Deformation and Geologic Structures

Structural Geology

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. This understanding of the dynamics of the stress field can be linked to important events in the regional geologic past; a common goal is to understand the structural evolution of a particular area with respect to regionally widespread patterns of rock deformation (e.g., mountain building, rifting) due to plate tectonics.

Introduction

• The lecture is devoted to a review of “geologic structures”• such as folded and fractured rock layers resulting from deformation

– their descriptive terminology

– and the forces responsible for them

• Deformation – refers to changes in the shape or volume (or both) of rocks as a result of stress

Stress and Strain

• Stress – a force per unit area– Since it’s difficult to directly observe stress, geologists study the effects of

past stress when bed rock is exposed after uplift and erosion at the Earth’s surface

– The principal directions of stress can be determined by our observations

• Strain – the change in size (volume) and/or shape, in response to stress

Stress and Strain

Three types of Stress:– A compressive stress is caused by forces pushing together, or squeezing from opposite

directions.• Compressive stress is common along convergent plate boundaries

• Typically results in rocks being deformed by a shortening strain; either by bending and/or folding.

– A tensional stress is caused by forces pulling away from one another in opposite directions.

• Tensional stress is produced at divergent plate boundaries and results in a stretching or extensional strain.

Stress and Strain

– A shear stress is due to forces parallel to one another by in opposite directions along a discrete surface, such as a fault.

• A shear stress results in a shear strain parallel to the direction of the stresses.

• Shear stresses are notable along transform plate boundaries and actively moving faults.

Stress and Strain

Folding Thrust/reverse fault

Normal fault

How Rocks Behave

Behavior of Rocks to Stress and Strain

Rocks behave as elastic, ductile, or brittle, depending on:– the amount and rate of stress applied

– the type of rocks

– and the temperature and pressure

– The rock behaves elastically if after deformed, it returns to its original shape (e.g. a rubber band)

– A rock behaves in a ductile or “plastic” manner if it bends while under stress, but doesn’t return to its original shape after relaxation of the stress

– A rock exhibiting brittle behavior will fracture at stresses higher than its elastic limit

Stress and Strain

Behavior of Rocks to Stress and Strain

Structures as a Record of the Geological Past

Strike and DipAccording to the principle of original horizontality, sedimentary rocks are deposited as

horizontal beds or strata• Where these originally horizontal rocks are found tilted, it indicates that tilting must have

occurred after deposition and lithification

– Strike is the compass direction of a line formed by the intersection of an inclined plane with a horizontal plane.

• The strike is measured in reference to the northerly direction by degrees from 0o – 90o east or west.

– Dip is measured downward from the horizontal plane to the bedding plane and perpendicular to the strike.

• The dip is always measured at a right angle to the strike.

Strike and Dip

Exercises

Exercise (cont.)

FoldsGeometry of Folds

Folds are usually associated with compressive stresses along convergent plate boundaries but are also commonly formed where rock has been sheared along a fault.

• Determining folds have important economic implications.• Used to determine movement of tectonic plates.

– An anticline is an upward arching fold; layers dip away from the hinge line (or axis) of the fold.

– A syncline is a downward arching fold; layers dip toward the hinge line.

– Each anticline and adjacent syncline share a limb.

– An axial plane is an imaginary plane containing all of the hinge lines of a fold.

• The axial plane divides the fold into it two limbs.

It’s important to realize that anticlines and synclines are not necessarily related to ridges nor synclines to valleys. This is because valleys and ridges are nearly always erosional features.

Monocline

Anticline and Syncline

Anticline and Syncline

Anticline and Syncline

In an area that has been eroded to a plain, the presence of underlying anticlines and synclines is determined by the direction of dipping beds in exposed bedrock.

– Determining the relative ages of the rock layers, or beds, can tell us whether a structure is an anticline or a syncline.

• The oldest exposed rocks are along the hinge line of the anticline.• The youngest exposed rocks are found along the synclinal hinge line.

Plunging Folds

• Plunging Folds – folds in which the hinge lines are not horizontal.– In nature, anticlines and synclines are frequently plunging folds.

– On a surface leveled by erosion, the patterns of exposed strata resemble V’s or horseshoes rather than the striped patterns of non-plunging folds.

– A plunging syncline contains the youngest rocks in its center or core.• The V or horseshoe points in the direction opposite of the plunge.

– A plunging anticline contains the oldest rocks in its core, and the V points in the same direction as the plunge of the fold.

Dome and Basin

• A structural dome is a structure in which the beds dip away from a central point.– In cross section, a dome resembles an anticline.

• In a structural basin, the beds dip toward a central point.– In cross section, it is comparable to a syncline.

Domes and basins tend to be features on a grand scale, formed by uplift somewhat greater (for domes) or less (for basins) than that of the rest of a region.

Dome near Casper, WyomingPhoto by D. A. Rahm

Folds

Interpreting Folds

– Open folds have limbs that dip gently• The more open the folds, the less intense the stress involved

– Isoclinal folds have limbs that are parallel to one another, implies intense compressive or shear stress

“Click to view animation”

Folds

Interpreting Folds– Overturned folds – if the axial plane is inclined to such a degree that the fold limbs dip in the

same direction, the fold is classified as an overturned fold• Imply that unequal compressive stresses or even a shearing stress caused the upper limb of the fold to

override the lower limb

– Recumbent folds – are overturned to such an extent that the limbs are essentially horizontal• Indicate compressive and/or shear stresses were more intense in one direction and probably record

shortening of the crust associated with plate convergence.• Found in the cores of mountain ranges such as the Canadian Rockies, Alps, and Himalayas

Fractures in Rock

Joints

Joints are one of the most commonly observed structures in rocks.

– Where joints are oriented approximately parallel to one another, they are called a joint set.

• Joints can often indicate the direction of compressive stress.

• Vertical joint sets are often associated with tectonic uplift of a region.

If a rock is brittle or if the strain rate is too rapid for deformation to be accommodated by ductile behavior, the rock fractures.

– Commonly, there is some movement or displacement.

When there is no shear displacement, a fracture or crack in bedrock is called a joint.– If the rock on either side of a fracture moves, the fracture is a fault.

Vertical Joints in Sedimentary Rock of Colorado PlateauPhoto by Frank M. Hanna

Fractures in Rock

Faults

Fractures in bedrock along which movement has taken place.– Geologists describe fault movement in terms of direction of slippage.

• In a dip-slip fault, movement is parallel to the dip of the fault surface.• A strike-slip fault indicates horizontal motion parallel to the strike of the fault surface.• An oblique-slip fault has both strike-slip and dip-slip components.

“Click to view animation”

Fault in Big Horn Mountains, WyomingPhoto by Diane Carlson

Fractures in Rock

Dip-slip faults

Normal and reverse faults, are the most common types of dip-slip faults.– These two types of faults are distinguished from each other on the basis of the relative movement of

the footwall block and the hanging wall block.• The footwall is the underlying surface of an inclined fault plane.• The hanging wall is the overlying surface of an inclined fault plane.

– In a normal fault, the hanging-wall block has moved downward relative to the footwall block.• A normal fault results in extension or lengthening of the crust

• When there is extension of the crust, the hanging-wall block moves downward along the fault to compensate for the pulling apart of the rocks.

• Graben• Horst

Exercise

Fractures in Rock

“Click to view animation”

Fault in Volcanic Ash Layers, OregonPhoto by Diane Carlson

Fractures in Rock

Dip-slip faults

– In a reverse fault, the hanging-wall block has moved upward relative to the footwall block.

• Horizontal compressive stresses cause reverse faults.• Reverse faults tend to shorten the crust.

– A thrust fault is a reverse fault in which the dip of the fault plane is at a low angle ( < 30o) or even horizontal.

• Typically move or thrust older rocks on top of younger rocks.

• Result in an extreme shortening of the crust.• Commonly form at convergent plate boundaries to

accommodate shortening during collision.

“Click to view animation”

Fault in Volcanic Ash beds, OregonPhoto by Diane Carlson

Fractures in Rock

Strike-slip Faults– A fault where the movement (or slip) is predominantly horizontal and parallel to the strike

of the fault.• The displacement along a strike-slip fault is either left-lateral or right-lateral and can be

determined by looking across the fault.• Right-lateral fault –when movement on other side of fault line is to the right.• Left-lateral fault – when movement on other side of fault line is to the left

San Andreas Fault, CAPhoto by C. C. Plummer

What’s type of this fault ?

What’s type of this fault ?

What’s type of this fault ?

What’s type of this fault ?

What’s type of this fault ?

Exercises

Exercises (cont.)

Structures as a Record of the Geological Past

Understanding and mapping geologic structures is also important for evaluating problems related to engineering decisions and environmental planning

Geologic Maps and Field Methods

A geologic map uses standardized symbols and patterns to represent rock types and geologic structures for a given area.

– Different colors and patterns on a geologic map represent distinct rock units

– Type and distribution of rock units

– Structural features

– Ore deposits

Map symbols

Symbols of geologic ages

Structures as a Record of the Geological Past

Strike and DipA specially designed instrument called a Brunton pocket transit is used by geologists for measuring

the strike and dip• The Brunton contains a compass, a level, and a device for measuring angles of inclination

Strike and dip symbols are drawn on a geologic map for each outcrop with dipping or tilted beds

• The long line of the symbol is aligned with the compass direction of the strike

• The small tick, which is always drawn perpendicular to the strike line, is put on one side or the other, depending on which of the two directions the beds actually dip

– The angle of dip is given as a number next to the symbol on the map

– Beds with vertical dip require a unique symbol

Geologic Cross Section

A geologic cross section represents a vertical slice through a portion of Earth

• On a geologic map, cross sections are constructed by projecting the dip of rock units into the subsurface

HomeworkUsed concepts:- Strike & dip- Geologic symbols- True & apparent dip- Cross section mapping