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Why do normal faults dip away from a horst?

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tectonic basins and rift valleys Escarpments of the Great Rift Valley rising above the plain north of Samburu Game Preserve, central Kenya. Beisa oryxes graze in the foreground.

A horst is an elevated block of the Earth's crust created by lateral tension, which causes the crust to extend and fracture. These blocks are bounded on both sides by steeply dipping normal faults.

The faults dip away from the horst because of the mechanics of crustal extension. In a normal fault, the hanging wall moves downward relative to the footwall. When a series of these faults occurs, the blocks between them shift to accommodate the stretching of the crust. For a central block to remain as a raised horst while the adjacent blocks (or grabens) drop down, the fault planes must incline outward, away from the central elevated mass. Conversely, the faults bounding a graben dip toward each other, allowing the central block to subside relative to the surrounding terrain.12