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