Lesson 16: Transverse Ranges and Continental Borderland
California's Transverse Ranges trend roughly east-west and cut across the northwestern alignments of other provinces—such as the Sierra Nevada and Great Valley—that formed in response to Mesozoic subduction. Based on the principle of cross-cutting relationships you would infer that the Transverse Ranges are younger than those Mesozoic provinces, and that's exactly the case. The Transverse Ranges and Continental Borderland developed during late Cenozoic time as movement on the San Andreas fault squeezed and rotated crustal blocks along the western margin of North America. These fault movements have uplifted several large mountain ranges, including those whose summits stick above sea level to form the Channel Islands (Anacapa Island, right) and others whose slopes expose ancient metamorphic and igneous rocks. Movements along the San Andreas have also created deep basins where thousands of meters of sediments have accumulated and entombed tiny marine organisms whose remains produced rich deposits of petroleum and natural gas. The Transverse Ranges province is still very active tectonically, and even though the adjacent San Andreas fault has not produced a large earthquake since 1857 related faults beneath the Los Angeles Basin have taken heavy tolls in lives and property during the 20th century. We'll only have a chance to touch on the diverse geology of this dynamic region this week, but the links below offer additional information if you would like to learn more.
In chapter 16 Harden describes the geologic structures that accomodate compression in the Transverse Ranges, reviews the record of uplift and deposition recorded by the sediments that fill the region's basins, and discusses how block rotations have led to local crustal extension and bimodal volcanism. She also describes the variety of pre-Cenozoic rocks—from the Pelona schist to the Proterozoic crystalline rocks of the San Gabriel and San Bernardino Mountains—that form the "basement" of the province and have been exposed by Cenozoic uplift and erosion. Finally, she explains how petroleum has formed and migrated in to traps within the region, and outlines some of the environmental problems that petroleum extracton has caused. For this week's exercise you'll be writing the abstract of your second article by pulling together what you learned from both your first abstract and your second outline.
As you read about the geology and geologic history of the Transverse 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:
- Identify the common structures in Transverse Ranges and adjacent Borderlands that indicate local crustal compression (e.g. folds, uplift, thrust/reverse faults) versus local extension (e.g. bimodal volcanism, subsidence, normal/detachment faults). Briefly explain how both types of stress have arisen from the shearing and block rotation related to the San Andreas fault.
- Infer the relative rates of subsidence versus deposition in a sedimentary basin from a knowledge of whether younger rocks are deposited in progressively shallower or deeper water settings.
- Infer the environment of deposition in a marine basin—deep versus shallow and small versus large input of land-derived sediment—in which a diatomite such as the Monterey Formation is accumulating.
- Briefly describe how the Cretaceous basement rocks of the northern Transverse Ranges (e.g. Santa Ynez Mountains) differ from those of the southern Transverse Ranges (e.g. Los Angeles Basin) and indicate which part of the original Mesozoic margin of North America each corresponds to (i.e., accretionary wedge = Franciscan Formation; forearc basin = Great Valley sequence; and volcanic arc = Sierra Nevada batholith).
- Cite two independent lines of evidence that suggest the crustal "blocks" of the Transverse Ranges and Borderlands have been detached from the western margin of North America and rotated clockwise more than 90° due to shearing along the San Andreas fault.
- Briefly contrast the typical basement rocks of the San Bernardino and San Gabriel Mountains in terms of their ages and the geologic settings in which they were formed. (Note that there are two types in the San Gabriels: one above and one below the Vincent thrust fault. In the San Bernardinos only focus on the most widespread type.)
- Outline the four conditions necessary for petroleum to form and be concentrated into an exploitable deposit, and use this knowledge to predict the likely locations for petroleum "traps" in a geologic cross-section that shows the subsurface structure of source and reservoir rocks.
Reading and Browsing Assignment
- Read Chapter 16, focusing on the topics outlined in the learning objectives above.
- First, to get a better sense of how the southwestern margin of California has been fragmented and rotated by shear along the San Andreas, watch this animation of the past 20 million years of southern California's tectonic history. Notice how blocks of crust from near San Diego (the Peninsular Ranges province) have been pulled away from the mainland and rotated clockwise to form the Transverse Ranges province and its submerged counterpart, the Borderlands. This animation, like those below, was created by Tanya Atwater and her students at U.C. Santa Barbara.
- To see how the lithosphere behind one of these rotated blocks is stretched and thinned by extension, watch this animation of Miocene extension of the Santa Monica Mountains. Notice that as the crust is thinned by faulting the hot asthenosphere below (green) flows towards the surface, partially melts due to decompression, and produces the red lavas of the Conejo Volcanics.
- Next, to see how the rocks deposited in an extensional basin are folded and thrust faulted by later regional compression, check out this animation of Pliocene-Pleistocene deformation in the Santa Clara area. Notice that the folded and faulted rocks never got as high as shown in the first part of the animation because erosion was operating at the same time as deformation.
- Finally, to get a sense for how petroleum can migrate upward from reservoir to source rocks in the deformed sedimentary basin like the Santa Barbara basin, watch this animation of petroleum migration.
Exercise 16: Abstract of Second Article (Due by 9:00 AM on 9-May-2011)
Once you recieve your outline back it will be time to prepare your second abstract. To get started, go to the Writing Assignment page.
During week 14 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 16 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 16: Transverse Ranges (Due by 9:00 AM on 9-May-2011.)
After you feel you have met the learning outcomes outlined above please complete Quiz 16 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the California's coast and the processes that are shaping it, and each is worth one point. If you can answer all of them correctly it means that you know your way around the geology of the Transverse Ranges-Continental Borderland province pretty well and are ready to start learning about the Penninsular Ranges 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.