Cleavage Front Research Articles

A Paleomagnetic Inspection of the Paleocene-Eocene Thermal Maximum (PETM) in the Southern Pyrenees

Rock magnetic properties can be useful determining paleonvironmental changes. A dramatic climate change as occurred in the PETM modified the environment and hence the magnetic properties recorded in the sediments. New paleomagnetic data from the marine record of the PETM in the Southern Pyrenean Zone shows the variation on the magnetic parameters in four different sections. The magnetic signal reveals a positive excursion of magnetic values starting before the onset of the marly interval of La Faja de las Flores Mb (beginning of the PETM record, Ilerdian) in the Carriata section with a maximum value in the marly interval. A similar magnetic signal is observed in Bujaruelo section (~10 km south of Carriata at PETM times) directly related to the marly interval of Faja de las Flores Mb. However more to the south, the PETM interval does not appear in the sedimentary record, therefore in the southern Gallisue section no magnetic excursion occurs, and in the southernmost studied Entremon section a magnetic positive excursion occurs in a thin marly interval unrelated to the PETM, and in two lower intervals of the column. All sections were subjected to later deformation during the pyrenean orogeny and the three northernmost are in the regional cleavage front where pressure-solution and remagnetizations have been described. A postfolding remagnetization component is found in the three northern sections.

Sixty years of anisotropy of magnetic susceptibility in deformed sedimentary rocks

The use of the Anisotropy of Magnetic Susceptibility (AMS) has become a rather common practice in Earth Sciences since the pioneer note by Graham (1954). The versatility of the technique, and the rapidness in obtaining and processing AMS data largely improved in the past thirty years, and has generated a wealth of literature, notably on mudrock fabrics. The assessment of the current trends in magnetic fabric studies reveals that AMS has one of its largest potential in sedimentary rocks from structural settings where the ductile component of deformation is cryptic or hindered by the brittle component. Abundant evidence provided by AMS data reveal that deformation extents beyond the deformation or cleavage front in contractional settings, including fold-and-thrust belts and active accretionary prisms, configuring magnetic fabrics as a standard method for fabric quantification in deformed sedimentary rocks.

Magnetic fabrics in the Western Central-Pyrenees: An overview

In the Western Central-Pyrenees numerous investigations during the past years have yielded an exceptional high density of localities (more than 700 sites) where the AMS and rock magnetic properties have been determined. This unique AMS dataset helps in understanding the orogenic evolution of the Pyrenees and its foreland basins. Processes related to AMS development in different structural units permit to identify: (i) an early recording of strain related to the transtensional or transpressional stages during the Early Triassic and Early Cretaceous, (ii) a moderate shortening related to the fold-and-thrust belt and foreland basin evolution during the Cretaceous–Tertiary orogenic stage, and (iii) a relatively strong deformation that produced cleavage related to the emplacement of basement thrusts in the Axial Zone during the Middle–Late Eocene. The main achievement of this contribution is the integration in cross-section and map-view of magnetic fabrics (orientation and parameters) in the western Central-Pyrenees, which allow constraining the processes affecting magnetic fabrics at the orogen scale. AMS results indicate: (i) a dominant bedding-controlled foliation, very sensitive to subtle layer parallel shortening (LPS) processes, especially in the foreland basins; (ii) magnetic lineations parallel to fold axes or strike of thrusts outside the cleavage front that can be deflected by vertical-axis rotations in particular areas; and (iii) an overall decrease of the definition of AMS fabrics (magnetic lineation or the anisotropy of the ellipsoid) associated with incipient cleavage and intersection lineation fabrics in the internal zones of the orogen. Therefore, AMS is a sensitive indicator to delineate the strain patterns at the orogen-scale, in particular, in those areas undergoing a modest degree of deformation.

On the origin of cleavage in the Central Pyrenees: Structural and paleo-thermal study

Abstract The present work provides a combined structural and paleothermal (fluid inclusions, illite content in mixed layers illite–smectite, Kubler index and vitrinite reflectance) investigation of the Alpine cleavage domain affecting the western South Pyrenean Zone (Central Pyrenees). Integration of structural and paleothermal data allows us (i) to determine the position of the cleavage front in the sedimentary sequence and (ii) to define mechanisms and temperature conditions for cleavage development. Cleavage in the eastern part of the study area is approximately limited to the forelimb of the Gavarnie hanging wall anticline, but to the West it extends progressively southwards, affecting younger turbiditic units. At regional scale, the cleavage front is oblique to the strike of bedding and the overall trend of macrostructures. Pressure-solution is the main mechanism for cleavage development, and a strong thermal control on the position of the cleavage front is inferred. The transition between sedimentary and tectonic, compressional fabric is marked by vitrinite reflectance values of 1.12–1.14%, Kubler index values between 0.71 and 0.74°Δ2θ and the transition between R1 and R3 mixed layers illite–smectite. A lower thermal boundary of 140–160 °C is suggested for incipient cleavage development in the shaly beds of the turbiditic sequence, as well as an upper boundary constrained by a maximum measured value of about 215 °C.

Transition from extensional to compressional magnetic fabrics in the Cretaceous Cabuérniga basin (North Spain)

The transition between extensional and compressional-driven magnetic fabrics in sedimentary rocks is explored in this paper through the study of an example of the Basque–Cantabrian basin. In the area where extensional structures prevail and no superimposed deformation is observed, except for gentle large-scale folds, the magnetic fabric is interpreted as extensional, in consistency with mesostructural (tension gashes) and macrostructural (large-scale faults) data. Compressional tectonic fabrics are unequivocally interpreted in the area with cleavage development related to the buttressing of the syn-rift sequence against faults located near the northern basin margin. In this area, kmax is oriented according either to the intersection lineation or the dip direction of cleavage planes. In the area located in-between, where no macroscopic evidence of either compression or extension exist, there is a transitional fabric between compressional (resulting from the modification during inversion of a previous sedimentary or extensional fabric) and extensional (inherited from the extensional stage) magnetic fabrics that correlate with subtle evidences at the microscopic scale (pressure shadows, deformation and re-orientation of nodules). Therefore, the magnetic fabric is revealed as an exceptionally sensitive marker of deformation in sedimentary rocks. This transition in the magnetic fabric occurs within a length of 6.25 km along the cross-section that correlates with a thickness of 200 m of the stratigraphic pile. These results indicate that even in the absence of clear structural markers of compressional deformation, extensional magnetic fabrics can be only interpreted when there is a minimum separation (in the vertical or the horizontal) to the cleavage front.

Propagation of a cleavage crack front across a field of persistent grain boundaries

The nonuniform propagation of a cleavage front across a field of persistent grain boundaries is analyzed. When a cleavage crack advances in a field of grains, some of the grain boundaries cannot be directly broken through, which interrupts the crack growth process. When the crack front bypasses such persistent grain boundaries (PGB), the overall crack growth driving force must be increased so that the local stress intensity can overcome the local fracture resistance. A theoretical model is developed based on the R-curve analysis. A closed-form expression of the critical stress intensity factor is given as a function of the line content of PGB.

Nonuniform cleavage cracking across persistent grain boundary

The nonuniform characteristics of cleavage cracking across high-angle grain boundaries are analyzed in considerable detail. To break through a grain boundary, a cleavage front would first penetrate across the boundary at its central part, with the side sections being locally arrested. Such a front behavior causes a strong crack trapping effect and a large increase in required crack growth driving force. Eventually, as the persistent grain boundary areas are separated apart, the crack front bypasses the grain boundary. The critical condition of the unstable crack propagation is determined by both the local fracture resistance and its increase rate with respect to the expansion of the break-through window. The grain boundary toughness is dominated by the effective grain boundary ductility.

Lithology-dependent reliability of AMS analysis: A case study of the Eocene turbidities in the southern Pyrenees (Aragón, Spain)

Fine-grained sediments are generally better strain-markers in the outer part of orogenic belts and foreland basins because they are more prone to cleavage formation than coarse detrital rocks in diagenetic to very-low metamorphic environments. This also commonly applies to magnetic fabric analysis, thus favoring sampling in fine-grained rocks. In this article, this general assumption is tested sampling for anisotropy of magnetic susceptibility (AMS) analysis along different levels of Bouma sequences in turbidites from the Eocene South-Pyrenean turbiditic basin, above and below the cleavage front. Application of other techniques (LTAMS, AGRM and hard-APARM) indicates that the cleavage imprint in AMS fabrics is carried by paramagnetic particles, in this case, by clay minerals. The results obtained show that: (i) directional results of AMS are similar in the different levels of the Bouma sequence; (ii) the paramagnetic contribution to the susceptibility is independent from the lithology; and (iii) the equivalent paramagnetic contribution to susceptibility (detrital sedimentary particles) does not show differences between a to d (sandstones) and e-level (siltstones and shales).

Transposition of structures in the Neoproterozoic Kaoko Belt (NW Namibia) and their absolute timing

Three structural profiles across the Coastal Terrane, the Boundary Igneous Complex and the Orogen Core have been studied in the Kaoko Belt of northwestern Namibia. The oldest known Si fabric is inherited from an older tectono-metamorphic event. It occurs in the Coastal Terrane only and the extent of its reworking increases from south to north. The S1 foliation reactivates or folds Si fabric in the Coastal Terrane and appears as an early planar fabric in granitoids of the Boundary Igneous Complex and migmatites of the Orogen Core domain. Superimposed subvertical S2 fabric corresponds to axial plane cleavage of upright close to isoclinal folds and the extent of its development also increases from south to north. Active migration of partial melt during S2 development in the Orogen Core dates the onset of this deformation at ~550 Ma. Distribution of F2 fold axes and L2 stretching lineations suggests pure shear–dominated deformation associated with development of N–S trending S2 cleavage preserved in the central profile, followed by sinistral simple shear–dominated deformation on newly developed NW–SE trending pervasive cleavage in the northern part of the area. Such spatial variation in the deformation record is attributed to the irregular shape of the Congo Craton indenter that is reflected by heterogeneous development of the S2 cleavage front in the Coastal Terrane and the Boundary Igneous Complex. Common orientation of L1 and L2 stretching lineations and solid-state reworking on both S1 and S2 planes suggest single event of sinistral transpression since 550 Ma with strain partitioning into domains of oblique thrusting (reactivated S1) and transcurrent sinistral shearing (S2 and S3). Such succession of deformation structures suggests that major subvertical shear zones in the Kaoko Belt do not correspond to early crustal discontinuities, but rather reflect late strain localization during cooling.

Resistance to cleavage cracking and subsequent shearing of high-angle grain boundary

In a previous experimental study, it was observed that the break-through process of a cleavage front across a high-angle grain boundary can be highly nonuniform. While the central part of the boundary can be cleaved quite smoothly, the rest parts must be sheared apart. In this paper, the trapping effect of grain boundary shearing is analyzed in considerable detail. Before the shearing is completed, the crack flanks are locally pinned together and a bridging stress must be provided. The bridging stress has a negative contribution to the local stress intensity at the cleavage front segment that penetrates across the grain boundary, and thus the crack growth driving force must be increased. A closed-form equation is derived to relate the overall fracture resistance to the fracture mode through an energy analysis.

Resistance of grain boundary array to cleavage cracking in free-standing thin film

In a previous experimental study on free-standing silicon thin film, it was observed that cleavage front transmission across a through-thickness grain boundary could be considerably constrained by film surfaces. As a result, the boundary toughness was much lower than its bulk counterpart. In this study, inspired by the observation of crack front behaviors at triple grain boundary junctions, we perform a theoretical analysis on the fracture resistance of a regular grain boundary array in a thin film. The result indicates that as the cleavage front breaks down into a number of sections by the grain boundaries, the overall fracture resistance can be increased by nearly 60%. However, if the grain size is too small, the fracture resistance may decrease. The optimum grain size is around 1/5 to 1/4 of the film thickness. This finding provides a scientific basis for further experimental investigation on advanced processing techniques.

Cleavage cracking across triple grain boundary junctions in freestanding silicon thin films

This article is focused on a fractography study of cleavage cracking at triple grain boundary junctions in freestanding silicon thin films. At a triple junction, as the crystallographic orientations of the two grains ahead of the crack are different by only a few degrees, the cleavage front advance becomes quite jerky. The crack first enters the grain of smaller boundary toughness and then turns into the other grain from the lateral direction. Consequently, the overall fracture resistance cannot be analyzed in the framework of line-average theory. The nonuniform characteristic of crack behavior can be attributed to the increase in local stress intensity. A few typical crack front advance modes are identified.

Grain boundary separation in transgranular cleavage cracking

The energy balance associated with the transmission of cleavage front across high angle grain boundary is analysed theoretically. On the one hand, if the distance between adjacent break-through points is small, their crack trapping effect tends to be pronounced. On the other hand, decreasing this distance promotes the separation of persistent grain boundary areas. The effective grain boundary toughness is also dependent on the profiles of penetrating front sections and the surface free energy of grain boundary. The most energetically favourable crack front transmission mode can be reached through a self-adjusting process.

Interaction of cleavage ridges with grain boundaries in polysilicon films

The morphology of cleavage surfaces across grain boundaries in free-standing silicon thin films was investigated. Three ridge–boundary interaction modes were identified. If the cleavage ridge was relatively deep, it could directly bypass a grain boundary. If it was relatively shallow, it would act as a stress concentrator promoting cleavage front transmission. It was also observed that due to the smoothening effect, the surface could become effectively less rough across a grain boundary.

Characteristic length scale in cleavage cracking across high-angle grain boundary

The factors that govern cleavage cracking across high-angle grain boundaries are investigated theoretically. According to previous experimental observations, a cleavage front overcomes the resistance of a high-angle grain boundary by first penetrating across it at a number of break-through points (BTP) and then separating apart persistent grain-boundary islands (PGBI). In the current study, this process is modeled as a competition between grain boundary shearing and crack front transmission. The numerical calculation shows that at a large grain boundary there exists an optimum BTP distance at which the grain boundary toughness is minimized, and when the BTP distance is relatively large its influence is secondary, fitting well with the experimental results.

Secondary cracking at grain boundaries in silicon thin films

In this article, we report irregular cleavage front transmission at grain boundaries in free-standing polysilicon thin films. When the orientations of two adjacent grains are correlated, the crack may bypass the boundary via a “tunneling” process. Similar behavior can also be achieved if the crack path curves in the grain-boundary-affected zone. Moreover, the separation of crack flanks can be aided by secondary cracking. These irregular modes of crack front behavior tend to lower the effective boundary toughness.

Energy equilibrium of cleavage front transmission across a high-angle grain boundary in a free-standing silicon thin film

In this article, the crack growth driving force and the resistance to cleavage cracking associated with crack front transmission across a high-angle grain boundary in a silicon thin film are analyzed, and a closed-form solution of grain boundary toughness is obtained. It is noticed that the fracture resistance of the grain boundary is a function of the film thickness. This size effect is attributed to the nonuniform nature of cleavage front advance.

Mixed-mode cleavage front branching at a high-angle grain boundary

In a previous study it was observed that a cleavage front can penetrate through a high-angle grain boundary in either regular or irregular mode. In this article, we report the third mode, which has a self-similar characteristic. In this mode, the cleavage front branches into a number of segments and the breakthrough points are in clusters, which can be regarded as the combination of regular and irregular processes. A first-order analysis is performed to estimate the boundary fracture resistance.

Crack trapping effect of persistent grain boundary islands

ABSTRACTIn the polycrystalline Fe–Si alloy, when a cleavage front transmits from one grain to another, it first penetrates stably across the grain boundary at a number of breakthrough points (BTPs) that distribute along the front quasi‐periodically. As the critical energy release rate is reached, unstable crack jump occurs and the persistent grain boundary islands (PGBI) between the BTPs are left behind the verge of propagating, bridging across the crack flanks, which leads to a 10–30% increase in fracture resistance. In this article, this process is investigated through an energy analysis. The influence of the size/spacing ratio of PGBI on the grain boundary toughness is discussed in detail.

Crack Trapping Effect of Recalcitrant Grain Boundary Islands in Fe-3wt%Si Alloy

AbstractIn the polycrystalline Fe-Si alloy when a cleavage front transmits from one grain to another, it first penetrates stably through the grain boundary at a number of break-through points that distribute along the front quasi-periodically. As the critical energy release rate is reached, the unstable crack jump occurs and the recalcitrant grain boundary islands (RGBI) between the break-through points are left behind the verge of propagating, bridging across the crack flanks. Eventually, the RGBI are sheared apart, which leads to a 10%-30% increase in fracture resistance. In this paper, this process is investigated through an energy analysis. The influence of the size/spacing ratio of RGBI on the grain boundary toughness is discussed in detail.