Lesson 12: Coast Ranges
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:
- Briefly describe how the rocks of the Salinian Block differ from those in the Franciscan complex, and explain how these differences are related to the original tectonic settings in which each of these parts of the Coast Range province were formed during Mesozoic time.
- Recognize the major rock types that comprise the Franciscan Complex (greywacke-shale sequences; pillow basalt (greenstone); chert; limestone; blueschist; eclogite; and serpentinite) from a description or annotated photograph, and explain what each is telling us about the environment in which it formed.
- Explain how graded bedding forms in a turbidite sequence, and use such bedding to determine if a sequence of turbidity current deposits is right side up or upside down.
- Describe what a morro is and explain how these features, which are found in the Coast Ranges near San Luis Obispo, originated.
- Estimate the sense of offset and average rate of motion along the San Andreas Fault based on a correlation of volcanic or sedimentary units of known age across the fault.
- Interpret the subsurface sedimentary record of the San Francisco Bay in terms of the fluctuations of sea level that occurred in response to the Pleistocene glacial and interglacial episodes.
- Briefly explain how changes in the following factors are likely to affect if a slope will fail in a landslide: steepness of a slope, strength of the rock or soil, addition of water, and intensity of earthquake shaking.
- Determine if a given mass movement is best described as a/an: rockfall, slump (rotational slide), rockslide (translational slide), earthflow, or debris flow based on the descriptions given in chapter 19—especially Figure 19-7 (p. 508).
Reading and Browsing Assignment
- Read Chapter 12 and the part of chapter 19 (p. 504-512) about landslides, and focus on the topics outlined in the learning objectives above.
- To get a sense of what a turbidity current looks like in action, check out this video of a small turbidity current created in a laboratory tank at Middleton College.
- To see what some typical Coast Range rocks look like in the field, browse through the Del Puerto Canyon virtual field trip from CSU Stanislaus, and view this video of a fellow describing an outcrop of deformed chert layers from the Franciscan Formation. (Note that his explanation of how the layers were deformed is not correct!)
- To get a much clearer picture of how the San Andreas Fault developed as the Pacific plate began to grind along the western margin of North America than can be conveyed by still pictures (e.g., Figure 12-2, p. 282) watch this animation of the past 38 million years of California's tectonic history put together by Tanya Atwater and her students at U.C. Santa Barbara.
- Finally, to find out a bit more about the volcanic morros that dot the central Coast Ranges check out this page from Cuesta College.
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.