Structural Geology Exam 1

Tectonic Structures

created after rock is formed in response to forces caused by plate motions and plate interactions

décollement

a basal sliding plane that separates independent structural deformation in rock masses above and below the fault.

How can we classify tectonic structures?

scale, geometry, cohesiveness, strain significance, distribution

Non tectonic structure

geologic feature that forms during the original formation of the rock

Mega scale

orogenic, regional deformation, plate boundaries

Macro scale

mountain, regional fault system

Meso scale

outcrop scale, bedding cleavage, normal fault

Micro scale

Microscopes, thin section

Planar elements

faults and veins

• fabrics like slatey cleavage

Curviplanar

folded layers, warped

Linear elements

lineation

• fault striations

Fault striations

parallel linear grooves or scratches son a fault plane caused by friction and abrasion of rock bodies sliding past each other

Brittle structures

fractures, veins, faults

• seismogenic (earthquake producing)

Ductile structures

folds, shear zones, fabrics

Fabric/ foliation/ cleavage

bucket term for all foliation, synonymous

distribution

discrete, localized, penetrative, continuous

penetrative

fabric development in mylonitic gneiss

strain occurs uniformly down to the grain scale

Continous

fault breccia with fractured clasts down to micron size

• all the way to bigger and bigger

Kinematic

describes the motion, displacement, and deformation (changes in shape, size, or orientation) of rocks through time

Where do we study geology?

• space

• airplanes and drones

• outcrops, trenches, rock deformation labs\

types of stress

Compression, tensional, shear

Why is structure important to science?

maps and explains how the earth’s crust deforms, folds, and fractures under immense tectonic forces

why is structure important to industry

allows companies to locate sub surface resources safely

• oil rigs, building infrastructure

mining and mineral exploration

ground water management

why is structure important to nature?

it governs how landscapes evolve, where natural resources form, and how we protect life from geological hazards

why is structure important to society?

natural disaster mitigation, ground water (wells for houses), safe construction for houses and buildings humans use.

Joint definition

a natural fracture or crack in a rock body where there has been no visible or measurable movement parallel to the surface of the break

• usually upper crust

• tectonic stress, cooling and shrinkage, unloading and weathering

• allow water and oil to pass through

Joint surface characteristics

• plumose structures

no lateral displacement, zero slicken sides, slicken lines or mullions.

Joint sets

family of parallel, evenly spaced fractures

no visible or measurable lateral movement

joint relationships to principal stress orientations

parallel to max stress (o1)

and perpendicular to min stress (o3)

Orthogonal Joints

mutually perpendicular joint sets

• dihedral angle = 90 deg

• imagine even brick road or concrete square pavement

Conjugate system joints

dihedral

• cross cross (conjugate)

• 60 deg angle

exfoliation dome

spherical joint surfaces of intrusive/metamorphic rocks

columnar joints

cooling joints

• literal columns

sheeting joints

(exfoliation joints)

• common with igneous /metamorphic rocks

• particularly in rocks that lack strong foliations

Joint spacing considerations

perpendicular distance between adjacent fractures within a specific joint set

• decided by combo of lithologic characteristics , stress dynamics, and mechanical boundaries.

joint analysis

study of natural planar fractures in rocks with no lateral movement (joints)

applications of joint studies to real world problems

public safety, tunnels and dams, slope and landslide prevention or safety, oil and natural gas, fracking

define veins

sheet-like body of crystallized minerals that fills a fracture, crack, or fault within a pre-existing host rock

common mineral infill

quartz, calcite, various clay minerals

non systematic vein arrays

non planar veins that vary in width and orientation

En Echelon vein arrays

short, offset, sub parallel veins

• lie between two parallel enveloping surfaces

• inclined at an angle to enveloping surfaces

Pure shear

just flattening, and no rotation, and pure dilation perpendicular to the maximum stress direction

what do en echelon veins tell us

the direction, type, and history of surfaces that deformed a rock layer

Simple shear

card deck like shearing environment but dilation is still perpendicular to max stress direction

• tension gashes will rotate during shearing

pascals principal

pressure is transmitted equally throughout a confined liquid

• multiplies force

• water is compressible when under pressure, it will transfer that pressure

hydraulic fracturing

important rule of fluids and fluid pressure in cracking rocks

• creating veins and fracking for oil underground

• if fluid press is high enough rocks can crack by tension even if buried deeply and surrounded by compressive stresses

conjugate vein arrays

intersecting or paired sets of veins whos geometry is defined by specific geometric angles

veins and their fluid history significance

Earth's subsurface archives, providing a tangible record of ancient fluid movement, stress conditions, and chemical evolution

how can veins reveal kinematics

they directly record the history of stress, fracturing, and fluid movement that occurred after the host rock formed

dikes vs veins

• dikes are formed by molten magma

• veins are formed by mineral-rich water solutions

joints vs veins

• joints are open or unfilled fractures in rock with no measurable sliding or displacement and represent tension or stress release

• veins are fractures that have been filled and sealed with mineral crystals

σ 1

maximum stress (strongest)

σ 2

medium/ intermediate stress

• live on the intersection of faults

σ 3

minimum stress, least strong

slickensides

sheared surface, measure strike and dip

slicken lines

striations on the sheared surface

mullions

large grooves and corrugations on the sheared surface

types of faults

Normal: hanging wall moves down

thrust fault (reverse): hanging wall moves upwards

strike slip: rock blocks move horizontally

fault separation

measured distance or offset between two parts of a single disrupted marker features across a geological plane

true slip

exact total displacement vector representing the relative movement of two rock blocks along a fault place measures in both distance and direction

listric fault

fault with a curved, concave upward fault plane where the dip angle is steep near the surface and flattens out at a greater depth

planar fault

a geological fault that occurs along a flat, two-dimensional plane rather than a curved or irregular surface

Parallel fault arrays

multiple faults run side by side in the same general direction

• usually sharing the same stress regime

Anastomosing fault arrays

Fault surfaces branch out and reconnect around intact rock lenses, creating a braided, network-like pattern.

En Échelon fault Arrays

Short, overlapping, or step-like individual faults are arranged along a staggered line oblique to the overall trend of the broader zone

Congugate fault arrays

Two intersecting sets of faults develop at acute angles to each other under the same principal stress axes, commonly showing complementary sense of displacement

fault length vs total displacement

• The total distance a fault extends from one tip or termination to the other along its trace or strike dimension.

• • The maximum or cumulative relative movement between two originally adjacent points on opposite sides of the fault surface, typically greatest near the center and tapering to zero at the fault tips.

relationship between brittle failure and depth in the crust

crust is cooler than mantle or lower depths which prevents rocks from flowing easily, so they crack and break.

anderson's theory of faulting

classifies geological faults based on how the three principal stress axes (σ₁, σ₂, σ₃) align with the Earth's surface

fault plane

the flat or planar fracture surface inside the Earth's crust where rocks slide past each other

fault slip fibers and their kinematic significance

fibrous mineral growths—commonly calcite, quartz, or chlorite—that form on fault planes during incremental fault slip in the presence of mineralizing fluids

• show slip direction, sense of movement and incremental strain

fault drag

he bending or deflection of rock layers, beds, or marker lines in the immediate wall rocks adjacent to a geological fault

fault bend folds

geologic structures that form when rock layers bend as they move along a thrust fault that changes dip angle from a flat surface to an inclined ramp

Fault propigation fold

a bend in rock layers that forms when an underground thrust fault stops moving forward and pushes its remaining squeeze energy upward to warp the strata at its tip

Fault gouge

clay sized fragments inside a fault zone

• ultra pulverized fault rock

Fault Breccia

fragmented non cohesive highly shattered/ broken

• not as much as gouge

Pseudotachylite

frictional melting along fault plane

• fault glass (usually black and opaque)

• paleo earthquake

Cataclastie

broken, crushed shattered etc

• cohesive rock that is well indurated and solid despite being fragmented in character

damage zone

volume of rock around the fault that is affected

thrust fault

reverse fault where a low-angle fault plane pushes older rock layers upward and over younger rock layers due to intense horizontal compression

imbricates

an overlapping, shingle-like arrangement of rock fragments or tectonic slices

duplex geometry

a stack of small, fault-bounded rock slices sandwiched between two main thrust faults

fold and thrust belt

a series of mountainous or elevated structural regions formed by tectonic compression where layers of sedimentary rock are folded and pushed over one another along major thrust faults.

thrust ramps

inclined sections of a thrust fault where the fault surface cuts across rock layers instead of running parallel to them

fault flat

A fault flat is a sub-horizontal fault segment parallel to bedding layers, whereas a fault ramp is a steeper segment that cuts across rock layers

back thrusts

a secondary thrust fault that dips and transports material in the opposite direction of the region's dominant tectonic transport

• thrust one way and it thrusts back the other way

blind thrust

a hidden underground reverse fault that stops before reaching the Earth's surface, making it invisible to standard surface mapping

triangle zone

a wedge-shaped structural feature found at the outer edges of foreland fold-and-thrust belts

klippe

an isolated geological remnant of an overthrust rock mass (a nappe) that remains surrounded by younger, underlying strata after erosion has removed the connecting portions

window

an opening or hole in a sheet of displaced rock that exposes the normal, underlying rock layers beneath it

how do we know thrust sheets move uphill

fluid pressure

• water can be compressed so it moves block upwards, lubricates surface of rock to slide

extensional duplexes

a geological structure made of fault-bounded rock blocks (called horses) that form between overlapping or linked normal faults under extension

fault smears

thin layers of ductile, clay-rich rock or sediment that get pulled and stretched into a fault zone during normal faulting

hydrocarbon sealing significance

prevents oil and gas from escaping their reservoirs or containing environment

synthetic fault

a small minor fault that moves in the same direction and has the same sense of displacement as a larger, associated major master fault

antithetic fault

a minor secondary fault that has a sense of displacement and a dip direction opposite to those of the major or master fault with which it is associated

dextral vs sinistral fault

differ based on the relative direction the blocks move past each other

• strike slips

• right moving and left moving respectively

restraining bend fault

a structural curve or bend along a strike-slip fault where the relative movement of the fault blocks forces them into each other, creating intense compressional stress and crustal uplift

releasing bend

a curved segment or stepover along a strike-slip fault where the blocks of crust pull apart from each other during lateral movement

flower structures geology

a complex assembly of upward-splaying faults that branch out from a deeper, subvertical strike-slip fault zone, resembling the petals of a flower in cross-section