14. Question: Why would thick, buoyant continents not entirely prevent subduction?
Response: This question is motivated by a statement in a paper by P. Molnar entitled “Continental tectonics in the aftermath of plate tectonics,” Nature 335, 131-137, 1988, available without cost here:
…the buoyancy of thick continental crust keeps it afloat. If continental lithosphere were strong enough to maintain its integrity at a subduction zone, the buoyant continental crust would not only resist being subducted, but the subducting plate would abruptly grind to a halt when the continental “passenger” reached the trench.
The paragraph containing this quote is the first paragraph in a section entitled “Differences between continents and oceans” and focuses on the contrast in buoyancy. The next paragraph focuses on their contrast in strength. It reads,
The strength of the continental lithosphere also contrasts with that of the oceanic lithosphere. The strongest part of the oceanic lithosphere seems to lie in the mantle, between 20 and 60 km depth, between a brittle upper part and above its increasingly ductile lower part, which grades downward into the asthenosphere (Fig. 3). In the same depth range where oceanic lithosphere is strongest, however, continental lithosphere consists of crust, not mantle. At temperatures typical of the lower crust (400-700 °C), the minerals comprising the crust appear to be much weaker than olivine, the strong mineral that comprises most of the upper mantle. Consequently, continental lithosphere could be much weaker than oceanic lithosphere. Oceanic lithosphere behaves as a virtually rigid plate because of its strong core, but, as the late C. Goetze noted in the mid-1970s, continental lithosphere might consist of three layers: a brittle upper-crustal layer, a weak lower crust and a stronger uppermost mantle, which, nevertheless, would not be as strong as the strongest part of the oceanic lithosphere. This jam-sandwich-like rheological profile (Fig. 3) is also suggested by the frequent occurrence of earthquakes (brittle deformation) in the upper crust, their nearly complete absence in the (presumably weak, ductile) lower crust, and their occasional presence in the underlying upper mantle.
The point of this paragraph is that in regard to overall strength continental lithosphere contrasts strongly with the oceanic lithosphere. Indeed, the strength profile of continental lithosphere has frequently been referred to as a “jam sandwich” because of the weakness of the warm lower crust. So the original quote, taken in its context, does not suggest or imply that continental lithosphere is strong enough to maintain its integrity at a subduction zone and therefore that subduction should abruptly grind to a halt. It is just the opposite. The author in the following paragraph is providing reasons why continental lithosphere is weak and deformable and why this grinding to a halt state of affairs does not generally take place. This observation applies equally to UPT and CPT.