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