GLG345H1 Final Exam Notes

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University of Toronto St. George
Earth Sciences
Rebecca Ghent

GLG345 Text Notes: Final Exam Chapter 12: Foliation and Cleavage Definitions – fabric describes penetrative and distributive components of rock masses (linear, planar, random), primary fabrics formed during deposition, while tectonic fabrics develop in deformed metamorphic rocks (tectonites – L = linear, S = planar) A tectonic foliation is a planar structure formed by tectonic processes, such as cleavages and schistosity. Also, a foliated rock is by definition cohesive, but may split along foliation. Spaced cleavage – distance between planar elements are over 1mm, continuous cleavage – spacing less than 1mm. Spacing generally increases with temperature and burial depth. Cleavage is the low-T version of foliation and is best developed in rocks with abundant platy minerals. First stage of cleavage formation begins with compaction, resulting in a compaction cleavage, stylolitic or pressure solution cleavage. Initial onset of tectonism will produce pencil cleavage through crustal shortening, or where two tectonic cleavages intersect. Continued shortening will develop into slaty cleavage (also a form of pressure solution), with the development of mineral domains (QF – qtz & fels, M – mica) and can also be called domainal cleavages. Disjunctive cleavage describes the early tectonic cleavage developed in a previously unfoliated rock. Compaction cleavage Æ pencil cleavage Æ slaty cleavage Æ phyllitic cleavage Æ schistosity Slaty cleave is dominated by grain rotation and wet diffusion, while schistosity is dominated by recrystallization. Crenulation cleavage occurs when previously existing foliation is folded and can be symmetric or asymmetric. It can also be zonal, if the previous foliation can be traced continuously, or discrete. Can have a wavelength, similar to a train of folds, governed by the domain thickness. Axial plane cleavage splits a fold along the axial surface and is parallel to it. It typically varies between layers due to contrasting viscosities, location on the fold structure – cleavage refraction. Neutral points are where the cleavage disappears and develops due to conformation with outer arc stretching. Final cleavage pattern is affected by prior layer-parallel shortening and flexural shear during folding. Most cleavages approximate the XY-plane of the strain ellipsoid and are characterized by substantial volume loss. Pressure solution is largely responsible for oblate strain ellipsoids associated with cleavage fm. Shear bands develop as a result of simple shearing, also called extensional crenulations cleavage. Transecting cleavage and folds transect not only the axial surface, but also the fold hinge. Transected fold – lineation makes an angle to the hinge line Absence or lack of micas will result in the formation of quartzitic banding, instead of cleavage (can also have gneissic banding). They form by transposition into transposition foliation consisting of transposed layering. Chapter 13: Lineations A lineation is a fabric element in which one dimension is considerably longer than the other two. Types of tectonic linear structures include elongated physical objects, lines of intersection between two sets of planar structures, and geometrically defined linear features. Penetrative/surface lineations are restricted to a surface, building up a linear (L-) fabric, while geometric lineations are non-physical, such as fold axes. Minerals and mineral aggregates can form a linear fabric by means of (synkinematic) recrystallization, dissolution/precipitation or rigid rotation. In addition, ppt of qtz in pressure shadows or strain shadows is a common way to facilitate mineral growth in a preferred direction. Cataclasis, pressure solution and recrystallization all contribute to change the shape of minerals and mineral aggregates during deformation. Typically, the final shape of a deformed grain gives a qualitative impression of the strain ellipsoid. Stretching lineations and shape fabric refer to lineations defined by the shape of deformed objects, common in gneisses. Stretching of minerals and mineral aggregates into a penetrative stretching lineation forms the most common type of lineation in deformed metamorphic rocks. Other mineral lineations include mineral fiber lineations and rodding (elongated mineral aggregates) Intersection lineation occurs when two or more planar surfaces intersect, typically related to folding and running parallel to the axial trace and hinge line. Crenulation lineation refers to numerous mm- to cm-scale fold hinges of low-amplitude folds, such as in schists. Boudins define a lineation based on their geometry, mullions are restricted to interface between a competent and incompetent rock, pencil structures are similar to pencil cleavage. Brittle regime lineations include fiber lineations, slickenlines (mechanical – striations, fibrous growth – slip fiber lineations) with groove lineations, geometric striae, intersection lineations, and slickolites (in limestone). Slickenlines, striae and several other lineations associated with faults parallel the movement direction, but do not in themselves reveal the sense of shear. Stretching lineations commonly indicate the direction of transport when projected onto the shear plane. Folds can form with axes at any angle to the transport direction and typically rotate toward this direction as strain accumulates and folds tighten. Chapter 11: Folds and Folding Geometric terms – hinge and limbs, hinge zone, kink bands, chevron folds, concentric folds (bluntness), amplitude and wavelength, harmonic and disharmonic (stack of folds differ in wavelength and shape along axial trace or die out), hinge point and line, fold axis, cylindricity, mapping, projection, conjugate folds, polyclinal (axial surface with variable orientation), upright fold (vertical axial plane and horizontal hinge line), recumbent fold (horizontal axial plane and hinge line), antiforms and synforms, anticline and syncline, synformal anticline and antiformal syncline, doubly plunging (non-cylindrical upright antiform), dome and basin (has four-way dip closure), monoclinal fold, tightness and interlimb angle, enveloping surface Dip isogons are drawn by orienting the fold so that its axial trace is vertical and are drawn between points of equal dip on the outer and inner boundaries of a folded layer. Three main types: class 1 (dip isogons converge toward the inner arc), class 2 (dip isogons parallel the axial trace), class 3 (dip isogons diverge toward inner arc). Class 1 can be further subdivided into 1A (thinned hinge zones), 1B (parallel folds with constant layer thickness), and 1C (slightly thinned limbs). Symmetry of folds can be orthorhombic or monoclinic and symmetric folds are sometimes called M- folds. Asymmetric folds are referred to as S-folds and Z-folds. Vergence is the term used to describe fold systems with consistent asymmetry and describes the displacement of the long limb wrt the short one. Fold asymmetry may relate to position on a lower-order fold, sense of shear or orientation of the folded layer relative to the strain ellipse. Active folding/buckling is a fold process that can initiate when a layer is shortened parallel to the layering (class 1B). It requires a contrast in viscosity with the folding layer being more competent. Buckling implies that there is layer parallel shortening and a viscosity contrast involved, and also irregularities on which folds can nucleate. Flexural folding is the term used to encompass the simplest mechanisms involved in buckling and includes orthogonal flexure, flexural slip and flexural flow. Folding usually initiates in thin layers and move on to thicker layers with progressive stress strength. Passive folding is where the layering exerts no mechanical influence on folding and the passive layers only serve as a visual expression of strain (class 2 or shear folds). Passive folds generated by simple shearing are perfectly similar folds. Bending happens when forces act across layers at a high angle, like passive folding but more directly forced upon the layers by bounding rocks. A classic example is a forced fold blanketing faulted rigid basement blocks. It is a boundary condition- or external load-related model rather than a strain model. Other examples include fault-bend folds, differential compaction, forceful intrusion, and boudins. Flexural slip occurs along layer interfaces or very thin layers during folding (1B). Slickenlines on folded weak layers and constant bed thickness reveal flexural slip. Flexural shear or flow occurs when the strain is more evenly distributed in the limbs as shear strain. Pure flexural folds have no neutral surface, and strain increases away from the hinge zone. Orthogonal flexure (1B) has specific conditions where all lines originally orthogonal to the layering remain so throughout the deformation history. Thus stretching occurs in the outer part and shortening in the inner part of
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