Plot the magnetostratigraphic data (stratigraphic height, VGP latitude) contained in file ex6.1 using plotxy. Use the program jackstrat to investigate the robustness of the polarity pattern and to calculate J.
Solution
Type:
% plotxy < ex6.1.com
and see mypost.
The program jackstrat expects polarity information (positive or negative numbers) in stratigraphic order. For this, you can use either the inclinations, if they are steep enough to uniquely determine polarity, or, preferrably, VGP latitudes. In order to extract the polarity zonation from the file, we need to select the second column (the VGP latitudes) from the file ex6.1 using the awk command. These can then be piped directly to jackstrat, whose output in turn can be piped directly to plotxy:
% awk '{print $2}' ex6.1 | jackstrat | plotxy
The output of the magnetostratigraphic jackknife is shown in mypost.
Example 6.2
1) Sometimes specimens are so weak that it is advantageous to measure them several times and take some sort of average. A Fisher mean (see Chapter 3) ignores the intensity information, so a full vector average (instead of unit vector) average is more appropriate. Use the program vspec to calculate vector averages of multiple measurements at a single treatment step for the data in ex6.mag.
Solution
The format of the .mag files is space separated:
specimen_name treatment_step CSD Intensity Declination Inclination
where CSD could be the circular standard deviation of the measurement, but can also be any dummy variable.
Now type:
% vspec < ex6.mag > ex6.vmag
to get the averaged data ex6.vmag.
Unique specimen/treatment steps are simply copied over and the vector average of multiple measurement replaces those data. The CSD field is replaced with a parameter similar to Fisher's R, but takes into account the intensity. It is the resultant divided by the sum of all the intensities and ranges from zero to one, one resulting from perfect alignment of multiple measurements.
It is handy to have a program to help enter orientation data. In Chapter 3 of Paleomagnetic Principles and Practice, we discussed three methods of orienting samples, using the ``hand sample'' (or ``cube'' in the following) method, the azimuth and plunge of the direction of drill (or ``drill direction'') method and the ``sun compass'' method. There are many more. What is required by programs such as di_geo is the orientation of the X1 direction of the coordinate system that the measurement directions are in. It is assumed that X2 is horizontal and X3 follows the ``right-hand rule '' discussed in Chapter 3.
1) Use the program mk_nfo to enter the orientation data for the specimens in the previous example. These are:
Sample | NB Azimuth (Strike) | NB Plunge (Dip) | Stratigraphic height:
tn07a | 250 | 22 | 15.6
tn07c | 275 | 51 | 15.6
tn07d | 245 | 42 | 15.6
Put the orientation data calculated by mk_nfo into a file called ex6_c.nfo These samples were taken with the ``cube'' method, so notebook (NB) values for ``azimuth'' and ``plunge'' are actually the strikes and dips of a perpendicular face (see Figure 3.2 of Paleomagnetic Principles and Practice). The sediments are consistently oriented with a strike of 240 and a dip of 20 (dip to the right of strike), but stratigraphic position is of interest, so these data should also be entered. The samples were collected from the East African Rift valley at 1 N, 35 East in 1985 and according to the results of the igrf program, the magnetic declination was negligible (less than 1 degree).
2) Use the program mk_nfo to make an .nfo file for specimens drilled from a lava flow (listed in the Table below). The orientation device was the commercially available ``Pomeroy'' orientation device which measures the azimuth of the direction of drill and the angle that the drill direction made with the vertical down direction. Also available are sun compass information for the azimuth of the drill direction. These data came from the Island of La Palma in the Canaries at 29 N and 18 W on December 31, 1994. The reference field for these data (from igrf) has a declination of 351 2#2| NB azimuth | NB plunge
lp01a | 192 | -2 lp01b | 221 | -6 lp01c | 296 | -36 lp01d | 160 | -2 lp01e | 135 | -58
Solution
mk_nfo is a big and complicaticated program, so let's start with the help message generated by the mk_nfo -h command or see documentation to mk_nfo.
1) In order to carry out the first part of the program, we can enter the data into a file with the fields:
sample NBaz NBpl height
and put in the orientation type (c), bedding orientation (240 20) as command line arguments:
% mk_nfo -HtBf c 240 20 ex6_c < ex6.mk_nfo.c
This creates a file named ex6_c.nfo
Look at the ``output'' format in the documentation for the key to what these fields mean.
2) To make a .nfo file for the La Palma samples, we need to specify notebook Plunge: [ 0.0] that the type is `p', that the magnetic declination is -9 degrees and that there are sun compass data from the correct latitude and longitude (29, -18) and with the difference between the measurement time and Greenwich mean time, which in this case is 0. We can either put the data in a datafile, or enter the data in an interactive way (keyboard input). For the purposes of illustration, we will do it the interactive way. Data in brackets will be retained by pressing return.
% mk_nfo -ktDs p -9 29 -18 0
Sample name: [AB123a ]
Control-D to quit
lp01a
Notebook Azimuth: [ 0.0]
12
Notebook Plunge: [ 0.0]
88
yyyymmdd: [ 19560126]
19941231
hhmm: [ 0300]
1314
Shadow angle: [ 0.0]
359
lp01a 0.0 c 12. 88. 3. -88. 0. -9. 0. [magnetic declination]
lp01a 0.0 c 12. 88. 359. -88. 0. -9. 0. [sun compass dec.]
Sample name: [lp01a ]
Control-D to quit
lp01b
Notebook Azimuth: [ 12]
41
Notebook Plunge: [ 88]
84
yyyymmdd: [ 19941231]
just hit return to keep this value!
hhmm: [ 0300]
1318
Shadow angle: [ 0.0]
28
lp01b 0.0 c 41. 84. 32. -84. 0. -9. 0.
lp01b 0.0 c 41. 84. 29. -84. 0. -9. 0.
Sample name: [lp01b ]
Control-D to quit
and so on.
If we had specified an output file basename (e.g. lp), the program would have created two files, lp.nfo and lp.snfo. The magnetic compass information goes into the .nfo file, while the sun compass data go into the .snfo file. These are not always the same as you can see by comparing the LABaz fields in the two examples shown above. The user must choose between the two. One way to do this is to concatenate the two files using cat and then sort them:
% lp.nfo lp.snfo | sort > lp_all.nfo
Then you must go through line by line and choose which method gives the most reliable azimuth.
3) Finally, we must do the third part of the problem which is to supply user defined affines for the NBaz and NBpl variables. With a little thought, we find that, if NBpl is negative (we were drilling downwards), then we must add 180 to the NBaz to get the LAB_az correctly, so az_add = 180 and az_mult = 1. The LABpl = -90 - NBpl = -1 (NBpl + 90), so pl_add = 90 and pl_mult = -1. (If the drill direction was up, it is another whole ball game.) In this excercise, all the drill directions were down (NBpl < 0), so we will use az_add = 180, pl_add = 90 and both multipliers = 1. Having put the orientation data into a file ex6.mk_nfo.u, we type:
%mk_nfo -t u 180 1 90 -1 < ex6.mk_nfo.u
and get:
lp01a 0.0 u 192. -2. 12. -88. 0. 0. 0.
lp01b 0.0 u 221. -6. 41. -84. 0. 0. 0.
lp01c 0.0 u 296. -36. 116. -54. 0. 0. 0.
lp01d 0.0 u 160. -2. 340. -88. 0. 0. 0.
lp01e 0.0 u 135. -58. 315. -32. 0. 0. 0.
Example 6.4
Use the program mag_dat to convert the file created in Example 6.2 (ex6.vmag) into what we will call a .dat format. Use the .nfo file created in Example 6.3 ex6_c.nfo to rotate the vectors into geographic and tilt adjusted coordinates.
The syntax of mag_dat is quite simple; it reads from standard input, writes to standard output and the .nfo format file is specified as a command line argument, following the switch -n:
% mag_dat -n ex6_c.nfo < ex6.vmag
to which the response is:
tn07a 15.6 0.00 2.3 0.8420E-04 81.6 -10.1 83.6 -2.3
tn07a 15.6 150.00 2.4 0.6500E-04 89.2 -14.2 92.0 -3.9
tn07a 15.6 300.00 2.3 0.2930E-04 104.7 -19.6 107.7 -5.1
tn07a 15.6 400.00 1.7 0.1260E-04 115.7 -25.4 119.1 -8.5
tn07a 15.6 450.00 0.7 0.4140E-05 153.8 -48.5 152.9 -28.6
tn07a 15.6 500.00 1.2 0.4190E-05 66.6 1.6 65.7 3.7
tn07a 15.6 550.00 1.1 0.3650E-05 145.2 -28.2 145.7 -8.3
tn07a 15.6 600.00 0.8 0.2230E-05 142.0 -54.0 144.3 -34.1
The help message will inform you that these fields are:
sample, position, treatment, CSD, Intensity, geographic D and I, tilt adjusted D, and I.
Suppose you have collected a set of samples from two margins of a dike (nominally East and West). These were measured and you calculated the s data using the techniques discussed in Chapter 5 in the book and are listed in dikeexample/e.s and dikeexample/w.s. Several dike orientation measurements were made and the dip directions and dips were: (102/70;98/67;108/82). Also, some outcrop flow lineations were noted as declination inclination pairs: (20/38;20/36;24/30). Finally, there are two tectonic rotations necessary to restore the dike to its original position (strike # 1 = 270, dip # 1 = 10; strike # 2 = 205, dip # 2 = 20).
Use the program plotdike to analyze these data. Plot the bootstrapped eigenvectors instead of ellipses and use a site parametric bootstrap.
Solution
Place the Eastern and Western margin data in files called e.s and w.s respectively. Put the dike orientations (dip direction, dip) in a file called dike.dd and the lineation data (azimuth, plunge) in lin.di. Put the structural corrections (strike, dip, strike, dip) in a file called struct.dat. It is best if each dike has its own directory (in this example, it is called dike_example) in order to avoid hopeless confusion and overwriting of files.
Now type:
% plotdike -Pv | plotxy
and enjoy the mypost file. [an error occurred while processing this directive] [an error occurred while processing this directive]
