cover rocks. This paper proposes to show that the domed structures in the Rum Jungle area, and the emplacement of Middle Proterozoic granites in the Pine Creek Geosyncline were related, and caused by diapiric intrusion of granites, in a solid state, into basement complexes and cover rocks. Structural and metamorphic evidence in support of diapiric intrusion in the Rum Jungle area includes: pebble deformation within steeply dipping beds of quartz conglomerate; disappearance of polyphase fold structures away from the basement complexes; bending of folded country-rock strata into concordance with the complex—sediment contact; and metamorphic and metasomatic alteration of sediments in contact with the basement complexes. Gravity data show mass deficiencies in the Archaean complexes which possibly coincide with young granite diapirs.

A Study of Pebble Deformation in the Precambrian Rocks of Singhbhum District, Bihar

Abstract The deformed pebbles in the conglomerate show a large variation in the orientation of their longest axes as well as in the values of the axial ratios a' and b'. This has been interpreted as due to a nonspherical nature of the original pebbles. The effect of deformation has been separated from the effect of the original shape. It has been shown that a superficial analysis of the shape of deformed pebbles may give a wrong idea about the state of strain in the rocks, and the method by which the true amount of strain can be computed from dc:formed pebbles has been discussed.

Pebble deformation and thrusting in the Bygdin area (Southern Norway)

The overthrusting of the Bygdin Conglomerate by the Upper Jotun Nappe and the pebble deformation are broadly coeval. The regional variation of the irrotational component of the pebble strains can be represented by two parameters which define the amount and the symmetry of the strain. Strains caused by the nappe thrusting are apparently absent in the conglomerate and minor structural evidence suggests that most of the pebble deformation post-dated the nappe emplacement and was produced by a static flattening of the conglomerate under the weight of the nappe. The Jotun Nappe also seems to have undergone a comparable flattening deformation. The maximum elongation strain has a northwest-southeast trend and the conglomerate probably flowed out in this direction underneath the nappe front.

Deformation of Pebbles in Lower Old Red Sandstone Conglomerates Adjacent to the Highland Boundary Fault

AbstractThe deformation produced by the pre-Upper Old Red Sandstone movements on the Highland Boundary Fault in a zone close to the fault was sufficiently intense to rupture the pebbles in conglomerates. The patterns of pebble fracturing are similar in type and orientation over a wide area and are comparable with the patterns obtained in the experimental deformation of brittle materials. In experimental deformation a direct relationship has been established between the fractures and the causal stresses and so from a statistical orientation of the fractures in the pebbles the directions of the principal stresses can be deduced.The principal compression in the vicinity of the Highland Boundary Fault acted N.W.–S.E. varying in places to N.N.W.–S.S.E., normal to the trend of the fault, while the least principal stress acted parallel to the strike of the fault.The orientation of the least principal stress parallel to the direction of the fault in the conglomerates arises through increased stress in a vertical direction due to the nature of the downbuckling of the rocks on the southern side of the fault.The orientation of the maximum compression confirms the Highland Boundary Fault as a reverse fault of late Caledonian age rather than a Proto-Armorican wrench fault produced by N.N.E.-S.S.W. compression.

The Lithology and Structure of a Boulder-Bed in the Dalradian of Mayo, Ireland

Summary Exposures of a boulder-bed resembling those of Donegal have been found in North-West Mayo. The boulders are granitic in aspect, though severely affected by cataclasis, and the matrix is foliated and generally semi-pelitic. The probable succession in North-West Mayo and the position of the boulder-bed in it have been determined, but no correlation with other areas has yet been established. For the most part the boulders are about equidimensional, but locally they are intensely deformed. Many of the deformed boulders are markedly elongated and flattened. The elongation is parallel to the regional lineation, which generally plunges to the south-east. Although several analyses of pebble deformation in other areas are interpreted as indicating a main movement parallel to the elongation, it is considered that the Mayo boulders have been rolled and thus extended in b . This conclusion is strongly suggested by more than one line of evidence. Firstly, certain mullion structures (with a south-easterly plunge corresponding to the direction of boulder elongation) were found to have their peripheries determined by complex folds that have required considerable movement at right angles to the mullion trend. Secondly, mapping in North-West Mayo has revealed large-scale folding that plunges to the south-east. Thirdly, the shapes of many of the deformed boulders are not such as would be expected if they had been extended in a , though they might well have been produced by rolling and flattening. Petrofabric investigations have been carried out on some of the boulders, and the relationship between fabric orientation and deformation is briefly discussed in the paper.

Quartzite pebble deformation in central Vermont

Quartzite pebble deformation in central Vermont