Tectonic subsidence

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

Where the lithosphere undergoes horizontal extension at a normal fault or rifting center, the crust will stretch until faulting occurs, either by a system of normal faults (which creates horsts and grabens) or by a system of listric faults. These fault systems allow the region to stretch, while also decreasing its thickness. A thinner crust subsides relative to thicker, undeformed crust.

Lithospheric stretching/thinning during rifting results in regional necking of the lithosphere (the elevation of the upper surface decreases while the lower boundary rises). The underlying asthenosphere passively rises to replace the thinned mantle lithosphere. Subsequently, after the rifting/stretching period ends, this shallow asthenosphere gradually cools back into mantle lithosphere over a period of many tens of millions of years. Because mantle lithosphere is denser than asthenospheric mantle, this cooling causes subsidence. This gradual subsidence due to cooling is known as "thermal subsidence".

The adding of weight by sedimentation from erosion or orogenic processes, or loading, causes crustal depression and subsidence. Sediments accumulate at the lowest elevation possible, in accommodation spaces. The rate and magnitude of sedimentation controls the rate at which subsidence occurs. By contrast, in orogenic processes, mountain building creates a large load on the Earth's crust, causing flexural depressions in adjacent lithospheric crust.

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