In simple terms, sedimentary basins are holes in the Earth's surface into which sediment is deposited. Many different types of sedimentary basins can be found at various localities around the world today. However, all of these basins can be explained by one of two simple basin-forming mechanisms; stretching or flexing of the lithosphere. Presumably, the same mechanisms have existed throughout geological time.
As described in the previous section, when lithosphere is stretched it thins to form an extensional basin. The upper crust deforms by brittle failure, forming large faults and tilted blocks. The underlying mantle behaves in a ductile fashion and forms a 'necked' or thinned region, which is slightly wider than the brittle faulted zone. Subsidence in these basins occurs in two distinct phases; rift subsidence as a result of active stretching and thinning of the lithosphere, and thermal subsidence due to passive cooling and sinking of hot, upwelled mantle (thermal 'sagging').