How do tectonic basins and rift valleys form when Earth’s crust stretches?

Tectonic basins and rift valleys form primarily through the extension of the Earth’s crust, which causes the brittle rock to fracture. As large crustal blocks or lithospheric plates diverge, the crust thins and stretches, creating a gap.12

This process typically involves normal faults, where the hanging wall moves downward relative to the footwall. A central block of crust, known as a graben, slides down into the resulting space between the surrounding fault blocks. This down-dropping creates the characteristic flat floor and steep, mountainous sides of a rift valley or tectonic basin.31

While many rift valleys are symmetrical, others are asymmetrical, featuring one steep, fault-bounded wall and a gentler side formed by the flexing and tilting of the lithosphere. In some regions, this extension results in horst and graben topography, a series of alternating raised and lowered blocks. Additionally, some rift valleys form over large domes where upwelling asthenosphere heats and stretches the overlying lithosphere, often leading to associated volcanism.23