A graben is a depressed block of the Earth’s crust bordered by parallel faults, typically forming a rift valley or basin. Grabens are significant in understanding extensional tectonics, where the Earth’s crust is stretched and thinned. They are often associated with volcanic activity and the formation of basins that may host sedimentary deposits.
Reference: McClay, K. R. (1990). Extensional Fault Systems in Sedimentary Basins: A Review of the Geometry, Processes, and Evolution. In: Extensional Tectonics and Stratigraphy of the North Atlantic Margins. AAPG Memoir 46.