Transcript Chapter20 Structures

Chapter 20
Geologic structures
How rocks respond to tectonic
forces!
Tectonic forces
• Forces generated inside the earth are
called tectonic forces.
• Tectonic forces are expressed in terms of
Stress and the resulting Strain.
• Stress: force applied per unit area.
• Strain: resulting change in size or shape or
both.
• Stress: 3 types
– Compressive: when a body is pushed
together or squeezed from opposite
directions.
– Tensional: when a body is pulled away in
opposite directions
– Shear: when two bodies slide past one
another.
How rocks respond to stresses?
• Rock behavior depends on;
– Amount and rate of applied stress
– Type of rock
– T&P conditions
• Rocks behavior is of 3 types;
– Elastic: deformed body recovers its original
shape after the stress is released.
– Ductile: deformed body does not return to its
original position and its bends.
– Brittle: deformed body will fracture at stresses
higher than the elastic limit.
Geologic field methods
Geologic maps
• What is a map?
– Representation of the 3D world on a 2D flat
surface.
• Maps are designed to show where things
are.
• A geologic map shows the distribution of
geologic features, including different kinds
of rocks and structures.
What is Strike and Dip?
• Strike is the compass direction of a linear
or planar feature.
– N30°E
– N45°W
• Dip is the angle an inclined plane makes
with the horizontal.
– 30°NE
Geologic structures
• They may be;
– Folds
– Faults
– Joints
• Folds: bends or curved features in layered
rocks. Best displayed in sed. rocks.
• Folds are the result of ductile deformation.
• Fold geometry:
– Anticline: arching upwards
– Syncline: arching downwards
• Parts of a fold:
– Limb: side of a fold
– Hinge point: highest point on a fold
– Hinge line: line joining hinge points. Also
called fold axis.
– Axial plane: plane joining all hinge lines.
• Dome and basin:
• Dome: a structure in which beds dip away
from a central point.
• Basin: beds dip towards a central point.
Types of folds
•
•
•
•
Open
Isoclinal
Overturned
Recumbent
• Open: limbs have gentle dips indicating
moderate stresses.
• Isoclinal: limbs parallel to one another
indicating intense stresses.
• Overturned: axial plane inclined and the
limbs dip in the same direction. Indicate
unequal compressive stresses.
• Recumbent: the limbs and axial planes are
essentially horizontal. Extreme stresses
are indicated.
Geologic structures and OIL
• Oil and gas are concentrated in the
core of an anticline where they are
trapped by an impermeable layer.
Faults
• Fracture in rocks along which
there has been some movement.
• Parts of a fault:
–Hanging wall
–Footwall
–Fault plane
Types of faults
• 3 types:
–Dip slip:
–Strike slip:
–Oblique slip:
• Dip-slip faults: movement is
parallel to the dip of the fault plane.
Two types of dip slip faults:
normal and reverse
–Normal fault: H.W. moves
downwards relative to the F.W.
–Reverse fault: H.W. moves
upwards relative to the F.W. A
thrust fault is a low angle reverse
fault.
• Graben and Horst:
• A graben is a block of rock which
is moved downwards due to two
normal faults.
• A horst is an uplifted block of
rocks due to two normal faults.
• Strike-slip faults: movement is
parallel to the strike of the fault
surface.
• San Andeas fault in CA.
Joints
• Joints are regular fractures along
which there has been negligible
movement.
Joints
Joint sets form where they are
oriented parallel to one another.
• Joints indicate the direction of
applied stresses.