Structural Geology- Exam 2

In-situ measurements

overcoring, flat-jack, hydrofracturing

Laboratory measurements

fault plane solutions, and paleopiezometry

overcoring

drilling a hole and adding a second hole around the borehole to allow the relief of stress from original borehole

Elasticity

E (elastic strain) = σ (stress) / Ɛ (strain)

Elastic Deformation due to stress relief which will cause rock to expand . The limitation is that the measure of elasticity is not in all directions. Need to find the orientation of sigma 1 and 3

Flat Jack

a technique that directly measures stress in-situ. there are 4 holes and measure fluid pressure necessary to bring it back to its original position. Limitation is it is one direction of stress field, need multiple to figure out the orientation of stress

borehole stress measurement

collected by using fluid pressure to determine the stress necessary to induce cracking or reopen existing fractures

hydrofracturing

calculations and imaging will allow the calculation and orientation of sigma 1 and 3. It measures tensile stress and the amount of fluid and pressure it takes to keep a crack open. (also look at hydeofracturing notes for diagram)

Borehole breakouts

can figure out the orientations of sigma 1 and 3 by fractures and borehole breakout. view image

focal plane solutions

based on seismic data provide locations and orientations of movement along a fault. black and white circle thingies. Black represents pulling and white represents pushing.

Convergent Margins

Ocean-ocean, ocean-continent, continent-continent. All three contain thrusting, reverse faulting, and folding

reverse and thrust faulting

reverse is high angled contractional faults with generally small displacement

Thrust faults are low angle contractional faults with generally large displacement. Common in compressional environments

what are nappe, klippe, and fensters

nappe- a block of older material thrust over younger material

Klippe (cliff)- isolated hanging wall blocks

Fensters (windows)-areas where erosion has exposed the footwall

Look at image

Duplex

form from the hinterland towards the foreland

Horses

convergent margins and thrust faults (notes)

Roof thrust

convergent margins and thrust faults (notes)

Sole thrust

convergent margins and thrust faults (notes)

decollement

convergent margins and thrust faults (notes)

back thrust

develop to accomidate strain when forethrusts cannot relieve stresses

frontal thrust vs lateral vs oblique

view image

Hinterland

basement involved in deformation

foreland

deformation primarily in overlying sedimentary cover

thin vs thick skinned thrusts

thin is located above basement rock whereas thick skinned cuts across basement rock

detachment folds

folds as overlying units above a decollement fold. weak sedimentary layers that buckle

subduction zones formation

while unsure what initiates subduction. One plate sinks into the plate boundary due to density (cold) & old oceanic crust is denser than younger oceanic crust (warmer)

slab rollback

negative buoyancy of downgoing plate exerts a pulling force that causes the rest of the plate to sink (look at picture)

orogenic wedge

develops on thrust faults thicken the material brought to the subduction zone. Snow plow instance

accretionary prism

made of sediments from both planes

Critical taper angle

is related to the material properties of the wedge. friction at base and strength of material in wedge. The thicker the wedge, the faster the erosion.

load of wedge: row x g x H x sin(a+b)

load of water: row x g x D x sin(b)

Friction in wedges

high friction at base increases thickness of wedge

Overthickening in wedge

leads to collapse structures (slumps / normal faults)

orogenic collapse

as a bulldozer, it makes a thicker and thicker wedge and wedge becomes unstable and begins to collapse. If to thick, internal material starts to deform

Structure of the slab graph

convergent margins and thrust faults lecture

deep focus earthquakes

metamorphic reactions cause earthquakes. This includes volume change and water (water is released causing high fluid pressure, causing earthquakes). Shallow DFE, serpentine dehydration (breaks down). Deep DFE, olivine to wadsleyite to ringwoodite (aka changes in crystal structures). Structural transformation that makes the phases denser (change in volume)

Continental Arcs

one oceanic plate subducting beneath a continental plate

Island Arc

two oceanic plates one subducting beneath another

Continent-continent collision

one continental plate subducting beneath each other. Usually develops thick sedimentary deposits off of stable margins

Rift

belt of continental lithosphere that currently undergoing extension or has underwent extension. Extension of continental crust and is the 1st part of the wilson cycle. Normal faulting and flood basins occur.

terraine

chunck of continental crust from a continent that pulls apart

what causes rifitng

pulling apart of crust and a stretch of lithosphere.

Thermally activated rifts

develop in response to asthenospheric upwellings, chains of hot spots which makes the lithosphere stretch and thin

flexure related rifts

develop in response to beinding of plates. Form in downgoing slabs

graviational collapse rifitng

develop in response to extrusion of hot (soft) lower crustal rocks. Form in young orogenies

Back arc basin rifts

develop in response to slab pull cause by slab rollback. Subducting plate goes to fast causing suck which pulls on surrounding lithosphere.

Continental rifts

develop in response to slab pull and localize at or near old suture zones and is from subducting plates. Example- farrilon plate slides under continental plate and cooled causing pulling

pull apart rifts

in response to bends of faults (releasing bends)

Structures in rifts

normal faults, horsts, grabens, sometimes listric detachment faults. Folds can develop to accommodate for strain

antithetic

opposite direction related to sense of shear

synthetic

same as the sense of shear on fault

modles of crustal rifting

pure shear, simple shear, delamination, and hybrid (mix of pure and simple shear) (look at rifitng lecture)

Basin and Range in US

formed by rollback of shallowly subducting farralon plate

evolution of rifting

two antithetic rift zones begin to interact with each other which form an accommodation zone. New fault zone forms that causes similar extensional deformation. Thinning of teh lithosphere causes upwelling and decompression melting of mantle. Upwelling evoles to form a new trough which becomes an oceanic spreading center

Pressure

sigma v (horizontal and vertical stresses) = pgh

differential stress

the difference between maximum and minimum principal stresses and is referred to as strength of material.

different environments related to stress (vertical)

Maximum compressive stress in extentional environments is vertical in image

different environments related to stress (horizontal)

max compressive stress in compressive environments

Pore Fluid Pressure

ratio of pore fluid pressure to vertical stress= Pp (pore fluid pressure) / sigma v (vertical stress)

Look at horizontal and vertical stresses mors circle

stress in crust lecture

Transfer faults

strike slip faults that transfer one kind of displacement to another. Two seperate faulting and it connects those two faults together. also called transform boundaries. Connect themselves from faulting

How do transfer faults and transform boundaries form

wing cracks (mode 1 cracks) connect extensional boundaries

San Andreas Fault

also known as a ridge-ridge boundary. connected by two spreading centers. One is from the farralon plate and goes to extensional boundary (spreading center)

transcurrent faults

strike slip faults that do not connect other boundaries. Oceanic crust coming in at an angle and moving the plate over, cause a strike slip.

oblique folding

view image

restraining-releasing

compressional structures in restraining belts and extensional structures in releasing bends

flower structures

series of normal or reverse faults observed in releasing or restraining bends. Negative in extension (goes down) and pos. in compression (goes up)

Rigid body strain

transltion and rotation and involve no internal deformation

Non rigid body strain

involves internal deformation of the body (distortion). Common in lower crust during ductile deformation

Homogenous strain

when original straight lines remain straight, parallel lines remain parallel, and circles become ellipses.

finite strain

the final state of deformation

incremental strains

are intermediate steps that lead to finite strain. may have different steps to lead to finite strains

noncoaxial

material lines do not remain principle strain axes (simple shear, top)

coaxial

material lines remain principle strain axes (pure shear, bottom)

transpression

more vertical exaggeration (diagonal) Simple shear, y=tan (angular shear)

transtension

elongation of circle, pure shear, k= 1/kx

longitudinal strain

e= (Lo-L)/Lo

strain states

1. general strain

2. axially symmetric extension (hot dog)

3. axial symmetric shortening (hamberger)

4. Plane strain

5. Simple (uniaxial) shortening (no change in x and y but change in z)

Look at strain lecture

Flinn diagram

Look at strain lecture

Creep

1- primary creep (deceleration)

2- secondary creep aka steady state (constant rate of acceleration)

3- tertiary (accelerated)

Look at graph (stress and strain)

permanent strain

Look at graph (stress and strain)

linear reheologies

if ratio of stress/strain or sress/strain rate is constant

Elastic vs viscous

stress is applied but goes back to original shape. Viscous deforms under any pressure

viscoelastic

do not initially respond lineraly to the applied force but recover all strain after removal of applied force

elastico-viscous

materials are initially elastic then behave viscously and have recoverable portion of strain due to the elastic behavior