Structural Analysis of the Basin and Range / Colorado Plateau Transition
Julie B. Willis, BYUI Department of Geology
Introduction:
Southwest Utah straddles the transition from the Colorado Plateau to the Basin and Range physiographic province. This transition also coincides with the leading edge of the Late Cretaceous to early Tertiary Sevier orogeny (Biek, et al 2009). The exposures of rock in the region are excellent and provide an excellent opportunity to study overprinted compressional and extensional structures within an important geologic transition.
Outcomes:
Increase your ability to
- describe the occurrence and geometric characteristics of structures.
- gain experience reading geologic maps.
- collect, analyze, and display quantitative structural data.
- gather regional kinematic and dynamic information from structures.
- Summarize findings in a scientific analysis
- work with others to organize data, analyses, figures, and conclusions into a scientific report.
Assignment:
As a class we will investigate some of the structural differences between the eastern edge of the Basin and Range(BR) province and the western edge of the Colorado Plateau (CP)in southern Utah. Some of the general questions we want to answer include:
-What are the types and orientations of geologic structures across the transition? Do those in the CP province differ from those in the BR province?
-Which structures formed during the Sevier orogeny (~100 to ~45 mya) and which are due to Basin and Range tectonics (~17 mya to present). Did any structures form at other times?Which structures are possibly active?
-What are the directions of regional shortening and extension and have those directions changed over time? Howhave the orientations of the regional principal stresses changed over time?
You will individually analyze two of the structure types. You will collect and summarize data for your assigned structures and write a report with figures, captions, tables and appropriate references. Your work will be peer reviewed by others in the class.Then you will join a team that willprepare a final report in which you will include yours and others previously written analyses of each structure type, analyze consistencies between structure types, and address the general questions and draw conclusions.
Data:
-Geologic Map of St. George 30x60 quadrangle (paper and kml)
-BasinRange_ColoradoPlateau.kml: contains a polygon of the study area and placemarkersof important locations. (The study area iseast of Ivins, UT (long -113.6797°W)south of Toquerville, UT (lat 37.2535°N), and north of Blommington, UT (lat 37.0467°N)).
Part 1: Analyze the primary active fault.(Everyone will complete this analysis and report.)
In the study area, the Hurricane fault (which is active) marks the eastern edge of the Basin and Range province and the start of the Colorado Plateau. Use the map to learn a little more about this fault and write a discussion. Turn in your written paper for grading.You may want to discuss:
-thetype of fault and its geometry -- average trend, dip direction, and dip;the surface expression of the fault (simple strand or complex fault zone); can you pinpoint en echelon or anastomosing faults? Is the Colorado Plateau on the foot wall or hanging wall?Estimate the vertical offset and explain how you figured it out.
-the “relay ramp” placemark in the ‘BasinRange_ColoradoPlateau folder in Google Earth’
-your certainty of the fault being present in areas where it is dotted
-the tectonic setting and associated tectonic event; the probable orientation of the principal stresses (indicate which stress is vertical and the azimuthal orientation of the horizontal stresses)
General Deformation: describe the level of deformation on each side of the Hurricane fault; use strike and dip of bedding to support your description
Format for all papers: Calibri 11 pt. font, right-justified, single-spaced, 7.5” lateral spacing. Primary level- headings are bold and centered. Figures must be numbered in the order they appear and have informative captions.Tables are numbered separately from figures and have a title not a caption.
Part 2:Analyze one type of structure to contribute to the final report. Complete only the analysis that you are assigned. You will peer-review papers on each of the other 3 analyses.
- Joints/fracturesAnalysis: The Navajo Sandstone is athick, strongly cross-bedded sandstone deposited during the Jurassic. Joints in the Navajo have evolved into many of the famous slot canyons in Zion National Park. Compare the frequency and style of joints in the Navajo Sandstone exposed in the NW corner of the study area (‘Navajo SsBR’ Google Earth placemark) and in the SE corner of the study area (‘Navajo SsCP’ placemark). The joints in these areas are clearly visible in Google Earth (with the St. George geologic map overlay “off”); in some areas they are covered by dunes. Be careful with zoom or you might start counting cross-beds as joints. Prepare figures (rose diagrams and/or stereo nets). Write your analysis and turn it and your figures in for peer review.Here are some questions you may want to consider in your analysis:
-Are there differences in style and frequency of jointing in the Basin and Range versus the Colorado Plateau? (For example, determine for each: Do the joints have a preferred trend? Is there a pattern to the occurrence (location and spacing)? Is the spacing regular? Do joints occur evenly or are they in clusters? Is there a common length? Is there more than one direction of jointing? What are the relative ages of the joints? Is there any relative motion of the two sides of a typical joint?)
-Are the joints related to compression or extensional stress?What are the likely principal stress orientations at the time the fractures formed?
-How might stratigraphic and/or compositional differences affect the joint patterns? How might past tectonic events affect the joint patterns, frequency and spacing?
- Other faults (Thrust fault Analysis and Normal fault Analysis):Discuss the faulting on both the east (Colorado Plateau) and west side (Basin and Range) of the Hurricane faultin the study area. Organize your data in a table and prepare diagrams(rose diagram, stereonet). Write your analysis and turn it and your figures in for peer review.Here are some questions you may want to consider in your analysis:
-Describe the geometry of the faults – trend, direction of fault plane dip, dip of the fault plane if it can be determined, etc.
-Describe fault kinematics -- what was the direction of regional compression or extension? Is the direction consistent for all faults of a given type?
-Describe the tectonic setting at the time of formation andconnect it with a tectonic event; include the likely orientation of the principal stress axes; Evaluate whether the principal stress orientation is consistent for all normal faults? Is it consistent for all reverse faults?
-Determine which types of faults are likely still active
- FoldsAnalysis:Discuss the folding on both the east (Colorado Plateau) and west side (Basin and Range) of the Hurricane fault in the study area. Organize your data in a table and prepare diagrams (rose diagram, stereonet). Write your analysis and turn it and your figures in for peer review.Here are some questions you may want to consider in your analysis:
-Describe the location of folds relative to the B&R and the CP.
-Are the folds related to any faults?
-Describe the geometry of the folds and the orientation of the fold axes; what was the direction of compression or extension? What was the likely orientation of the principal stress axes at the time of formation?
-Are the Virgin Anticline and Kanara Anticline the same fold cut by the Hurricane fault or are they structurally two different folds?(You will need to extend your analysis outside of the study area to include the Kanara anticline.) Use an on-line stereonet program to create a pi diagram of these two folds and interpret the diagram.
Part3: Write a final paper. Divide into teams of 4 with one person from each analysis type. As a team writeapaper that includes all of the analyses with any corrections mandated by your peer reviewers.Make the paper scientific: write an introduction, analysis, conclusion, and abstract.In your analysis, compare information gathered from all of the assignments and answer the general questions (see page 1 of the assignment). Look for and discuss consistencies and inconsistencies across data types. Explain what you see. Figures should have a caption and a number and be referred to in the text by number. Figure captions should be are should be informative and descriptive. All figures and their captions can be placed at the end of the report. Submit both a digital and a paper copy of the report. Attach all earlier analyses with the peer-reviewed comments to your paper copy.
Grading for each analysis and for the final paper is based on how well they meet the following criteria:
- Is the paper written at a level that is appropriate for an upper division college-level science course?
- Are figures clear, numbered in the order they appear in the text, and have well written captions?
- Is the discussion well-organized and complete? Are the questions addressed and are data or figures provided to support the answers?