EPS 116 - Midterm 1

geometric models

descriptive models of what you can see

kinematic models

how does it get there/move?

mechanical models

why did it get there? What are the mechanics behind it?

what determines if a rock behaves ductiley or brittley?

• Temp

• Pressure

• deformation rate

• rock type (mafic vs felsic)

where does the transition from brittle to ductile occur?

~300-350 C

how do seismic velocities change with depth?

lower velocities at depth (higher temp)

how old is the oldest continental and oceanic crust, respectively?

• 4 Ga

• 200 Ma

left or right lateral?

right

what kind of stress do the dark and light part of beach balls represent?

dark = compression

light = tension

what do normal vs reverse beach balls look like?

what forces drive plate tectonics

• slab pull

• ridge push

• trench suction

• bottom drag

• hot spots

what force is behind ridge push

gravity. uprising magma pushes plates upward at MORs and gravitational potential energy causes the to fall apart

how does trench suction work

the convection in the mantle drags the subducting plate down

bottom drag

the friction between the plate and the mantle, slowing down the movement of the plate

what relationship exists between rate of plate motion and frequency of earthquake (if any)?

first order correlation

how has plate motion changed in recent geologic history?

remained fairly consistent the past 2 Ma

primary structures (and examples)

structures that form prior to tectonics (ie during or shortly after deposition or crystallization)

• cross bedding

• load casts

• graded bedding

• desiccation cracks

• etc

secondary structures (examples)

structures that form after tectonics

• fractures

• folds

• lineations

• foliations

what is rake?

lineation on planes (the angle of linear feature from horizontal or strike)

True or false: strike line is always horizontal

true

true or false: dip angle is the maximum slope on a plane?

true

true or false: angular values for strike and dip uniquely describe the attitude of a plane

false: you need dip direction

true or false: plunge is another term to describe slop of a plane

false: it is used to describe linear features

thrust fault symbol

triangles

normal fault symbol

what are histograms and rose diagrams?

• histograms: used to quantify frequency or number of features vs. orientation (strike or trend)

• rose diagrams: angular histograms (looks like a dial)

equal angle vs equal area stereonet projection and what are they better for?

• equal area: plotting distributions of fold axes, lineations, etc.

• equal angle: determining geometric relationships (plotting angular relationships, focal mechanisms, etc.)

what does the first motion on vertical seismograph look like for push and pull stresses? how does this correspond to beach ball?

• push: up, dark

• pull: down, light

mode 1 fractures

extension (aka joints)

mode 2 fractures

shear (aka faults)

mode 3 fractures

tear

what do joints do?

accommodate extension across fractures

plumose tecture

feathery texture on joint surfaces. occurs in the direction of the crack opening

dips of normal faults

~60 degrees

dip of thrust faults

shallow ~30 degrees

dip of reverse faults

steep, greater than 45 degrees

true or false: faulted planar features are unique indicators of fault slip

false: they don’t always reflect motion happening outside plane

back arc extension

basin located behind convergent fault (phillippines)

out rise extension

tension and fractures on the subducting plate from large amounts of bending (doesnt occur everywhere)

forearc extension

extensional stresses in front of volcanic arc at subduction zones. could be do to rollback, or gravity

how do we get extension in continental collisions?

plates move perpendicular to compressive force to accommodate

transpressional transform bends

when strike slip faults curves in a direction that opposes relative motion of tectonic plates (happens at san andreas every time time we step left)

strike slip faulting in continental collision

as the plate is trying to move perpendicular to collision to make room for colliding plate, we get shearing

what force is behind brittle structures?

stress

force

rate of change of momentum

traction

force per unit area on a surface of particular orientation

normal stress

internal force acting perpendicular to material’s cross sectional area

• can be compressive or tensile

3d stress tensor

6 independent components fully described by 1 normal and 2 shear stresses on 3 mutually perpendicular planes

what convention do we use for stress tensors

on/in convention. the force is acting ON a plane, IN a direction

principal stresses

the maximum and minimum normal stresses that act on orthogonal planes within a material where all shear stresses vanish

σ₁

largest (positive) principal stress

σ₂

second biggest stress (between 1 and 3)

σ₃

smallest principal stress

what is θ on mohr diagram

between max principal stress axis and the normal of a plane. angles are double those in physical space. angle between σ₁ and σ_n

where is maximum shear stress at in mohr circle

θ=45 (top of circle)

positive vs negative shear stress represented

positive: arrows pointing to top left and bottom right

negative: arrows point to bottom left and top right

how do fluids affect stress and mohr circles

makes it more likely to fail, moves circle left

temperature gradient of earth

+ 25 degrees C per km

set

many fractures in same area with same orientation

systematic joints

regular parallel orientations and spacing

non-systematic joints

curved, irregular joints. nearly always terminate against older joints

sheet joints

sub-parallel to topography. characterized by rounded topography, late formation. also form from plutonic cooling

columnar joints

separated into hexagonal or pentagonal columns

gash fractures

mineral-filled s- or z- shaped extensional fractures formed in ductile shear zones. forms either pinnate/feather features en echelon

stress directions in mode 1 fractures

minimum principal stress is normal to the joints, max in direction of trajectories

stress directions in mode 2 fractures

shear stress perpendicular to plane of fracture

stress directions in mode 3 fractures

shear stress parallel to plane

hackle fringe

describes a set of extension fractures that are aligned en echelon and rotated away from joint axis

rib marks

curved features perpendicular to the lines of hackle of the fracture face

are principal stresses always orthogonal to each other?

yes

is shear stress maximum on planes perpendicular to the max compressive principal stress

no, shear stress is 0.

what does the top of a mohr circle represent

max shear stress

is there a particular coordinate system in which the off-diagonal components of the stress tensor are zero?

yes, always

where does max shear stress occur

on planes oriented 45 degrees from principal stresses

what terms describe stress?

• tension (tensile stress)

• compression (compressive stress)

what terms describe strain

• extension

• contraction (or shortening)

what is T₀. what does a line through it represent? what happens if a mohr circles cuts across it

• tensile strength of a material

• a line through it indicates the tension fracture envelope

• in an unstable stress state, failure criterion

where do tensile (mode 1) fractures form?

at θ=90 (parallel to max compressive principal stress)

why do we care about both shear and normal stress in compressive faults?

because compressional faults form at θ=60, NOT θ=45 (where shear stress is maxed out). if we only cared about shear stress, we would get failure at top of mohr circle (or at θ=45)

coulomb failure criterion

stress conditions at which faulting occurs

what happens to the principal stresses and mohr circle at higher pressures?

the mohr circle gets bigger and the difference between σ₁ and σ₃ increases

what is ϕ?

angle of internal friction (~ 30 degrees)

at what angle from σ₁ do faults occur. how does this explain what we see in nature?

at around 30 degrees. this explains why thrust faults form at around 30 from horizontal (because σ1 is also horizontal). and why normal faults form at 60 (because σ1 is vertical)

what is byerlees law

the amount of shear stress required to slide rocks past each other in pre-existing faultsd

what does the y intercept represent for byerlees law?

cohesion (where σn = 0) and any shear stress could cause failure

what happens to the normal stress if you increase water pressure?

it decreases in all directions

what do earthquakes indicate about a fault?

it is stick slip

what determines fault behavior

• fault zone material (mineralogy major component of creep vs stick slip)

• fluid pressure

• stress state

• rate and history

• more

does mode of slip reflect frictional strength (friction coefficient)

not necessarily

how does strain relate to stress in an elastic material? what law relates them?

strain is proportional to stress. hooke’s law

what are some true statements about elastic strain

• it is linearly related to stress

• it’s instantaneous

• it’s reversible

young’s modulus

a mechanical property that measures the stiffness or resistance to elastic deformation of a solid material when stretched or compressed

how to calculate mean normal stress

(σ₁+σ₃)/2

how to calculate max shear stress

(σ₁-σ₃)/2

what rules must slickenlines (linear features) follow to make sense with the measured strike and dip

• rake must always be within 0-90 degrees

• plunge can never be greater than the dip

what are the types porphyroclasts with tails and what do they look like

1. horst

2. graben

3. half grabens

4. synthetic faults (in same direction as main fault)

5. antithetic faults (in opposite direction as main fault)

6. detachment fault

7. listric faults

listric faults and roll over folds visual

fault propagation fold visual

fault bend fold visual