Lesson 9: Klamath Mountains
The Klamath Mountains, which stand just west of us here in Weed, are one of California's most complex geologic provinces. This rugged landscape of ridges and deep valleys (view across the Trinity Alps, right) has been etched into a patchwork of far-traveled terranes by the Klamath and Trinity Rivers. The terranes that comprise the Klamaths are distinct blocks of lithosphere—mostly offshore volcanic arcs—that have been carried to the edge of North America by subduction and then "accreted" during a series of collisions that spanned a period of about 250 million years. This week we'll review the geologic history of the Klamath Mountains, look at what fossils tell us about where some of its terranes have come from, and consider the processes that have produced the rich deposits of chromite and gold that have made the range a source of mineral wealth.
In chapter 9 Harden briefly describes the origins and accretionary histories of the terranes that comprise the Klamath Mountains. She goes on to explain what ophiolites are and what they tell us about the structure of the oceanic lithosphere, and concludes with a brief discussion of how deposits of chromite were formed by magmatic processes in the mantle parts of Klamath ophiolites. Finally, this week's exercise will challenge you to turn the outline of the article you read two weeks ago into a concise, informative abstract.
As you read about the geology and mineral resources of the Klamath Mountains it will be useful to take detailed notes. A lot of information that bears on this week's learning objectives is presented in the chapter, and 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. 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 what an accreted terrane is and outline the history of terrane accretion and subsequent deformation and erosion that have shaped the Klamath Mountains. Do not worry about the ages of specific terranes or accretion events, but know—in general—which tectonic setting most Klamath terranes were formed in, broadly when most were accreted, and how their locations are related to their ages and the ages of the accretion events that added them to the province.
- Use the relative dating principles you learned about earlier in the semester to infer the time of a terrane's accretion given: (1) its age; (2) the age of the terrane it is accreted to; and (3) the age of a pluton that cuts the suture between these two terranes or a bed of sedimentary rock that overlies this suture.
- Explain what the fossil community preserved in the McCloud limestone tells us about where the offshore volcanic islands on which this limestone was deposited once lay. Specifically, were these islands at high, middle, or low latitudes and did they lay close to or far offshore from North America during Permian (latest Paleozoic) time?
- Briefly describe what an ophiolite is and tell which rock types you would look for to determine if you were in the lower, middle, or upper part of an ophiolite.
- Contrast the origins of the chromite and gold deposits found in the Klamath Mountains. (Remember, because the terranes of the Klamath Mountains are very similar to those in the northwestern Sierra Nevada, the origins of local gold deposits are very similar to those in the Mother Lode.)
Reading and Browsing Assignment
- Read Chapter 9, being sure to focus on the topics outlined in the learning objectives above.
- For another perspective on the geology of the Klamath Mountains (especially the eastern part of the range) check out the Geologic Overview of the Eastern Klamath Mountains I wrote as a "text" for the College's short-course on the Klamaths. You'll already be familiar with most of the content, but there are a couple of important topics—including the development of the Trinity detachment fault—that are not included in our text.
- Browse through Wikipedia's brief intoduction to terranes and follow as many of the links as you would like for background. Although this site covers the same topics as our text, it provides a little different perspective on this complex subject.
- If you would like to see a detailed map of the accreted terranes of the Klamath Mountains and the "stitching plutons" that cut across their boundaries and enable us to place minimum ages on when they were accreted, check out this geologic map of the Klamath Mountains from the U.S. Geological Survey. Also, to see how terranes in Klamaths are related to those in the northern Sierra Nevada check out this companion terrane correlation map.
- The Josephine ophiolite is one of the best preserved and exposed sections of oceanic lithosphere in the world. To get a better sense of what it looks like in the field, take the virtual field trip along the Smith River posted by College of the Redwoods. (If you really want to learn more about this ophiolite, check out the Josephine Ophiolite page posted by Mike Strickland at University of Oregon.)
- Finally, to learn a little more about how chromite deposits are formed check out this page on magmatic ore deposits from Earth Science Australia.
Exercise 9: Abstract of First Article (Due by 3:00 PM on 15-Mar-2011)
Once you recieve your outline back it will be time to prepare your first abstract. To get started, go to the Writing Assignment page.
During week 7 you completed the first two steps of this assignment, and this week we'll be completing step three. Working from your outline, write an abstract of your article's major points following the format of the "sample abstract" linked to the writing page. For the purpose of this abstract, act as though you are the author of the article. In this assignment format is very important, and the easiest way to make sure your abstract is formatted correctly is to download the "abstract template" linked to the writing page (it's a Word document). Print one copy for reference and then simply "type over" each part of the digital copy on your computer with the appropriate information relevant to your article (title, author information, citation, abstract body, etc.) That way each part of the abstract will be formatted and positioned correctly on the page. (Note that this template is in Word format. If you are using another word processor check the link on the writing assignment page for abstract formatting specifications such as column widths, margins and so on.) When you are done, save your abstract in a place (and with a name) that you'll remember. This is the file you'll send a copy of to me.
The abstract template is a two column document, so if your text in the left-hand column is too long it will "push" the text in the right-hand column down. If this happens, shorten your abstract and remove any blank lines at the top of the right-hand column so that the first line of of text is flush with the title at the top of the left-hand column. Be sure to include your PIN ( last five digits of your Etudes user id) and the course name and date at the appropriate places in the right-hand column. Also, pick three keywords that someone searching for your abstract might be expected to use in a search engine. Proper names (e.g., Mount Shasta) and accepted phrases (e.g., global warming) are okay, but otherwise try to stick to single words. Be sure to only capitalize keywords that are proper names.
Write your abstract in complete sentences and be sure that it is well-organized and coherent. It's a good idea to check out the abstract grading rubric that is also linked to the Writing Assignment page so that you can see what criteria I will use for evaluation. Complete your abstract and send it to me as an attachment in MS Word format to Exercise 9 in the Etudes "Assignments, Tasks, and Tests" tool. I will score your abstract and return it to you with a copy of the grading rubric in about one week. Your abstract is worth twice as many points (20) as a typical weekly exercise, so take your time and work carefully as you write it.
Quiz 9: Klamath Mountains (Due by 9:00 AM on 14-Mar-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 9 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the geology of the Klamath Mountains and each is worth one point. If you can answer all of them correctly it means that you know your way around the terranes of the Klamath pretty well and are ready to start learning about California's water resources 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.