Measuring Neenach-Pinnacles Distance and Fault Motion Analysis

Activity 2.2 A: 1. Measure the distance between Neenach and Pinnacles along the San Andreas Fault. Because the fault is not a straight line on the map, you might want to use a string in the measuring process. Carefully position the string along the curved trace of the fault. Mark the average positions of Neenach and Pinnacles on the string. Straighten the string along a ruler and measure the distance between the two points on the string in millimeters. Each millimeter on the map represents 4 km on Earth's surface. Approximate distance from Neenach to Pinnacles: 300 km 2. The age of the Neenach-Pinnacles Volcanics is between about 19.0 and 24.1 Myr. Based on your answer to the previous question and making the first-order assumption that faulting began soon after volcanism ceased (around 19.0 Myr), estimate the average rate at which Neenach and

Pinnacles have moved apart since 19 Myr. Estimated average rate of displacement since 19 Myr: 15.79 km/Myr R = 300 km / 19 Myr R= 15.79 km / Myr 3. There are three permanent GPS sites very near Neenach and two GPS sites near Pinnacles. Based on data processed for those sites by UNAVCO and posted on March 2, 2019, Pinnacles and Neenach are currently moving apart at a rate of about 21.2 mm/yr (-21.2 km/Myr). If we assume that Neenach and Pinnacles have moved apart at a constant rate of 21.2 km/ Myr-a first- order assumption that might not be accurate estimate when the faulting that split them apart might have begun. (Hint: Use the distance you measured from Neenach to Pinnacles.) Estimated age of faulting: 14.15 Myr Time = 300 km / 21.2 km/Myr Time = 14.15 Myr Activity 2.2 B: 1. Velocities measured at several GPS stations in southern California are shown in Fig. A2.2.2. The length of the arrow provides the speed of the GPS site, measured in a reference frame called NAM08 that is fixed to the stable interior of the North American Plate-the area east of the Rocky Mountains. Longer arrows indicate faster motion, and an arrow that is 25 mm long on the map represents a speed of 50 mm/yr. Use the velocity vectors on either side of the San Andreas Fault

to estimate how much faster the Pacific Plate is moving relative to the deforming western edge of the North American Plate in southern California. You may give your answer as a range of velocities. Answer: 26.44 - 10.57 mm/ yr The Pacific Plate is moving about 2 times faster. 2. Add half-arrows along the San Andreas Fault to show the sense of motion across the fault based on your best interpretation of the difference in GPS vectors across the fault. Look at Fig. A2.2.1 for an example of the half-arrow symbol used to map faults that have horizontal slip. Activity 2.2 C: REFLECT & DISCUSS Many of the GPS velocity vectors that are shown east of Malibu on Fig. A2.2.2 appear to be directed toward a part of the San Andreas Fault near the center of the map, where the fault is labeled. This segment is called the "Big Bend" in the San Andreas Fault. Vectors on the north (upper) and the south (lower) parts of the map seem to be more nearly parallel to the fault. Describe one or more ways that the crust along the Big Bend might be affected by this difference in motion compared with the areas along the fault to the northwest and southeast. One way that the crust along the Big Bend might be affected is by creating a collision. A collision zone can occur when two continental plates collide while moving toward each other and have equal densities. As a result of the collision zone big earthquakes can occur, while no land is formed or created. Activity 2.4 A :

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1. If both the Emperor and Hawaiian Islands Chains developed as a result of the same mantle hotspot, what is a possible reason that the hotspot trail changes direction at ~42 Myr? The possible reason can be the force and the acceleration changing the direction of the hotspot trail. 2. What was the rate of Pacific Plate motion relative to the Hawaiian hotspot as it was developing the 2,300 km-long Emperor Seamount Chain from 65 Myr to 42 Myr? Express the rate in millimeters per year (mm/yr). In what direction was the plate moving (north-northwest or south-southeast) relative to the hotspot during that time interval? Rate = 2300 km / 65 Myr - 42 Myr = ((2300 km) (1000 m) (100 cm)) / 23 x 10^6 yrs

= 10 cm/yr or 100 mm/yr 100 mm/yr to the north-northwest 3. What was the rate of Pacific Plate motion relative to the Hawaiian hotspot from 5.1 to 0.8 Myr, expressed in mm/year? Rate = 400 km / 5.1 Myr - 0.8 Myr = 400 km / 4.3 x 10^6 yr = (400 km)(1000m)(100cm) / 4.3 x 10^6yr = 9.30 cm or 93.0 mm 93 mm/yr to the northwest 4. Using Lo'ihi Seamount as the current location of the Hawaiian hotspot, what was the rate of Pacific Plate motion relative to the Hawaiian hotspot from 0.8 Myr to today, expressed in mm/yr? Distance =230 km Time = 0.8 Myr Rate = 10^6 mm = x mm (10^6 mm)(230km) = (1 km)(x) 1 km 230 km 230 x 10^6 mm = x Rate = 230 x 10^6 mm / 0.8 x 10^6 yr Rate = 287.5 287.5 mm/yr to the north-northwest 5. The map locates each GPS station with a green dot and a yellow line that extends outward in the direction that the GPS station is moving relative to the NNR reference frame. GPS station HNLC is located on Oahu. (a) How does the current motion of HNLC on Oahu compare to the direction of Pacific Plate motion relative to the Hawaiian hotspot over the past ~42 million years? Now the islands are moving more toward the north compared to the past. (b) GPS station HNLC on Oahu has the following component velocities relative to the NNR reference frame as of March 5, 2019: moving north at 34.607 ± 0.038 mm/yr and moving west at 62.814 ± 0.041 mm/year. Use the Pythagorean Theorem to find the current speed of the Pacific Plate at Oahu relative to the NNR reference frame. Show your work. (34.607)^2 + (62.814)^2 = 5143.243045 Square root of 5143.243045 = 71.7 71.7 mm/yr

6. REFLECT & DISCUSS Based on all of your previous work, explain how the direction and rate of Pacific Plate movement changed over the past -70 million years. The direction and the rate of Pacific Plate movement changed significantly over the past ~70 Myr in a way that the Pacific Plate has moved northwest relative to the plate that holds North America, and relative to the hotspot coming up through the mantle from below the plates. Activity 2.4 B : 1. What does this progressive chain of volcanic centers indicate about the possible origin of the active volcanism at Yellowstone? Support your answer with evidence. The progressive chain indicates that the North American Plate is moving southwest from the Yellowstone HotSpot. We can support this statement by looking at the image above which shows that the oldest deformation zone which is 13.8 Myr is southwest of the hot spot which is 0.6 Myr which indicates that the oldest deformation zone has moved southwest away from the hot spot. 2. Based on the map, what was the average speed and direction of North American Plate motion at Yellowstone relative to the hotspot, since 13.8 Myr? 10^6 mm = x mm x = 600 x 10^6 mm 1 km 600 km

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Distance = 600 x 10^6 mm Rate = 600 x 10^6 mm = 45.45 mm/yr Time = 13.8 - 0.6 yr) x 10 ^6 yr 13.2 x 10^6 yr The average speed and direction of North American Plate motion is 45.45 mm/yr to the southwest. Add an arrow (vector) and rate label to the map in Fig. A2.4.1 to show this movement. 3. Plate Boundary Observatory GPS station P717 near the east gate of Yellowstone National Park had the following component velocities relative to the NNR reference frame as of March 5, 2019: moving south at 8.155± 0.123 mm/yr and moving west at 14.905 ± 0.094 mm/yr. (a) Use the Pythagorean Theorem to find the current speed of the North American Plate at P717 relative to the NNR reference frame. Show your work. (8.155)^2 + (14.905)^2 = 288.66305 Square root of 288.66305 = 16.99 16.99 mm/yr (b) How do the present-day direction and speed of P717 relative to the NNR reference frame compare with your estimate of the average direction and speed of the North American Plate relative to the hotspot during the past 13.8 Myr? The current direction of P717 relative to NNR is similar to the trend of the calderas that extend west-southwest of Yellowstone, but the speed calculated from GPS data is much slower with 16.99 mm/yr compared with 71.7 mm/yr. Activity 2.4 C : REFLECT & DISCUSS How do hotspots help us understand plate tectonic processes and rates? Hotspots help us to understand the plate tectonic processes and rates in a way that it allows us to track the direction of the plate’s motion and how fast the plates are moving. Activity 2.6 A :

1. Using a pencil, draw a line on the seafloor to show where new ocean crust and lithosphere is forming now (zero millions of years old). Using Figs. 2.1, 2.3, and 2.12 as guides, label the segments of your line that are the Juan de Fuca Ridge and Gorda Ridge (divergent plate boundaries). Then label the segments of your pencil line that are transform fault plate bound- aries. Add half-arrows to the transform fault boundaries to show the motion of the two plates relative to the transform fault. 2. What has been the average rate and direction of seafloor spreading in mm per year (mm/yr) west of the Juan de Fuca Ridge, from B to A? Show your work. The distance in km between A and B: (2.5 cm × 100 km/cm) = 250 km and covers an age of 8 Myr. 250 km x 100 cm/km= 25000 cm 25000 cm / 8000000 yr = 3.125 cm/ yr 3.125 cm/yr is 31.25 mm/yr The average rate of seafloor spreading is 31.25 mm/yr toward west-northwest

3. What has been the average rate and direction of seafloor spreading in mm per year (mm/yr) east of the Juan de Fuca Ridge, from B to C? Show your work. The distance between points B and C: (2.35 cm × 100 km/cm) = 235 km and covers an age of 8 Myr. 235 km x 100 cm/km=23500 cm 23500 cm / 8000000 yr= 2.9375 cm/yr 2.9375 cm/yr is 29.375 mm/yr toward east-southeast 4. Notice that rocks older than 11 million years are present west of the Juan de Fuca Ridge but not east of the ridge. What could be happening to the seafloor rocks along line segment C-D that would explain why rocks older than 11 million years no longer exist on the seafloor east of the ridge? The reason why rocks older than 11 million years no longer exist on the seafloor east of the ridge may be that the seafloor rocks have been subducted beneath the North American Plate when it was moving westward. 5. Notice the black curve with triangular harbs just east of point C: (a) If you could take a submarine to view the sea floor along this line, what feature would you expect to see? (Hint. See Fig. 2.1A.) The feature that I expect to see along the line of the point C is a seafloor along the Cascadian trench. (b) Based on Fig. 2.1, what lithospheric plate is located east of the black barbed curve? The lithospheric plate located on the east of the black barbed curve is the North American Plate. (c) Based on Fig. 2.1, what lithospheric plate is located west of the black barbed curve? The lithospheric plate located on the west of the black barbed curve is Juan de Fuca Plate. 6. REFLECT & DISCUSS Notice the line of volcanoes that form the Cascade Range, extending from northern California to southern Canada. These are active volcanoes, meaning that they still erupt from time to time. What sequence of plate tectonic events is causing these volcanoes to form? The sequence of plate tectonic events that causes these volcanoes to form is a subduction line along the points C and D, which is subjected to heating along the geothermal gradient. The water acts as an inflow to the overlying rocks of the mantle wedge and eventually causes them to undergo hydration and partial melting. The resulting magma rises up to form the Cascade Range of volcanic mountains. Activity 2.7 A :

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1. What is the average speed at which Bna drifted away from the ridge at A during the past 67.7 Myr expressed in mm / yr? 133800000 cm / 67700000 yr = 1.97637 cm or 19.7637 mm/ye The Average speed of Bna relative to A since 67.7 Myr is 19.8 mm/yr What is the average speed that Baf drifted from A? 19.5 mm / yr. 132000000 cm / 67700000 yr = 1.94978 cm/yr or 19.4978 mm/yr Which plate moved faster relative to the ridge over the past 67.7 Myr, if either? North American Plate (BNA) 2. The area between Cna and Bna consists of oceanic lithosphere added along the Mid- Atlantic Ridge to the North American Plate between 154.3 and 67,7 Myr. What is the average speed at which new lithosphere was added to the North American Plate along that line? Show your work. 15.63 mm/yr 154.3 - 67.7 x 10^6 yrs = 86.6 x 10^6 yrs = 1.563 cm/yr 1.563 cm/yr is 15.63 mm/yr

Do the same analysis for the lithosphere between points Caf and Baf. What is the average speed at which new lithosphere was added to the African Plate along that line ? 14.76 mm/yr. 127900000 cm / 86600000 yr = 1.476 cm/yr or 14.76 mm/yr Which plate moved faster relative to the ridge between 154.3 and 67.7 Myr, if either? North American Plate BNA Activity 2.7 B : Given your answers in part A, did the North Atlantic Ocean Basin develop by adding lithosphere symmetrically along the mid-ocean ridge, or was new lithosphere added more rapidly to one side than the other? New lithosphere was added along the North Atlantic Ridge in the vicinity of point A in 154.3 Myr. If spreading was asymmetric, which plate had more lithosphere added, or did the asymmetry vary from plate to plate over time? North American Plate. Activity 2.7 C : Use the rates that you calculated above and the map scale to estimate when the coastlines of North America and Africa might have last touched before they were separated by the opening of the North Atlantic Ocean Basin. Between 177 Myr and 191 Myr Opening based on the North America: 154.3 Myr + 36.3 Myr = 190.6 Myr or 191 Myr Opening based on the Africa: 154.3 Myr + 22.9 Myr = 177.2 Myr or 177 Myr Activity 2.7 D : REFLECT & DISCUSS Based on the rates you calculated above, estimate the number of meters that Africa and North America have moved apart since the United States was formed in 1776. Discuss what you did to accommodate the uncertainty in your estimate. 2023-1776=247 yrs North American Plate: 247 yrs x 19.8 mm/yr = 4890.6 mm 4890.6 mm x 0.001 m/mm = 4.8906 m African Plate: 247 yrs x 19.5 mm/yr = 4816.5 mm 4816.5 mm x 0.001 m/mm = 4.8165 m Africa and North America have moved apart by about (4.8906 m + 4.8165 m) = 9.7071 m.

Activity 2.8 A :

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Activity 2.8 B:

1. What kind of plate boundary is shown in your cross-section? Convergent 2. Draw a curve in the cross-section to show your interpretation of where the top surface of the subducting plate is located. 3. Label the part of your cross-section that probably represents earthquakes in the subducting slab. Then label the part of your cross-section that probably represents earthquakes in the South American Plate above the subducting Nazca Plate. 4. At what depth does magma probably originate below the volcanoes, just above the subducting plate: 100 - 150 km. How can you tell? I can tell it because most earthquakes occurred at these depths. 5. REFLECT & DISCUSS What is the deepest earthquake plotted on your cross-section? Why do you think that earthquakes occur at hundreds of kilometers depth along subducting slabs but not elsewhere in the mantle at that same depth? The deepest earthquake plotted on the cross-section is 620 km. I think the reason may be the different temperatures. While the mantle is very hot and heat causes the plates to move, the subducting slabs are very cold and therefore earthquakes occur

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here when the plates move toward each other and crush to each other.

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