A structural basin is a large-scale structural formation of rock strata formed by tectonic warping (folding) of previously flat-lying strata into a syncline fold. They are geological depressions, the inverse of domes. Elongated structural basins are a type of geological trough. Some structural basins are sedimentary basins, aggregations of sediment that filled up a depression or accumulated in an area; others were formed by tectonic events long after the sedimentary layers were deposited.
Basins may appear on a geologic map as roughly circular or elliptical, with concentric layers. Because the strata dip toward the center, the exposed strata in a basin are progressively younger from the outside in, with the youngest rocks in the center. Basins are often large in areal extent, often hundreds of kilometers across.