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 OILGeologic structures and OIL

• Oil and gas are concentrated in the Oil and gas are concentrated in the core of an anticline where they are core of an anticline where they are trapped by an impermeable layer.trapped by an impermeable layer.

FaultsFaults

• Fracture in rocks along which there has Fracture in rocks along which there has been some movement.been some movement.

• Parts of a fault: Parts of a fault: –Hanging wallHanging wall–FootwallFootwall–Fault planeFault plane

Types of faultsTypes of faults

• 3 types:3 types:

–Dip slip: Dip slip:

–Strike slip:Strike slip:

–Oblique slip:Oblique slip:

• Dip-slip faults: movement is Dip-slip faults: movement is parallel to the dip of the fault plane.parallel to the dip of the fault plane.

Two types of dip slip faults: Two types of dip slip faults: normal and reversenormal and reverse–Normal fault: H.W. moves Normal fault: H.W. moves

downwards relative to the F.W.downwards relative to the F.W.

–Reverse fault: H.W. moves Reverse fault: H.W. moves upwards relative to the F.W. A upwards relative to the F.W. A thrust fault is a low angle reverse thrust fault is a low angle reverse fault.fault.

• Graben and Horst:Graben and Horst:

• A graben is a block of rock which is A graben is a block of rock which is moved downwards due to two normal moved downwards due to two normal faults.faults.

• A horst is an uplifted block of rocks A horst is an uplifted block of rocks due to two normal faults.due to two normal faults.

• Strike-slip faults: movement is Strike-slip faults: movement is parallel to the strike of the fault parallel to the strike of the fault surface.surface.

• San Andeas fault in CA.San Andeas fault in CA.

JointsJoints

• Joints are regular fractures along Joints are regular fractures along which there has been negligible which there has been negligible movement. movement.

JointsJoints

Joint sets form where they are Joint sets form where they are oriented parallel to one another.oriented parallel to one another.

• Joints indicate the direction of Joints indicate the direction of applied stresses.applied stresses.