THE STRUCTURE AND EVOLUTION OF CONTINENTAL-CRUST

The use of seismic reflection profiling techniques to explore the deep structure of continental crust has revealed images that indicate the response of the crust to stresses of various kinds. In extension the upper crust responds by brittle fracture along individual faults with intervening blocks sliding against each other. The lower crust, however, behaves in a more ductile fashion, extending by pure shear. In contrast, under compression the continental crust behaves as an entity, shortening and thickening by crustal-scale imbrication of thrust sheets. Such processes, recognised as aspects of plate tectonics active at the present time, appear to have been operating for at least the past 2000 million years.