3 - Theory of Elastic Plates

Summary

Flexure is an important feature of deep-sea trench–outer rise systems. In particular, the topographic rises seaward of deep-sea trenches can be explained by vertical and horizontal loads that act on the approaching oceanic lithosphere. While the source of these loads is not always clear, they combine to flex the lithosphere downwards by a few kilometres at the trench and upwards by several hundreds of metres in the region of the topographic rise. Many of the differences in the bathymetry of deep-sea trench–outer rise systems can be explained by variations in Te of the approaching plate. Others may be due to variations in the state of stress that exist across the plate boundary. There is evidence that the flexure is large enough at some trenches to locally weaken the lithosphere and cause it to fail through normal faulting. Seamounts and oceanic islands that ‘ride’ the topographic rise will experience uplift and then subsidence as they are carried by plate motions into the trench. This will also be the case for continents that arrive at a trench because of the subduction of oceanic crust. Uplift is in the process of affecting New Caledonia and other neighbouring islands, and flexure seaward of trenches may explain several erosional unconformities that are observed in the ancient rock record.