Rift is here treated as synonymous with megashear (Carey, 1958) or great fundamental fault (de Sitter, 1964). Rifting does not necessarily include fissuring, crustal extension, or the formation of graben (rift valleys). The terms graben and rift valley are not necessarily synonymous, although recently the distinction is not preserved.
Since the study of the dynamics of these structures began with recognition of their physiographic expression, rift valleys are first discussed. The morphotectonic name rift valley establishes its nature as a tectonic landform or oceanic form, the surface expression of endogenic (geodynamic) processes (Fig. 1). Rift valleys are found in two crustal settings: continental, as a primarily tectonic landform, which can be modified by exogenic processes, an elongate depression or valley formed by subsidence of the block lying between subparallel, opposed normal faults and floored by the surface of the block (Figs. 2 and