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Lesson 12: Coast Ranges

Blueschist block in Coast Range melange (photo by M Miller)Returning to the geologic or topographic "layout" of California we discussed last week, this week we'll explore the westernmost of the state's large northwest-trending provinces—the Coast Ranges. Like the Great Valley and the Sierra Nevada, the alignment of this province developed during the Mesozoic Era when California's geology was dominated by a subduction zone that ran the length of the state. As you can see in Figure 12-13 (p. 292), rocks to the west of the Great Valley were part of the Franciscan complex—an accretionary wedge formed by sediments and pieces of oceanic lithosphere that were scraped off the sinking Farallon plate and piled up against the western margin of North America. Rocks in the wedge were deformed and mixed by shearing, and slivers of ocean lithosphere that had been dragged deep into the Earth as part of the sinking plate were thrust back up along faults within the wedge as blocks of high-pressure metamorphic rocks (blueschist block in mélange, right). About 28 Ma subduction of part of the Farallon plate brought the North American and Pacific plates into direct contact and formed the San Andreas fault. This transform boundary, which has grown longer since that time, shuffled rock bodies in the Coast Ranges and changed where the Great Valley's rivers drained into the sea. Step-overs (Chapter 1) between faults within the San Andreas system have also produced local areas of extension where magmas rise into the crust and sustain both geothermal (The Geysers) and volcanic activity (Clear Lake). Although much of its geologic framework was formed during Mesozoic time, California's Coast Range province is a very dynamic region even today.

In chapter 12 Harden describes the Mesozoic "basement" of the Coast Ranges—the Franciscan mélange and granitic rocks of the Salinian Block—and then outlines the episodes of marine sedimentation, volcanism, and alternating alluvial and estuarine deposition that have shaped this complex region during Cenozoic time. In the middle part of chapter 19 (p. 504-512) the author outlines the conditions that promote slope failures and describes the different types of mass movements ("landslides") that occur in California. Finally, because the mélange that comprises much of the Coast Ranges is a very weak material, landslides are a constant threat throughout the province. This this week's exercise introduces you to the factors that geologists consider when they are asked to decide whether a site is stable enough for construction or not.

As you read about California's Coast Ranges be sure to take careful notes on the topics covered by the learning objectives below. As during previous weeks, you'll find that writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Don't hesitate to post any questions you have to the Discussion Board so that your classmates or I can help you figure them out. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. 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:

Reading and Browsing Assignment

Exercise 12: Landslide Hazard Assessment (Due by 9:00 AM on 11-Apr-2011)

Because the rocks of accretionary wedges like the Franciscan complex are commonly sheared by faulting as they are thrust together above a subduction zone, landslides are a constant threat in these regions. To learn a little about how geologists assess landslide hazards, load up your Hazard City CD and start version 1 of the Landslide Hazard Assessment. Work through the exercise carefully, and note that you are really answering two questions about each building site: is it stable or unstable with respect to landslides; and, independently, is it suitable for construction? You can answer the first question by referring to the table on the Introduction page, but you will need to review the soil data to answer the second. When you are all done evaluating the five properties print the worksheet on the "Report" page, fill it out, and answer the questions on Exercise 12 in the "Assignments, Tests, and Surveys" tool. Let me know if you have any questions as you work through this exercise.

Quiz 12: Coast Ranges (Due by 9:00 AM on 11-Apr-2011.)

After you feel you have met the learning outcomes outlined above please complete Quiz 12 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the geology of the Coast Ranges and each is worth one point. If you can answer all of them correctly it means that you know your way around the Coast Ranges pretty well and are ready to start learning about earthquakes and seismic activity in California next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.