Structural Geology

Erosion that occurs as a result of uplift

(from faulting, folding and mountain building) can be both gradual or episodic

dip has 2 components

*an angle of inclination measured from a horizontal planar surface *general direction which must be measured perpendicularly to the strike direction

2 main types of folds

*anticline *synclines

3 types of directional stress

*compressional stress *tensional stress *shear stress

all sedimentary rocks are oriented ______ before they are folded w/ the _____ unit on the bottom & the ____ on the top

*horizontally *oldest *youngest

evidence in the field that suggests that faulting has occurred at some time in the past include

*offset, repeating & missing beds *fault gouge, shear zones and fault breccias *slickensides

factors that determine how rocks deform

*pressure-temp in the environment *physical strength of the rock type *time or how long a rock has been subjected to stress *the rate at which it has been subjected to stress

normal faults are a result of ______ and are associated with ______ boundaries and occur in pairs forming _______ & ________

*tensional stress *divergent plate boundaries *grabens and horsts

3 types of unconformities which are classified based on

*the type of rock that lies below the buried erosion surface *the spatial relationship of the rock above the buried erosion surface to those below the buried erosion surface

the minimum amount of dip would be _____ for horizontal beds and the max amount of dip would be ________ for vertical beds

0 degrees 90 degrees

for any strike direction, such as north-south there are

3 possible dip directions: east, west and vertical

Deformation of rocks can classified into

3 types: *elastic *plastic *brittle

as stress increases so does the amount of strain & if the stress is removed, the material returns to or recovers its original shape

Elastic deformation

at great depths in the earth where temp & confing pressures are relatively high, rocks under go

Plastic deformation

the amount of strain in material may continue even if stress increases only slightly or not at all

Plastic deformation plastic deformation is permanent

strain

a change in shape &/or volume in response to stress or pressure

joint

a fracture along which there has been little to no displacement

faults, joints and fracture can act as

a passage way for groundwater and a host for valuable mineral deposits as ares of gold, silver and copper

Orogeny

a period of deformation, regional metamorphism,igneous activity and regional uplift and mountain building

angular unconformity

a series of sedimentary rocks form, next the rocks are deformed by folding or tilting, next a non deposition or massive erosion event occurs, then a series of younger horizontal sedimentary rocks are deposited

disconformity

a series of sedimentary rocks form, then the terrain is uplifted w/o any internal deformation, next a period of non deposition occurs, then an series of younger horizontal sedimentary rocks are deposited

basin

a special category of a syncline in which all beds are dipping down and toward a central point rather than an axial plane. In map view basin will have a circular outcrop w/ the youngest bed in the center

dome

a special category of an anticline in which all beds are dipping down and away from a central point rather than an axial plane. In map view, a dome has a circular outcrop pattern w/ oldest bed in the center

synclines

are down folds in which the beds are dipping down and toward the axial plane and the youngest bed is in the center of the fold on the earth's surface after erosion

Strike slip faults

are faults along which the dominant sense of displacement has been horizontal or parallel to the strike of the fault

geologic cross sections and block diagrams

are interpretations of what occurs below the earth's surface based on what we can observe on its surface and a little imagination and geologist's artistic license.

folds

are produce by compressional stress during mountain building at active convergent boundaries usually occur at depth during Plastic deformation

Anticlines

are upholds in which the beds are dipping down and away from the axial plane and the oldest bed is in the center of the fold on the earth's surface after erosion

Why are thrust faults important?

b/c large blocks of the earth's crust have been transported up to 50km along them during mountain building leaving older rocks positioned over young rocks as a result

hanging wall

block of rock that lies above the fault plane

footwall

block of rocks that lies below the fault plane

at shallow depths in the Earth where temp and confing pressure are relatively low, rocks under go

brittle deformation brittle deformation is permanent

faulting

can be episodic and gradual

fold axis

can be horizontal or plunging, is the line of intersection of max curvature of any bed w/ the axial plane

reverse faults are a result of ________ and are associated w/ _____________

compressional stress convergent plate boundaries

compressional stresses are associated with

convergent plate boundaries and cause most permanent folding. (plastic deformation) & reverse or thrust faulting

to determine the sense of relative motion along a dip slip fault, one must visualize the fault in a

cross-sectional perspective

at greater depths where confing pressures and temps are higher, rocks tend to

deform plastically ex: non recoverable, permanent folding

How do rocks deform?

deformation usually occurs in the form of folding & faulting, which happens concurrently with regional metamorphism, igneous activity, regional uplifting, and mountain building.

stress or pressure can be

directional (shear, compression, tension) or standing, such as confine pressure, which is a stress caused by the load or weight of rocks overlying other rocks

tensional stresses are associated w/

divergent plate boundaries and cause normal faulting

at shallow depths breakage or brittle deformation occurs when the

elastic limit is exceeded

Dip Slip fualts

faults along which the relative motion has been essentially up and down along the dip surface to the fault plane

exfoliation joints

form as a result of stress or pressure release usually sub-horizontal

tectonic joints

from compression, tension, or shear looks like little squares begging pushed together

Columnar joints

from contraction during cooling usually vertical looks like tall columns

topography is more a

function of differential weathering and erosion than geologic structure

unconformities can be used to mark

geological time boundaries for eras, periods, and epochs

rocks that tend to fracture or behave brittlely at shallow depths

granite gneiss quartzite mass sandstones

thrust faults

have the same sense of relative motion as reverse faults but differ in that the fault plane has a sub-horizontal inclination usually 15o or less form the horizontal

nonconformity

intrusive igneous or metamorphic rock form, next a massive erosion event occurs, then a series of younger sedimentary rocks are deposited

Unconformity

is a buried surface of erosion or non deposition which represents a break in the rock record

normal fault

is a dip slip fault in which the hanging wall block moves down relative to the footwall block

reverse fault

is a dip slip fault in which the hanging wall block moves up relative to the footwall block

fault

is a fracture along which there has been significant displacement sudden movement along faults causes earthquakes

axial plane

is a plane that divides a fold into 2 equal & symmetrical halves

geologic map

is a representation of the distribution of rock units and geologic structures such as faults and fold based on field observations and reconnaissance

stress

is an applied force per unit area & is measured in the same units as pressure

folding

is gradual

parts of folds

limbs axial plane fold axis

plastic deformation strain is

not proportional to applied stress, and the deformation is not recoverable

left lateral

on must look at his left to see the same feature not eh opposite side of the fault

right lateral

one must look at his right to see the same feature on the opposite side of the fault

the uplift and subsequent erosion of regions allows us to see

parts of the earth in 3 dimensions and to better understand deep crustal processes and materials

2 types of strike slip faults

right lateral left lateral

rocks that flow plastically at very shallow depths

shale rock salt rock gypsum

rocks tend to deform brittlely (fracturing, faulting) or elastically (recoverable gentle folding) at

shallow depths where confining pressure and temp are relatively low

strike slip faults are a result of _________ and are associated with _______

shear stress transform fault plate boundaries

rocks become deformed when the under go

strain

stress causes

strain

Elastic Deformation

strain (the change in shape &/or volume) is proportional to applied stress, and the deformation is recoverable. occurs in shallow depths where temp and confining pressures are low

Shear stress

stress that causes a rock body along an finite number of parallel planes & slide along those planes no change in shape or volume

tensional stress

stress that pulls a rock apart or extends it increases the rock's length

confining stress

stress that pushes inward or downward uniformly from many directions. confining stress causes a reduction in volume and an increase in density of the materials under stress

compressional stress

stress that pushes together on a rock body or compress it makes the rock short

the 3 dimintional orientations of beds of rock & geologic features can be defined by their

strike & dip

joint can form as a result of

tensional/extensional stresses, shear stresses, contraction related to cool in and stress release

Dip

the acute angle of inclining ion of the planar surface of a geologic feature measured form any horizontal planar surface which must be measure perpendicular to the direction a strike

Strike

the geographic bearing of a line, defined by the intersection of any inclined planar geologic feature w/ any horizontal planar surface

Elastic Limit or Yield Point

the max stress that a substance can withstand and still show elastic behavior

confining stress occurs anywhere there is a substantial load or weight on earth material, such as at depth in the earth's crust and the mantle. It decreases ..

the possibility of brittle deformation and increase the tendency for materials to deform plastically

the type or deformation that occurs when strain in a particular material exceeds its elastic limit depends on

the pressure and temp to which that material is subjected, time, rate, and physical properties of the material

there are 3 types of directional stress that cause deformation in rocks &

these are closely associated w/ the 3 types of plate boundaries

shear stresses are associated with

transform fault plate boundaries and cause strike slip faulting

oil and natural gas are formed and found

trapped in subsurface folds