Lecture 3 - Folds, Principles, Geological mapping Flashcards
What are folds the product of?
ductile deformation and the resulting bending of a planar structure
What are the different parts (elements) of fold?
Hinge
Limb
Hinge line
Fold axis
Axial plane
What is the hinge in folding?
point/ zone of greatest curvature
What is the limb in folding?
connects 2 hinge points
What is the hinge line in folding?
Line through hinge along 1 layer
What is the fold axis in folding?
any line parallel to the hinge line
What is the axial plane in folding?
divides two limbs, passing through the hinge lines of overlying layers
How are folds classified?
axial plane and plunge of hinge line
What are the most common folds?
- Upright
- Plunging upright
- Horizontal inclined
- Recumbent
What are the different fold tightness’s and their matching interlimb angle?
Gentle 180-120
Open 120-70
Close 70-30
Tight 30° - 0
Isoclinal 0°
What are monoclines?
folds with 1 limb
How are monocline produced?
By deeper faults that have not reached the surface (blind faults)
What are kink bands?
sharp angular folds bounded by planar surfaces (normally small scale)
How do chevron folds form?
in layers with a regular alternation of contrasting (soft – rigid) competences (e.g. claystones and sandstones)
What are the characteristics of chevron folds?
- “V” shape
- straight limbs
- sharp hinges
- 60° interlimb angle
What are the characteristics of concentric folds?
- Rounded shape
- Curved limbs
- Broad hinges (hinge zone)
What are the characteristics of box folds?
- Box shapes
- Three limbs
- Two hinges and axial planes
- ~90° interlimb angle
What are sheath folds?
Highly non-cylindrical folds formed in high-strain shear zones, hence deep levels crust
where rocks deform plastically
Where do parasitic folds form?
in layers with different competence (soft and rigid layers alternate)
What are the different types of parasitic fold?
Z-shaped
S-shaped
M-shaped
What do Z shape parasitic folds indicate?
fold core is located to the right
What do S shaped parasitic folds indicate?
fold core is located on the left
Where do M shaped parasitic folds form?
Near fold hinge
What is the difference between antiform and synform?
Both fold form:
Antiform =convex up
Synform =convex down
What is the difference between anticline and syncline?
Describe the age distribution in the fold:
* Older rocks in fold core = anticline
* Younger rocks in fold core = syncline
What is flexural slip?
When a layered rock unit is folded, adjacent
layers within each limbs experience a simple shear, which has opposite sense of slip in the two limbs.
What are slickenlines?
Scratches parallel to the main stress responsible for the folding may form above the layers surfaces
What is flexural flow?
Essentially the same as slip but with deformation distributed more evenly
within the limbs.
What conditions does flexural flow more commonly occur in?
It commonly occur in more plastic conditions (higher temperature)
What 3 ways can folding occur?
Buckling
Bending
Passive folding
What is buckling? (folding)
Coming together and producing rounded folds when a material is between high and low ductile materials
What is bending? (folding)
forces applied at a high angle to the
layers
What is passive folding?
layering plays no mechanical role and therefore has no influence on the shape of the fold
What is the basis for the principle of Uniformitarianism?
Present is the key to the past
What are the uses of the principle of uniformitarianism?
look at modern geological systems (sedimentary, mountain-building, volcanoes etc.) explain past geological processes
can look at ancient events (e.g., climate changes) to understand current processes
What is an example of an age where modern tectonic processes occurred?
The Jurassic (170 Ma) i.e. Fossil oceanic detachment fault 130 Ma (South Tibet)
What is the principle of superposition?
Younger sediments are deposited horizontally over older sediments in a “layer cake” style
What is the principle of cross cutting relationship?
Younger rocks cut across older rocks (faults)
What are the different types of unconformities?
Angular unconformity
Non-conformity
Disconformity
What is an angular unconformity?
Older rocks below were tilted and eroded before the deposition of the younger layers above. Angle between the beds
What is non-conformity?
Younger layers were deposited over metamorphic/igneous basement where layering is absent
What is a disconformity?
Uplift and erosion of layered
sedimentary rocks is followed by down thrown and new deposition: all beds are parallel
What do unconformities indicate?
time gap in deposition (or hiatus) i.e. deformation, uplift, burial, erosion, and metamorphism
What are contour lines?
lines connecting points of equal elevation
What is an individual contour line?
the intersection between a 3D object (i.e., topography) and a horizontal plane with a given elevation (in meters)
What are the lines dividing areas of different colours called on a geological map?
Unit boundaries
How will lines (unit boundaries or faults) usually be shown on a geological map?
As curved lines (curve controlled by dip and topographic profile)
What is true thickness of rock unit?
visible only along a section perpendicular to its layering/bedding.
What is apparent thickness of a rock unit?
when observing rock thickness from ant perspective then perpendicular to its bedding
What is the apparent thickness like compared to true thickness?
always larger
What is apparent thickness controlled by?
angle at which the topographic surface cut the the beds
How can true dip of a rock unit be observed?
along a section normal to the strike (or
parallel to the dip direction)
What is apparent dip of a rock unit?
Observing a rock unit dip on any angle other then parallel to dip direction
What will apparent dip be like compared to true dip?
Shallower
what 3 geometries can faults and unit boundaries appear as on geological maps?
Vertical plane - straight cross cut line
Horizontal plane - curved parallel to specific contour
Inclined plane - Curved line cutting across contour
How will dip be shown on a geological map?
It will be a T like shape with the long side being strike and the short being dip with an accompanying angle (drawn on cross section with protractor)
How are folds in geological maps represented?
a repetition of units, which are symmetric
with respect to the hinge line
What will an upright fold produce on a geological map?
map unit boundaries (lines) that are parallel to each other
What will plunging folds produce on topographical maps?
V-shaped unit boundaries
What will the strike be like on the 2 limbs of a plunging fold?
Will be different
What are the 2 outcomes if a plunging fold forms chevron shaped unit boundaries?
Plunging antiform: plunge is in the direction of the chevron
Plunging synform: plunge towards the core of the chevron
What will faults do in geological maps?
cut and offset unit boundaries
What are structure contours?
connect points of equal elevation on a geological surface (bedding plane etc)
What do structure contours run parallel to?
Strike
How can structure contours tell you if a surface in planar?
if the contours are straight, parallel and equidistant
What does it mean if the structure contours are curved?
the strike of the bed varies
If spacing of structure contours vary so does bed dip
What is the formula to calculate dip?
a=tan*-1(VI/HE)
VI= vertical elevation interval between contours
HE= horizontal spacing between selected structure contours
How can structure contours be used to find true thickness? (equation)
t = HE *sin(a)
HE= total map distance horizontal meters between equal elevation structure contours
(a)= known dip of structure
