Layer thickness and the shape of faults

[1] We analyze from a conceptual point of view, the role of layer thickness on fault vertical restriction, size distribution and shape evolution. We propose a model of fault growth from an initially circular/elliptical shape of individual ( but kinematically interrelated in a coherent fault zone) fault segments to a final rectangular shape. Faults ( or fault segments) that do not interact strongly with the stratigraphy grow vertically as nonrestricted with circular to elliptical shapes. Linkage of those faults leads to elliptical shapes for large composite faults. In contrast, faults ( or fault segments) whose vertical growth is restricted by the layer thickness evolve from circular to elliptical with progressively larger aspect ratios. In a sequence with heterogeneous strength and varying layer thickness, individual faults within each layer are restricted, producing a complex irregular tipline for the aggregate large fault. The irregular geometry will persist as long as the tendency for horizontal fault growth exceeds that for linkage; once linkage across layers occurs, then more regular elliptical tiplines will evolve. Because longer faults have larger displacements, then for brittle strain to be equal in layers of different thickness, more faults are needed in thin layers for the total displacements, and thus the strains, to balance. As a result, irregular fault shapes can evolve as more new faults grow in thinner layers into more rectangular shapes.