Geology 151 Lab- Paleomagnetism and Crustal Motions
Because of the proximity of an active plate boundary, the western part of North America is a geologically active
and complex region. Many portions of it are thought to have been transported from distant areas and accreted to
the western margin of the continent by plate tectonic processes. These accreted pieces of continental crust are
known as exotic terranes and are geologically different from the continental crust around them. Many other areas
in western North America may have originated at or near their present locations, but have been moved or deformed
by tectonic activity along this plate margin. One of the most useful tools for investigating and quantifying
large scale motions of the Earth's crust is paleomagnetism. By examining the Earth's magnetic field as recorded
in rocks, we can often determine where an exotic terrane originated and/or the type and amount of deformation a
crustal block has undergone. This lab will introduce you to paleomagnetic techniques and allow you to investigate
the tectonic history a series of Miocene-aged, basaltic lava flows.
The Rocks
Six to seven paleomagnetic samples were taken from each of several sites in the Miocene aged Yakima Basalt Subgroup
along the Klickitat River near Goldendale, WA. These lava flows are part of the larger Columbia River Basalt Group
(Figure 1). Samples labeled BOF13 and BOF14 are from the Grande Ronde Basalt which was erupted between 14 and 16.5
million years ago (Hammond, 1980). BOF15 is from the Wampanum Basalts erupted between 12 and 14 million years ago
(Hammond, 1980). (Note different ages given by Reidel et al.)
Paleomagnetic Analysis
For this lab, we will use standard paleomagnetic techniques to determine the orientation of the thermal remanent
magnetization recorded in these flows. This direction will be compared to the direction we would expect for rocks
of these ages to determine if the rocks have been translated longitudinally or rotated about a vertical axis since
their formation.
Groups of 4-5 students will be responsible for preparation and analysis of a single paleomagnetic core sample. I
will prepare and analyze a sample to show you how this is done. We will focus on one site in order to generate
enough magnetic directions to calculate a statistically significant mean (average) direction for the site. The
instructor will provide detailed instructions on the techniques and instruments you will use for the analysis.
Each research group will have their sample analyzed by class next Tuesday. Bring the results of your analysis to
class so you can share data with other groups for use in lab later that day. Each group MUST sign up for a 3-hour
time period in the paleomagnetic lab to perform their analysis. Make sure ALL group members can be in the lab for
the procedure. I want all of you to use the instruments and programs.
You'll Need to Know
Sample Site Location: latitude = 46.1° N longitude = 121.4° W
Expected Magnetic Direction: D = 358° I = 63°
Paleopole (VGP) Location: 88° N, 101° E
Lab Write-up
This lab is due in lab two weeks from today. You will each turn in the following:
- a copy of your demagnetization study results and resulting magnetic direction for your sample
- each group will share it's data with others, and each individual will turn in your calculation of a siteÊmean
direction for the site
- a comparison of your determined mean direction with the expected direction and what it indicates about motions
of these rocks since they were formed
- suggestions as to why tectonic movements (if any are indicated) may have taken place
- a formal ABSTRACT for this study
REFERENCES
Hammond, P.E., 1980, Reconnaissance geologic map and cross sections of southern Washington Cascade Range: Portland,
OR, Portland State University, Dept. of Earth Sciences, 31 p., 2 sheets, scale 1:125,000.
Reidel, S.P., Tolan, T.L., Hooper, P.R., Beeson, M.H., Fecht, K.R., Bentley, R.D., and Anderson, J.L., 1989, The
Grande Ronde Basalt, Columbia River Basalt Group; Stratigraphic descriptions and correlations in Washington, Oregon,
and Idaho: in Reidel, S.P. and Hooper, P.R., eds., Volcanism and Tectonism in the Columbia River Flood-basalt
Province, GSA Special Paper 239, pp. 21-53.
Geology 151 Lab- Paleomagnetism and Crustal Motions II
Paleomagnetic samples were collected from Miocene aged rocks 1 km. north of Motionville, CA. Analysis of their Thermal Remanent Magnetism was performed. Results of paleomagnetic analyses are shown below along with a simplified geologic map of this area. The Snotmai Fault is Oligocene in age.
1) Arrows indicate the direction of the "h" (horizontal) component of the magnetic field recorded in these rocks at the indicated times (My = million years). In other words, they point at the theoretical north magnetic pole at the time they cooled through their Curie temperatures.
a) Assuming the pole did NOT actually change position during the formation of these rock units, how could motion on the Snotmai Fault during and since the Miocene have resulted in this effect? Put arrows along the fault to indicate the direction of motion across the fault and explain your answer.
b) How might this fault motion have also resulted in the younger rocks in the sampled units exhibiting a TRM with a steeper inclination than the older ones? Assume that the sampled rock layers have never actually been tilted (eg- they are still horizontal; with the same attitude as they were when they cooled).
c) Why is there a larger declination difference between h and N in older rocks than younger ones?
2) Results of thermal demagnetization studies are shown on the J/Jo vs. Temp. graph below. Magnetic minerals which are present in these rocks and may carry the magnetic signal are listed below with their Curie Temperatures. Which minerals probably hold the bulk of the magnetic signal in these rocks? Explain.
Native Iron 770°C
Magnetite 580°C
Pyrrhotite 320°C
Troilite 305°C
Hematite 680°C
Geothite 120°C
3) You DO NOT need to do any formal write-up for this lab. Answer the questions above and turn them in next week in lab.
Mike Valentine
Contact: mvalentine@ups.edu
Copyright © 2008 Last update: Monday, June 24, 2002
