Flexing is not stretching: An analogue study of flexure-induced fault populations

Abstract

Flexure-induced fractures are predicted to form along the axis of maximum tensile stress within a bending brittle plate. The mechanics of this process differ from extensional fault growth in response to lithosphere stretching, where a distributed set of simultaneously growing fractures evolves through elastic interaction. To simulate extensional fault growth during lithospheric flexure, partially solidified plaster layers resting on a foam rubber substrate were depressed by a linear load and fractured in analogue models. The length- and spacing-frequency distributions of the resulting crack populations were analyzed for a series of nine thin (5 mm) and ten thick (15 mm) layer experiments. Previous analogue stretching models predict power-law length-frequency distributions and clustered spacings ( C v > 1) at low strains (< ~ 10%), evolving toward an exponential distribution and more regular spacings ( C v < 1, often termed anticlusted) at larger stains. Crack populations formed at low strains during these bending experiments, however, exhibit length-frequency distributions that are not well described by either a power-law or exponential distribution model, being somewhat better fit by the exponential model in the thin layer experiments and somewhat better fit by the power-law model in the thick layer experiments. One-dimensional spacing-frequency distributions are well described by an exponential distribution model, and crack spacing can be characterized as anticlustered within both the thin and thick layer experiments. Although similar spacing patterns may develop when fracture growth is limited by mechanical layer thickness, the characteristic spacing does not scale with the layer thickness in these flexural experiments. Alternatively, the development of power-law (fractal) populations may be inhibited by the growth history of flexure-induced faults, whereby nucleation is localized spatially due to the distribution of stresses within bending plate. These analogue experiments may be relevant to the outer-rise regions of subduction zones, where the oceanic plate is flexed downward, and the abyssal flanks adjacent to fast-spreading mid-ocean ridge crests, where recent models for axial high development suggest that the plate is unbent as it rafts away from the axis.