构造地质学双语9 lineation

Chapter 5 Lineation and foliationLineationPrimary (secondary ) lineationPrimary ( secondary )foliation线理原生线理（次生线理）原生（次生）面理Stretching lineationsphenocrystCrenulation LineationBoudins拉伸线理斑晶皱纹线理布丁构造（石香肠构造）Mineral LineationxenolithIntersection LineationMullionsRodding矿物生长线理包体交面线理窗棂构造杆状构造axial planar cleavageSchistosityCrenulation cleavageM-domainsQ-domains轴面劈理片理褶劈理M域Q域FoliationCleavagespaced cleavageMicrolithonDisjunctive面理，剥理，页理辟理间隔劈理微辟石不连续劈理dismembered foldkinkingflow foliationslaty cleavagefabric无根褶皱膝折带流面板理（板劈理）组构Foliations and lineations are mesoscopic (handspecimen to outcrop) penetrative (occur throughout)features that characterize volumes of rocks and are related to larger-scale mappable features such as folds and faults. They are clues to large scale geometry, kinematics, strain at the microscopic level, and conditions of deformation.Part one: LineationLineation is parallel alignment of elongate linear fabric elements in a rock body. We use these features because they contain the most kinematic information. In other words, the most strain and stress history of the rock is most recorded in these structures. According its causes, lineation can be classified into primary and secondary lineation. Primary lineation is those formed during the formation of rocks, such as ropy structure in lava flows and linear alignment of phenocrysts and xenoliths (flow lineation) in plutonic rocks. Secondary lineations are those formed after the formation of rocks. The lineations discussed here are secondary lineations. From the viewpoint of observation, lineation can be classified as small scale and large-scale lineations.1. Small scale lineation Stretching lineations: An important type of lineation is formed by the parallel alignment of a set of objects that have acquired an elongate shape as a result of deformation. Individual detrital grains or fragments of any size may be deformed and/or rotated, to define a lineation. Extension lineations are generally inclined to the related fold axes at an angle close to 90 (transverse lineations) butlocally may be parallel to the fold axis. In a given area it is commonly one or the other. Crenulation Lineation: is expressed as bundles of very small, closely spaced fold hinges (crests and troughs). Best developed in strongly anisotropic rocks, such as phyllites and schists. Intersection Lineation: The intersection of any two planar structures forms a line. If many such intersections are present in a rock they constitute an intersection lineation. Common intersections include bedding-cleavage intersections and intersections between two foliations of different generation. In cylindrically folded rocks, intersection lineation are subparallel to the fold axis. Mineral Lineation: Metamorphic minerals often grow with a preferred crystallographic and dimensional orientation, i.e. with their long axes in parallel alignment. Mineral lineations are delineated by the long axes of individual, elongate or platy crystals (for example amphibole crystals) or mineral aggregates aligned and sub-parallel within a foliation plane. They are a penetrative element of the rock fabric, commonly parallel to other types of lineation, and serve to reinforce them. Mineral lineations may be parallel or inclined to the axes of related folds.Sg/Sq iriterssctiiIntersection LineationStretching lineationsCrenulation LineationMineral Lineation2. Large scale lineations Boudins - sausage-shaped segments of extended competent layers surrounded by less competent matrix. If ductility contrast is large, boudins will be subangular with rectangular forms. As ductility contrasts diminish, boudins become more lens-shaped in profile.Boudin, boudinage, from a French word for sausage, describes the way that layers of rock break up under extension. Imagine the hand, fingers together, flat on the table, encased in soft clay and being squeezed from above, as being like a layer of rock. As the spreading clay moves the fingers (sausages) apart, the most mobile rock fractions (ore-making fluids) are drawn or squeezed into the developing gaps. In that simple analogy for rock deformation for which the term was coined almost a hundred years ago, is a model for mineral exploration which is as simple as it is controversial, and which is ignored by the consensusBoudinage depicts the periodical segmentation of pre-existing bodies, generally more competent than the rock surrounding them, when they are inhomogeneously stretched during deformation. Typically, a strong layer or dyke is broken up into a series of elongate and aligned blocks (whose cylinder-like shape motivated the name boudin). Boudin profiles are variable, with rectangular, rhomboidal shapes being common. In low-grade rocks, boudins are usually separated and form a pull-apart structure or gap, which is generally mineralised. At higher grades, and in unconsolidated rocks, the competent layers have generally not broken through; narrow, thinned necks separate and alternate with boudins of relatively still, thick layers and the resulting structure is known as pinch-and-swell. Pinch-and-swell and pull-apart structures may be combined at any level since they really depend on the ductility contrast between the strong bed and its matrix.Boudins are commonly linear and aligned parallel to the axes of related folds. However, stretching may take place in two directions in the plane of layering. Segmentation in these two directions produces nearly equidimensional boudins rather than the elongate forms. This process is referred to as chocolate-tablet boudinage.Structures similar to boudins and pinch-and-swells may occur in certain zones of homogeneous strongly foliated rocks with no apparent lithological contrast between the boudins and the host rocks. These generally long lens-shaped structures are described as foliation boudinage.Boudins (and mullions) tend to be large in size and are commonly restricted to certain layers or, in the case of mullions, are restricted even to certain surfaces in a deformed sequence. Thus at outcrop scale they are a non-penetrative feature.A boudin axis can be measured, like a fold axis, as the nearest approximation to a line that, if moved parallel to itself, generates the boudin form. The neckline connects points of minimum layer-thickness. The length of a boudin is measured parallel to the boudin axis. The width and the thickness are dimensions orthogonal to this axis. Pencil structure - formed by the intersection of bedding parallel foliationand cleavage, such that the rock breaks up into elongated, square sided pencils. Pencil structure forms in weakly deformed shales or mudstone, representing an early stage in the development of slaty cleavage Mullions are coarse structures formed in the original rock material as opposed to segregated or introduced material. The mullion is a columnar corrugation of the surface of a competent layer, at any size. These long features are remarkably cylindrical. They have a ribbed or grooved appearance, often cuspate in shape with broad smoothly curved convex surfaces separated by narrow, sharp, inward-closing hinges. The individual surface features are very persistent along the length of the mullion. Rodding: is a morphological term for elongate, cylindrical and monomineralic bodies of some segregated mineral (quartz, calcite, pyrite, etc.) enclosed in metamorphic rocks of all grades. In profile, rods may have any outline, from elliptical to irregular to that of a dismembered fold.3. Observation and measurement of lineations(1) Differentiate the primary and secondary lineation(2) Determine the types of lineation.(3) Measure its orientation. And find out its relationship with associated large scale structures. Be sure that only those measured on the foliation plane is the real lineation.(4) As one of the important kinematic marker, lineation can show the movement direction of materials during deformation as well as the strain state. In most cases, the directions of stretching lineation and mineral lineation are parallel to the X axis of strain ellipsoid. And the directions of boudins ,mullions, intersection lineations ,pencil lineation and crenulation lineation are parallel to the Y-axis of strain ellipsoid.Part two: foliationFoliation are any type of planar fabric in rock, including bedding, cleavage, schistosity. Foliations are penetrative (occur throughout) in samples at 10s of cm scale. Thus faults are not foliations, nor are fractures and joints because the latter are simply fractures and not related to internal structure of rock.According to the formation and evolution procedure, foliation can be divided into two basic types: Primary foliation includes layers in sedimentary rocks and flow banding and flow foliations in igneous rocks. Secondary foliations are usually associated with deformed metamorphic rocks and include (in increasing grade and grain size) slaty cleavage, phyllitic structure, schistosity and gneissic foliation.Cleavage is a secondary foliation formed under low grade metamorphic conditions (or less) that allows the rock to split along planes. Here ,we will discuss mainly on cleavage. Other types of foliation, such as phyllitic structure and schistosity will be introduced in petrology course.1. The structure of cleavageCleavage is a foliation that forms in relatively low-grade metamorphic rocks. One of the most important feature of cleavage is its domain structure. That is, the cleavage consists of cleavage domains and microlithons, each of which has a unique composition and geometry Cleavage domains ( M-domains) are thin zones of concentrated, strongly aligned, platy minerals(mica) or insoluble oxide and clay residue. Microlithons(Q-domains) are lenticular or tabluar zones with less abundant platy minerals that exhibit a weak to strong alignment. It is usually the result of concentration of quartz.2. Types of cleavageThere are many different kinds of classification of cleavages. Here we will only introduce the Powells classification. According to Powells classification scheme, cleavage is broken out into two main categories: continuous and spaced, depending on whether there are distinguishable, distinct cleavage domains in the rock: if no distinct domains, if distinct domains are present spaced. Scales of cleavage development from about .01 mm to approaching 1 m , 1 mm = Spaced(a) continuous fabric: the lines represent a planar fabric elements that continuous to be visible no matter how small your field of view(at least down to the scale of individual grains.(b) spaced fabric: the rock between the fabric elements does not contain the fabric.Fig.: The distinction between continuous and spaced fabrics. The circled areas represent the enlarged view.(1) Continuous cleavage is where the domains (microlithons) between the cleavage surfaces are too fine to observe with the naked eye. These are further broken down into fine and coarse.(2) Disjunctive cleavage obviously, is the opposite where the microlithons between the cleavage surfaces are large enough to be seen plainly. However, there are two general categories of spaced cleavage: spaced and crenulation. Spaced cleavage consists of an array of parallel to anastomosing, stylolitic to smooth, fracture-like partings common in slightly deformed sedimentary rocks. Crenulation spaced cleavage cuts a preexisting continuous cleavage inherent to the rock.Crenulation cleavage is characterized by microscale kinking of an earlier fabric. In symmetric crenulation cleavage the boundaries between adjacent zones are approximate planes of symmetry. In sigmoidal or asymmetric crenulation cleavage, the relict earlier foliation is bent into a sigmoidal shape. Pressure solution (压溶)can also enhance crenulation cleavage, resulting in a concentration of mica in the cleavage domains (M-domains) and a concentration of quartz in the hinge zones of microlithons (Q-domains).a- CFTI：3 cm3 cma. Sutured domainb. Planar domainc. Wavy domaind. An anastomosing array of wavy domaine. The description of domain based on domain spacefd)闾b)(c)IHIIIIN1訓糊酬1細旧阳1miIQ ClT|11 cm1 mm 0.1 mmSpacedCOrtlifivouscFeavagecleavciuFig l.Spaced cleavagea. Crenulation cleavageb. Symmetricc. AsymmetricFig.2.crenulationcleavage.The arrowsindicatea possiblecomponent shear.Bndoudinsdeveloped on limbs3. Observation and measurement of cleavage Distinguish the layer and cleavage Observe carefully the geometrical pattern and its structure. Measure its orientation. Determine the strain state of deformed rocks. In general, cleavage is perpendicular todirection of maximum shortening, or in other words , parallel to the maximum flattening of strain ellipsoid, that is , the XY plane. For example, the axial planar cleavage is always approximately parallel to axial surfaces of folds, showing that the cleavage is perpendicular to direction of maximum shortening.Determine the generations of different phases of cleavage. Usually, we use the SO to stand for the sedimentary bedding , S1 the earliest developed cleavage, and so on.