Re: good text on plate tectonics

On Thu, 28 Jun 2007 20:15:50 GMT, J. Taylor
<nchiwana@xxxxxxxxxxxxxxxxxxxxx> wrote:

The real problem GPS
http://sideshow.jpl.nasa.gov/mbh/series.html

does not agree with the direction this shows
http://www.ngdc.noaa.gov/mgg/image/crustageposter.jpg


Just by looking at the two pictures I cannot tell how much agreement
or disagreement is between the two sets of data, especially since the
web image of the ocean floor age is of poor quality. Personally, I am
inclined to state there is a rather fair agreement between the GPS
horizontal velocity fields and the geologically derived plate
velocities, but that's just me. If you are willing to take it upon
yourself and somehow quantify the differences, it would make for a
very interesting discussion.

From what I know, the geodetic horizontal velocities reflect short
term, instantaneous (geologically speaking) rates of deformation and
include contributions from more than one type of deformations:
long-term plate movements, earthquake slips, elastic strains, aseismic
creep, etc. For each particular case, the relative contribution of
each factor must be accounted for before making interpretations.

One the other hand, plate tectonic velocities are derived from a
combination of geometrical constructions on transform fault
directions, spreading rate variations, and focal mechanism solutions
and reflect long-term plate deformations.

Given that the geodetic and geological velocities measure different
things (short-term and long-term deformations, respectively), perfect
agreement between the two sets of data is rather unlikely.

Mircea
.