ANOTHER COMPLICATION

             NOTE THE ANGLE OF SUBDUCTION

Nick has amply demonstrated the fascinatingly complicated nature of tectonic activity along the western margin of North America during late Mesozoic and Tertiary time. Zentnerds will of course know exactly what I am talking about; all others Google “Nick Zentner geology lectures” and go from there.  This little blog is meant to add a bit more complication, as if any more were needed.

If western North American tectonic history were as simple as the original Dickinsonian model a lot of present-day academic geologists (including, probably, me) would have spent their lives earning an honest living as bricklayers, plumbers, and other useful people.  Fortunately, it wasn't..  

To slightly oversimplify, the early model called for monotonic eastward subduction of the Farallon plate beneath North America, resulting in formation of the “California triad”; from east to west: magmatic arc, forearc basin, sedimentary subduction complex.  The earlier model also called for creation of the San Andreas Fault a few tens of million years ago, as the Pacific plate impinged on North America.  Zentnerds will know this stuff backwards and forwards; the rest of you should study up.  Anyway, this model – which works well for California proper -  is owing to vital early geophysical work by Tanya Atwater and others, and rendered into geological dogma by Bill Dickinson and his followers.  As I said earlier, it works pretty well for the southern portion of the Cordilleran tectonic belt – if, of course, you ignore Baja B.C.

However, north of Cape Mendocino things get very  much  more complicated.  Nick has done a masterful job of describing what might be termed “accretionary tectonics”; that is, growth of the continental margin by adding large scraps of foreign lithosphere to the craton.  Again,  Zentnerds will know exactly what I mean; anyone else still with me will have even more to study.

So, did you follow up on the bit I wrote about triple junctions?  If you did, you will know that some of them are mobile, and can go sliding along the continental margin.  That introduces some secondary complications, particularly with timing.  For instance, as the Mendocino TJ tracked northward from southern California it should have altered the margin tectonic setting systematically, from subduction to transform.   This ought to show up ,in the geologic record.  Does it?  Damned if I know.

Well, finally, my additional complication.  Magmatic arcs appear above the point at which certain pressure-temperature conditions are encountered.  This, obviously, depends mainly on the angle of subduction.  Most models depict that angle as about 45 degrees, but in the real world it can range from nearly vertical to nearly horizontal.  For the most part the actual subduction angle seems to be determined by the age of the subducting lithosphere.  Old, cold lithosphere is “negatively buoyant”, hence tends to sink at a steep angle, pulling its trailing plate along with it.  This kind of subduction is a positive plate-driving force.  Conversely, young lithosphere is hot, positively buoyant, and has to be forcibly shoved under to be subducted.  Such subduction zones retard plate motion, and can produce super-large earthquakes, like those found in Chile – and, unfortunately, our own Cascade forearc!

Finally, what determines the sink/not sink properties of a slice of oceanic lithosphere?  Well, pretty obviously: how far it is from the ridge that gave it birth, and how fast spreading at that ridge is taking place.  (Cooling rates seem to be fairly constant).   So, the additional complication I threatened earlier is this; potentially, the location of subduction-related magmatism depends largely on the location and activity of an offshore ridge – and these  can, and do, change.  So, poentially, the location of the magmatic arc likewise can skip around.  Hasit?  Again, damned if I know.