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Eric H Christiansen
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Main Idea Deformed rocks are a record of Earth’s tectonic system and reveal how it works. Joints Faults Folds
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Rock Deformation Stress is the pressure or force applied to rocks that cause deformation to occur Uniform (confining) stress is equal in all directions Rocks are confined by the rock around them Differential stress is not equal in all directions This is what deforms rocks
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Rock Deformation Three types of differential stress
Tensional - pulling apart Compressional - squeezing together Shear - slipping, twisting, or wrenching
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Rock Deformation = Strain
Definition: Strain is the change in the shape or volume of a rock that results from stress.
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Deformation?
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Deformation?
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Brittle deformation Fracture Ductile deformation
Stress exceeds the brittle strength Irreversible break Ductile deformation Irreversible change in size and/or shape Volume and density may change
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Geometry of Rock Structures
Structures may be defined by the orientation of planes Dip – the angle of inclination downward from a horizontal plane Strike – the compass bearing of a horizontal line where the inclined plane intersects an imaginary horizontal plane
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Strike and Dip
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Joints Fractures created in brittle rocks
No shear or displacement has occurred Form as overburden is removed, confining stress reduced Form by cooling of igneous rocks Often occur in sets
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Joints Fractures created by in brittle rocks
No shear or displacement has occurred Any joints here?
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Why are joints important?
Fluid flow Ground water Oil Ore deposits Weathering and erosion
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Why are joints important?
Fluid flow Ground water Oil Ore deposits Weathering and erosion
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Faults Blocks on either side have moved
Fractures along which displacement has occurred Blocks on either side have moved Most faults are inclined at some angle measured from horizontal The dip angle of the fault Two blocks are defined, one on either side of the fault
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Faults Fault geometry Imagine a horizontal tunnel cutting through a fault in a cross-section Horizontal Surface Dip angle Hanging Wall Foot Wall Fault plane
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Three Types of Faults Normal faults Reverse faults Strike slip faults
Hanging wall moves down relative to foot wall Block slides down the dip angle Reverse faults Hanging wall moves up the dip angle Hanging wall moves up relative to foot wall Reverse to what seems normal Strike slip faults Displacement along fault is horizontal Parallel to the strike of the fault plan
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Which fault is “normal”?
B C
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Which fault is “reverse”?
B C
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Types of Faults Normal Reverse or thrust Strike Slip
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Fault Types Normal faults
Hanging wall moves down relative to foot wall Block slides down the dip angle Hanging Wall Foot Wall
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Normal faults are created by tension
Rifts are created by normal faults
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Fig. 7.8b. Normal faults produce grabens & horsts
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Normal Faults Normal faults are created by tensional forces Rifts are created by parallel normal faults dipping toward each other The block in the center which drops down is a graben The Rio Grande valley in New Mexico is a rift graben
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Reverse Faults Compressional stress usually causes reverse faults to form Reverse faults are common at convergent plate boundaries Reverse faults cause a thickening of the crust as rocks are piled up Older rocks may be found above younger rocks
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Reverse Faults Thrust faults are a special kind of reverse fault
Shallow dip angle, > 45o Common in large mountain ranges Horizontal displacement may be many tens of kilometers Evidence of thrust faults in sedimentary rocks is seen when a sequence of the same rocks are repeated
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Reverse faults Hanging wall moves up relative to foot wall
Block moves in the reverse direction to what seems normal created by compression Hanging Wall Foot Wall
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Reverse Faults Thrust faults are a special kind of reverse fault
Shallow dip angle, > 45o Common in large mountain ranges
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Strike-Slip Faults Strike-Slip faults Principle movement is horizontal
Left or Right Lateral Little or no vertical movement Caused by shear stress Indicated by abrupt changes in drainage patterns
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Fig. 7.8e. Strike-slip faults offset drainage
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Strike-Slip Faults Strike-Slip faults Principle movement is horizontal
Left or right Caused by shear stress
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Movement Along Faults Rarely exceeds a few meters in a single event
Small movements, cm scale, may occur on a regular basis Total displacement may be km, but does not occur in a single event
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Wasatch Fault: Our Fault
What type of fault is it? How much displacement at each event? What is this event called? How many events if altitude is 2,500 m? Is that an accurate assessment of the total amount of displacement?
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Fault Breccia
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Fig. 7.8a. Easily recognized displacement
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Folds Warps in rock layers due to ductile deformation
Generally indicate horizontal compression Multiple generations of folding may exist Folds are described by: strike of their hinge line The angle of dip of their limbs
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Folds Folds are described by: The strike of their hinge line
The hinge line is the intersection of the hinge plane with the folded layer Hinge lines may be inclined in a plunging fold The angle of dip of their limbs
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Fig Fold geometry
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Folds Three simple fold forms exist Synclines warp downward
Anticlines warp upward Monoclines dip in one direction
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Fig Types of folds
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Anticlines & Synclines
The sequence of ages of strata indicate the geologic structure in folds Anticlines have the oldest layers exposed at the center of the fold along the axial plane Synclines have the youngest strata exposed along the axial plane
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Fig. 7.15a. A series of anticlines & synclines
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Synclines have the youngest strata exposed along the axial plane
Anticlines have the oldest layers exposed at the center of the fold along the axial plane Synclines have the youngest strata exposed along the axial plane (a) Youngest rock (b) Syncline Anticline Monocline Oldest rock
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Fold Belts (folded mountains)
Orogenic belts are a long linear series of folds Fold geometry is not overly complex Pattern of outcrops may appear complex Complex folds: Re-folded Cut by thrust faults
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Orogenic belt with complex folding
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Complex Folds Folds may be very complex Application of shear stress
Multiple folding events Complex forms are created
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Complex Folds Folds may be very complex Application of shear stress
Multiple folding events Complex forms are created
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Complex Folds Plunging folds occur when the folds axis is dipping or plunging Limbs of some folds are not the same, one dips more steeply than the other Some folding is so extreme that beds are turned upside-down
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Fig. 7.15d. A plunging anticline
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Domes & Basins Generally occur in continental interiors
Broadly warped regions Roughly circular pattern of outcrops
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Fig. 7.15b. A small dome
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Complex Folds Diapirs Less dense salt layers may rise up
Some overlying strata may be pierced Salt diapir has an inverted teardrop shape Strata above diapir are domed upward
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Unconformities
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Stress and Structure Geode II 818 847 819
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Structures and Plate Tectonics
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End of Chapter 7
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