## 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.