The landers earthquake shear zone , California

The landers earthquake shear zone , California

“CRUST-BUSTING” FAULTS PROJECT: GEOS 304, Spring 2016

INTRODUCTION:

“Crust-busting” faults, whether active or inactive, are regional-scale faults with trace lengths commonly in the range of hundreds of kilometers. Crust-busting faults emerge directly from plate-tectonic forces, and commonly record histories encompassing tens of millions of years. The characteristics we see today, when examining a crust-busting fault, is the ‘finite’ result of progressive deformation over time. In some cases a crustbusting fault may become dormant, only to become ‘reactivated’ hundreds of millions of years later. Some crust-busting faults may reveal exclusively brittle fabrics, others ductile fabrics, and still others a combination of brittle and ductile. It all depends on what depth-level of faulting (or shearing) is today exposed at the earth’s surface. Crust-busting faults are wonderful devices to integrate facts and knowledge. And this is the intention of the project. We want you to gain experience describing fundamental physical and geometric characteristics; describing the kinematic properties; interpreting the mechanics of faulting/shearing; and interpreting the faulting in relation to plate tectonics. ASSIGNMENT: First, you will choose a crust-busting fault from a list provided. If you wish to study one that is not on the list, you will need to gain approval from me in advance. I want to be certain that the fault truly qualifies, and that it is connected with a helpful literature of good articles and lots of information. Second, you will prepare a 7-page report. As in Project X, you will mount the pages in a Poster format. The contents are as follows: STRUCTURE-TECTONIC MAP (page 1): Based on maps that you discover in the literature, you will draw a geologic map that emphasizes structural geology. Don’t just Xerox a map/figure and cut-and-paste. Instead, draw it by hand, and color it by hand, so that the nature of the crust-busting fault becomes apparent. Somewhere near the base of this page you need to show a small index map of location, north arrow, and scale. You also will show the literature reference (source) for the map (e.g., Coney, 1998), and then post the full reference on p. 6, References Cited. Your map must be accompanied by an explanation of rock formations. You will ‘lump’ formations together into specific thick assemblages, e.g., Precambrian basement, Paleozoic sedimentary formations, Mesozoic sedimentary formations, Cenozoic sediments and volcanics, etc. The explanation will also explain structural symbols: folds, faults, bedding or layering, etc. STRUCTURE SECTION (page 2): You will hand draw and color a geologic cross section (structure section) showing the fault relationships. Be sure to note the orientation and scale of the cross section. This will not be a cut-and-paste of a figure from the literature, but your own rendering, typically simplifying the published section(s) in ways that underscore the key relationships. 2 FAULT DESCRIPTION (page 3): Through bullet-points, and in an organized fashion, you will describe the key geometric attributes of your crust-busting fault: shape, length, breadth, orientation, map pattern, associated structures (e.g., folds and faults), nature of fault rocks, rocks assemblages affected, truncations and offsets, etc. Be sure to cite sources for your information. KINEMATICS (page 4): Through bullet points, and in an organized fashion, you will describe the type of fault (normal, thrust, strike-slip, oblique), magnitude of slip, direction of slip, sense of slip, nature of strain, sense-of-slip indicators, rotations (if any), etc. Place the kinematics in a time frame: i.e., what happened when? In your own hand, draw diagrams/pictures that show in cross-section or map view the progressive kinematic development. For active faults include focal mechanisms and GPS data. Be sure to cite sources of your information. DYNAMICS AND MECHANICS (page 5): Through bullet points, and in an organized fashion, describe the depth/temperature/pressure/rate conditions under which your crust-busting fault evolved. Also describe the orientations of the principal stress directions that can explain what is seen, including any possible variations in principal stress directions over time. Be sure to cite sources of your information. TECTONICS (page 6): In your own hand, draw a map (and if useful a cross-section) showing the plate conditions and configurations that gave rise to your crust-busting fault. Then, through bullet points, describe the tectonic origin, evolution, and significance of your fault. Be sure to cite sources of your information. REFERENCES (page 7): Use the journal Geology as guide to citing references. Your document should include 3 or 4 references. Do not take all of your information and maps/sections from just one resource. We don’t want to see Wikopedia or other sources for which it is difficult to evaluate validity. Reference articles and books in the way I reference material in the textbook, at end. THE POSTER SESSION ITSELF This will be held in the Lobby of the Gould-Simpson Building from 10:30 to 12:30 on May 10th. As I did for Poster X, I will invite faculty and graduate students to come and view your work, and to engage in conversations to learn what you found and concluded. There will be sections of wall devoted to a particular ‘class’ of faults: strike-slip, thrust & reverse, and normal. We will all ‘mill around’ and look at as many of the products as possible. Then we will have a brief discussion re/ the diagnostic characteristics of strikeslip versus normal faults versus thrust faults versus reverse faults.

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