North Sea Basins
Concealed beneath the blanket-like sag of the North Sea basin is a complex of older basins and rift valleys (grabens) between elevated 'highs' and platforms. In the southern and central North Sea, the thickest sediments are in the Permian basins and in the deep Central Graben. The northern North Sea is dominated by the sediment-filled Viking Graben. Were the graben empty, Mount Everest would just about fit into it - upside-down. F27 shows the Permian basins and buried grabens as they would appear if all rocks younger than 285 million years were stripped away. The early rifting stage of basin formation lasted until about 125 million years ago, and was followed by the main sagging stage. The Earth's crust is thinned symmetrically across the entire width of the northern North Sea. Two further important influences on the structure in the central and southern North Sea are deformation produced by mobile salt masses (F28) and 'inversion' of basins accompanied by erosion (see F15).