Structural Geology Flashcards
The branch of
geology that deals with the form, arrangement,
and internal structure of the rocks, and esp. with
the description, representation, and analysis of
structures, chiefly on a moderate to small scale.
structural geology
defined as a geometric feature in a
rock whose form, shape and distribution can be described.
These features are separated into primary, secondary,
tectonic and non-tectonic structures
geological structure
structures that are acquired during the
genesis of a rock.
primary structures
locally driven or small scale features
not immediately due to tectonic interaction
non-tectonic structures
structures that are acquired after the genesis of the rock
secondary structures
according to which slow incremental changes, such as erosion, brought about all the Earth’s geological features. also known as gradualism
uniformitarianism
theorizes that the Earth has largely been shaped by
sudden, short-lived, violent events, possibly worldwide in scope.
catastrophism
states that layers of sediment are
originally deposited horizontally under the action of gravity
principle of original horizontality
it states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time
law of superposition
states that the geologic feature which cuts another is the
younger of the two features
principle of cross-cutting relationships
states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.
principle of lateral continuity
grading of particles from coarse to
fine from bottom to top in bands, several feet thick which are repeated with great regularity through a formation
graded bedding
– is an arrangement of laminar transverse
to the bedding plane in straight sloping lines or concave forms
cross bedding
is found in widespread and thinner
sandstones and is indicative of shallow-water
conditions of deposition
current bedding
type of analysis that Interprets deformational movements that formed the structures
kinematic analysis
type of analysis that recognize, describe structures by measuring their locations, geometries and orientations
* Break a structure into structural elements - physical & geometric
descriptive analysis
type of analysis that Interprets forces and stresses from interpreted deformational
movements of structures
dynamic analysis
types of kinematic analysis
distortion, dilation, rotation, translation
means slope or inclination
dip
The angle of inclination of a
bedding plane with horizontal
plane
amount of dip
direction along which the
inclination of the bedding plane
occurs
dip direction
measures the direction angle
clinometer
measure the dip direction
compass
defined as the direction of a
line formed by the intersection of
a bedding plane and a horizontal
plan
strike
strike is always at _________ with the true dip direction
right angle
When the dip layer is measured in a direction that is essentially at
right angle (90 deg) to the strike of that particular layer.
true dip
types of angle of dip
Horizontal - 0 deg
Inclined - 0-90 deg
Vertical - 90 deg
When the dip of layer is measured in any other direction
which is not at right angle (90 deg) to its strike direction
apparent dip
When sedimentary rocks are deposited in an originally sloping basin they are also inclined in the same fashion. Thus, if original slope of the basin of deposition is anything between 5-10
degrees the sedimentary formations accumulating over there in the period of time, will also have the same direction, such dips are
called Depositional Gradient
Primary Dip
in inclination induced in the strata after its
deposition due to the Tectonic Force. Secondary dip may range in
value up to vertical (90). Most folded mountain ranges of the world strata exhibit this kind of dip
secondary dip
are inclinations of the rocks exposed in a limited area of
observation. They may be primary or secondary or may not show
any appreciable variation laterally or with depth
local dip
an averaged general inclination of a series of
formation exposed over a wide area . These rocks may show
varying local dips, both in respect of degree and directions of
inclination.
regional dip
Force applied over an area
stress
observable deformation in the rock
strain
the result of rapidly applied
high stress which “break the rock”
brittle deformation
contact between two rock units
unconformity
the result of slowly applied,
constant, low stress which “bend the rock”
ductile deformation
Preferred orientation of planar rock bodies
and/or minerals
foliation
Zones of deformed rock that have accommodated
movement
shear zone
Preferred orientation of linear minerals and
rocks
lineation
are usually erosional contacts that are parallel to the bedding planes of the upper and lower rock units
disconformity
the contact that separates a younger sedimentary rock unit from an igneous intrusive rock or metamorphic rock unit
nonconformity
the contact that separates a younger,
gently dipping rock unit from older
underlying rocks that are tilted or
deformed layered rock.
angular unconformity
Once the elastic limit (strength) of a rock is
surpassed, it either flows or fractures.
brittle deformation
fractures along which no appreciable displacement has occurred
joints
often accomplished by gradual slippage
along planes of weakness within the atomic structure of mineral grains
ductile deformation
joint that share a
similar orientation in the same
area. Or it is a family of parallel
evenly spaced joints
joint set
two or more
intersecting joint sets in the
same area
joint system
which the block
have separated for the small
distance to right angle to the
fracture surface
open joints
there is no such
separation. Joints may be
capable of allowing fluid (gas
and water ) to pass through the
rock
closed joints
small in their
extension confined to only one
part of layer. It is called as
discontinuous joints.
small joints
existence of asperities between discontinuity
suggest stability of shearing
roughness
distance between discontinuity surface
separation
in the case of UG excavation, discontinuity is considered continuous if _________ than the dimension of the excavation
its length is greater
In tunnels, GW in liters per minute per _______ of the excavation
should be determined
10 meters
fractures at the scale of centimeters or less
shear fractures
A fracture or break in a rock along which a
movement occurs
fault
Shear Fractures at the scale of a millimetre or
less which may be visible only under a microscope.
micro faults
fragmented rock produced by the
grinding action of a fault at low pressure (low
depth) activity
cataclasite
recrystallized rock generated by deep
(high pressure) fault motion
mylonite
types of faults according to attitude of the fault surface
high angle fault - fault dip > 45 deg
low angle fault - fault dip < 45 deg
fault zone rocks that contain
mylonites that deform in a ductile manner
ductile shear zones
3 criteria for recognizing faults
Features intrinsic to faults themselves
Effects on geologic or stratigraphic units
Effects on physiographic features
are linear features characterized by sharp
increases in the topographic slope; they suggest the presence of faults
scarps
- are continuously linear breaks in slope that result directly from displacement of topography by a fault
fault scarp
a smoothly polished surface caused by
frictional movement between rocks along the two sides of
a fault. This surface is normally striated in the direction of
movement.
slickensides
relative displacement parallel to the dip of the fault plane (up-down movement)
dip-slip fault
are linear topographic features
characterized by an anomalous decrease in slope. They form where a fault displaces an originally smooth slope so that a strip of shallower slope results, or where erosion of
the less resistant rocks in a fault zone produces a shallower slope than is supported by the surrounding, more resistant rocks
fault benches
relative displacement parallel to the strike of the fault plane (left and right movement)
strike-slip fault
movement along the direction of the strike and slip (combination movement)
oblique-slip fault
type of fold in the form of an arch rock dip away from the axis and the oldest rocked exposed at the centr
anticline
type of fold that is uniform direction of strike but variable angle of dip
monocline
a reverse fault that has a dip of <45 deg
thrust fault
type of fold in the form of a through. rocks dip towards the axis and the youngest rocks exposed at the center
syncline
non curved parts of a fold
limbs/sides/flanks
curved portion of fold
hinge zone
limbs dip in the opposite direction at the same angle (symmetrical)
vertical
limbs dip in the opposite direction at different angle (asymmetrical)
inclined
