Plate Tectonics

Proposed revisions to Plate Tectonics: How it Works.
.


Overall strategy is to (1) replace stereonet techniques with
modern computer techniques and include relevant programs on a CD
ROM with the book, and probably on an associated website; (2)
replace schematic data with the real data that resulted in the
plate tectonic revolution; and (3) to shorten the book by removing
some of the material (i.e. much of Chapters 3, 5, 6 and 9) and
creating new integrated chapters.




Chapter 1: The Revolution.


This will be changed to include the iconic data that led to
plate tectonics, including the Bullard et al. (1965) fit of
Atlantic margins (and its effect on APW paths); the Raff-Mason
(1961) magnetic stripes in the Juan de Fuca area; Vine?s
(1968) correlation of these anomalies with the reversal time scale;
the Eltanin 19 profile from Pitman and Heirtzler (1966); and the
Barazangi and Dorman (1967) earthquake map (or an update similar to
Fig. 6-1). The schematic data shown for ridge, trench, and
transform boundaries will be replaced with figures of real data to
show students accurately what we know.




Chapter 2: Velocity Space.


This chapter is basically excellent, so it will be kept pretty
much as is, although the notation describing relative motion will
be changed to the modern usage, 1st named moves relative to 2nd
named. Also, R, F and T will only be used for types of plate
boundaries rather than used sometimes for plate names. The triple
junction stability criteria discussion can be improved from the
marbles rolling along boundaries analogy, and will be expanded to
include the cases of oblique and asymmetric spreading on ridges, so
that students will know how to do problems like 2-1i and 2-2b,
which show unstable RRR junctions contrary to the statement on p.
78 that all RRR junctions are stable. Fig. 2-17 will be replaced
with the more accurate McKenzie and Morgan (1969) geometry, because
this is not only iconic but a great discussion figure.




Chapter 3: Rotation Poles.


Part of the old chapter 3, including Mercator projections, will
be merged with the old chapter 4, with the addition of a short
discussion of spherical trigonometry, and the replacement of
stereonet rotation techniques with computer programs. A simple
computer program will be provided that, given the appropriate
rotation pole, calculates direction and rate of relative motion at
any point on a plate boundary. There will also be a table of the
best poles, currently NUVEL-1A, which could be updated in future
editions of the book so the most up-to-date version could always be
included.


The forward problem is easy, the inverse problem is harder --
given a set of observations, how do you find a rotation pole? The
direction of motion comes from transform faults, so some good
bathymetry will be provided (or perhaps the Sandwell and Smith
gravity map on a CD or website?). The rate of motion comes mostly
from magnetic anomalies (although I?ll add some discussion of
GPS results), and I will include the magnetic anomaly modelling
program developed in my lab (building on earlier work by Talwani
and Vine). This program, Magbath, calculates predicted anomalies
given real bathymetry (if available) and an age distribution based
on a magnetic reversal time scale, which will also be provided -- I
currently use Cande and Kent (1995). This will enable students to
analyze real data such as the Eltanin 19 profile. Another exercise
will be to analyze a profile from the Galapagos area containing one
easily recognizable ridge jump, which Magbath was designed to
analyze (these jumps, caused by propagating rifts, will be
important in the discussion in the final chapter).


At this point, it would be great if one other computer program
could be provided. The program I and my students have used to
derive rotation poles was written and provided to us by Bernard
Minster and Tom Jordan (1974; 1978); a somewhat different version
based on Chase (1972) was used by the NUVEL group (DeMets et al.,
1990; 1994). Although I have a working copy, someone else wrote it.
We will approach the authors and discuss the possibility of
including the source code, global data set, fit of their model, and
set of poles derived from these rates and azimuths that best fits
the data in a least-squared sense.


This considerably revised chapter will replace the current
chapter 3, half of chapter 4, and much of chapter 8. The other half
of the current chapter 4 will be included in the next chapter on
triple junction analysis.




Chapter 4: Triple Junction Analysis.


The Cartesian coordinate discussion from the current chapter 3
will be merged into this chapter. The book currently fails to note
a problem resulting from an ambiguity in the tangent function when
using Cartesian coordinates to calculate a pole given the other two
poles at a triple junction (longitude = tan-1 (wy/wx) is ambiguous,
the same number corresponds to 2 different longitudes 180 degrees
apart). I?ve found that most students don?t notice
this, apply the technique as given on p. 146-147, and get the wrong
answer half the time. This problem (and its solution) will be
discussed, and another simple computer program to always get the
correct answer can be added.




Chapter 5: Finite Rotations.


This corresponds to the old chapter 7. Here it would be great if
a map program could be provided, capable of doing plate tectonic
reconstructions given stage poles or total reconstruction poles.
The one that I?ve used was written by Jason Morgan 35 years
ago, Bob Parker?s Supermap program is equally ancient, and
students are now used to seeing fancier reconstructions.
Unfortunately the best reconstruction programs are now commercial,
e.g. Chris Scotese?s PALEOMAP. Perhaps he would allow a link
to his website in exchange for co-authorship etc. Or perhaps
students could be directed to the reconstruction animations on his
website: http://www.scotese.com/newpage13.htm. Alternatively, the
equations and rotation matrix given in the current chapter 7 can be
used to rotate a grid or outline point-by-point, and perhaps a
simple program that can do this would still be sufficient for
students. We will approach various authors about the possibility of
including their programs.




Deleted chapters.


I don?t know how useful the present book is for
seismologists or paleomagnetists, so I haven?t included
updates of those chapters. I no longer use most of chapters 3, 5,
6, and 9 when I teach the course, although I incorporate parts of
them into the new Chapter 6 discussion. For example, the concept of
paleomagnetic pole cusps is important in understanding the driving
mechanism, so this discussion will be included. Similarly, students
need to know about plate boundary earthquakes and how to interpret
seismic beachballs. I do use a lot of material in the present
chapter 8 in the rotation pole chapter (new Chapter 4), to show how
rotation rates are determined.




Chapter 6: Driving Mechanism.


This chapter works best if rearranged. Previous chapters use
relative plate motions, but ?absolute? plate motions
are necessary for the driving mechanism analysis (Figs 10-7, 10-8,
10-9, 10-10, and 10-11). These figures now precede, but in the
revision will follow, a discussion of hotspots and how they provide
this useful ?absolute? reference frame, similar to but
intuitively simpler than the no-net-torque reference frame, and
they will also be updated, e.g. the Pacific plate velocity is ~20%
slower than presently shown. As part of this final chapter, in the
discussion of ridge push, slab pull etc., the parts of current
chapter 6 discussing what kinds of earthquakes would be expected in
each case will be included (Figs. 6-16 and 6-17).


Because ridge propagation is an important constraint on the
driving mechanism and style of convection, when I teach I show our
(Hey and Wilson, 1982) computer graphics animation of Juan de Fuca
evolution by propagating ridges
http://www.soest.hawaii.edu/~hey/movie.html, and would be happy to
add it (and perhaps the computer code used to analyze complicated
areas like this).




In Summary, if the book were revised this way, it would be more
useful to people who want to learn how to do plate tectonics. It
could also evolve naturally through the years ? if someone
writes a better program, it could replace the old one, and the
co-authors could change accordingly. This would ensure that
students and researchers always have the modern tools necessary to
do plate tectonics. Perhaps the title should be modified to reflect
this, something like Plate Tectonics: How it works, and How to do
it. I think such a book could be both shorter and more useful than
the present edition..


Dr. Richard N. Hey.


University of Hawaii.


Honolulu, HI 96822.


(808) 956-8972.


hey@soest.hawaii.edu.


http://www.soest.hawaii.edu/~hey