Structural geology Flashcards
Joints
Most common type of tensile fracture.
Form near Earth’s surface.
Control the bulk strength of the rock (coal cleat)
Important fluid conduits (groundwater and hydrocarbons)
Sigma 1 parallel to joint.
Often control weathering and subsurface fluid flow.
Systematic joint sets due to both regional and local stresses during deformation.
Joint spacing controlled by bed thickness.
Orthogonal joint sets require flip of principal stresses.
Rapid unroofing causes joints
Cooling and contraction causes joints
Fracture modes
Mode I - opening and tension [fractures]
Mode II - sliding and shear, strike slip
Mode III - tearing and shear
[Veins and Faults all three - veins must have mode I]
Vein systems
Array (collections) of fractures filled by mineralisation. Leads to bulk volume increase. Common in low grade metamorphic rocks (PT conditions). Contain many important mineral deposits (quartz reef - thick vein with gold).
Brittle failure, sigma 1 parallel to vein (like fractures)
Shape depends on fractures - can be folded (with at least 2 stages of deformation [1 - extension -> fracture, 2 - compression -> folding]
Veins - opening mode fractures filled with new minerals. Crystal growth often controlled by the progressive opening of the vein.
Opening perpendicular to crack
Mineralisation perpendicular to extension direction.
Oblique opening
Mineralisation at an angle to the opening (due to shearing from faults).
Episodes of cracking and opening
Syntaxial veins - grow inwards from wallrock
Antitaxial veins - appear to grow from a median suture line towards walls (outward from walls)
Ataxial veins - formed by repeating opening and sealing of fractures
En Echelon Veins in Shear Zone
Veins that step down - can determine how the rock was sheared. Shearing in opposite direction to what you would assume, in order to keep the steps. In class exercise.
Faults
Fractures with shear displacements.
Brittle failure, sigma 1 must have some obliquity to fault plane (sigma 1 bisects the acute angle)
Dip - angle from horizontal to fault plane
Hade - angle from fault plane to 90
Dextral = right lateral
Normal fault - horizontal component called heave and vertical component called throw
Reverse fault - horizontal component called contraction and vertical component called throw
Thrust - low angled fault <45
Block diagrams - beware
Often misleading, as they give a false impression of the 3D form of a fault. Faults do not extend laterally forever - they are discontinuous with a finite length. Somewhere along this length they have a max displacement and at ends have 0 displacement.
True offset
Oblique-slip fault - has dip-slip (DS) and strike-slip (SS) components
As rocks are often horizontally layered, DS usually easier to measure than SS.
To measure SS, need either steeply inclined beds or dyke.
To accurately calculate the total displacement vector of a fault, need different dipping beds/layers/intrusions to be faulted. Knowing the slip direction can be helpful.
In map view, offsets can look strike-slip, but in reality, are normal.
Faults - movement indicators from outcrop
1) slickensides (fibrous mineral growth) and striations (scarpes) on fault planes parallel to the direction of slip.
2) fault drag folds (normal drag)
3) fault plane striations
Fault rocks
Faults often grow by the superimposition of many slip events on the fault surface. Each slip causes an earthquake. Depending on PT, different types of rocks form.
Shallow depths - forms by fragmentation; fault breccia (>30% rock fragments visible cohesive/incohesive) and fault gouge (<30% rock fragments visible and rock incohesive)
Deep - rocks become cohesive (lithified with some recrystallization). Matrix formed by tectonic reduction of grain size. Protocataclasite - cataclasite - ultracataclasite (grain size decreasing)
-cataclasties still considered to be formed under brittle conditions.
Deeper - ductile (recrystallization dominates) -> mylonites (strong foliation)
-very rapid movement -> partial melting of rock -> pseudotachylite (black glass)
Folds
Folds form in already-layered rock masses (bedded sediments, layered/foliated metamorphic rocks or in igneous rocks in discrete layers). They may occur on any scale (scale usually depends on thickness of layers they deform). Thick layers produce larger folds.
Fold geometries
Straight hinge line - cylindrical fold (doesn’t match reality)
Bowed up/down hinge line - non-cylindrical (reality)
Synform - downward closing fold (concave up)
Antiform - upward closing fold (convex up)
Cline - need to know ages
-anticline - oldest in middle
-syncline - youngest in middle
Monocline - one limb
Fold hinge - joins points of max curvature
Axial surface - contains fold axes of folded layers. Defines plane of flattening (xy plane of strain ellipsoid)
Crest line - the line which lies along the highest points in a folded layer
Trough line - the line which lies along the lowest points in a folded layer
Plunging folds - axis plane dipping
Fold axial trace - intersection of axial plane with present land surface
Fold classification
Links to Fleuty fold classification (axial plane dip vs plunge dip)
Upright fold - axial plane is vertical
Inclined fold
Overfold - one limb is overturned
Recumbent fold - axial plane is horizontal
Symmetric folds - limbs have same dip, same shape
Asymmetric folds - verge to one direction, one limbs is steep and one is shallow
Overturned folds - verge to one direction, shallow limb and overturned limb
Vergence direction suggests shear direction (right angles to limb)
Fold tightness
Interlimb angle - draw tangents to folded surface at points of inflection Flat lying - 180 (least strain) Gentle - 120-180 Open - 70-120 Tight - 30-70 Isoclinal - 0-30 (most strain)
Fold hinge shapes
Folds can have varying hinge geometries due to the nature of the faulted sequence or the conditions of folding. Ex: chevron and kinds often formed in thin multi-layered sequences and more typical of relatively shallow depths of formations
- Chevron folds - straight limbs and angled hinges
- Kink folds - horizontal, angled, horizontal
- Cuspate folds - go up to a point (soft rocks)
- Box folds - more rounded than kink
- Disharmonic folds - beds have different fold shapes, common
Facing direction
If beds are upright (beds young up the axial plane) we have upward facing folds (Anticline: concave up and oldest in middle; Syncline: concave down and younger in middle).
If beds are overturned, then have downward facing folds (beds young down the axial plane). Antiformal syncline (concave up, youngest in middle) and Synformal anticline (concave down, oldest in middle)
-Syncline = younging direction. Antiformal = shape.
Fold description method
Axial surface dip Hinge dip Fleuty description Interlimb angle and fold tightness Symmetry Harmonicity Hinge shape Fold class Depth of formation
Non-cylindrical folds/pericline
Reality of folds - they die out along their length (periclines). Amplitude changes across the fold.
Often developed as en-echelon structures, as one dies out laterally, another grows laterally. The zone between two en-echelon periclines, where strain transfers from one fold onto another, is a relay
Sheath folds
Periclinal folds have curvilinear fold axes. If a fold axis curves by >90 it is called a sheath fold. Found in very highly deformed zones (geometry tells us about their deformation).
Classes of folding - quantitative
Based on fold profiles (cross-sections) of a folded layer. Classes of folds distinguished by:
-relative curvature of upper and lower bounding surfaces of layer
-relative thickness of the folded layer in the hinge vs limbs
Class 1 - isogons converge (fan outwards); shallow/brittle
-a = strongly convergent (bottom bed tighter than top)
-b = parallel
-c = weakly convergent
Class 2 - isogons parallel; deep/ductile
-similar
Class 3 - isogons diverge (fan inwards); deep/ductile
-divergent (top tighter than bottom)
Dip isogons - a line that connects a point on top and bottom bed with the same dip
Parallel folds (class 1b)
No thickness change in hinge or limb, uniform orthogonal layer thickness. Dip isogons perpendicular to fold limbs.
Typical of strong layers within a weaker matrix. Spacially inefficient and cannot extend for great distances. Shallow in crust.
Similar folds (class 2)
Thickened hinges and thinned limbs, uniform thickness parallel to axial plane. Dip isogons parallel to fold trace.
Typical of weaker rocks/greater depths. Spacially very efficient (exactly equal curvature) so the rock can extend for great distances.
Harmony
The extend to which folding of multilayer sequence is consistent through the sequence.
- disharmonic fold - dies out within a couple of half wavelengths
- harmonic fold - continuous along its axial trace for many multiples of its half wavelength (each bed doing the same thing)
- polyharmonic fold - harmony on various multiples of half wavelengths within the multilayer sequence.
Parasitic folds
Polyharmonic folding generates parasitic folds. Occurs on many scales. Can describe the collective geometry of parasitic folds with respect to their enveloping surface.
Parasitic fold geometries are predictable in relation to associated main fold (map vergence)
Fold vergence
Characteristic of parasitic folds
Indicates the direction of movement and rotation during deformation.
Can identify the location of major folds using minor folds.
Z (LHS of fold)
M (top of fold)
S (RHS of fold)
Powerful field mapping tool, especially in areas that are relatively poorly exposed.
Fold-cleavage relationships
Axial planar cleavage - cleavage that is parallel to the axial plane
Planes defined by fractures or mineral alignment parallel to a folds axial plane
-bedding steeper than cleavage = overturned limb
-cleavage steeper than bedding = upright limb
Assume:
-bedding folded and right way up
-cleavage parallel to axial plane
-axial plane bisects interlimb angle
Foliation
Alignment of minerals, forming cleavage or schistosity. Usually the plane of flattening xy plane of strain ellipsoid.
Folding mechanisms
Brittle deformation - buckling, bending, flexural slip and kinking
Ductile deformation - passive folding, flexural shear/flow, oblique shear
Buckling
Horizontal forces
Sinusoidal (smooth wave) folding of a single strong layer within a weaker matrix by lateral compression (layer maintains thickness). Tends to form parallel folds with uniform orthogonal layer thickness. Shallow crust.
Strain accommodated parallel to layering (tangential longitudinal strain). Most strain accommodated in hinge (nothing happening in limbs).
Outer arc extended; normal faults, extension veins, boudinage, horizontal cleavage
Inner arc compressed and shortened; thrust faults, stylolites, vertical cleavage.
Between them is a neutral surface where no deformation is taking place.
Bending
Often hard to separate from buckling
Vertical forces
Achieved when F act across the layer (at 90) and may involve more than one mechanism.
Look at context - diapir (salt or intrusion rises into layers and pushes them up), faults (beds respond to vertical motion - reverse and normal), boudinage *layers drop into folds)
Flexural slip
Where many thick, strong and still layer in between which is some material with low cohesion (weak boundary). Length of beds does not change, slip between (eg: bending a book).
All action in limbs and nothing in hinge.
Sense of shear changes across hinge zones - consistent between anticlinal and synclinal limbs.
Produces parallel folds - constant bed thickness.
Slickensides and striations are lineations and oriented at right angles to fold axis.
Slip at layer boundaries form bedding-plane faults (thrust) characterised by grooving and mineral growth parallel to movement direction (slickensides or striations).
Within the competent layer, veins often open at 45 angle to the shear directions of individual beds. Sigmoidal shape of cracks due to progressive lengthening of veins with rotation.
Kinking
Produces folds with straight limbs and very sharp hinges - chevron folds or kink bands.
Occurs in strongly layered or laminated sequences where have strong mechanical anisotropy between very competent layers and thin incompetent layers (associated with thin bedded sandstones interbedded with shales; slates and schists). Resultant folds show flexural slip and are parallel folds. MIX of buckle and flexural.
Straight limbs, angular hinge
Usually parallel folds - shallow crust.
Chevron folds
Kinking
See very intense layer-parallel slip on incompetent layers (discontinuities between layers)
gaps in hinge zones = saddle reefs (can fill with mineralisation)
lock up when inter-limb angle ~60
Kink bands
Kinking Smaller scale (mm) than chevron Two parallel sided, very well defined axial surface. Layering deflects suddenly across the 1st kink plane then returns to its original orientation across the 2nd kink plane Ideally, each kink has an inter-limb angle of 120 and is symmetric around axial plane
Passive folding
Produces harmonic folds where the layering plays no mechanical role and has no influence on the fold shape.
Can form in response to any kind of ductile strain (simple shear, subsimple shear, transpression, pure shear).
Oblique shear/flow
Distortion of layering by shear on an infinite number of closely spaced parallel planes that are inclined perpendicular/at a high oblique angle to layering. Displacement on shear planes varies along the layer so it folds. Deformation = heterogeneous simple shear -> class 2 (similar) folds. Rocks must be very ductile and have low viscosity contrast - deep in crust or in particularly weaker rocks (salt).
Flexural flow/shear
Modification of folding layer by simple shear that occurs parallel to the layer and is evenly distributed within the layer.
Occurs in layers with an initial uniform fabric (shale) and is geometrically similar to flexural slip folding thousands of infinitesimally thin layers.
Distortion in limbs, no strain in hinge.
Strain becomes more evenly distributed in limbs, shear increases down limbs (change in orientation), pure flexural folds have no neutral surface and strain increases away from hinge. Do not see individual slip surface.
Controls on fold wavelength - buckle folds
Two main controls - viscosity and thickness (both proportional to wavelength but t more significant due to 2t vs cube rooted n)
Viscosity = resistance to flow, high viscosity (most competent rocks).
Increased viscosity ratio = longer wavelength (proportional)
-high viscosity ratio = high viscosity layer encased in low viscosity rock
-low viscosity ratio - folds are poorly developed (short wavelength), layer thickens by pure shear
Thicker layers = longer wavelength
Controls on fold wavelength - parasitic folds
Interplay between layer thickness and viscosity contrast on intensity of folding (wavelength, A, number of folds) explains how parasitic folds might form.
Thinner beds with lower viscosity contrasts -> short wavelength; incorporated within larger wavelength folds defined by thicker layers
