What happens in volcanic trenches and arcs

What happens between trenches and volcanic arcs (the fore-arc area) has important implications for the evolution of continents, and for the understanding of earthquakes. International deep-sea drilling and related investigations have revealed a number of processes which occur in response to a subduction. Accretion occurs where sediments on the under-thrusting plate are thick: their upper layers are stripped off and folded and faulted against the front of the fore-arc, giving rise to net outward growth of the over-riding plate.

Examples are the Pacific fore-arc of Mexico, and the Barbados fore-arc. Sediment subduction is where sediments are carried below the fore-arc, perhaps to be recycled in the Earth’s mantle, so that the over-riding plate is essentially unmodified by subduction (for example, in the Quaternary fore-arc of Guatemala). Tectonic erosion, a rather surprising discovery, occurs where the under-thrusting plate abrades the over-riding plate, causing a retreat of the outer margin of the fore-arc (such as in the Mariana fore-arc). A fourth process, underplating, is a type of accretion in which sediments on an under-thrusting plate move under a fore-arc, and are plastered onto the over-riding plate from below. This process has been found, in association with frontal accretion, by a French – Japanese team working off the coast of Japan, and by a British – Pakistani team working in Baluchistan, Pakistan.

As well as diverging and converging, plates also slide past each other –  at conservative (or strike-slip) margins. In terms of total length of each type of margin worldwide, these are the least important. However, some of the most destructive of all earthquakes occur along them. San Francisco, site of a huge earthquake in 1906, sits on a strike-slip margin of the North American and Pacific plates; geologists expect another major earthquake there soon. Small sedimentary basins arranged along the complex, braiding fault zones which characterize some strike-slip margins can accumulate disproportionate thicknesses of sediments, and have considerable significance in oil exploration.