Structural geology

Hinge line (zone) (fold axis)

maximum curvature on the fold, described by trend and plunge

Boit Ramburg equation

Layer thickness is highly variable and as layers get thinner, wavelength gets shorter.

Flexural Slip

Slip between beds, visible, contraction and extension at hinge points, veins on outer arc

Flexural flow

No visible slip between beds, no contraction or extension, slip is in the fold.

Forced Folds/Bending

Fold shapes required (forced) to form as the result of other geologic processes, forces act across layer. Intrusion forces beds to fold, or could be dripstone in river or ocean. 

Shear folding

Axial plane (surface)

Inclined (dip), recumbent (horizontal), upright (vertical)

Antiform

upside down U shaped

Synform

U shaped

Anticline

convex towards younger rock

Syncline

convex towards older rock

Monocline

Step like fold, only one limb

Dip Isogon

parallel dip

Class 1B

Constant unit thickness

Class 2

Outerarc and Innerarc angles are the same, limbs are thinner

Vergence

direction of motion, how was the fold moved through time

Z folds, S folds

Parasitic, second order folds

Cylindrical

hinge line is straight

non cylindrical

hinge line is curved

gentle inter limb angle

180-120

open inter limb angle

120-70

tight inter limb angle

70-30

isoclinal inter limb angle

30 and less

Window

Erosional exposure of the rock unit underlying a Nappe

Vein

extensional fracture filled with minerals

Klippe

an erosional remnant of a thrust Nappe

Ribs and rib marks

Elliptical structures occurring on extension fractures, centered on the nucleation point of the fracture They are ridges or furrows where the fracture has a slightly anomalous orientation and are perpendicular to hackle marks

allochthon

Tectonic unit that has been transferred too far for direct correlation with the substrate

autochthon

lithologic unit in or along an orogenic belt that has not been tectonically transported

Structural Geology

The geometries of rocks after a process has altered their appearance from an original state. Rock deformation.

Tectonics

The regional processes and motions that generate a set of structure in an area (observer defined)

The composition of Earth and Crust (whole earth)

Fe-35%, O-30%, Si-15%, Mg-13%, NI,Ca,S,Al

Crust composition

Plagioclase-39%, Alkali Feldspars, Quartz, Pyroxenes-12% each, amphiboles, micas, clays

Why is crustal composition important?

Explains faulting and fracturing

Compositional layers of the earth

Crust, mantle, core

Rheological layers

lithosphere, asthenosphere, mesosphere, outer core, inner core

Continental crust

40 km thick on average, oldest rock in 4ga, less dense, 5-10x thicker than ocean crust, avg. age 2.6-2.7 ga

Composition an characteristics of oceanic crust

Basalt and gabbro, younger - no older than 200ma, thinner

Lithosphere vs. asthenosphere

l - rock, implying strength; plates70-10km thick in ocean, 200-225km thick continental a- 660km, plasticky

Tectonic plates of earth

deepest earthquakes happen at subduction zones, earthquakes trace plate boundaries, plates can be oceanic and continental

Plate motion and driving mechanics

Bimodal distribution of elevation, density contract, this is because of fractional melting and water. Mechanisms are ridge push, slab pull, mantle flow, plume push, crustal thickening, gravity is main driver.

Difference between primary structures and secondary structures

primary structures develop during the formation of a rock body, we want to know this so we can figure out what a rock looked like in its original state, this helps u orient ourselves.

Sole/tool marks/flute casts

Ripple marks

Cross bedding/cross stratification

Graded bedding

desiccation/mud cracks

Buttress unconformity

younger sedimentary rocks are deposited against an older, topographically irregular surface. This results in the younger rocks abutting or "buttressing" the older rocks, creating a distinct angular relationship between the two sets of layers.

nonconformity

sedimentary rock on top of igneous or crystalline base rock

discomformity

sedimentary on sedimentary rock, bottom layer is eroded

angular unconformity

horizontal eroded, tilted, meet at an angle

paraconformity

time missing

Characteristic fauna and flora of Cambrian

Trilobites

Characteristic fauna and flora of Paleozoic

brachiopods

Characteristic fauna and flora of Mesozoic

ammonites

Characteristic fauna and flora of Cenozoic

gastropod shells

Difference between sigma 1 and sigma three

sigma 1 is greatest, sigma 3 is least

Griffith cracks and tensile crack formation

In the lab ideal material fracture at 10gpa, real rocks have micro defects - micro fractures

Griffith fracture theory

Joints due to unloading

removal of overlying rocks, rocks expand in vertical direction, contract in horizontal direction, form nearly vertical and horizontal arrays

Joints

Fracture with little displacement, abundant on surface of earth, tensile feature, formed by dilation

Joint Arrays

Often observed as sets or arrays, systemic, multiple sets 90 degrees from each other, vertical or horizontal, refract, change orientation or disappear along rock layers (thickness matters too)

Plumose Structure

Hackles

Form where joint propagates rapidly

Modes of Fracture

1. Opening fractures (joints)

2. Sliding fractures

3. Tearing fractures

Exfoliation joints 

1. Thermal contraction during pluton cooling “pulls in” country rock

2. Stress is relaxed, pluton is unroofed, rocks crack=joints

3. Joint orientation is parallel to surface and margins of pluton

Hydrofracturing

Fracking

Joints related to thermal contraction 

Horizontal Jointing maximizes surface to fracture length ratio (basalts near surface)

Why do joints form near the surface

Stresses become more comprehensive with depth to the points where rocks can’t pull apart

Faults/Shear Fractures

Compressive shear fractures with appreciable displacements

Fault zones

Zone of faults, scale dependent

Slip

Relative movement on opposite sides of the fault, actual total displacement

Seperation

Offset of a marker (i.e. sedimentary beds, dikes, etc), apparent displacement

Normal Fault 

If shallow dip, less than 30 degrees it is a detachment, steeper angle is normal fault, generally placed younger rocks and or lower grade rocks (in the hw) on top of older and or higher grade rocks (in the fw), omits stratigraphy, accommodate extension.

Reverse Faults 

Shallow dip, thrust fault, older higher grade rocks on younger lower grade rocks, repeats stratigraphy, accommodate shortening

Slicken lines

Provide direction of slip

Chatter marks

Imbricate fan

Contractional faults in the foreland of an orogenic zone typically form an imbrication zone, imbricate fan. It is a series of similarly oriented reverse faults that are connected through a low angle floor thrust.

Duplex

An imbricate fan that is bound by a roof structure

Fault Bend Fold

The moment a ramp is established and the hanging wall starts climbing above it, the hanging wall layers are deformed into a fault bend fold.

What are fold and thrust belts? What are the geometric rules of fold and thrust belts

1. Thrusts cut up section in direction of transport

2. Thrusts place older on younger

3. Thrusts are shallow ~10 degrees to almost flat in weak rocks

4. Thrusts are oblique ~30 degrees in strong rocks

5. Thrusts break at lower level, usually young towards foreland

6. Thrusts belts expressed as ramps and flats

Decollement

Undeformed basement

Detachment

Low angle normal fault

Normal fault Domino

Horst and Graben

Passive Folding

Class 2 folds, limbs thinner than hinge, same interlimb angle of inner and outer arcs

Active folding

Layers are active in the folding process, class 1b folds, Baraboo

Critical wedge theory

Antiformal stack

Buckle Folds

Active folding due to end unloading, high grade metamorphic rocks (more plasticky). Requires layer parallel shortening and a strong viscosity contrast between layers and matrix. Mechanical instability.

Rift Setting of Normal Fault

a. symmetric rift setting: Broad extension to accommodate thinning, b/c symmetric we encounter Horst and Graben style normal faults primarily

b. Asymmetric rift settings: mostly domino style normal faulting

Orogenic Belt setting of normal faults

Orogenic Collapse

Rocks collapse from their own weight due to gravitational potential energy. Rocks ‘flow’ downhill How do you collapse the thick portion of the plate? Normal faulting.

Gravitational Potential Energy

density * gravitational constant * height

Stress on Normal Faults

Anderson’s theory of faulting

Stress on Reverse Faults

Anderson’s theory of faulting

Heave and Throw

Heave - horizontal component of slip, Throw - vertical component of slip