PLATES H: Transform faults

 

I was all set to conclude this series of blogs about “growing up” (geologically) in the formative stages of the plate tectonics revolution with an attempt to describe an advanced stage – wherein it got all mathematical.  I have been dreading this task for days – not that the subject matter is at all boring; quite the contrary.  Fascinating and important it is, for sure, but darned hard to explain.  Maybe I can get Nick to do a lecture on it someday,  with pictures.  But in the meantime . . . .

I have inexcusably failed to pay tribute to one of the most important figures of the time: J. Tuzo Wilson, of the University of Toronto.  Tuzo not only named, described and analyzed the third kind of plate boundary, the transform fault, he was a nice man who took the trouble to explain it all to a sniveling graduate student.  Me.

So what, then, is a transform fault?  The short answer is that it is a plate boundary in the form of a strike-slip fault that connects two other plate boundaries; see the illustration above.  Not all strike slip faults are proper transforms, but the most famous are.  For instance, the San Andreas Fault can be understood as a transform that connects the East Pacific Rise at its southern terminus with the fracture zone delimiting the Gorda Plate to the north.  

Also, if you look carefully at maps of oceanic fracture zones you will see that in large part they consist of offset segments of ridge (spreading centers) connected by short segments if strike-slip faults.  An important aspect of this configuration, pointed out by Wilson (and others) is that strike-slip earthquake activity is limited to the segments between the bits of ridge, and is in the opposite sense that one would predict from the relative position  of the ridge segments!  For instance, the upper pair of apparently offset ridge segments appears to be right-lateral, whereas the seismic activity all is left lateral.

Try to figure out why, Patrick.

So, now I must attempt to explain such non-intuitive concepts as Euler poles, triple junctions, and plate motions as vectors in three-dimensional space.  Give me a few days!