Geological maps

What are the main stratigraphic principles?

• Original horizontality

• Stratigrapgic superposition

• Lateral continuity

• Cross-cutting relationships

Define contact and deposition.

Contact: The boundary between 2 lithologies

Deposition: Material carried by fluid/gravity and settled on a substrate

What is emplacement?

Forcible intrusions or eruption of magma (subsurface) or lava (surface) or veins from hot fluids

What are thress measurements used for planar features?

• Strike

• Dip

• Dip direction

Define dip

The steepest angle between an inclined planar feature and a horizontal plane - record as an angle

Define strike

The compass direction of the intersection between a dipping plane and a horizontal plane. - recorded as a three-digit (000)

How is dip direction related to strike?

    Dip direction is always 90 degrees from strike. [ Use the left-hand rule: index finger points along strike, thumb points in dip direction.]

Name examples of geological features which can have strike and dip measurements.

• Bedding

• Faults

• Cleavage

• unconformities

• fold axial planes

• lithological contacts

• metamorphic fabrics

What is the difference between true dip and apparent dip?

true dip is always perpendicular to strike whereas apparent dip is based on the perspective/viewing direction

What is the Rule of V’s?

A tool to work out dip direction using maps. A planar feature intersecting a valley will make a V shape pointing in the direction it is dipping.

What determines outcrop patterns?

• Defined by geology interacting with topography

• predictable once you measure planar features

• steeper features make less exaggerated patterns

• shallower features make more exaggerated outcrop patterns

What is deformation and what does it tell us?

The change in shape, size, or position of an object resulting from an applied force

Allows us to interpret the structural history and the deformation conditions

What is rheology?

The study of material responses to deformation. Environmental conditions like pressure, temperature, and fluids partially govern rheology.

What is stress and what are its units?

Stress = Force / Area

Units include:

• Pascal (Pa) N/m²

• MegaPascal (MPa)

• 100MPa = 1kbar

What are the three principle stresses?

• σ1 = Maximum principal stress (compressive/squashing axis)

• σ2 = Intermediate principal stress

• σ3 = Minimum principal stress (extensional/stretching axis)

Define different types of stress

• Mean stress: Average of principal stresses

• Differential stress: σ1 - σ3 (difference between the max and min principal stresses

• Deviatoric stress: represents `distortion/ deformation relative to mean stress

• Total stress: σ1 + σ2 + σ3 (combination of volume and shape changes)

     

What is lithostatic stress?

• Mean stress equivalent

• Gives estimate for stress/pressure at given depth

• Does not account for anisotropic nature of stress

• lithostatic stress = p g h

What is hydrostatic stress?

• Pressure exerted by fluid contained within geological formations

• Isotropic like lithostatic

• hydrostatic stress = pgh

What happens with pore-fluid pressure/overpressure?

• Fluids get trapped in rock pore space become pressurised

• Can lead to hydrofracturing

• Results in: mineral vein formation, earthquakes and fracking

How does differential stress operate on tectonics?

• Slab pull and ridge push mechanism

• governed by density difference

• graviational push of cooling oceanic crust with age as it moves from MOR

• Cold, dense ocean crust sinks into mantle at sunduction zone

What are the three main tectonic stress regimes?

1. Vertical σ1 (maximum):

   • Vertical compression - gravity and/or upwelling mantle

   • Results in normal/extensional fault systems

   • Creates extensional basins

2. Vertical σ3: minimum principle stress

   • Horizontal compression

   • Creates folds and thrust belts

   • Forms mountain belts and plateaux

     3. Vertical σ2:

     • Both compression and extension are parallel to Earth’s surface

     • Results in strike-slip regimes and shows translation of geological markers obvious in map view

     • Development of strike-slip fault systems

What is strain?

• Size or shape changes to an object in response to deformation

• Differs from rigid body deformation (rotation/translation without changing volume/shape)

What are the two main types of rock fabrics?

1. L.tectonites

   • Linear tectonic fabric

   • Looks like a fistful of pencils

   • Examples: amphibolite fabrics, stretch pebbles

2. S-tectonites

   • Planar tectonic fabrics

   • Sheet-like

   • Examples: mica-defined/gneissose banding

Measuring strain in 1D

• Linear strain causes a change in length or geological materials

  • extension/contraction

  • the change in length of a stretched object e.g fossil in a fault or shear zone

Measuring strain in 2D

• Shear strain causes change in shape or orientation of geological materials

• If the initial shape of the object is know → we can measure the angular rotation between known linear strain markers

  • particularly in fossils

• Can also measure the x, y and z dimensions 2-D and possible 3D strain

Assume it they are perfect spheres → not real life

What is fold anatomy?

• Form where initially planar features become curved/bent

• Consists of hinge flanked by two limbs

• Hinge point = point of maximum curvature

• Hinge zone = volume of high curvature material around hinge point

• Hinges extrapolated along hinge lines

What’s the difference between cylindrical and non-cylindrical folds?

• Cyclindrical folds: hinge projects as straight (like a toilet roll)

• Non-cylindrical folds: hinge projects as curved line (can indicate redeformation or dome/basin structure)

Define fold wavelength and amplitude

Wavelength: measured between two hinges

Amplitude: measured between hinge and inflections

What is an axial plane/surface? Starred

• Connects the hinge lines of multiple folded layers

• Can be measure as a planar feature (strike/dip/dipdirection)

• Creates axial trace where it intersects Earth’s surface

Explain the difference between -forms and -clines

Antiform : oppositely dipping limbs

Synform: limbs dipping towards eachother

Anticline: oldest rocks in core, younging outward

Syncline: rocks in core, younging inward

What determines fold tightness?

• Determined by size of interlimb angle

• Qualitative way of determing how much strain a folded layer has experienced (Arms out and being pushed inwards → the more strain on your arms the smaller the angle would be.)

• Interlimb angle can measured with protractor or compass clino

• Isoclinal folds: both limbs orientated parallel

What are some descrptive terms for the curvature of a fold

• Low radius of curvature = sharp fold

• High radius of curvature = rounded fold

How is the orientation of a fold classified?

• Classified based on the inclination of the hinge line

• an uptight fold has a vertical axial surface

• Inclined folds have inclined axial surfaces

• Increased inclination can lead to one limb becoming overturned as the axial surface becomes flatter

• A flat axial surface = a recumbent fold

• Rotation of the axial surface >90o from upright produces an inverted fold (synformal anticline, antiformal syncline)

What are the three main folding mechanisms?

• Buckling: active folding by forces acting parallel to layers of different competency (squash it on its side

  

• Passive folding: layers of comparable competency are passively sheared (when in lower crust and it is hot and sticky the layers will flow passively due to the stress)

• Bending/Flexure: Warping of layers in response to orthogonal stress (salt or magmatic loading)

What are the controls of buckling?

• Viscosity/ competency contrast between the layer and the surrounding layers

• Layer thickness - thicker layers produce folds with longer wavelengths

What causes bending/flexure?

• Monoclinal fold forming during rifting

  • can occur in settings where you may have a buried fault beneath some ductile layers

• Salt or magmatic loading

  • the crust can be loaded by putting a big suite of magma.salt dome and cause the layers to flex around it

• Boundinage

  • One less competent unit bends into a broken more competent unit

What are the three main mechanism of flexure?

• Flexural slip

  • like bending book pages

  • Frictional sliding at layer interfaces

  • Low-temperature at layer interfaces

  • Low-temperature conditions

  • Shows striations on bedding surfaces

• Flexural flow

  • Internal ductile deformation within layers

  • May occur at high temperatures

  • Common in weaker materials (mudstones )

    

• Orthogonal flexure

  • How the fold, folds at the hinge

  • Causes tension in outer arc

  • causes compression in the inner arc

What are parasitic folds?

• Form when thin layers buckle earlier than thick layers

• Become asymmetric on fold limbs (Habe a long limb and a short limb)

• Long limb lean toward hinges of larger structures

• Form predictable S,Z and M- fold patterns

What is cleavge and how does it form?

• Rock cleavage is penetrative foliation governed by minerals with themselves have a strong cleavage

• Is is a strain fabric produced during deformation

• Can form where minerals align under tectonic stress

• Cleavage = low grade metamorphism

What is crenulation cleavage?

• Forms when original cleavages are deformed by later events

• Creates crepe-like paper texture

• common in metamorphosed mudstones

• Shows folding/crinkling of initial cleavage

How does cleavage help predict fold structure?

• If cleavage forms at the same time with fold → the cleavage will form parallel to axial planes

• If the cleavage is axial planar, you can measure the angle between the bedding and the cleavage to find where you on the fold

  • fold hinge = 90 degrees

  • fold limb: <90 degrees

What shows the true geometry of a fold?

Only a profiles taken 90 to the hinge line/axial plane (only cross ections parallel to the true profile)

How do upright vs plunging folds appear on maps?

• Upright folds show barcode/traintrack patterns

• PLunging folds show fold closures:

  • Antiforms close down-plunge

  • Synforms open down-plunge

How can you identify folds on maps when closures aren’t visible?

• You can identify folds through:

  • Reflected stratigraphy - mirrors of the same strata)

  • Reversal of younging direction across fold axes

  • Reversal of dip markers - but not reliable for overturned folds

How do structure contours appear in folded rocks?

• They are parallel on the limbs for upright folds with horizontal axes.

• They increase elevation towards antiform hinge

• They decrease in elevation towards synform hinge

• Spacing varies due to dip direction:

  • Wider spacing in hinge

  • Narrower spacing on limbs

How are fold axial traces shown on maps?

• Axial traces connect hinge points

• Antiforms: chevrons pointing away from axial trace (not coloured in)

• Synforms: chevrons pointing toward axial trace

• Anticlines/Synclines: same as above but coloured in

• Arrows indicate plunge direction

• Two opposite arrows indicate a dome

What is vergence and how is it shown?

Vergence shows the way a fold leans and it will affect the spacing of structure contours on each limb

• Shown with a single chevron indicating lean direction

• Affects structure contour spacing:

  • steep limb = narrow structure contours

  • Shallow limbs = wide structures contours

How can you determine fold hinges from structure contours?

• Match up equivalent structure contours

• The point where they meet = hinge points

• Joining successive hinge points from other structure contour intersections = hingeline

• This methos works for intersections of any two planr features.

How do you distinguish between topographic features and actual folds

• If curvature of contact is associated with curvature of topographic contours, it’s likely a feature dipping into valley (not a fold).

• If curvature of contacts occurs where topographic contours are not curved it indicates structural variation

What is elastic behaviour in rocks?

Rocks behave elastically in the upper crust, where deformation is fully recoverable until applied stress is relaxed to a point.

Important factors include:

• strain rate

• temperature,

• cohesion/lithification of rock

What happens when rocks reach their yield point?

When rocks reach their yield point, they can’t appreciate any more strees and fracture in the brittle/frictional regime of the crust. This immediately relieves stress but causes irreversible damage to the rock

What are cracks and how does stress act on it?

They are 3D ellipsoids - not square or lines

Stress concentrates at the tip of a crack

The more elliptical, the more stress concentrates

What are the main types of fractures?

• Mode i (extension fractures)

  • propagate along σ1 and σ2 axes

  • Open and widen along σ3 axis

  • Son’t show any sense of offset

• Mode ii (shear fractures)

  • Accommodate slip and offset geological markers

  • Form ~30 degrees from σ1

  • Can from conjgate pairs ( dipping in opposite directions

  • when shear fractures grow they create faults

What is a fault zone?

• A tabulate volume of rock undergoing deformation with a localised slip zone

• must have brittle or frictional deformation mechanisms, as ductile behaviour in mid-lower crust inhibits fracturing.

Define hanging wall and foot wall

Hanging wall: the block you would hang a lamp on

Footwall: the block you would stand your foot on

What are the three types of fault displacement?

1. Strike slip

   • Fault slip is parallel to the strike of the fault

   • Side-to-side motion across the fault

2. Dip slip

   • Fault slip is 90 degrees to the strike of the fault

   • Up-and-down motion across fault

   • opposite to strike slip

3. Oblique slip

   • a mixture of the two

   • Diagonal motion across fault

What is the slip vector and the pitch?

• The slip vector represents the magnitude at which a fault has moved along a particular direction.

  • Only a cross-section parallel to the displacement vector will show true displacement of offset markers

• Pitch is the angle between the fault strike and the slip vector

What are the main types of dip-slip faults?

1. Normal faults:

   • hanging wall downthroan relative to footwall

   • down-dip slip vector

   • Usually high angle (~60)

   • Accomodates stretching/extension

2. Reverse/thrust faults:

   • Hanging wall upthrown relative to footwall

   • Up-dip slip vector

   • Usually 45- 60 dip angle (thrust <30 - shallow)

   • Accomodates shortening/contraction

What are the types of strike-slip faults?

1. Sinistral (left-lateral):

   • Left-lateral displacement

   • Near-vertical dip

   • Accommodates translation

2. Dextral (Right-lateral):

   • Right-lateral displacement

   • Near-vertical dip

   • Accommodates translation

What is Andersonian faulting?

• Stress configurations relative to Earth’s surface produce particular fault geometries

  • σ1 vertical: extensional regime, normal faulting

  • σ2 vertical: compressional regime, thrust faulting

  • σ3 vertical: translational regime, strike-slip faulting

What are slickensides, slickenlines and slickenfibres?

• Slickensides: polishes fault surfaces resulting from abrasion/frictional wear

• Slickenlines: grooves/lineations onslickensides indicating abrasion pathway of clast/minneral

  • can measure plunge and trend of slickenlines to approximate fault slip vector

• Slickenfibres: during slips, gaps open along fault planes and develop fibrous textures

What are the main types of fault rocks and where do they form?

• Brittle fault rocks (up to 10-15km depth)

  • Grain size is reduced by fracturing and frictional wear. incohesive or cohesive

    • Fault gouge or falt breccia

    • cataclasite

    • Pseudotachylite

• Ductile fault rocks (>10-15km depth)

  • Mylonites - rocks deform plastically in shear zones

  • deformation achieved by diffusion of atoms

How do fault rocks form?

From physical damage, chemical alteration and fluid-rock interaction

What is a fault trace?

The intersection of a fault plane with the Earth’s surface

What is a fault scarp?

An exposed fault plane with marked relief change

What’s the difference between slip and separation?

• Slip: quantitative measurement of magnitude and direction of fault motion (slip vector has dip-slip and strike-slip components)

• Separation: Displacement of a marker horizon along strike or up-down dip surface

What are heave and throw?

Heave: Horizontal component of dip separation

Throw: Vertical component of dip separation

How can fault mechanics be determined from mapping?

• Map symbols

• Offset of oppositely dipping markers

• Fault dip direcction

• Stratigraphic knowledge

What happens to stratigraphy around faults?

Faults can result in the repetition or omission of stratigraphy either down a borehole or in outcrop patterns

What are piercing points?

• Sawn off, diplaced narkers which can be found located at either side of the fault (incredibly rare)

How does erosion affect fault identification?

Erosion often removes topographic evidence of fault scarps, leaving only patterns that show offset of geological markers

What effect do fault zones have on geomorphology?

• Faulting physically and chemically damages and alter rocks

• Fault zones are often easily eroded - greating gullies and valleys

• Can cause vegetation changes by drainage

How can fault dip direction be determined?

• Using the V-in-the-valley rule

• Lookung at how exaggerated the outcrop pattern is

• Constructing structure contours on a fault

What is the Law of Superpositions role in fault interpretation

Young rocks must come down to be in contact with older rocks

What are the characteristics of conformablesequences on maps?

• Rock units initially deposited continuously

• No erosion or time gaps

• Right-way-up strata young in direction of dip

• Geological contacts do not cross cut one another

• Conformable layers are not repeated (unless faulted)

How can you recognise unconformable contacts?

An unconformity surface truncates or cuts into older geological features.

What are the characteristics of angular unconformities?

• Demonstate change in orientation of strata across their interface

• Visually striking in field and on maps

• Signified by abrupt changes inn dip and/or strike of units

How do disconformities differ friom angular unconformities?

• Disconformities are subtle features in the field and on maps

• They represent a time gap or erosion in parallel rock units

• Strike and dip values may be identical or very subtly different

• Look for evidence of missing stratigraphy and major environmental changes across contacts

What is a flat planar unconformity?

• produced by peneplanation

• represents mature landscape with very low relief above sea level

• Found in mature river courses and shallow marine settings

• Shows evenly spaced, parallel structure contours

What are irregular unconformities?

• REpresent buried landscape with preserved paleotopography (hills and valleys)

• Sedimentary/volcanic rocks above may be highly variable in thickness and lithology

• Common in alluvial and aeolian deposits

• Difficult to structure contour

What is subcrop?

• Defines the intersection of features beneath an unconformity with the unconformity surface

• If both unconformity and subcrop features are planar - their intersection forms a line

• Can be projected beneath unconformity to determine in subsurface

• Pattern can be defined by joining intersection points if same elevation using structure contours