Problem Set #4: Folds and Folding Stereonet Analysis Due Tuesday, Nov. 22

Transcription

1 Geol 360 PS #4 Name Problem Set #4: Folds and Folding Stereonet Analysis Due Tuesday, Nov. 22 Supplies: a. Data: i. Compilation of bedding attitudes at Mt Baldy; BaldyBedding.txt ii. Compilation of bedding attitudes at French Cabin Creek; FCC_beddingplanes.txt iii. Compilation of fold axis attitudes at French Cabin Creek; FCC_Fold_Axis.txt iv. Compilation of fold plane attitudes at French Cabin Creek; FCC_foldplanes.txt v. Compilation of fold plane attitudes at French Cabin Creek; FCC_Foliations.txt b. OSX Stereonet program on computers in back of class (or you can download to your computer: for windows for macs) Part I: Attitude of Mt. Baldy Anticline: Early in the quarter, the class went to Mt. Baldy and recorded a number of strikes and dips of lava-top planes along the northern limb of an anticline. By plotting the attitudes of the lava-tops of the northern limb, as well as the attitudes of lava tops on the other limb of the fold (collected by other people, and included in BaldyBedding.txt), you will be able to determine the attitude of the fold. Since you will be plotting many planar attitudes, you will be plotting poles-to planes. Note that these poles to planes all lie on a plane, and will plot as a great circle on your stereonet. The pole to this plane is the trend and plunge of the fold axis.

2 To better understand this, the photo shows a mine adit conveniently following the core of a fold (I imagine that they were probably mining a folded coal layer). The support timbers are approximately poles to bedding, and like the poles you will plot should define a common plane, or great circle on the stereonet. Note also that the pole to this plane parallels the fold axis of the fold (as does the main timber in the photo). Picture half a wagon wheel: the spokes are poles to the rim and the hub is the pole to the plane of the wheel. The wheel is a fold and the hub is its axis. 1) Finding the fold axis of Mt. Baldy Anticline: plotting data a) Using the stereonet program, Plot all the bedding attitudes as POLES TO PLANES. Fit a great circle to your data. What is the attitude of this great circle? b) Plot the pole to this great circle and mark it prominently with a "+". What is the attitude of this pole? c) What is the orientation of the fold axis? d) Look back in your field book- Did you record our field guess for the trend of the fold? What was it? How close were we? 2) Finding the best estimate of attitude of limbs of the fold: Contouring Data Our data should be concentrated in two regions of the π circle. These represent the steep limb and gentle limb of the fold. We want to reduce these to two representative points. The best way to do this is to contour the concentrations of data points to find the region of highest data density. This can by computer. a. What are the resulting poles to limbs (i.e. the center of each central region)? b. Plot the planes (great circles) that they are the poles to. What is the strike and dip of each limb? c. Look back in your field book, did you record your estimate for the attitude of the northern limb? What was it? How close were you? 3) Finding the best estimate of attitude of the axial plane. a) Now fit a great circle to the two fold-limb-poles, and count the angle between

3 them. What is this interlimb angle? b) Mark the point half way between the poles along the great circle connecting them (this is the fold bisector). Fit a great circle to fold bisector and the fold axis. This plane approximates the axial surface for folds that are not too asymmetric. What is the strike and dip of the axial surface of the fold? c) Using your field map and analysis, make an accurate geologic cross section of the Baldy structure. Do not make a topographic profile, just draw the structural cross-section to scale. Keep the horizontal and vertical scale the same. d) Lastly, describe the type of fold you mapped (your description should use almost all of the words covered in class) Part II: Attitude of Folds in the Cascades: Just a few weeks ago, the class went the Cascades, near French Cabin Creek, and recorded a number of strikes and dips of folded meta-sedimentary rocks, as well as attitudes of fold axis and axial surfaces. The present day structure of the Washington Cascades is dominated by a broad, deeply eroded north-south anticlinorium. This young structure upwarps a system of older, tighter, northwest-southeast folds and faults (part of the system that formed the Mt Baldy anticline). In this part of the problem you will determine the present-day attitude of folds in the French Cabin Creek region of the Cascades. By comparing the attitude of these multiply deformed folds to the attitude of the Mt Baldy fold, we will be able to talk about the younger event. 1) Finding the fold axis of folds in the French Cabin Creek region: plotting data a) Using the stereonet program, Plot all the bedding attitudes as POLES TO PLANES. Fit a great circle to your data. What is the attitude of this great circle? c) Plot the pole to this great circle and mark it prominently with a "+". What is the attitude of this pole?

4 d) What is the estimated orientation of the fold axis? e) Using the stereonet program, plot the attitudes of the fold axis that were taken in the field. Contour these. What is the attitude of the best-fit field-fold axis? How close are the field attitudes and the calculated attitude for the fold axis? 2) Finding the best estimate of attitude of the fold limbs and the axial plane. We never really got to see the other limb of the macroscopic fold, so we can t just look for high density attitudes on the stereonet to find the two limbs. However, by looking at the poles-to-bedding, you may get a hint of two limbs. As a budding geologist, you re willing to test the idea that you can, in fact, use the poles to bedding to estimate the axial plane. a) Given that we were able to take only a couple strikes and dips of northern limbs (of mesoscopic folds)- what do you think are the attitudes of the two poles to limbs? b) What are the attitudes of the two limbs? c) Now fit a great circle to the two fold-limb-poles, and count the angle between them. What is this interlimb angle? d) Mark the point half way between the limb-poles along the great circle connecting them (this is the fold bisector). Fit a great circle to fold bisector and the fold axis. This plane could approximate the axial surface of the fold. What is the strike and dip of the axial surface of the fold?

5 3) Another way to get a handle on the axial plane. a) Plot the pole to the axial surface that you determined is step 2 Now plot the poles to fold planes that were measured in the field, and the poles to bedding. Notice that all of these poles fall near a great circle- thus they all have about the same strike, but just have different dips. Draw a great circle through the cluster of poles. Now, determine the pole to the circle. The trend of the pole is the approximate strike of all these planes. What is the average strike of the measured fold planes? What is the range of dips of the fold planes? b) Now, look at the attitude of the axial plane you determined in part 2 above. Do the two attitudes match? Why or why not? If not, rethink step c-d above, and recognize that there are two interlimb angles. Now, redo steps c-d, using the other interlimb angle. What is the new calculated attitude of the axial plane? c) Looking at your plot of poles to bedding, and poles to the fold planes, again see how they all fall near a great circle, they are all just planes with different dips. Now, try to visualize parasitic folds on a larger fold, imagine how the axial planes of these parasitic folds could have the same strike, but different dips. Try and draw a picture of this. b) Now make another plot with all of the above poles, and add the poles to foliation. Knowing the relationship between foliation of fold planes, make another sketch showing the attitudes of parasitic folds, fold axis, and foliation. Part III: Describing the most recent folding in the Cascades The attitude of the Mt Baldy fold is somewhat representative of folds formed in the Neogene. This period of folding affected rocks throughout the region, including rocks in the Cascades. Subsequently, another folding event has affected the rocks in the Cascades. By

6 recognizing the pole of rotation between the two folds, we can describe the second stage of folding a) plot the pole to the Mt Baldy axial plane, and the range of poles to the axial planes in the French Cabin Creek area. b) What is the rotation axis that best rotates the older fold attitude into the younger fold attitude? c) How much rotation occurred? Was it clockwise, or counterclockwise? (looking down the rotation axis) d) Given the young age of this second folding event (just a few million years ago), what regional tectonics could be responsible for the folding?