Lesson 12: San Andreas Fault and Other Transform Boundaries
This week's lesson is our last to focus on parklands that have been shaped by geologic activity along a plate boundary. In this case, we'll consider transform (shear) boundaries where one plate is sliding laterally past another. One of the best known transform boundaries on a continent is California's San Andreas Fault, and we'll explore how offset on this structure is reflected in landscapes that stretch along almost the entire length of the state. Also, because the fault runs through or near several major metropolitan areas, we'll look at how geologists assess seismic hazards in our weekly exercise.
As you read through this chapter and the supporting websites please take careful notes so that you can keep track of major points and recall them more easily when we refer to them later in the semester. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Infer the correct sense of motion (left- or right-lateral) on a transform fault from observations of the offsets of displaced features (or, displacement arrows if they're given).
- Calculate the average rate of plate motion along a transform boundary given the age of the an offset feature and its present displacement. (Hint: v = d/t, so to obtain the average velocity divide the distance of offset by the time since the displaced feature formed.)
- Explain how the following features are related to the subduction of part of the Farallon Ridge that began about 25-27 Ma (millions of years ago) and the subsequent development of a shear boundary between the Pacific and North American plates: San Andreas fault; Mendocino and Rivera triple junctions; Juan de Fuca and Cocos plates; Transverse Ranges (including the Channel Islands); and the Salinian block.
- Predict how a bend in a transform fault will produce either a region of local extension (a "releasing" bend) or compression (a "restraining" bend) depending on whether the bend "steps over" in the same direction or the opposite direction as the fault is moving.
Reading and Browsing Assignment
- Read Chapter 7, focusing on the topics outlined in the learning objectives above.
- Download and view the animations of California's tectonic history created by Tanya Atwater and her colleagues at U.C. Santa Barbara. In particular, you'll want to see "N.E. Pacific and W. North American Plate History, 38 Ma to Present" and "Plate Tectonic History of Southern California, 20 Ma to Present"; both can be found at the Regional Plate Tectonic and Geologic Histories section of the EMVC download page. These animations bring the still images in our book (Figures 7.5 and 7.16) to life and really help clarify the tectonic history of the San Andreas transform. They are large (multi-Mb) files, however, and will take a long time to download if you are on a dial-up connection. Simply clicking on the download page brings up a "thumbnail" of each animation that you can watch even if you don't have time to download the full-size version.
- To learn more about how transform faults behave at bends and step-overs (places where one fault strand ends and another, a short distance away, picks up the offset) check out this illustration from an article by McClay and Bonora. Notice that the two drawings in part (a) show bends in a right-lateral fault. If the bend is in the same direction as the motion on the fault a region of local extension will develop, whereas if the bend is in the opposite direction as the motion on the fault a region of local compression will occur (notice the heavy black arrows). For example, if you imagine yourself standing on one end of the fault in the top picture and looking down it towards the bend. In this case, the piece of the fault on the other side of the bend lies towards your right. Because this right-lateral fault and has a right bend, the bend will be a region of local crustal extension. Now check out what happens in the second drawing in part (a). Here, a right-lateral fault has a left bend, and the bend becomes a region of local compression. This is analogous to what the San Andreas fault is going through at the "big bend" in California's Transverse Ranges and why Los Angeles is in such danger from thrust (compressional) faulting.
- California is one of the most seismically active places in the world. You can see a near-real time map of the the past week's earthquake activity in the state by visiting the Recent Earthquakes Map. (Click on a quake and see what happens!) Also, check out the California Earthquake Forecast map based on recent aftershock activity.
Exercise 12 (Due by 9:00 AM on Monday, 12-Apr-2010)
To learn a bit about how geologists assess the hazards posed by earthquakes that occur along transform faults and other plate boundaries please load your Hazard City CD and work through version 3 of the Earthquake Hazard Assessment exercise. This is a fairly involved project, so allow at least an hour to complete it. Be sure to jot down any notes on data and procedure as you work through the exercise. Printing the form provided, filling it in, and then adding notes on how you arrived at your answers may be helpful. Be sure to have your final results in hand before you go to the Etudes "Assignments, Tasks and Tests" tool to complete Exercise 12.
Quiz 12 (Due by 9:00 AM on Monday, 12-Apr-2010)
After you feel you have mastered the learning outcomes outlined above, please complete Quiz 12 in the Etudes "Assignments, Tasks and Tests" tool. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around transform faults pretty well and are ready to move on to learn about parklands formed at oceanic hotspots next week.