# Chapter 9: Structures & Mountain Building

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Chapter 9: Structures & Mountain Building
PowerPoint Presentation Stan Hatfield . SW Illinois College Ken Pinzke . SW Illinois College Charles Henderson . University of Calgary Tark Hamilton . Camosun College Copyright (c) 2005 Pearson Education Canada, Inc.

Chapter 9: Structural Deformation
Forces Origin Nomenclature

Structural Geology: A Study of Earth’s Architecture
Earth is a dynamic planet. Some rock units in the Canadian Rockies have been thrust for over 100 kilometres Structural geologists study the architecture and processes responsible for deformation of Earth’s crust A working knowledge of rock structures is essential to our economic well-being for hazards & resources Copyright (c) 2005 Pearson Education Canada Inc.

Caledonides formed Late Paleozoic with the closing of Iapetus Ocean, forming Pangea

Deformation Deformation involves
Stress - force applied to a given area Types of stress (differential stress that is applied unequally in different directions) Compressional stress – shortens a rock body Tensional stress – tends to elongate or pull apart a rock unit Shear stress – produces a motion similar to the slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom Copyright (c) 2005 Pearson Education Canada Inc.

Compression (convergence) Tension (uplift) Shear (lateral rotation)

Deformation How Rocks Deform
Rocks subjected to stresses greater than their own strength begin to deform usually by folding, flowing, or fracturing Weaker rocks deform more easily (lithology, bedding) Fluids and Heat affect lower strength Pressure increases rock strength General characteristics of rock deformation Elastic deformation – the rock returns to nearly its original size and shape when the stress is removed Once the elastic limit (strength) of a rock is surpassed, it either flows slowly (ductile deformation) or fractures quickly (brittle deformation) This depends on Strain Rate and Time/Duration Copyright (c) 2005 Pearson Education Canada Inc.

Deformed Lacustrine Strata, {Normal Fault} Palmdale, CA

Increasing Confining Pressure/Depth Brittle Ductile
S max S min S int

Shear reorients Foliation

Strike: intersection of dipping rock layer with horizontal surface & contact direction on map (azimuth relative to north) Dip: Angle below horizontal

Folds During crustal deformation rocks are often bent into a series of wave-like undulations called folds Characteristics of folds Most folds result from compressional stresses which shorten & thicken the crust: mountain belts Parts of a fold Limbs – refers to the two sides of a fold Axis – a line drawn down the points of maximum curvature of each layer Axial plane – an imaginary surface that divides a fold more or less symmetrically Asymmetry points in the direction of crustal transport, roll over, vergence Copyright (c) 2005 Pearson Education Canada Inc.

Strike & Dip Permits 3-D Visuals
Read the map symbols: Visualize the Cross Sections

Folds Common types of folds Anticline – upfolded or arched rock layers
Oldest in the middle Outwards directed, antithetic dips Syncline – downfolds or troughs of rock layers Youngest in the middle Inwards directed, synthetic dips Depending on their orientation, anticlines and synclines can be described as Symmetrical, asymmetrical, recumbent (an overturned fold), or plunging Antiform & Synform when age is unknown Anticlinorium or Synclinorium for continental size Copyright (c) 2005 Pearson Education Canada Inc.

Symmetric Anticline Opposing outwards dips Oldest beds in middle

Plunging Anticline Strike wraps around Nose points down plunge

East Verging Fold & Thrust Belt
 Vergence (transport) Direction 

Kink Folds, Sharp Axial Planes: Syncline Anticline

Plunging Folds: Tilted or Refolded Fold Belt

Doubly Plunging Anticline or Asymmetric Dome

Chevrons on flank of Monocline
Compression Drape Fold Reverse Fault in Basement

Folds Common types of folds Other types of folds
Monoclines – large, step-like folds in otherwise horizontal sedimentary strata Other types of folds Dome Upwarped displacement of rocks Circular or slightly elongated structure Oldest rocks in centre, younger rocks on the flanks Basin Downwarped displacement of rocks Youngest rocks in centre, older rocks on the flanks Copyright (c) 2005 Pearson Education Canada Inc.

Salt, Intrusion Or Central Uplift

Subsidence

Black Hills, SD Cretaceous (Laramide Orogeny) Deforms Mesozoic through Precambrian Rocks
Mz Pc Pz

Late Paleozoic Michigan Basin

Faults Faults are fractures in rocks along which appreciable displacement has taken place Brittle/Shallow in the upper crust Ductile/Deep in the lower crust Sudden movements along faults are the cause of most earthquakes usually deeper than 5 km to about 660 km Along faults, rock is often broken into breccia, pulverized into gouge or polished as slickenslides Faults are classified by their relative orientation & movement which can be Horizontal, vertical, or inclined Strike Slip, Dip Slip or Oblique Slip Copyright (c) 2005 Pearson Education Canada Inc.

Normal Fault (Tension)

Faults Types of Faults Dip-Slip Faults: (Normal, Reverse & Thrust)
Movement is mainly parallel to the dip of the fault surface May form in either compression or tension Normal faults thin the crust & miss out some strata Reverse & Thrust Faults thicken the crust & double some strata May produce long, low cliffs called fault scarps Active if fault cuts to surface Resequent if erosional & controlled by strata Parts of a dip-slip fault include the hanging wall (rock surface above the fault) and the footwall (rock surface below the fault) Copyright (c) 2005 Pearson Education Canada Inc.

Normal Fault: (Extension)

Normal Fault: (Extension)
Hanging Wall Falls Section Thins

Faults Types of dip-slip faults Normal Faults
Hanging wall block moves down relative to the footwall block Accommodate lengthening or extension of the crust Many are small like landslides with displacements of a metre or so Larger scale normal faults are associated with Mid Ocean Ridges, Rifts & Fault-block mountains, Horsts & Grabens Reverse & Thrust Faults Hanging wall block moves up relative to the footwall block Accommodate shortening or compression of crust Larger scale thrust faults are associated with edges of Fold & Thrust Mountain Belts Copyright (c) 2005 Pearson Education Canada Inc.

Faults: Horst & Graben as in Rifts or Basin & Range
Diagrammatic sketch of downfaulted (graben) and upfaulted (horst) blocks. Note that there are places where if you drilled there is missing stratigraphy due to extensional faulting. Copyright (c) 2005 Pearson Education Canada Inc.

Development of a Normal Fault

Most Normal Faults “sole-out” and become Listric with depth.

Faults: Thrust & Reverse
Types of dip-slip faults Reverse and Thrust Faults Hanging wall block moves up relative to the footwall block Reverse faults have dips greater than 45o and thrust faults have dips less than 45o Most thrust faults have flat soles and arise from a common surface called a decollement Accommodate shortening of the crust Strong compressional forces Common in mountain belts like the Alps and Rockies An isolated outlying remnant of a thrust sheet is called a klippe (old rocks surrounded by younger rocks, Teeth point inwards) Copyright (c) 2005 Pearson Education Canada Inc.

Reverse Fault: (Compression)
Hanging Wall Rises Section Thickens Angle > 15°

Thrust Fault: (Compression) Laramide Orogeny Rockies
Section thickens Older over younger Thrust Vergence direction

Crowsnest Mountain Klippe: Thrust Outlier
Paleozoic Limestone Cretaceous Shales

Right Lateral Strike Slip Fault: Like Queen Charlotte & San Andreas

Columnar Joints: Thermal Cooling

Joints from from Decompression