Structural Geology

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Lecture 2 24/01/017

Folds form from a ductile deformation process. This is typically a very slow process. Folds are very self-similar - the small scale structure reflects the macroscale structure.

Hinge - zone of maximum curvature. Hinge line - line of maximum curvature.

Inter limb angle - how tight a fold is.

Isoclinal - 0 degrees

Tight, close, open, gentle (between 120-180 degrees).

Profile plane - a line straight through the fold. Planar surface perpendicular to the hinge line. Measure inter limb angle from this plane.

Inflection point - where the sense of curvature changes from one fold to the next

Axial plane- exactly bisects the fold. Joining up the hinge points also defines the axial plane. The axial trace is where the axial plane intersects the Earth's surface.

Anticline - old rocks in the core of the fold. Syncline - young rocks in the core. 'cline' when you know age of rocks. 'Form' when you only know shape.

If folds are downward facing then there must have been at least 2 separate deformation episodes. 1. Turned the beds upside down. 2. Folded them.

Lecture 3 25/01/17

Primary planar structures - undeformed

Secondary planar structures - modification of original rock

Secondary planes cross cut primary planes.

Foliation - result of metamorphism.

Measuring a planar structure:

Strike and Dip

Strike - flat horizontal line, Dip - line perpendicular to the strike

Dip value must be between 0 and 90 degrees. Two figures = dip value

The strike line is the orientation towards the azimuth of a horizontal line on a planar structure.

The dip is the angle of inclination of a planar structure perpendicular to the strike and therefore the maximum possible dip amount.

Dip direction is the compass direction in which the planar structure is dipping. (N,E,S,W)

Lecture 4 26/01/17

True vs Apparent Dip

Line of section at 90 degrees - true dip.

Parallel - flat

Oblique - less inclined

Rule of V's - if bedding is horizontal then the outcrop of the beds will follow the contours.

If bedding is vertical the outcrop of the beds will cut straight across the contour lines. Inclined beds.

Diamond symbol on map = top of an antiform.

X symbol on map = top of synform.

Rock units that follow contour lines must be flat.

Outcrop parallel to valley - no V.

The V shape of outcropping beds nearly always points in the direction that the layers are dipping in.

Lecture 5 27/01/17

Lecture 6 30/01/17

Foliations and Lineations

Foliations can form due to compression (planar structure)

Lineation - structures that are just lines. Primary - magma flow Secondary - tectonic deformation.

Primary lineation structures eg in igneous rocks. Crystals align in flowing magma and define a lineation.

Primary lineation structures in sedimentary rocks e.g. currents form scour marks in deposited sediments (pebbles) These structures can be measured as lineations to determine the paleoflow direction.

Secondary lineation structures e.g. slickenlines form due to friction on fault surfaces.

Crenelation cleavages from where a planar structure has been folded by a second deformation event.

A lineation is simply a set of lines produced by primary or secondary processes. A crenelation is the intersection of microfolds with a planar surface. Eg foliation or bedding plane.

Crenelation - more than one phase of deformation because there is a foliation which has been deformed. Foliations doesn't tend to occur in sandstones as easily as in mudstones. Cleavage always forms at 90 degrees to the compression direction.

Plunge is always 2 figures. Trend is 3 figures.

Lecture 8 01/02/17

Recap of lecture 7:

Cleavage will always form at 90 degrees to the applied stress. Hinge line will be at 90 degrees to the applied force. Minerals align at 90 degrees to the compression direction. The cleavage plane is parallel to the axial plane of the fold. So if the intersection of the axial plane and the fold is the hinge line, the cleavage plane is parallel to the hinge line.

Axial planar cleavage - fine planar fabric parallel to the axial plane of the folds.

Geological Faults

What is a fault? a structural fracture in a rock across which there is notable displacement. Important structures in the upper crust controlled by brittle deformation.

The displacement across faults is directly proportional to their length

Joints - everything crystallises towards each nucleation point.

Why is 120 degrees found in nature

Fracture pattern that is random - just cracksJoint- fractures which have no appreciable displacement across them.

Scottish faults- Moine fault, great glen fault, highland boundary fault, southern upland fault.

Recognising a fault - offsets

Faults propagate upwards

Rocks in the ductile zone - mylonites.

1. Dip slip fault - slip parallel to the dip of the fault surface

2. Strike slip fault - no vertical movement but there is horizontal movement e.g. san andreas

3. oblique slip - both vertical and horizontal movement.

NORMAL FAULT - hanging wall block moves down relative to the footwall. When the hanging wall moves down it is a normal fault. This happens as a consequence of extension. Blocks are pulled apart - gravity will force the hanging wall to move down hence a normal fault.

REVERSE FAULT - hanging wall block moves up relative to the footwall as a result of compression of the earths crust = thickens the crust.

Normal faults horst and graben structures

Moine thrust is a REVERSE fault - hanging wall block moves up relative to the footwall as a result of compression of the earth's crust. Steeper than thrust faults.

Strike slip - sinistral when strata on the opposite side of the fault plane is displaced to the left, dextral when strata on the opposite side of the fault plane is displaced to the right.

Wrench fault - a vertical strike slip fault

Transcurrent fault or transform fault - lithospheric scale strike slip fault. e.g. San Andreas, California. Links rift segments in plate boundaries.

Thrust - teeth triangles show the direction the plane is dipping towards, normal - tick marks, strike slip - arrows

Faults as conduits - formation of hydrothermal ore deposits on the seafloor from fluid transport along faults.

Faults as mineralisation zones, faults as traps.

Stress and Strain

Stress- force per unit area. It is a pair of equal and opposite forces acting on a unit area of a body. Hydrostatic - pressure is equal in all directions. Differential of deviatoric stress

Difference between stress ellipsoid and strain ellipsoid.

Brittle deformation - the cracking and fracturing of a material subject to stress

Ductile deformation - the

Strain is caused by tectonic forces. Compression = thrust faults = mountain building. Extension = normal faults = rifting.

Conjugate faults - faults in pairs dipping in opposite direction.

Deformation style is dependent on rheology. Ductile faults are often called shear zones.

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Mountains and Metamorphism

Lecture 1: Continental Deformation and Mountain Building

Eclogites are rocks that form deep within subduction zonesConvection, ridge push.

when tectonic plates collide the stress applied during collision causes STRAIN.

Compressive stres, tensional (extensional) stress, shear stress (oblique).

Fossils are useful strain indicators

Kinks along a fault can lead to convergence or divergence = space or mountains!

Lecture 4: Classification of metamorphic rocks, minerals, textures. 09.02.17

Texture describes the size, shape and alignment of grains in a rock modified during metamorphism as minerals recrystallise.

Foliated - minerals are aligned and define a planar pattern. Rocks have been subjected to differential stress.

Non-foliated - minerals have been recrystallised but do not form a planar or linear alignment.

Limestone metamorphoses to marble.

Foliated rocks:

Slate, phyllite, schist, gneiss. Increasing grain size, low grade to high grade.

Non-foliated:

Quartzite, marble.

Slate - parallel alignment of sheet minerals e.g. clays, chlorite. Produces easily split fissile slate.

Phyllite - sheet silicates white mica and chlorite. the rock will have a micaceous sheen.

Schist - considerable growth and alignment of new mica minerals. Chemical differentiation into different domains that may accommodate the foliation. Results in silica rich and Al rich domains.

Gneiss - compositional alternating bands of quartzofeldspathic and mafic minerals. Bands represent extreme differentiation at high T and P. Often highly folded and extremely deformed.

Migmatite - the rock starts to melt. Granitic melt pods/ layers separate from mafic pods. Deformation within in the banding is common. Pockets of white - parts of the rock that have begun to melt.

Hornfels -elavated heat causes new minerals to grow. Lack of pressure = randomly orientated minerals.

Triple point - 500 degrees, 3.5kbar

Only high pressure - kyanite.

Lecture 5 - Contact, Regional and subduction zone metamorphism.

Contact metamorphism - Hornfels

Regional - organic belt metamorphism, subduction zone metamorphism.

Shock metamorphism

Metabolites - a metamorphic rock where the protolith was very rich in clay minerals (i.e. mudstone)

psammit - sedimentary protolith - sandy mudstone.

Geothermal gradient- 30 degrees per km

Mudstone (pelite) - 1. Chlorite 2. Muscovite 3. Biotite 4. Garnet 5. Staurolite 6. Kyanite 7. Sillimanite. (Burrovian sequence)

550 may scotland and england were both in the southern hemisphere departed by the lapetus ocean.

Andalusite is part of a different PT trajectory, Pressure has to increase slower and temp faster to get andalusite.

Chlorite - slates or phyllite

Biotite - phyllite or schists that contain biotite and chlorite.

Barrow's Zonea - core of the himalayas

Buchan zones - chlorite, biotite, cordierite, andalusite, sillimanite.

Contact occurs in rocks adjacent to igneous intrusions. Metamorphosed area is known as an Aureole.

Mafic magmas are hotter than felsic. Aureoles around felsic intrusions are much larger. Granite contains some proportion of water - the addition of fluids can promote the convection of heat and allow reactions to occur at lower temperatures.

Caledonian granite with well studied aureole. Granite intruded in to ordovician.