PLATES K: Triple Junctions

                              AFAR:  AN RRR TRIPLE JUNCTION

I have left this topic for dead last.  I might pretend that it is of relatively little importance, but that wouldn’t be strictly true.  The real treason for putting it off  probably is that essential details of  this topic – triple junctions – have been the hardest plate tectonic concepts I have ever been forced to to wrap my brain around.  The heroes here – I almost said villains – are Dan McKenzie (Cambridge University, UK) and W. Jason Morgan (Princeton).  Together they wrote a seminal paper entitled:

McKenzie, Dan P. and Morgan, W. Jason, Evolution of triple junctions; Nature, v. 224, pp. 125-133, 1969

which I have seriously tackled at least three times over the past five decades.  I am still not certain I am quite up to speed but – what the heck – let’s go.

Okay, so a triple junction is a point at which three lithospheric plates touch one another.  Expressed otherwise, a TJ is a spot at which three plate-bounding structures meet.  Following Mc & M, we will call the plate boundaries R (a spreading center, as in the mid-Atlantic Ridge), T (an oceanic trench, our subduction zone) and F (a transform fault).  Thus, an FFF triple junction is a point on the earth’s surface at which three transform faults intersect, RRR is the juncture of three spreading centers; TFR then would denote the intersection of a subduction zone, a  transform fault, and a spreading center. 

Obviously, TJs are relatively rare, yet there are a few dozen of them currently scattered about.  No stable examples of FFF exist nor can exist; Mc& M demonstrate that, because of brute geometry, they are impossible.  RRR, on the other hand, is always stable; stays put and keeps chugging away.  Other species of TJ – there are a dozen of them - are stable under certain restrictive circumstances, unstable otherwise.

By stable, above, I mean that they maintain their geometric configuration.  For instance the famous Afar triangle is an RRR triple junction where spreading centers forming the Red Sea, the Gulf of Aden, and the East African Rift Zone meet at a point.  Being RRR, it will be around for a long time, and may succeed in prizing a chunk of East Africa away from the rest of the continent.

RRR is always stable, but another important example – the Mendocino TJ -  is stable only under specific geometric conditions.  The MTJ formed about 30 Ma years ago when a transform fault between the Farallon and Pacific plates intersected the margin of North America, which previously had been the locus of Farallon-North America subduction.  (See Tanya Atwater*).  This brought the Pacific plate into contact with North America, and resulted in the creation of an FFT triple junction.  One stability condition of FFT is that one F be collinear with T.  This condition was satisfied here because the newly created F (our San Andreas fault) was collinear with the trench still existing to the north.  (Too many acronyms, I know – draw yourself a diagram!)  A geometric consequence of this configuration is that the TJ migrates along the direction of the common, collinear boundary.  In the Mendocino example, the TJ will appear – seen from North America – to slide steadily northward, thereby lengthening the San Andreas system.  Now, however, at Cape Mendocino the smooth, this linear plate boundary has abruptly disappeared.  My guess is that the MTJ has become unstable and is in the process of breaking up; hence all the seismic excitement around Eureka, CA.

Don’t let me leave you with the notion that triple junctions are dynamic elements that move blocks of crust around willy-nilly.  They aren’t; like Euler poles, they are merely useful descriptive devises.  Geophysical forces, ultimately arising from  geothermal energy and gravity, are what move plates.  Triple junctions, like  Euler poles, are simply mathematical constructs that help us understand geological history.  

*Atwater, Tanya, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, GSA Bulletin, v. 81, pp. 3513-3536, 1970.