EESA06 Chapter 11.doc

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EESA06 - Chapter 11
Structural Geology: branch of geology concerned with the shapes, arrangemnet, and
interrelationships of bedrock units and forces that cause them
Stress and Strain in the Earths Lithosphere
-Tectonic forces move and deform parts of the lithosphere, particularly along plate mar-
gins
-Stress: a force per unit area (can only observe stress of rock bodies that are exposed,
difficult for buried rocks)
-Strain: change in size (volume) or shape, or both, in response to stress
-Compressive stress common along convergent plate boundaries and result in rocks
being defromed by a shortening strain (changes in perpendicular direction to applied
stress)
-Tensional stress: caused by forces pulling away from one another in opposite direc-
tions, results in stretching or extension of material (changes in parellel direction to ap-
plied stress)
-Tensional stress occurs at divergent plate boundaries (since rocks are very weak when
pulled apart, fractures and faults are common)
-Shear stress: produced when stresses act parallel to a plane (like putting deck of
cards in your hands and then shearing the deck by moving your hands in the opposite
direction)
-Shear stress occurs along actively moving faults
-Elastic: if a deformed rock body recovers its original shape after the stress is reduced
or removed (like elastic band)
-Most rocks behave in elastic way under low stress, but after elastic limit is reached, the
rock is deformed permantently
-Ductile: (or plastic manner) rocks behaving this way will bend under pressure and
don’t return to original shape
-Brittle: some rocks will fracture at stresses higher than its elastic limit or greater than
strength of the rock (happens more at Earth’s surface, which forms faults and joints)
-Sedimentary rock at Earth’s surface is brittle
-Understanding and mapping geological structures is important for pertroleum, mining,
and for evaluating problems related to engineering decisions and environmental plan-
ning (bridges, nuclear reactor, buildings)
Geologic Maps and Field Methods
-Geologic map: uses standardized symbols and patterns to represent rock types and
geologic structures, is typically produced from the field map for a given area
-On this map are rock types and distribution, occurence of structural features, ore de-
posits and more
-Many sedimentary rocks and some lava flows and ash falls are deposited as horizontal
beds or strata (tilting occurs after deposition or lithification)
-3D modelling used for geologic structures on Baffin Island
-Strike: compass direction of a line formed by the intersection of an inclined plane with
a horizontal plane
-Angle of dip: measured downward from the horizontal plane to the bedding plane
(and inclined plane)
-Direction of dip: compass direction in which the angle of dip is measured (always
measured to the right angle to the strike)
-Brunton pocket transit used by geologists for measuring the strike and dip
-Geologic cross- section: represents a vertical slice through a portion of the Earth
(helps visualize geology in 3 dimensions)
-Folds: bends or wavelike features in layered rock (rock that is folded shows it behaved
as a ductile material, but it is generally brittle)
Geometry of Folds
-Geometry of folds important because oil, gas and metallic mineral deposits are local-
ized in folded rocks.
-Also important in showing how a rock was strained and how it might relate to the
movement of tectonic plates
-2 basic fold geometries are common: anticlines and synclines
-Anticline: an upward-arching fold
-Syncline: a downward-arching fold
-Rock layers dip away from the hinge line (or axis) of the fold
-Each anticline an adjacent syncline share a limb
-For each anticline and the syncline, the hinge lines are contained within the shaded
vertical planes. Each of these planes is an axial plane, an imaginaery plane containing
all of the hinge lines of a fold. The axial plane divides the fold into 2 limbs
-Anticlines are not necessarily related to ridges nor synclines to valleys because valleys
and ridges are nearly always erosional features
-Oldest rock layers exposed along anticline and youngest rock layers exopsed along
syncline
-Plunging folds: folds in which the hinge lines are not horizontal (these semeble Vs or
horseshoes)
-Structual dome: structure in which the beds dip away from a central point (resembles
an anticline, an unside down bowl)
-Structural basin: bed dip toward a central point (resembles a syncline, a bowl)
-Domes and basins are features on a grand scale (hundreds of kilometres across)
Interpreting Folds
-Open folds: have limbs that dip gently (more open a fold is, the less it has been
strained by shortening)
-Isoclinal fold: limbs parellel to each other, implies larger shortening strain or shear
strain
-Overturned fold: in which the axial plane is inclined to such a degree that the fold
limbs dip in the same direction (in this fold, youngest rocks on top and oldest on bottom,
superposition, doesn’t apply)
-Recumbent fold: overturned to such an extent that the limbs are essentially horizontal
(form core of mountain ranges such as Canadian Rockies, Alps, and Himalayas)
-Joint: a fracture or crack in bedrock with essentially no displacement occurs
-Fault: a fracture with a rock on either side of a fracture moves parallel to the fracture
surface
Joints
-Joint set: where joints are oriented approximately parallel to one another
-Geologist sometimes find valuable ore deposits by looking at orientatin of joints
-Info on joints also important in planning and construction of large engineering projects
(dams and reservoirs)
Faults
-Faults are fractures in bedrock along which sliding has taken place
-Most faults are no longer active
-Dip-slip fault: fault movement is parallel to the dip of the fault surface
-Strike-slip fault: indicates horizontal motion parallel to the strike of the fault surface
-Oblique- slip fault: has both strike-slip AND dip-slip components