Where plates converge, subduction recycles the oceanic crust and erases the evidence for the existence of old oceans, although not always completely. Thus it is in former subduction zones and collision sutures that the only remains of ancient ocean floors are preserved. Together with paleomagnetic and paleontological data, these are our only sources of information regarding the vanished oceans of the past.

Narrow strips of deformed, metamorphosed and often deeply eroded rocks, called mobile belts (Figure 8.1), are common on all continents where they surround stable cores of great antiquity, the cratons. The oldest mobile belts are early Precambrian in age and their significance is not yet fully clear (Section 13.5), but those of the late Precambrian and Phanerozoic represent converging plate boundaries, either extinct like the Appalachian, Alpine and Scottish–Norwegian ranges, or active like the Andes or Himalayas. All contain andesitic volcanic rocks and intensely metamorphosed and folded sediments, often intruded by granites. Therefore continental margins bordered by subduction zones are known as active margins.

Plate-tectonic theory says simple things about what must happen at converging plate boundaries, but as we have gone on to apply the theory to ever more real mobile belts, the difficulties have increased and reality has overtaken theory.

SCENARIOS OF SUBDUCTION

The simplest case of convergence is a collision between two oceanic plates.