Half-graben

Newark Basin Cross Section

Graben and half-graben

Rift segmentation

Half-graben sedimentation

Riogranderift albuquerquebasin

Half-graben is a geological structure formed due to extensional tectonics, characterized by a block of the earth's crust that has dropped down between two or more normal faults. It is a type of rift or rift valley that is asymmetrical in cross-section. The term "graben" comes from the German word for ditch or trench, and the prefix "half" indicates that the structure is bounded by faults on one side only, whereas a full graben is bounded by faults on both sides.

Formation

Half-grabens are formed by the extensional forces acting on the Earth's crust and lithosphere. This extension causes the crust to crack and form faults. A half-graben is bounded by a major normal fault on one side, where the crust has moved downwards relative to the other side. The opposite side of a half-graben is typically bounded by a less significant fault or by a monoclinal flexure where the crust has bent rather than broken. The downthrown side accumulates sediments that are eroded from the uplifted side, as well as from the surrounding areas.

Geological Significance

Half-grabens are significant in the study of tectonics and sedimentology because they are sites of active sedimentation and can preserve a detailed record of geological processes. They are often associated with the early stages of rift development, where the earth's crust is being stretched and thinned. Understanding the formation and filling of half-grabens can provide insights into the history of continental break-up and the formation of ocean basins.

Half-grabens are also important for hydrocarbon exploration. The thick sequences of sediment that can accumulate in these structures may contain oil and natural gas reservoirs. The geometry of half-grabens, with their tilted fault blocks and adjacent sedimentary basins, can create traps where hydrocarbons can accumulate.

Examples

One of the most well-known examples of a half-graben is the Basin and Range Province in the western United States, which consists of a series of north-south trending half-grabens and horsts (the uplifted blocks). Another example is the East African Rift, a complex rift system where the African continent is being pulled apart, forming a series of half-grabens.

See Also

• Graben
• Horst
• Rift valley
• Normal fault
• Tectonics
• Sedimentology

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