Lab 7: Plate tectonics, numerical dating and the Pacific Northwest

Transcription

1 Geology 103 Name: Lab 7: Plate tectonics, numerical dating and the Pacific Northwest Goal: In this lab, you will use the structural geology and relative age relationships of the formations you learned about in the previous lab to describe the plate tectonics-based geologic history of the Pacific Northwest. One idea I hope you get out of this is: why are there so many Eocene-age rocks in western Washington and not so much of other ages? 1. Below are three cross-sections of the Skookumchuck Formation near Centralia. To determine the age of the formation, what material would you submit for numerical dating? Circle the layers on any/all of the columns that contain datable materials. Hint: see the explanation. 2. From the explanation on the Snavely, 1958 geologic map of the Centralia area, why would the Northcraft Formation provide radiometrically datable material?

2 3. The accelerator mass spectrometer is an instrument that can separate and measure atoms of various isotopes. For potassium-argon (K/Ar) dating, it can measure atoms of potassium-40 ( 40 K) and atoms of its daughter isotope, argon-40 ( 40 Ar). The age equation for radiometric dating is: age of sample = t!/! P + D ln P where t1/2 is the half-life of the radioisotope used; P is the number of atoms of the parent isotope measured and D is the number of atoms of the daughter isotope measured. The half-life of potassium-40 ( 40 K) is 1,280,000,000 years ( yr). Calculate the ages of the following formations (the numbers are derived from Payne, 1998), based on the information in the table below: Formation name Number of 40 K atoms Number of 40 Ar atoms Age of rock (yr) Skookumchuck Northcraft Are the numerical ages consistent with the stratigraphic order from the field trip?

3 The story of the Centralia-Chehalis area The principal mechanism by which Washington state (and the entire North American west coast) grew in size was by terrane accretion. Below are two possible orientations of terrane accretion (note the direction of the slab movement). 4. Describe an observation a researcher could make to determine which of the two orientations deposited a particular accreted terrane, even long after the accretion has occurred. Hint: it is not an obvious surface observation.

4 5. Look on The Plate Tectonic Map of the Circum-Pacific Basin Region (1985). Trace the pattern of magnetic stripes 26 through 32B off the Aleutian Islands below. Note that the bend significantly. Include a north arrow on your map! 6. If these magnetic stripes are laid down parallel to spreading ridges and if spreading ridges are linear, why do magnetic stripes bend? Hint: Look at the southwestern part of the Cocos Plate for similar current activity. 7. Run the film of earth history backwards. What compass orientation would this new spreading ridge have had? Steve Dutch has an excellent reconstruction of this early Tertiary plate history (

5 8. The Columbia Embayment is a geographic feature of Eocene Washington, which was inferred from the extensive marine formations found of that epoch. The embayment brought the coast far inland, to present-day Walla Walla (see map by John Figge below). What could create such a major feature? Hint: where might the feature alluded to in the previous questions been located?

6 9. Examine the sequence of three maps set during the Eocene (all drawn by John Figge). On one of the drawings, label the Kula Plate and the Farallon Plate. 10. Note that the Kula-Farallon Ridge is offset by a series of transform faults. In the case of the transform fault closest to the coast (shown with a black arrow, we ll call it the Fraser Fault), what type of margin is on the west side of the fault? What type of margin is on the east side of the fault? 11. What is signified by the shadowed area to the north and east of the subduction zone shown? What kind of rocks would you expect to find there, even today? (And we do the Teanaway Formation is one example). 12. Given what type of rock a spreading ridge generates, which of the formations from the field trip are the direct result of the Kula-Farallon Ridge? 13. Given the huge amount of erupted material from the Kula-Farallon Ridge (up to twelve miles thick, according to Rowland Tabor), what would the weight of that much rock have done to the depth of the ocean off the Washington coast?

7 14. How would this affect the thickness of subsequent Eocene epoch sedimentary formations? 15. Given the proximity and composition of the rocks of the source area of the river that deposited some of these sedimentary formations, justify the presence of so much arkose in the Centralia area. In addition why would there be so many tuffaceous layers within these sedimentary formations? 16. Consider the faults that trend northwest-southeast (like the Kopiah Fault), and the faults that trend east-west (like the Salzer Creek Fault). In the previous lab, in question 15b, you stated that all these faults must be reverse faults, due to the compressional nature of the stress in this area. Using the cross-section A-A on the Geologic Sections of the Centralia-Chehalis District, Washington laminated sheets, is the Kopiah Fault a reverse fault? Is the Salzer Creek Fault a reverse fault? 17. In fact, there are quite a few normal faults in this supposed compressional stress area! However, there are a couple of explanations: first, the axial rocks in an anticline can sag down due to its own weight, like the falling keystone in an arch. Sketch a portion of one of the cross-sections that shows this keystoning leading to the production of normal faults. Label the diagram with the name of the structure. Hint: look at B-B.

8 18. Another explanation is the rollover anticline, where the fold literally shears into a lot of aligned blocks that topple such that normal faults develop between the blocks. The Salzer Creek Fault is such an example; sketch a portion of one of the crosssections that shows this point, and label the structure with its name. 19. But why are there two different fault orientations in this area? Referring to your answer to question 16b in the previous lab, which set of faults (the NW-SE or the W-E) is younger? 20. A current hypothesis suggests that the North American lithosphere to the west of the Cascades is broken up into a series of blocks, and that each block is rotating clockwise as time goes on. Show how a clockwise rotation of the lithospheric block that the Centralia area sits on would or would not explain the age relationship and orientations seen in the faults of the Centralia area. 21. What experiment could be run to confirm your answer to the previous question? Your answer should involve paleomagnetism. Hint: Check your answer at