Horizontal Foliation Research Articles

Effective count of square-tiled surfaces with prescribed real and imaginary foliations in connected components of strata

Abstract We prove an effective estimate with a power saving error term for the number of square-tiled surfaces in a connected component of a stratum of quadratic differentials whose vertical and horizontal foliations belong to prescribed mapping class group orbits and which have at most L squares. This result strengthens asymptotic counting formulas in the work of Delecroix, Goujard, Zograf, Zorich, and the author.

Large genus asymptotic geometry of random square-tiled surfaces and of random multicurves

We study the combinatorial geometry of a random closed multicurve on a surface of large genus g and of a random square-tiled surface of large genus g. We prove that primitive components $$\gamma _1, \dots ,\gamma _k$$ of a random multicurve $$m_1\gamma _1+\dots +m_k\gamma _k$$ represent linearly independent homology cycles with asymptotic probability 1 and that all its weights $$m_i$$ are equal to 1 with asymptotic probability $$\sqrt{2}/2$$ . We prove analogous properties for random square-tiled surfaces. In particular, we show that all conical singularities of a random square-tiled surface belong to the same leaf of the horizontal foliation and to the same leaf of the vertical foliation with asymptotic probability 1. We show that the number of components of a random multicurve and the number of maximal horizontal cylinders of a random square-tiled surface of genus g are both very well approximated by the number of cycles of a random permutation for an explicit non-uniform measure on the symmetric group of $$3g-3$$ elements. In particular, we prove that the expected value of these quantities has asymptotics $$(\log (6g-6) + \gamma )/2 + \log 2$$ as $$g\rightarrow \infty $$ , where $$\gamma $$ is the Euler–Mascheroni constant. These results are based on our formula for the Masur–Veech volume $${\text {Vol}}{\mathcal {Q}}_g$$ of the moduli space of holomorphic quadratic differentials combined with deep large genus asymptotic analysis of this formula performed by A. Aggarwal and with the uniform asymptotic formula for intersection numbers of $$\psi $$ -classes on $${\overline{{\mathcal {M}}}}_{g,n}$$ for large g proved by A. Aggarwal in 2020.

Giant silicic ignimbrites and comagmatic domes at the southeastern end of the Sierra Madre Occidental, México

The Sierra Madre Occidental (SMO) ignimbrite province includes giant ignimbrites related to large silicic calderas and dome complexes. Previous work in the SMO has paid little attention to the ignimbrites and domes exposed in the southeasternmost portion of the volcanic province, in the Guanajuato and Querétaro States. This work focuses on two widespread Oligocene ignimbrite sheets and numerous lava domes of this part of the SMO, and provides data on their distribution, stratigraphy, geochronology, textures, and geochemistry. Two large ignimbrite sheets and several domes were identified, which according to the stratigraphy they were classified in pre- and post-ignimbrites. We have named the ignimbrites as Victoria I and Victoria II. Victoria I is purple to light grey, with strong welding, horizontal foliation, and well-developed columnar jointing. It contains quartz, plagioclase and sanidine; fiamme is a common feature. Victoria II ignimbrite conformably overlies Victoria I. It is dark pink to brown, densely welded but less welded than Victoria I; in most locations it has developed columnar jointing. Victoria II's mineral assemblage includes biotite and hornblende in addition to quartz, plagioclase and sanidine; it has a higher content of lithics fragments and fiamme than Victoria I. A black vitrophyre forms base of each ignimbrite and marks the contact between them. These ignimbrites are stratigraphic markers due to their wide distribution and distinctive field and textural characteristics. We have obtained zircon U–Pb ages of 31.65 ± 0.17 to 30.57 ± 0.14 Ma (1σ) for Victoria I ignimbrite and 31.31 ± 0.21 to 29.73 ± 0.18 Ma for Victoria II ignimbrite. Silicic lava domes identified in the study area are either pre- or post-ignimbrite; the former are dacitic, and the latter are rhyolitic or rhyodacitic. We focus on the post-ignimbrite domes with the hypothesis that they were comagmatic with the Victoria ignimbrites. The post-ignimbrite domes are devitrified rocks with phenocrysts of plagioclase, quartz, sanidine, biotite and hornblende; and zircon U–Pb ages yielded 30.81 ± 0.21 Ma to 28.02 ± 0.19 Ma.

Using Continuum-Discontinuum Element Method to Model the Foliation-Affected Fracturing in Rock Brazilian Test

In this study, the continuum-discontinuum element method (CDEM) was used to investigate the tensile fracture mechanism of rock materials. An isotropic rock disk model and models considering different foliation inclinations were established, and three schemes were used to simulate the rock fracturing in Brazilian test. Then, the influences of the rock matrix and foliation strengths on anisotropy rock fracturing were investigated. Furtherly, simulation results were verified, and the rock fracture mechanisms were discussed. The results show that the rock fracturing in Brazilian test can be accurately simulated by CDEM, which is in accordance with the experimental results. For isotropic and horizontal foliation rock, the stress concentration in loading positions causes a local fracture of rock sample, and application of a local strengthening scheme can simulate the integral tension fracture of sample middle. As the foliation angle varies from 15° to 45°, the rock fracturing is affected by the stress concentration and foliation distribution. In splitting simulation, a strengthening scheme should be adopted to overcome this influence. As a result, the rock sample generates the sliding and compression-shear fracture. As the foliation angle changes from 45° to 75°, the foliation, rather than the matrix, dominates the fracture behavior of rock sample. For vertical foliations’ rock, as the middle foliation thickness is appropriately broadened, the simulation results are reasonable. In general, the tensile strength of anisotropic rock entirely decreases with an increase of foliation angle, and the effect of foliation strength on the tensile strength rock sample is larger than that of the rock matrix.

Structural Setting and Kinematic Analysis of the Halaban Region, Eastern Arabian Shield, Saudi Arabia

AbstractThe vorticity analysis technique was applied to measure the different lithological units, such as schist, metagranite and metavolcano‐sedimentary rocks, which are present in the Halaban region. This work aims to interpret the relationship between the different lithologies and the tectonic setting, in order to elucidate the nature of kinematic analysis in the Halaban region. The kinematic analyses were applied to feldspar porphyroclasts, quartz and hornblende for twenty‐six samples. The kinematic vorticity number (Wm) for deformed rocks in the study area ranged from ∼0.6 to 0.9. The direction of the long axes for finite strain data (X axes) revealed a WNW trend with shallow dipping. The direction of the short axes for finite strain data (Z axes) were represented by vertical with associated horizontal foliation. The results of the kinematic vorticity and strain analyses are characterized by simple shear with different degrees of deformation in the Halaban region. Furthermore, our finite strain data shows no significant volume change during deformation. The subhorizontal foliation was synchronized with thrusting and deformation. Furthermore, throughout the overlying nappes, the same attitudes of tectonic contacts are observable, the nappes in the orogens being formed from simple shear deformation.

Quadratic differentials, measured foliations, and metric graphs on punctured surfaces

A meromorphic quadratic differential on a punctured Riemann surface induces horizontal and vertical measured foliations with pole singularities. In a neighborhood of a pole, such a foliation comprises foliated strips and half-planes, and its leaf space determines a metric graph. We introduce the notion of an asymptotic direction at each pole and show that for a punctured surface equipped with a choice of such asymptotic data, any compatible pair of measured foliations uniquely determines a complex structure and a meromorphic quadratic differential realizing that pair. This proves the analogue of a theorem of Gardiner–Masur for meromorphic quadratic differentials. We also prove an analogue of the Hubbard–Masur theorem; namely, for a fixed punctured Riemann surface there exists a meromorphic quadratic differential with any prescribed horizontal foliation, and such a differential is unique provided we prescribe the singular flat geometry at the poles.

X-ray computed micro-tomography of spiral garnets: A new test of how they form

Garnet porphyroblasts with spiral shaped inclusion trails of two micaschist samples have been investigated with X-ray computed tomography. For each garnet crystal, the orientation of its internal Foliation Intersection/Inflexion Axes (FIA) was measured, its volume and shape orientation, the latter via calculation of best-fit ellipsoids. Additionally, image analysis of inclusion trails was performed in slices passing through garnet centres oriented normal to FIA. Our data demonstrate subvertical and subhorizontal preferred orientations of the inclusion trails and of the porphyroblast crystals themselves in both samples. This supports a genetic model in which garnets periodically overgrew a succession of vertical and horizontal foliations during alternating crustal shortening and gravitational collapse without significantly rotating. The fact that both samples come from areas previously identified as regional shear zones supports the idea that porphyroblasts are able to maintain stable positions even in such environments. Our study exemplifies the strength of X-ray computed micro-tomography compared to previous techniques based on serial thin sections for distinguishing multiple FIA sets present in single samples.

Metavolcanic rocks and orthogneisses from Porongos and Várzea do Capivarita complexes: A case for identification of tectonic interleaving at different crustal levels from structural and geochemical data in southernmost Brazil

The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Várzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D2 and D3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D1 structure is an alternation of non-mylonitic S1 and mylonitic-S1, developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L1) measurements along a great circle in stereoplots is interpreted to result from shearing along S1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma).

Transposition of foliations and superposition of lineations during polyphase deformation in the Nevado-Filabride complex: tectonic implications

Detailed structural analysis in a ca. 80 km2 area of the western Nevado-Filabride complex (Betic Cordillera) reveals a heterogeneous internal structure characterized by multiple cross-cutting foliations and lineations that locally transpose earlier ones. The large-scale geometry of these fabrics conflicts with continuous westward to south-westward tectonic transport related to thrusting or crustal extension, and mismatches a previously inferred extensional detachment in the area. Multiple crenulation lineations can be distinguished in the field and correlated with five foliation intersection axes (FIA1–5) preserved in garnet and plagioclase porphyroblasts of the western Sierra Nevada. These indicate crustal shortening in different directions associated with vertical foliation development and intermitted stages of gravitational collapse producing horizontal foliations. The large spread of lineation- and fold-axes trends in the Nevado-Filabride complex results from the mixed presence of multiple generations of these structures whose distinction is critical for tectonic models. The five principal FIA trends remarkably match successive vectors of relative Africa-Iberia plate motion since the Eocene, suggesting that deformation of the Nevado-Filabride took place during this period, although peak metamorphism in at least some of its parts was reached as late as the Middle Miocene.

Rel leaves of the Arnoux–Yoccoz surfaces

We analyze the rel leaves of the Arnoux-Yoccoz translation surfaces. We show that for any genus $g \geq 3$, the leaf is dense in the connected component of the stratum $H(g -1 , g -1)$ to which it belongs, and the one-sided imaginary-rel trajectory of the surface is divergent. For one surface on this trajectory, namely the Arnoux-Yoccoz surface itself, the horizontal foliation is invariant under a pseudo-Anosov map (and in particular is uniquely ergodic), but for all other surfaces, the horizontal foliation is completely periodic. The appendix proves a field theoretic result needed for denseness of the leaf: for any $n \geq 3$, the field extension of the rationals obtained by adjoining a root of $X^n-X^{n-1}-\ldots-X-1$ has no totally real subfields other than the rationals.

The onset of fabric development in deep marine sediments

Post-depositional compaction is a key stage in the formation of sedimentary rocks that results in porosity reduction, grain realignment and the production of sedimentary fabrics. The progressive time-depth evolution of the onset of fabric development in deep marine sediments is poorly constrained due to the limited quantity and resolution of existing data. Here we present high-resolution anisotropy of magnetic susceptibility (AMS) results from clay-rich deep marine sediments recovered at International Ocean Discovery Program Site U1438 (Philippine Sea). AMS is a petrofabric tool sensitive to the preferred orientation of grains in rocks. Down-section variations of AMS parameters, density, porosity and the inclination of magnetic remanences demonstrate that fabrics develop in response to compaction and dewatering but also that they do not develop progressively with depth below the mudline. Instead, a horizontal foliation first forms at 83 mbsf once the sediment load reaches an effective stress threshold for the onset of compaction and is then continuously enhanced down to 113 mbsf, defining a 30 m-thick ‘initial compaction window’. The magnetostratigraphic age model for IODP Site U1438 indicates a delay of 5.7 Ma in initial fabric formation following sediment deposition, with strongly defined fabrics then taking an additional 6.5 Ma to develop.

Topology of Foliations and Decomposition of Stochastic Flows of Diffeomorphisms

Let M be a compact manifold equipped with a pair of complementary foliations, say horizontal and vertical. In Catuogno et al. (Stoch Dyn 13(4):1350009, 2013) it is shown that, up to a stopping time $$\tau $$ , a stochastic flow of local diffeomorphisms $$\varphi _t$$ in M can be written as a Markovian process in the subgroup of diffeomorphisms which preserve the horizontal foliation composed with a process in the subgroup of diffeomorphisms which preserve the vertical foliation. Here, we discuss topological aspects of this decomposition. The main result guarantees the global decomposition of a flow if it preserves the orientation of a transversely orientable foliation. In the last section, we present an Ito-Liouville formula for subdeterminants of linearised flows. We use this formula to obtain sufficient conditions for the existence of the decomposition for all $$t\ge 0$$ .

Vertical limits of graph domains

We consider the limiting behavior of Teichmüller geodesics in the universal Teichmüller space T ( H ) T(\mathbb {H}) . Our main result states that the limits of the Teichmüller geodesics in the Thurston’s boundary of T ( H ) T(\mathbb {H}) may depend on both vertical and horizontal foliation of the corresponding holomorphic quadratic differential.

Graph manifold ℤ-homology 3-spheres and taut foliations

We show that a graph manifold which is a Z-homology 3-sphere not homeomorphic to either the 3-sphere or the Poincar\'e homology 3-sphere admits a horizontal foliation. This combines with known results to show that the conditions of not being an L-space, of having a left-orderable fundamental group, and of admitting a co-oriented taut foliation, are equivalent for graph manifold Z-homology 3-spheres.

Magnetic anisotropy reveals the depositional and postdepositional history of a loess‐paleosol sequence at Nussloch (Germany)

AbstractAnisotropy of magnetic susceptibility (AMS) is employed as a tool to unravel the depositional history of the 17 m thick Nussloch P8 Weichselian loess sequence located 10 km south of Heidelberg, Germany. Through an AMS study, the primary aeolian depositional origin of the magnetic fabrics is evaluated, and overprinting due to postdepositional reworking and/or deformation is identified. Primary fabrics along the P8 sequence are defined by near‐vertical KMIN axes and horizontal foliations. Eight intervals display secondary fabrics, characterized by either prolate orientation distributions or oblate orientation distributions with dipping foliation planes. These postdepositional fabrics are associated with laminated loess and tundra gley horizons. It is proposed that increased moisture (due to higher precipitation or enhanced snowmelt) and repeated cryogenic processes were able to reorganize and rework the accumulated loess. Primary aeolian fabrics are archived within 6.45 m of cumulated depth or 38% of the profile and dominantly within the Upper Pleniglacial units. Even though maximum susceptibility axes of primary fabrics are statistically well resolved at the specimen (ε12 = 10.1° ± 8.6) and population (ε12 = 6°) level, any inferred paleowind directions from the magnetic lineation remains speculative given the low concentration of ferrimagnetic minerals (<0.03 wt %). Tundra gley horizons upprofile display primary magnetic fabrics and no major changes in the degree of anisotropy or AMS orientation distributions. This suggests a weakening in gley‐induced diagenesis and therefore favorable environmental conditions needed (moisture and presence of permafrost active layer) to initiate their formation in loess deposits. Finally, pedogenesis has not played an important role in modifying the magnetic fabric since paleosols display the same magnetic fabrics observed in primary loess.

Structure, Timing, and Mechanism of the Pliocene and Late Miocene Uplift Process of the Ailao Shan‐Diancang Shan, SE Tibet, China

AbstractThe uplift of the Ailao Shan‐Diancang Shan (ASDS) along the Ailao Shan‐Red River (ASRR) shear zone is an important geological event in the southeastern margin of Qinghai‐Tibet Plateau tectonic domain in the Late Cenozoic, and it preserves important information on the structures, exhumational history and tectonic evolution of the ASRR shear zone. The uplift structural mode and uplift timing of the ASDS is currently an important scientific topic for understanding the ASDS formation and late stage movements and evolution of the ASRR shear zone. The formation of the ASDS has been widely considered to be the consequence of the strike‐slip movements of the ASRR shear zone. However, the shaping of geomorphic units is generally direct results of the latest tectonic activities. In this study, we investigated the timing and uplift structural mechanism of the ASDS and provided the following lines of supportive evidence. Firstly, the primary tectonic foliation of the ASDS shows significant characteristic variations, with steeply dipping tectonic foliation developed on the east side of the ASDS and the relatively horizontal foliation on the west side. Secondly, from northeast to southwest direction, the deformation and metamorphism gradually weakened and this zone can be further divided into three different metamorphic degree belts. Thirdly, the contact relationship between the ASDS and the Chuxiong basin‐Erhai lake is a normal fault contact which can be found on the east side of the ASDS. 40Ar/39 Argeochronology suggests that the Diancang Shan had experienced a fast cooling event during 3–4 Ma. The apatite fission track testing method gives the age of 6.6–10.7 Ma in the Diancang Shan and 4.6–8.4 Ma in the Ailao Shan, respectively. Therefore the uplift of the ASDS can be explained by tilted block mode in which the east side was uplifted much higher than the west side, and it is not main reason of the shearing movements of the ASRR shear zone. The most recent uplift stages of the ASDS happened in the Pliocene (3–4 Ma) and Late Miocene (6–10 Ma).

A multidisciplinary study on the emplacement mechanism of the Qingyang–Jiuhua Massif in Southeast China and its tectonic bearings. Part I: Structural geology, AMS and paleomagnetism

During the Cretaceous, the South China Block (SCB) experienced a widely distributed extensional event including numerous plutons emplacement and basin opening. Investigations on the tectonic regime coeval with pluton emplacement, and emplacement mechanism of the pluton remain relatively rare in the SCB. In order to address these questions, a multidisciplinary approach, including field structural and petrographic observations, anisotropy magnetic susceptibility (AMS) and paleomagnetic analyses, was carried out on the Qingyang–Jiuhua granitic massif which intrudes into the Lower Yangtze fold belt in the northeastern part of the SCB. The Qingyang–Jiuhua massif is composed of the granodioritic Qingyang and monzogranitic Jiuhua plutons dated by zircon U–Pb method at ca. 142Ma, and ca. 131Ma, respectively. Our structural observations show that the intrusion of the Qingyang–Jiuhua massif does not modify the fold strike. A weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism. In the contact aureole of the massif, the foliation follows the pluton contour, and the mineral lineation is rare. When present, it exhibits a down-dip attitude. Field and microstructural observations indicate isotropic magmatic textures in most parts of the massif. The AMS analysis of 93 sites reveals weak values for the anisotropy degree (PJ<1.2) and oblate magnetic fabric dominance (T>0) for most of the measured samples. Two principal foliation patterns are identified: horizontal foliations in the center of the plutons, and vertical foliations on the boundaries. Magnetic lineation strike is largely scattered, and weakly inclined at the scale of the entire massif. The paleomagnetic investigations indicate that (a) the younger Jiuhua pluton did not produce a remagnetization in the older Qingyang pluton, (b) no relative movement can be observed between these two plutons, (c) the entire massif did not experience any important relative movement with respect to South China, considering the paleomagnetic uncertainties. Integrating the newly obtained results with previous observations, our study favors a permissive emplacement mechanism for the two plutons, i.e. vertical magma intrusion into an opening space controlled by the NW–SE brittle stretching of the upper crust, which is in agreement with a weak extensional regional tectonic framework of the SCB.

Sucessões vulcânicas, modelo de alimentação e geração de domos de lava ácidos da Formação Serra Geral na região de São Marcos-Antônio Prado (RS)

In the Sao Marcos (RS) and Antonio Prado (RS), the Serra Geral Formation exposes at the base basalts of pahoehoe type, covered by basalts of the ´a´ā type. The first succession was generated by a low rate of eruption in a closed flow system allowed the flow to reach distances >; 100 km of the source. The ´a´ā lava flow types were generated by higher rates of eruption and transported in open channels where rapid cooling prevented long distances from the source to be reached. The two types of basalts are low-TiO2 tholeiitic and the morphology of flows is not related to variations in SiO2 and MgO contents. Above these rock types outcrop acidic volcanic rocks geochemically of Caxias Group (Palmas Subgroup). Dimension stones extraction exposed the inner portions of the acidic feeder dikes with vertical magmatic foliations. The lava domes have exogenous characteristics and horizontal foliations. We propose a model for the generation of domes involving the diapirically rise of acids magmas that become vesicular and more viscous, that stop near the surface. New magmatic pulses extracted pieces of the vesicular fraction generating autobreccias in the conduit and vertical structures that extend laterally toward the surface organizing the lava domes with vitrophyres in the base and in the top, with a thin massive phaneritic core. Magmatic textures of the domes are typical of effusive units and the identification of the feeder dykes in the area allows the understanding of the emplacement process of acidic flows in the Serra Geral Formation.

On the non-uniform hyperbolicity of the Kontsevich–Zorich cocycle for quadratic differentials

We prove the non-uniform hyperbolicity of the Kontsevich–Zorich cocycle for a measure supported on abelian differentials which come from non-orientable quadratic differentials through a standard orienting, double cover construction. The proof uses Forni’s criterion (J. Mod. Dyn. 5(2):355–395, 2011) for non-uniform hyperbolicity of the cocycle for \({SL(2, \mathbb{R})}\)-invariant measures. We apply these results to the study of deviations in homology of typical leaves of the vertical and horizontal (non-orientable) foliations and deviations of ergodic averages.

Fabric evolution at basement–cover interfaces in a fold-and-thrust belt and implications for décollement tectonics (Autochthon, Lower Allochthon, central Scandinavian Caledonides)

Microstructural and magnetic investigations (anisotropy of magnetic susceptibility, AMS) on sections across basement–cover interfaces (BCI) revealed a complex evolution in the crystalline basement rocks beneath and in the basal units of the Caledonian fold-and-thrust belt: (1) Pre-Caledonian mylonitic fabrics in basement granite relate to steep shear zones. (2) Palaeoweathering formed smectite and illite at the expense of feldspar and mica. Secondary Fe-bearing clay minerals and the intensity of the chemical weathering control the bulk susceptibility. Changing susceptibility and AMS relate to a (time) sequence from primary magnetite to secondary paramagnetic clay to pyrite and ferrimagnetic pyrrhotite. (3) Burial compaction with BCI-parallel fabrics. (4) Caledonian cleavage, overprinted by decollement zones with S–C–C′ fabrics. Decollement cataclasis overprinted pre-existing magnetic fabrics and produced horizontal magnetic lineations and subhorizontal foliations defined by the S–C–C′ fabrics. Clay mineral enrichment, together with subsequent, BCI-parallel compaction fabrics, decreased the shear strength in the basement rocks beneath the BCI. Detachments initiated at such low-strength zones and produced allochthonous units with their footwall within crystalline basement rocks, an observation of general importance for orogenic fold-and-thrust belts.