MUSINGS ON FLAT SLAB SUBDUCTION

 

More antique musings: 

What is this thing we call subduction?  Well, at its simplest, subduction arises because what we call the lithosphere becomes “negatively buoyant”. That is a fancy way of saying that it becomes more dense than the stuff it has been ‘floating” on, hence sinks under its own weight.  Such action actually will pull a plate along; in other words, it is a positive driving force.  Subduction zones bordering the northern and western Pacific basin are examples of this kind of behavior. 

So, then, why does an oceanic plate become negatively buoyant?  Cooling, that’s why.  When such a plate-edge is created at a spreading center it is thin and hot.  As such its overall density is lower than that of the stuff it over-rides, hence is buoyant – it “floats”.  However, as it ages it cools and becomes negatively buoyant – that is, it wants to sink.  Voila!  Subduction!

So why, then, are there situations in which the subducting slab is essentially plastered along the base of the over-riding plate – known as flat-slab subduction (FSS).  This condition generally arises in circumstances in which the over-riding plate is advancing rapidly toward the spreading center, thus narrowing the subducting oceanic plate and assuring that it will be forced down, regardless of its buoyancy.  In effect the over-riding plate forceably shoves the subducting plate below.  This situation naturally constitutes a negative driving force for both plates involved.  The Nazca plate grinding its way beneath the central Andes often is cited as an example.

Another way that FSS can come into play is the arrival at the trench of a thickened, relatively cool chunk of stuff – what we have come to regard as a terrane.  This can cause at least a temporary reduction in subduction angle – and may result in an “outboard leap” of the subduction zone itself.

So, who cares?  Well, FSS has been proposed by many – especially by Bill Dickinson and acolytes – as the reason we have the Rocky Mountains.  I have always been skeptical of this, mainly because I was puzzled by the reason for the subduction angle to shallow so abruptly at just the right time.  Now, maybe, I can convince myself that the arrival of the Stikinia at the trench was the culprit.  But I am still skeptical.