structural geology. +tectonic collision deforms crustal rocks producing geologic structures. ! folds...

Structural GeologyStructural Geology

Structural Geology+Tectonic collision deforms crustal rocks

producing geologic structures.! Folds

! Faults

! Joints and Fractures

+Tectonic collision deforms crustal rocks producing geologic structures.! Folds

! Faults

! Joints and Fractures

Deformation+All changes in the original location,

orientation or form of a crustal rock body.+Deformation common

at plate margins.+Deformation concepts…! Force

! Stress

! Strain

Force+Force – Mass x acceleration (F = ma) +The action that puts stationary objects in

motion or+Changes the motion of moving objects.

+Stress - Force applied to a given area. Determines the concentration of force.

+Differential Stress – Unequal in different directions.

+3 major types of differential stress! Compressional stress

! Tensional stress

! Shear stress

Stress

+“Push-together” stress.+Shortens and thickens crust.+Associated with orogenesis (mtn. building).

+“Push-together” stress.+Shortens and thickens crust.+Associated with orogenesis (mtn. building).

Compressional Stress

+“Pull-apart” stress.+Thins and stretches crust.+Associated with rifting.

Tensional Stress

Stephen Marshak

+Slippage of one rock mass past another.+In shallow crust, shear is often

accommodated by bedding planes.

+Slippage of one rock mass past another.+In shallow crust, shear is often

accommodated by bedding planes.

Shear Stress

+Changes in the shape or size of a rock body caused by stress.

+Strain occurs when stresses exceed rock strength.

+Strained rocks deform by folding, flowing, or fracturing.

Strain

+Elastic deformation – The rock returns to original size and shape when stress removed.

+When the (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

+Brittle behavior occurs in the shallow crust; ductile in the deeper crust.

How Rocks Deform

Stephen Marshak

+Factors controlling rock strength and deformation style.! Temperature and confining pressure

8Low T and P = brittle deformation8High T and P = ductile deformation

! Rock type – Mineral composition controls strength.

! Time – Stress applied for a long time generates change.

How Rocks Deform

Mapping Geologic Structures+Geologists describe and interpret rock structures.

! Structure usually determined from a limited number of outcrops.

! Mapping is aided by advances in aerial photography, satellite imagery and Global Positioning Systems (GPS).

! The most common and useful technique for geological mapping remains….

FIELD WORK !!

The Formation A mappable rock unit.

The Formation A mappable rock unit.

+Describing and mapping the orientation of a geologic structure or fault surface involves determining …

! Strike (trend)

!Dip (inclination)

Mapping Geologic Structures

+Strike (trend)! The compass direction of the line produced by the

intersection of an inclined rock layer or fault with a horizontal plane.

! Generally expressed an an angle relative to north.8N37°E

8N12°W

Mapping Geologic Structures

+Dip (inclination)! The angle of inclination of the surface of a rock

unit or fault measured from a horizontal plane.

! Includes both an angle of inclination and a direction toward which the rock is inclined.882°SE

817°SW

Mapping Geologic Structures

Folds+Rocks are bent by crustal deformation into a

series of wave-like undulations called folds.+Most folds result from compressional stresses

which shorten and thicken the crust.

+Rocks are bent by crustal deformation into a series of wave-like undulations called folds.

+Most folds result from compressional stresses which shorten and thicken the crust.

Stephen Marshak

+Parts of a fold! Limbs – The two “sides” of a fold.

! Fold axis or hinge line – A line connecting points of maximum curvature along a fold.

! Axial plane – An imaginary surface that divides a fold symmetrically.

+Parts of a fold! Limbs – The two “sides” of a fold.

! Fold axis or hinge line – A line connecting points of maximum curvature along a fold.

! Axial plane – An imaginary surface that divides a fold symmetrically.

Characteristics of Folds

+Anticline – Upfolded or arched rock layers.+Syncline – Downfolds or rock troughs. (Think “sink”) +Depending on their orientation, anticlines and

synclines can be described as! Symmetrical

! Asymmetrical

! Recumbent (an overturned fold)

! Plunging

Common Types of Folds

AnticlineAnticline

SynclineSyncline

Anticlines and Synclines are common in fold and thrust belts related to mountain belts.

+Monoclines – Large, step-like folds in otherwise horizontal sedimentary strata.

+Domes -Upwarped circular or slightly elongated structure. Oldest rocks in center, younger rocks outside.

+Basins – Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.

Common Types of Folds

FaultsFaults

+Breaks in rock that exhibit offset. +Exist at a variety of scales.+Sudden movements along faults are the cause

of most earthquakes.+Classified by movement…

8Horizontal8Vertical8Oblique

+Breaks in rock that exhibit offset. +Exist at a variety of scales.+Sudden movements along faults are the cause

of most earthquakes.+Classified by movement…

8Horizontal8Vertical8Oblique

Faults

+Faults grind rocks to create fault gouge.+Walls of a fault bear evidence of this grinding

as slickensides. +“Slicks” reveal

fault direction.

+Faults grind rocks to create fault gouge.+Walls of a fault bear evidence of this grinding

as slickensides. +“Slicks” reveal

fault direction.

Faults

+Dip-slip faults – Motion is parallel to fault dip.

+Strike-slip faults – Motion is parallel to fault strike.

Fault Types

+May produce long, low cliffs called fault scarps.

Dip Slip Faults

! Footwall (rock mass

below the fault)

! Footwall (rock mass

below the fault)

Dip Slip Faults

! Hanging wall

(rock mass

above the fault)

! Hanging wall

(rock mass

above the fault)

+Fault blocks classified as+Fault blocks classified as

+Two dominant types! Normal fault

! Reverse Fault8Thrust (a low angle reverse fault)

Types of Dip-Slip Faults

+Normal fault ! Hanging wall moves down relative to the

footwall.

! Accommodate lengthening or extension of the crust.

! Exhibit a variety of scales.

Types of Dip-Slip Faults

+Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).

+Normal fault bounded valleys are called grabens (Rhine graben).

+Normal fault bounded ridges are called horsts.

Normal Faults

Fig. 11.17b

W. W. Norton

+Reverse faults! Hanging wall block moves up relative to the

footwall block

! Reverse faults have dips greater than 45o and thrust faults have dips less then 45o

! Accommodate shortening of the crust

! Strong compressional forces

Types of Dip-Slip Faults

+Thrust faults - A special case of reverse fault.! Hanging wall block moves up relative to the

footwall block

! Thrust faults are characterized by a low dip angle (less then 45o).

! Accommodate shortening of the crust

! Strong compressional forces

Types of Dip-Slip Faults

Fig. 11.17a

W. W. Norton

U.S. Geological Survey

+Dominant displacement is horizontal and parallel to the strike of the fault

+Types of strike-slip faults! Right-lateral – as you face the fault, the block on

the opposite side of the fault moves to the right

! Left-lateral – as you face the fault, the block on the opposite side of the fault moves to the left

Strike-Slip Faults

+Strike-slip fault8Transform fault

– Large strike-slip fault that cuts through the lithosphere

– Accommodates motion between two large crustal plates

Strike-Slip Faults

+Joints are a very common

rock structure.+They are fractures with no

offset.+Result from tectonic

stresses on rock mass.+Occur in parallel groups.

Joints

+Chemical weathering tends to be concentrated along joints

+Many important mineral deposits are emplaced along joint systems

+Highly jointed rocks often represent a risk to construction projects

+Chemical weathering tends to be concentrated along joints

+Many important mineral deposits are emplaced along joint systems

+Highly jointed rocks often represent a risk to construction projects

Significance of Joints

QUESTIONS