Exploring the Basin & Range Province

In this activity, we will use geophysical data to understand the Basin & Range Province in the Western United States. This region is characterized by north-south trending mountain ranges with valleys (or basins) in between them. Why is this the case? Is this a typical geographic feature in the world? In fact, it is a fairly unique geography that can indicate a lot about the geologic processes taking place in this region. [Look at geography and geology of the Basin and Range through Google Earth or GeoMapApp.]

Notice in the two images below that the north-south trending mountain ranges are visible throughout Nevada and western Utah. Notice also that the rocks – as visible in the geologic maps – also display the stripes of the mountains interspersed with valleys in between them.

Figure 1. Satellite image of Nevada and Utah from Google Earth

Figure 2: Geologic map of Nevada and Utah in Google Earth

Part I. Telling the Story. Using GPS monuments to explain crustal characteristics. Something is going on in this region to cause the basins and the ranges. Typically the forces that can lead to deformation are pushes, pulls or lateral motion. Which is occurring here? What data can we use to explore crustal features and figure out whether this region is undergoing pushes, pulls or lateral motions? We will use GPS monument data to explore this region. [GPS monument intro – link to other resources.]

Goals:

Students will be able to perform calculations, develop models, or use computational skills successfully.

Students will be able to apply data analysis to draw and support conclusions.

Open UNAVCO GPS Velocity Viewer: This website, which is sponsored by NSF and NASA, displays the motion of GPS monuments over time. Most GPS monuments have been in place for 10-20 years and their movement over time is indicative of underlying plate motion. The arrows on the displayed map represent GPS monument motion where the tail of the arrow represents the location of the monument, the direction in which the arrow points is the direction the GPS monument is moving and the length of the arrow represents the speed at which the monument is moving.

http://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html

Once you have opened the GPS Velocity Viewer with the above link, zoom in on Utah and Nevada. In the right column, check the box for “Display Vectors”. Your data source should be “UNAVCO, NAM08: North America”. This will display the plate motion relative to a stationary central United States. Change “Vector length” to “4x”, and for “Sites displayed” select “Show half”. Then click on “Draw Map”.

Initial Observations:

1. What do you notice about the motion of the GPS monuments in Utah and Nevada? Which direction are the monuments moving? Are they moving generally in one direction or multiple directions?

2. Do the monuments in Utah and Nevada appear to be moving at the same rate? How can you tell?

3. What do you think is happening to the crust in Utah and Nevada based on your answer to question #2?

Data Analysis:

Now you will look at the data underlying the GPS vectors. On the right hand side of the UNAVCO GPS Velocity Viewer, click the box next to “Station names and data download” and then click on “Draw Map”. You will click on three designated stations in Utah near the Wasatch Mountains and three designated stations in central Nevada and fill in the tables below. Notice, from the image at right, how degrees correlate to direction of motion, so a direction of 315°, for example, correlates to crustal movement toward the northwest. In all cases, we are using a North American reference frame which means that motion is measured relative to a stationary central North America.

Utah Data:

Station Name Horizontal Speed

(mm/yr)

Direction of Motion

(Degrees)

Approximate Direction of Motion (East, West, North, South)

P086LMUTP108Average of 3 Stations:

Nevada Data:

Station Name Horizontal Speed

(mm/yr)

Direction of Motion

(Degrees)

Approximate Direction of Motion (East, West, North, South)

ELKOMINEP074Average of 3 Stations:

4. Do you see any trend in the horizontal speed or direction of plate motion for the Utah monuments?

5. Do you see any trend in the horizontal speed or direction of plate motion for the Nevada monuments?

6. Overall, can you say anything about how the rate of plate motion changes as you move from the Wasatch mountains toward western Nevada?

7. Calculate the average horizontal speed of the three stations in Utah and the three stations in Nevada. Write the results in the above tables. Does it make sense to average these data points? Why or why not?

8. What can you say about the rate of motion of the Wasatch Front versus the rate of motion of central Nevada? Can you deduce what is happening to the continental crust between the Wasatch Front and Central Nevada? Quantify what is happening here based on the above data.

9. Geologically speaking, one year or twenty years is not significant. One million years or 100 million years may be significant. Convert your answer is (8) to units of km/Ma.

10. It is fairly well accepted that the Basin & Range province has been stretching for 17 million years. Calculate how much stretching has occurred over 17 million years based upon your answer to question (9). Assume that the spreading rate has remained constant over the last 17 million years.

Show your work!! Note: your answer should be in kilometers.

11. In fact, geologists believe that the Basin & Range has extended somewhere between 60 km and 300 km over 17 million years. What is incorrect in the assumptions we made in answering question (9)?

Data Validation:

You have looked at GPS monument motion as summarized by UNAVCO. Now, you will validate that the motion listed for the ELKO monument is accurate. Go back to the UNAVCO GPS Velocity Viewer and click on the ELKO monument. Then click on “Download Data File”. This will open a data set. This data set can be imported into Excel. If you have difficulty importing the data into Excel, this dataset is provided for you on Canvas under the name “ELKO.pbo.nam08.xlsx”.

The data contained in the UNAVCO download is the rate of motion in a particular direction – either the direction east-west (positive east), north-south (positive north), or vertically up-down. We will focus on horizontal motion of the GPS monuments – so east-west motion and north-south motion in order to come up with a resultant velocity of the monuments in central Nevada. Velocity in this case is a horizontal vector where a vector contains information about both magnitude (in this case rate of motion or speed) and direction of motion.

In the Excel data set, select the first three columns of data – “Date”, “North (mm)”, and “East (mm)”. Notice that there is a data point for every day of the year starting on October 22, 1997 and going up to the present. Once these data are selected, click on “Charts” and choose “Scatter”. This will plot the date on the horizontal axis and the distance the monument has traveled in the north-south direction and the distance the monument has traveled in the east-west direction over the almost 20 years of data collection. You will end up with two data sets plotted on one axis.

For each data set, you want to have Excel draw a best-fit line to the data. Put your cursor over the north-south data (this should be on the top half of your plot) and click. This should highlight this particular data set. Now go to “Charts” on the menu bar and select “Add Trendline” and choose a “Linear” fit and then under “Options” choose “Display equation on chart”. Now you have the equation of a line in the form y = mx +b, where b is the y-intercept and m is the slope of the line. Do the same thing for the east-west data.

Paste your plot below with the best-fit lines and their equations.

12. Write the equation for the line for the way the ELKO GPS monument moves in the north-south direction over time:

Equation:

13. What is the slope of this line AND what are the units of this slope? [Note: slope is “rise over run”, so look at the units for rise on your plot (i.e., the y-axis) and the units for run on your plot (i.e., the x-axis) in order to determine the units of the slope.

Slope =

14. Write the equation for the line for the way the ELKO GPS monument moves in the east-west direction over time:

Equation:

15. What is the slope of this line AND what are the units of this slope? [Note: slope is “rise over run”, so look at the units for rise on your plot (i.e., the y-axis) and the units for run on your plot (i.e., the x-axis) in order to determine the units of the slope.

Slope =

What are we trying to do here? Validate the rate of motion and direction of motion that is listed on the ELKO monument summary plot. So, we need to get the overall rate of motion and its direction by adding two vectors. In the picture below, the blue vector is the east-west motion which happens to be negative in our case. However, once we draw this vector pointing to the left, the negative is taken care of by the direction of the arrow, so you don’t need to continue carrying the negative sign. This indicates the monument is moving in the westward direction. The red vector is the north-south motion for our monument. Do we have values for the

magnitude of the blue vector and the red vector? Yes!!!! Think about this from the previous questions.

16. Red vector = ____ ____________ [Remember units!!!]

17. Blue vector = _______ __________ [Remember units!!!]

Now you can calculate the green vector which is the sum of the blue vector and the red vector. You will need to calculate the magnitude of the green vector AND its direction. Remember that the direction you are calculating is the theta (θ) in the diagram. This is not the direction relative to north. Use the degrees marked on Figure 3 to adjust θ to get a direction relative to north.

Note: For a tutorial on computing a vector from its x- and y-components, please take a look at the following link:http://www.phys.ttu.edu/~batcam/Courses/semester%201/Readings/UNIT%2001%20READING%20B%20ONE-DIMENSIONAL%20MOTION%20GRAPHING%20AND%20MATHEMA.htm

18. Green vector magnitude =_____ ______________ [Remember units!!!]Show your work:

Green=√Red2+Blue2 (Pythagorean Theorem)

19. Green vector direction = ____________________ [Remember to set your calculator to degrees when you calculate direction.]

Show your work:

θ=tan−1( BlueRed ) (Basic Trigonometry)

Figure 3: Schematic of vector addition and angles

You will notice that this result does not match the values given for the rate of motion of the ELKO GPS monument. What is wrong? You have different units. Geologic motion

South (180°)

East (90°)

West (270°)

North (0°)

Θ

needs to be measured over a longer timeframe than days (which is what is contained in this dataset). You need to convert your green vector magnitude to distance per year (mm/yr).

20. Green vector magnitude = __________________ mm/yrShow your work:

21. Compare your result to the value of horizontal speed and direction of motion that you recorded in the table for ELKO. How close are the results?

Station Name Horizontal Speed

(mm/yr)

Direction of Motion

(Degrees)

Approximate Direction of Motion (East, West, North, South)

ELKO (from UNAVCO)ELKO (calculated)

22. Do you feel you have validated the values presented on the UNAVCO website? Why or why not? If your calculation is not exactly the same as UNAVCOs, can you stipulate some reasons why this discrepancy might exist?

Part II. Building a Theory. At this point, you should have convinced yourself that the Wasatch mountains are moving away from the Sierra Nevada mountains. Given that the Earth’s crust is rigid, what can happen to the crust as it is forced apart? As mentioned at the beginning of this activity, mountains tend to be created by pushes, pulls or lateral motion.

23. What crustal motion is creating the mountain ranges in the Basin and Range Province?

24. How can you make sense of pulling something apart and having it create a mountain range? You may want to explore this in your textbook or online. Try to convince yourself how a mountain range can be the result of pulling apart.

25. What type of geological evidence would you expect to see to validate this mountain growth?

Next we will look at earthquakes in the Wasatch to further understand the creation of the Basin and Range Province and the geological risks due to this type of mountain range.