Strike & Dip. Structural Geology I: Folds & Faults 2009 Lect. 18, J. Steven Kite, West Virginia University

Transcription

1 2 Apr Structural Geology I: Folds & Faults 2009 Lect. 18, J. Steven Kite, West Virginia University Steeply dipping axial plane cleavage. These folded sedimentary layers exposed near Sullivan River in southeastern British Columbia dramatically illustrate the awesome forces constantly at work reshaping the features of the earth. Photo by R.A. Price Strike & Dip

2 Strike & Dip Symbols N Strike Strike Strike & Dip Symbols N Dip Direction Strike & Dip Symbols N 30 = Dip Angle in Degrees

3 Force vs. Response Stress vs. Strain Rock Behavior Varies with Temperature Low Temperature, Pressure - Brittle Rocks - Faulting High Temperature, Pressure - Ductile Rocks Folding or Pull-Apart (Silly Putty or Taffy) Stress (Force) Red Arrows Ductile Strain (Response) Boxes

4 FOLDS Compression FOLDS Compression Rails Compressed During 1964 Earthquake Alaska National Earthquake Information Center neic.usgs.gov/neis/eqlists/usa/1964_03_28_pics_1.html

5 Types of Folds Anticline Syncline Monocline Eroded (or Breached) Folds Anticline Syncline Monocline Erosion Surface Younger Rx Older Rx Anticlines (A) & Synclines (S)? S A S A S A? Photo by R.A. Price

6 Strike & Dip Symbols N 30 = Dip Angle in Degrees

7 Home The Woods Anticline Syncline Anticline The River Grandmother s House Limb Axis = Line of Symmetry Parts of a Fold Plural = Axes Limb Axis = Line of Symmetry Axial Plane Parts of a Fold

8 Limb Axis = Line of Symmetry Axial Plane Parts of a Fold Axial Planes Parts of a Fold Non-Plunging vs. Plunging Folds Horizontal Axes Dipping Axes

9 Plunging Fold Breached (Eroded) Folds & Age of Rocks. Superposition: Youngest on Top Older Rocks Younger Rocks Where are older rocks exposed at the surface? Younger Rocks

10 Younger Rocks Older Rocks Older Rocks Where are Older Rocks, Younger Rocks? Remember Superposition MD Geological Survey Photo: gif/sidel.jpg Asymmetrical Anticline Symmetrical Syncline Asymmetrical Anticline Asymmetrical Syncline

11 Progression of Folding Symmetrical Asymmetrical Overturned Recumbant Thrust Fault Overturned Fold Up-Side-Down (Overturned) Limb Recumbant Fold Recumbant Limb

12 Thrust-Faulted Fold Thrust Fault Earthquakes & Faults Stress: Force Applied Strain: Response to Force Stress vs. Strain

13 Fault Plane of Weakness Along Which Movement Has Occurred Joint Break Without Significant Movement Go to this site to see animations of different fault types: Check Out Fig in Plummer Text Book, 10 th Ed. Dip-Slip Motion Mostly Up and Down

14 Fault Parts Normal Fault Before Stress Tension = Pull-Apart After Tension Normal Fault

15 Tension Normal Fault Normal Faults Tension Graben Down- Dropped Block Bounded by Two Normal Faults

16 Graben,, Death Valley, CA Reverse Fault Before Stress Compression = Squeezing After Reverse Fault Compression

17 Reverse Fault Thrust Fault Low-Angle (<30 o ) Reverse Fault May Have Great Displacement Displacement Reverse Fault Compression

18 Thrust Fault, Eastern Pennsylvania Strike-Slip Slip Motion Lateral Fault Right-Lateral Fault Map View - Before

19 Right-Lateral Fault Map View - After Stream Offset on San Andreas Fault Lineament Left-Lateral Lateral Fault Before After

20 Types of Faults Oblique Fault Before Stress Tension + Shear After L'Aquila, Italy Earthquake: 6.3 April 6th 2009 earthquake in Central Italy occurred as a result of normal faulting on a NW-SE oriented structure in the central Apennines. Focus was only 5 km (3 mi) depth. Sound Central+Italy April+6+01%3A32%3A42+UTC)&t=h&z=7 Seismologist Was Forced to Remove Italy Earthquake Warning From the Internet

21 2 Apr Active vs. Inactive Faults Active Faults: Moved at Least Once in Last 10,000 years OR Twice or More in Last 250,000 years Expected Earthquake Intensity 3 New Madrid Quakes Modified Richter (Total Energy) Magnitude Scale Sumatra 2004

22 Mercalli (Surface Intensity) Scale Fig Focus: Point of Faulting Epicenter: Point on Surface Above Focus What Causes Damage? No Opening of Earth & Gobbling up Cities Little Damage From Fault Offset Most Damage Caused by Seismic Waves (Including Tsunamis) & Landslides

23 Elastic Rebound theory Developed after 1906 SF Quake Earthquakes & Faults At Great Depths, Rock Does Not Fully Snap Back...

24 Earthquakes & Faults Elastic Rebound Earthquakes & Faults Elastic Rebound Earth quake

25 Elastic Rebound Marin County, 1906 Offset Fence Shows Strike Slip Motion Right or Left Lateral? USGS Photo by G.K. Gilbert USA/1906_04_18_pics_2.html

26 Good Friday 1964 Alaska Earthquake, Magnitude 9.2 National Earthquake Information Center neic.usgs.gov/neis/eqlists/usa/ 1964_03_28_pics_1.html This side rose 5 m (16 ft) Dip-slip motion Fault Scarp

27 Earthquake Damage: Landslides Anchorage Elementary School Destroyed National Earthquake Information Center neic.usgs.gov/neis/eqlists/ USA/ pics 1.html Earthquake Damage: Landslides 1964 Alaska RR Near Anchorage National Earthquake Information Center neic.usgs.gov/neis/eqlists/usa/ 1964_03_28_pics_1.html Earthquake Damage: Tsunami = Seismic Sea Wave (A.K.A. Tidal Wave ) National Earthquake Information Center neic.usgs.gov/neis/eqlists/usa/1964_03_28_pics_1.html

28 Logging Truck Wrapped Around Tree by Tsunami National Earthquake Information Center neic.usgs.gov/neis/eqlists/usa/ 1964_03_28_pics_1.html 18 April 1906 Quake Damage San Francisco City Hall _04_18_pics_2.html Stanford University neis/eqlists/usa/ 1906_04_18_pics_2.html

29 USGS California Seismic Risk Map USGS W.Va. Seismic Risk Map Largest Earthquake in West Virginia: 1969 November 20, Magnitude 4.5 Historic Earthquakes Southern West Virginia :00:09.3 UTC (Local 11/19) Magnitude 4.53 Intensity VI Largest Earthquake in West Virginia Minor damage occurred in Giles County, Virginia, at Glen Lyn and Rich Creek, and at three towns in southern West Virginia. At Glen Lyn, a few bricks were knocked from a chimney, windows were broken, and plaster was broken from most of the walls in an old house. At Rich Creek, plaster cracked and fell and windows were broken. A cornice reportedly was shaken from one building in Henry County, at Collinsville, Virginia. Windows also were broken in southern Mercer County, West Virginia, at Lerona, Oakvale, and Elgood. Felt over all or parts of nine States: Georgia, Kentucky, Maryland, North Carolina, Ohio, South Carolina, Tennessee, Virginia, and West Virginia. Abridged from Seismicity of the United States, (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.

30 At Some Point (T, P) Earthquakes Are Impossible Because Rock Is Not Elastic Roll of Fluids as Lubricants Water Nuclear Wastes in Colorado Quiz 7 Due in Class on 9 April OL101Quiz7Structures.pdf

31 What Develops in Rock Under Compression? Brittle Rocks: Reverse Faults Ductile Rocks: Folds Reverse Fault Reverse Fault

32 Compression of Ductile Rocks: Folds What Develops in Rock Under Tension? Brittle Rocks: Normal Faults Tension Normal Faults

33 11 Apr What Develops in Rock Under Tension? Ductile Rocks: Boudinage Boudinage Boudinage Force Brittle Nature of Rock Ductile Tension Normal Faults * Boudinage Compression Reverse Faults Folds * * Why Are Normal Faults & Folds Most Common?

34 Slaty Cleavage in Rocks First Step in Metamorphic Foliation Slaty Cleavage in Rocks Sandstone (Joints) & Shale (Slaty Cleavage Interpetation of Structures: Simple Case Horizontal Rocks (Law of Original Horizontality) Folded Rocks --> Deformation (Compression) Modern Topography Almost Always Is Related to Differential Erosion