Lab 7: STRUCTURAL GEOLOGY FOLDS AND FAULTS

Transcription

1 Lab 7: STRUCTURAL GEOLOGY FOLDS AND FAULTS This set of labs will focus on the structures that result from deformation in earth s crust, namely folds and faults. By the end of these labs you should be able to identify the different types of folds and faults as well as be able to interpret and draw the structures in block diagrams. Below is a very brief description of folds and faults. You should have more information in your lecture notes and in the textbook. On the following page is a table showing the most common types of deformational structures and the way they are represented on a map as well as the different ways to indicate strike and dip on a map. Folds Folds are geologic structures in which rock layers have been bent as the result of applied stress. The most common types of folds are anticlines, synclines, monoclines, domes and basins. An imaginary plane can be drawn through the fold where the layers of rock change the direction of their tilting. This is called the axial plane. It is noted on a map by drawing a line at the intersection of this plane and the surface. This line is called the fold axis. Based on the appearance of the fold in cross-sectional view, the fold may be upright (vertical axial plane), inclined (tilted axial plane with the limbs dipping at different angles), overturned (the limbs dip in the same direction), or recumbent (axial plane is horizontal). A. C. B. All of the problems we will be working in this section are shown in block diagrams (Figures B and C above). These diagrams represent a portion of the earth shown in three dimensions. The top view is called the map view and represents a bird s-eye view. The front view is called the cross section or profile view and represents what you would see in a road-cut view. Try to keep in mind that the map view also represents erosion, which removes much of the structures in question. Notice the difference between the anticline sketched in Figure A above and an anticline represented in a block diagram in Figure B. Faults Faults are fractures in the earth s crust along which rocks on opposite sides of the fault have moved parallel to the fault surface. Faults are the result of brittle deformation in which rocks break rather than bend as with folds. Faults can generally be classified as either strike-slip or dip-slip depending on whether movement is along the dip plane (dip-slip) or movement in a horizontal direction (strike-slip). Dip-slip faults are further classified as either reverse or normal depending on the relative motion of the hanging wall (block A in this diagram) and footwall (block B). These terms are used to describe either side of a fault system. The block of earth above the fault line is called the hanging wall and the block of earth below the fault line is called the 4

2 footwall. If, after movement, the footwall has moved up relative to the hanging wall, the fault is called a normal fault. If, after movement, the footwall has moved down relative to the hanging wall, the fault is known as a reverse fault. A thrust fault is a low angle (usually less than 40 ) reverse fault. Strike slip faults are further classified as either right-lateral or left-lateral depending on which way the rocks move on either side of the fault. The next set of tables provides you with the various symbols used on geologic maps to represent the structural orientation of rock layers. The first table provides you with the various symbols used to represent the strike and dip of rocks layers. Strike and dip is used to measure the orientation of rocks layers (or other planar features such as faults) with respect to a horizontal surface. More details will be given in lab on this concept. The other two tables provide you with the symbols used to represent various folds and faults on a map. 5

3 GEOLOGIC MAP SYMBOLS STRIKE AND DIP SYMBOLS Map Symbol Definition Explanation of Symbol strike and dip of beds other than horizontal or vertical strike (longer line) is horizontal line on bedding plane strike parallels nearby contacts between stratified rocks dip shows which way beds run downhill dip angle (number) is how much beds tilt down from horizontal horizontal beds strike and dip of vertical beds because the bed is horizontal, it strikes in all directions because the bed is horizontal, the dip is 0 strike (longer line) is horizontal line on bedding plane because the bed dips vertically (90 dip), it dips equally in both directions at right angles to the strike, so the dips are shown extending from both sides FAULT SYMBOLS Map Symbol Type of Fault Definition Type of Stress Geologic Associations zones of crustal extension Normal (Dip-slip) hanging wall down, footwall up Tension divergent plate boundaries edges of horsts and grabens Basin and Range province Reverse (Dip-slip) hanging wall up, footwall down high-angle (45 or more dip) Compression zones of crustal compression convergent plate boundaries Thrust (Low-angle Reverse) (Dip-slip) low-angle reverse fault (less than about 40 dip) hanging wall may appear to be on top of footwall Compression zones of crustal compression convergent plate boundaries hanging wall may appear to be on top of footwall Strike-Slip (Right Lateral) rocks on either side move horizontally to the right of each other Shear continental margin undergoing oblique (not straight on) plate convergence transform plate boundaries Strike-Slip (Left Lateral) rocks on either side move horizontally to the left of each other Shear continental margin undergoing oblique (not straight on) plate convergence transform plate boundaries 6

4 FOLD SYMBOLS Map Symbol Type of Fold Definition Appearance of Beds in Map View Syncline Plunging Syncline Anticline Plunging Anticline "down" fold (appears to "sink" in the middle) down fold with tilted axis "up" fold (appears to be an ant hill) up fold with tilted axis roughly parallel stripes dip toward center (axis) youngest rock is in the center (along axis) oldest rocks farthest from center roughly a U-shaped pattern on surface plunges in direction U opens youngest rock in the center oldest rock farthest from center roughly parallel stripes dips away from the center (axis) oldest rock in center youngest rocks farthest from center roughly a U-shaped pattern on surface plunges in the direction U points oldest at center (axis) youngest rock farthest from center Monocline strata tilted in one direction only all layers dip in the same direction appears to dip only on one side Basin Dome downward bulge in layered rocks upward bulge in layered rocks roughly a bull's-eye pattern on eroded surface everything dips toward center youngest rock in the center oldest rock farthest from center roughly a bull's-eye pattern on eroded surface dips away from the center oldest rock in center youngest rocks farthest from center IDENTIFYING GEOLOGIC STRUCTURES The diagrams on the following pages are block diagrams. The top of a block diagram is an oblique view of the earth's surface, in other words it is a geologic map that you are viewing at an angle from above. The vertical sides of a block diagram are cross-sections, cut-away views that show how the rocks and structures extend into the earth. Crosssections typically don't contain geologic map symbols. However, to make sure that we understand relative motion on faults, we will use half-arrows on each side of any fault in the cross-sections. All other map symbols appear only on the map (top) of the block diagrams. 7

5 HORIZONTAL AND TILTED STRATA Block diagram Add the missing strike and dip symbol to the map in block diagram. Number the layers oldest () to youngest (4). Have the rock layers in this diagram been deformed since they originated? Block diagram How would you describe the rock layers in block diagram? Are they tilted, folded, or faulted? Number the rock layers oldest to youngest. Based on the front cross-section, what is the angle of dip as measured with a protractor? Block diagram How would you describe the strata in block diagram? Are they tilted, folded, or faulted? Can you apply the principle of superposition to determine the age sequence of the beds in the diagram? 8

6 FOLDS Block diagram 4 Add an additional strike and dip symbol to the map to make it more complete. What is the name of the type of fold depicted in the diagram? What does the red line on the map indicate? Add what is missing to this symbol to make it complete. Block diagram 5 What is the name of the type of fold depicted in the diagram? What does the red line on the map indicate? Add what is missing to this symbol to make it complete. 9

7 Block diagram 6 Add an additional strike and dip symbol to the map to make it more complete. What is the name of the type of fold depicted in the diagram? What does the red line on the map indicate? Add what is missing to this symbol to make it complete. Block diagram 7 Add an additional strike and dip symbol to the map to make it more complete. What is the name of the type of fold depicted in the diagram? What does the red line on the map indicate? Add what is missing to this symbol to make it complete. 40

8 Block diagram 8 Add an additional strike and dip symbol to the map to make it more complete. What is the name of the type of fold depicted in the diagram? Add the appropriate missing symbol to the map. Block diagram 9 Add an additional strike and dip symbol to the map to make it more complete. What is the name of the type of fold depicted in the diagram? Add the appropriate missing symbol to the map. 4

9 Block diagram 0 What is the dip of the fault? What type of fault offsets the layers in the diagram? Add half-arrows on each side of the fault in the cross-section to indicate relative direction of movement. Add the appropriate fault symbols to the map. Block diagram What is the dip of the fault? What type of fault offsets the layers in the diagram? Add half-arrows on each side of the fault in the cross-section view to indicate relative direction of movement. Add the appropriate fault symbols to the map. 4

10 Block diagram There are two faults shown in the block diagram. What type of faults are they? Add half-arrows on each side of the fault in the cross-section to indicate relative direction of movement. There is one complete fault-bounded geologic structure depicted in the diagram. What is the name of the structure? 4

11 For each of the following block diagrams, you are provided with very simplified map patterns. Complete the cross section views. The numbers indicate different rock layers with being the oldest. Pay close attention to strike and dip symbols when present. Try to keep the thicknesses of the layers constant as you draw them in. Add all necessary map symbols (strike and dip symbol, fold symbols, etc.)

12 45 GEOLOGIC STRUCTURES FAULTS. Label the hanging wall (H) and foot wall (F) on each diagram.. Name the type of dip slip fault in blocks A F. Indicate with arrows in cross section view the direction of movement of each wall.. Measure the angle of dip for all of the faults where possible and add to the map view. B D E F A 4 C

13 For the following blocks, draw what is indicated next to the block. Make sure to have all appropriate map symbols present on the map view. Number all rocks layers and color your diagram when complete. Draw - 4 layers on each diagram. Horizontal beds (=strata=layers) Vertical beds Upright anticline Upright syncline 46

14 Plunging anticline Plunging syncline Normal fault with horizontal layers Reverse fault with horizontal layers 47

15 Normal fault with tilted layers Reverse Fault with tilted layers Right-lateral strike-slip fault Left-lateral strike-slip fault 48