Eric H Christiansen

Main Idea Deformed rocks are a record of Earth’s tectonic system and reveal how it works. Joints Faults Folds

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

Rock Deformation Three types of differential stress Tensional - pulling apart Compressional - squeezing together Shear - slipping, twisting, or wrenching

Rock Deformation = Strain Definition: Strain is the change in the shape or volume of a rock that results from stress.

Deformation?

Deformation?

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

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

Strike and Dip

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

Joints Fractures created by in brittle rocks No shear or displacement has occurred Any joints here?

Why are joints important? Fluid flow Ground water Oil Ore deposits Weathering and erosion

Why are joints important? Fluid flow Ground water Oil Ore deposits Weathering and erosion

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

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

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

Which fault is “normal”? B C

Which fault is “reverse”? B C

Types of Faults Normal Reverse or thrust Strike Slip

Fault Types Normal faults Hanging wall moves down relative to foot wall Block slides down the dip angle Hanging Wall Foot Wall

Normal faults are created by tension Rifts are created by normal faults

Fig. 7.8b. Normal faults produce grabens & horsts

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

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

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

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

Reverse Faults Thrust faults are a special kind of reverse fault Shallow dip angle, > 45o Common in large mountain ranges

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

Fig. 7.8e. Strike-slip faults offset drainage

Strike-Slip Faults Strike-Slip faults Principle movement is horizontal Left or right Caused by shear stress

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

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?

Fault Breccia

Fig. 7.8a. Easily recognized displacement

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

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

Fig. 7.12. Fold geometry

Folds Three simple fold forms exist Synclines warp downward Anticlines warp upward Monoclines dip in one direction

Fig. 7.11. Types of folds

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

Fig. 7.15a. A series of anticlines & synclines

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

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

Orogenic belt with complex folding

Complex Folds Folds may be very complex Application of shear stress Multiple folding events Complex forms are created

Complex Folds Folds may be very complex Application of shear stress Multiple folding events Complex forms are created

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

Fig. 7.15d. A plunging anticline

Domes & Basins Generally occur in continental interiors Broadly warped regions Roughly circular pattern of outcrops

Fig. 7.15b. A small dome

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

Unconformities

Stress and Structure Geode II 818 847 819

Structures and Plate Tectonics

End of Chapter 7