Earth 103-Structural Geology

structural geology

study of the geologic structures formed by deformation within the earth

Deformation: three ways

Translation, Rotation, Strain

second rank tensor

Stress-one vector is transformed into another

• Magnitude and direction changes based on the location on the object being acted on

• Brittle deformation:

• Fracturing and faulting

• Loss of cohesion, abrupt displacements

Ductile Deformation:

• No loss of cohesion

• Implies higher temperatures 

• Results in the development of tectonic fabrics (foliations and lineations)

Rock strength: definition and 6 things it depends on:

Definition: maximum differential stress that a rock can support before it fails

6 things:

• Pressure

• Temp

• Composition and grain size

• Differential pressure/Stress

• Water content

• Deformation rate 

Geometric description:

• Statement of what we can objectively see, akin to a still photograph 

• No interpretation of why and how; purely facts of what is observed

Kinematic description:

• interpretation about how a structure developed through time

• Describe how things form but not the cause

mechanical model

•  a simplified version of reality about why a structure formed the way it did 

Three principal types of plate margins and where they occur

• – Convergent margins (subduction and collision zones)

• – Divergent margins (mid-ocean ridges, continental rifts)

• – Transform margins (strike-slip boundaries)

Three types of continental margins:

• Atlantic-type

  • Passive

• Japan Sea-type

  • Extensional 

• Andean type

  • Convergent 

Sources of stress

• Tectonics

• magmatic/thermal

• Hydraulic forces

• Overburden 

Force vs. stress

• force=mass*acceleration

• stress=force/area

forces acting on a rock (2)

internal

external-surface force and bodyforce

a force applied to an area is a ___

traction

Maximum differential stress is

(sigma1-sigma3)

stress in 3D represented by (4):

• tensor notation

•Principal stresses

•Stress ellipsoid

•Mohr Circle

stress sign conventions (4)

tensile-negative

compressive-positive

counterclockwise (sinistral)-positive

Clockwise (dextral)-negative

hydrostatic pressure

all principle stresses equal

Uniaxial stress

2 principal stresses are zero, other is different

compression or tension depending on which stress is not zero

axial compression

two principle stresses are equal, one is higher

triaxial stress

all principle stresses are different

deviatroric stress

total stress-mean stress

pure shear

one direction of deformation same as the origin

Biaxial stress

biaxial (planar)-both stresses lie in a single plane and the out-of-plane stress is zero.

biaxial-two stresses non zero-one is zero

Fracture with no slip is__

• extensional fracture or joint

Fracture with slip is:

shear fracture or fault

Modes of fracture (3)

• Mode 1: extension

  • Rock relative motion perpendicular to fracture walls

• Mode 2: shear

  • Rock relative motion parallel to propagating tip

• Mode 3: shear (2)

  • Motion perpendicular to propagating tip

Brittle failure criterion depends on:

Confining pressure

• Fluid pressure

• Pre-existing weaknesses in the rocks

Tensile failure criterion states:

Tensile failure is a specific ___ on the ___ Mohr circle

•  that the effective least principle stress cannot be less than the failure value

• A specific point on the critical mohr circle

  • The point where an extension fracture is formed

  • Represented by a vertical line to the left of the origin 

  • Everything past this point is unstable

Andersonian Mechanics: what it assumes and faults that it predicts

• Assumes that:

  • One of the principle stresses is always vertical and the other two are horizontal

  • Rocks fail by either Tensile failure of Coulomb failure

• Predicts what the general orientations of all faults are:

-thrust fault: sigma1 horizontal; sigma3 vertical; sigma 2 horizontal and parallels the strike of the fault

-normal fault: sigma1 vertical; sigma 3 horizontal; sigma 2 horizontal parallel to the strike of the fault

-strike slip: sigma1 and sigma3 are horizontal; sigma 2 vertical and parallel to fault surface

What happens when the Mohr circle becomes tangent to the coulomb failure criteria line?

one surface has

the right amount

of normal and

shearing stress

to fracture

what is coulomb failure and coulomb failure predicts that: (think angles)

it is a Function of mean normal stress and cohesion of rock

predicts that fault normals will always be at about 60 degrees to the maximum compressive stress sigma 1

Or, the fault surface will be about 30 degrees to sigma 1

Griffith crack theory

rocks are a lot weaker than theoretical predictions due to tiny cracks in them

Poor fluid pressure equation (tensile stress):

Pf=sigma3-Tknot

Mode 1 fractures

Joints, fissures, dikes, veins

Mode 2 and 3 fractures

Faults, deformation bands, shear zones

Joint

• individual extension fracture that shows very small displacement normal to the fracture surface and no, or very little displacement parallel to the fracture surface 

joints form perpendicular to …

the least principle stress

Joints are produced by (4 things)…

• Exhumation and erosion

• Cooling (thermal contraction)

• Tectonic stresses

• Hydraulic pressure

sheet joints (formed by)

• Exhumation and erosion

• Curviplanar; parallel to erosion surface

Columnar joints and mudcracks (formed by)

• Hexagonal arrangement due to radial cooling/shrinking

En echelon pinnate fractures and tension gashes:

Associated with faults and shear zones

Tension (pinnate) fracture: form on either side of a brittle shear zone (fault) in an en echelon array

Gash fracture: mineral filled; form in ductile shear zone; rotated into sigmoidal shape

Younger joints ___ against older joints

terminate

3 features of Plumose structure (joints)

• Plume axis charts the direction of propagation

• Arrest lines indicate episodic propagation

• Twist hackle reflects rotation of the stress axes near mechanical boundaries

3 types of joints that are filled with something

dikes

fissures

viens

A ___ consists of a zone along which slip (shear displacement) has

occurred.

fault!

3 shear distribution scenarios

fault plane

fault zone

ductile shear zone

___ fractures tend to form at a ___ degree angle with respect to __ principle stress

shear; 30 degrees; sigma1

slip is…

slip connects…

a displacement vector

connects two points on either side of the fault zone that were

connected before faulting.

separation is…

an apparent displacement parallel to the strike and or parallel to the dip; may be a component of the slip

piercing points

a unique pair of “points” that were once connected, but have since been displaced along a fault.

Fault terms: rake; net slip; dip slip; strike slip; vertical throw; horizontal throw; heave

Rake: The angle (measured on the fault plane) between the net slip vector and horizontal

net slip-total slip

dip slip- the dip parallel slip component

strike slip- the strike parallel slip component

vertical throw-vertical component of the net slip

horizontal throw-the horizontal component of the net slip

heave-the apparent horizontal component of the net slip

fault rocks (4)

gouge-mostly clay sized, poorly consolidated material pulverized by fault slip.

cataclasite-generally cohesive fault zone rocks, with 30-100% matrix

breccia-angular, poorly sorted clasts up to boulder size that have been broken up by fault slip.

Pseudotachylite is dark glassy rock formed by frictional melting during earthquakes

5 ways to recognize faults in the field

-Polished, striated surfaces

• Fault zone rocks

• Drag folding

• Displacement of geomorphic features

• Truncation of layering by a planar surface

Two components of a thrust fault

ramp and flat

Normal faults terminologies and what they mean (2)

graben-going down

Horst-going up

Thrust fault terminology (2)

foreland-undeformed region in front of the fault

hinterland-region behind the fault

thrust fault duplexing

antiformal stacks of thrust faults that propagate upwards because the fault gets frictionally stuck