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A continent is considered to consist of a mosaic of crustal blocks each 500 to 1 000 km across, the blocks having different cratonization history. Within each block, gravity anomalies of wavelength 20 to 100 km are generally elongated and subparallel. The relative ages of the gravity trends in the adjacent blocks can be determined because those blocks with trends oblique to the boundary are likely to be older than the boundary, whereas those parallel to the boundary are likely to be younger. These major gravity trends are thought to be caused by large-scale folding initiated at the first major deformation of each block. The boundaries between Precambrian blocks commonly coincide with a gravity gradient between a major high and a major low anomaly, the total amplitude being more than 50 mGal. The dipole anomaly is interpreted as due to a large and abrupt change in mean upper-crustal density, the crust being in regional isostatic equilibrium. Within the crustal blocks, major cross fractures can be mapped using the combined gravity and magnetic data. The cross fractures cause minor gaps or offsets of the primary gravity and primary magnetic trends, and are coincident along part of their length with the axes of magnetic bodies. These cross fractures are commonly subparallel and are generally not at right angles to the major trends. Where basement geology is known, the cross fractures coincide with geologically mapped faults and, rarely, folds.

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