Earth_Structures_II_VL

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De Anza College *

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Apr 3, 2024

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Introductory Geology Laboratory 1 epending on the mechanical properties of a given rock and how extreme the temperature and pressure conditions, stress may cause it to bend rather than break. This bending or folding of strata within the crust of the earth commonly yields two distinct geometries. One is an arch-like shape (anticline ) or a trough-like shape ( syncline) . A fold can be divided into two halves or limbs that are separated by an imaginary plane called the axial plane . The fold axis is the line formed by the intersection of the axial plane and the bedding plane of strata. The axial trace of a fold is the intersection of the axial plane with the earth’s surface. D Block diagram illustrating the geometry of a hypothetical plunging anticline. Ages of strata is denoted by numbers, with 1 being the oldest. Axial trace with plunge direction and anticline symbol s is shown on upper surface of block along with strike and dip. Modified from a 3D model created by Dr. Stephen Reynolds as part of the Hidden Earth Project at Arizona State University. Earth Structures Part II: Outcrop Patterns, Geometry and Age Relationships of Folded Strata Virtual Geology Laboratory Edition Christopher DiLeonardo, Ph.D. Earth & Space Sciences De Anza College

Introductory Geology Laboratory Folds 2 Objectives By completing this activity you will be able to: • Recognize folded sequences by their outcrop pattern at the earth's surface. • Distinguish between anticlines and synclines. • Recognize plunging folds and determine the direction of plunge from outcrop patterns at the earth's surface. • Determine the age relationships of strata in a folded sequence. Materials: Pencil; color pencils; eraser; straight edge, millimeter/scale ruler, computer running QuickTime® and four-color pack of Play Doh® (if completing “Further Exploration” section below). 3-D Video Files: FO_Block01.mov; FO_Block02.mov, FO_Block03.mov; FO_Block04.mov QuickTime VR block models used in this exercise were designed by Dr. Steven Reynolds, Arizona State University as part of the NSF sponsored Hidden Earth Project. Activity 1a Folded Strata Open the two interactive block diagrams FO_01_Block.mov and FO_02_Block.mov by clicking on their links in the lab Module online . Each block rotates 360º and becomes transparent to see the internal layering. You can control this using the slider at the bottom of the video. Compare these two models depicting folded strata. The first model depicts an anticline (arch-like fold) while the second depicts a syncline. Use these two models to answer the questions below. Click on... FO_01_Block.mov and FO_02_Block.mov 1. For the top surface of the model of a syncline, does the youngest or oldest strata of the folded sequence appear in the trace of the axial plane.? For the anticline? 2. For the syncline, in map view, does the age of the strata become younger or older with distance from the trace of the axial plane? For the anticline? 3. Do the limbs dip toward or away from the fold axis of the syncline? For the anticline? The youngest strata of folded sequence appears in the trace of the axial plane. For the anticline the youngest strata appears. The age of the strata becomes older with distance with distance from the trace of the axial plane. For the anticline the age becomes older. The limbs dip away from the fold axis of the syncline. For the anticline the limbs dip away.

Introductory Geology Laboratory Folds 3 Activity 1b Plunging Folds Now open the two interactive block diagrams FO_03_Block.mov and FO_04_Block.mov . Each block can be rotated 360º and rendered transparent to see the internal layering. Compare these two models depicting plunging folds. The first model depicts a plunging anticline while the second depicts a plunging syncline. Use these two models to answer the questions below. Click on... FO_03_Block.mov and FO_04_Block.mov 4. Look at the outcrop pattern on the top of the two 3-D interactive block diagrams depicting plunging folds. How are they similar to each other? How do they differ from the outcrop patterns of nonplunging folds examined earlier? Plunging folds are folds in which the fold axis is not parallel to an imaginary horizontal plane at the earth’s surface. The fold axis of a plunging fold tilts into the crust, the compass direction of that tilt is considered the direction of plunge . 5. Do the age and structural (dip of limbs) relationships discussed for anticlines and synclines above apply to plunging folds as well? 6. The outcrop patterns for plunging folds are “horseshoe-shaped.” These shapes have one end that is connected and continuous (the closed-end of the horseshoe) and an unconnected end (the open-end of the horseshoe). Towards which end is the direction of plunge for a plunging syncline? Towards which end is the direction of plunge for a plunging anticline? They are similar because they both have a blue and green line. They are di ff erent because they FO3 block points inward and FO4 block points outward. Yes, age and structural relationships discussed for anticlines and synclines apply to plunging folds as well. The opened end is the direction of plunge for a plunging syncline. The closed end is the direction of plunge for a plunging anticline.

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Introductory Geology Laboratory Folds 4 Summary Examine your answers to questions above. Use your knowledge of folded strata to complete the table below by blacking out the wrong choice from those given and highlighting the correct one. Study the completed table and refer to it whenever you are dealing with folded strata. Relationship Syncline Anticline Dip of Limbs Limbs dip (TOWARDS; AWAY) from the axial plane of the syncline. Limbs dip (TOWARDS; AWAY) from the axial plane of the anticline. Age of Strata The (YOUNGEST; OLDEST) exposed strata in the folded sequence can be found in the axial trace of the fold at the earth’s surface. The folded strata becomes progressively (YOUNGER; OLDER) with distance from the axial trace on the earth’s surface. The (YOUNGEST; OLDEST) exposed strata in the folded sequence can be found in the axial trace of the fold at the earth’s surface. The folded strata becomes progressively (YOUNGER; OLDER) with distance from the axial trace on the earth’s surface. Direction of Plunge(?) If plunging The direction of plunge for a plunging syncline is towards the (OPEN; CLOSED) end of the horseshoe-shaped outcrop pattern in map view. The direction of plunge for a plunging anticline is towards the (OPEN; CLOSED) end of the horseshoe-shaped outcrop pattern in map view. Activity 2 Geologic Block Diagrams Complete the block diagrams below by finishing the front cross-section for each block diagram. Remember the contacts between rock units must connect from the front section to the map view and if needed to the perpendicular section along the side of the block. You should complete them using colors and label so that it is clear how the rock units connect in your block diagram. Note: Only three out of the four blocks represent anticlines or synclines, one of the four is not. Rules for Cross-Sections Rule 1 Always use the simplest solution that accurately completes a cross-section. Rule 2 Unless you have observations or data to the contrary, deformed units are neither overturned (upside down) or vertical. Rule 3 Even though thickness of units can vary over a region units in your cross-sections should maintain their thickness throughout. Rule 4 Lower numbers represent older units that are progressively younger with higher numbers. 1 is the oldest possible layer and missing numbers represent missing units. ammonium mm muumuus Mmmmm anammmm ummmm murmurman mum

Introductory Geology Laboratory Folds 5 Strike & Dip Symbol 2 2 3 3 4 6 2 3 5 4 Block Diagram 2.1 Block Diagram 2.2 Block Diagram 2.3 Soooo Bad nonnairreasate unannounced and

Introductory Geology Laboratory Folds 6 Activity 3 Geologic Structure Problem Geologic Map and Structural Cross-Section Below is a partially completed geologic map of a deformed area and accompanying topographic profile along a line X to Y on the map. You will follow the steps below to complete the simple geologic map by adding the requested structural data (attitudes = strike & dip; axial traces of folds). After completing the map you will be asked to complete a structural cross-section and determine the ages of units on the map. Step 1 Mark an appropriate attitude (strike & dip) at each location A through D on the map. Note the strike of units will be parallel to the general trend of the contacts in map view. The dip direction will simply show the general direction of dip This is always in one direction or the other perpendicular to strike. Your attitudes will not require an angle of dip so there won’t be a number. One strike and dip measurement was plotted for you on the map. Hint: The general direction of dip can be determined by the outcrop pattern where it crosses a stream canyon The direction of dip will be in the same direction as the v-shpaed outcrop pattern. If the contact across the stream canyon is an apparent straight line, then the outcrop here is vertical. Step 2 Draw in the axial traces of all folds on the geologic map using appropriate symbols for synclines and anticlines. Note direction of plunge on your axial trace with an arrowhead in the direction of plunge. Step 3 Using the methods originally described in the special paper you read on Earth Structures , and outlined on page 8, to complete the structural cross-section along the line X – Y. Step 4 Once you have completed the cross-section, determine the age of the units. Oldest __________________ Youngest __________________ Strike & Dip Symbol Block Diagram 2.4 rehonddahahahaharanaense

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Introductory Geology Laboratory Folds 7 Geologic Map & Cross-Section Complete the map and cross section below Using the steps listed above. M M A P P S A A S Stream Valley Stream Valley a b C d 58 X Y X Y 0 1,000 m Scale Map View if.pl I 1 1 vertical I I l i i t 1 l l l l l 1 I Diph I 1 I 1

© 2020 C. G. DiLeonardo Construction of geologic cross-section showing step-by-step approach to completing section. Illustration by C.G. DiLeonardo. Acknowledgements The visualizations used in this exercise were created by Dr. Stephen Reynolds as part of the Hidden Earth Project at Arizona State University. The Hidden Earth Project is funded by the National Science Foundation to help undergraduates in the geosciences better understand spatial relationships. More 3D visualizations of crustal structures (and other cool stuff too) can be found at Dr. Reynolds website: http://geology.asu.edu/~reynolds/ About the Earth Discovery Project The Earth Discovery Project is a collaborative effort to integrate hands-on discovery-based learning with modern research tools in undergraduate geoscience education. The approach is to develop and disseminate a comprehensive set of learning resources and experiences supporting systemic educational reform. The logo of the Earth Discovery Project portrays the earth as a three-dimensional puzzle. The globe used in the logo is from NASA’s Blue Marble Project . The Blue Marble is a unique view of the earth, which integrates numerous data sets to construct a “true-color” three-dimensional globe. Step 3 Step 2 Step 1 Step 4 Step 5 Construction of geologic cross- section can be done through a step-by-step approach. Note: each step is shown on the diagram. Step 1 Using a straightedge carefully project each place where a contact crosses the line of section to the corresponding topographic profile. Step 2 Between the projected contact are the individual rock units. Label the rock units just above their position on the profile. Step 3 The strike and dip symbols on the map indicate the orientation of rock units. Use those to mark the general orientation or direction of dip above the profile. Remember, these orientations are only valid right at the surface so you cannot project them indefinitely into the crust. Step 4 Using the surface dips as a guide and contacts as starting points project the contacts into the crust just below the surface. And then connect the same contacts in the subsurface as in the syncline and dashed connections across the top as in the eroded anticline. Step 5 Complete the cross-section by coloring and labeling the section to match the map units.

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