Downdropped

Downdropped is a geological term describing a crustal block that has moved downward relative to adjacent blocks along faults, creating a down-dropped block or basin within an extensional tectonic setting. It is commonly associated with grabens, which are elongated troughs bounded by parallel normal faults; the down-dropped block forms the basin floor while flanking blocks may be uplifted to form horsts.

Formation and characteristics: Downdropping occurs when tectonic forces stretch the crust, causing normal faulting and vertical displacement. The amount of drop can vary from a few hundred meters to several kilometers. The resulting topography often hosts thick sedimentary sequences that record basin infill, and the fault boundaries control drainage and groundwater flow. In some cases multiple downdropped blocks create a chain of basins.

Geologic significance: Downdropped blocks are important for understanding regional tectonics, crustal extension, and basin development. They can host mineral deposits, serve as hydrocarbon traps where impermeable seals flank permeable strata, and influence paleoclimate records preserved in sediments.

Examples: The Basin and Range Province of western North America contains numerous down-dropped basins between ranges. The East African Rift System features downdropped blocks and forming basins associated with active plate separation. The Rio Grande Rift in North America and the Danakil Depression in East Africa are also classic examples.

See also: Grabens, Horsts, Normal faulting, Extensional tectonics, Basin and Range Province.