Mechanical development of vein structures due to the passage of earthquake waves through poorly-consolidated sediments

Abstract

Vein structures are typically the earliest expression of brittle deformation within sediments. These mud-filled veins, which characteristically occur regularly spaced within bed-parallel arrays, form in sediments that possess a strong interlocking particle framework. Downslope creep has been proposed to explain the origins of vein structures, however, a recent suggestion that they are generated by the passage of earthquake shear waves through sediments explains aspects of their morphology, and their dominant occurrence at active convergent margins. Their coexistence with less disruptive “ghost veins” in Peru margin sediments, and their almost normal attitude to bedding, however, suggests that vein structures were formed by processes more complex than downslope creep, or seismically induced shearing alone. Experimental earthquake simulation was undertaken by laterally shaking a box containing crushed diatomite. Fractures were induced almost normal to the horizontal shaking direction, and to a lesser extent as antithetic Riedel shears, both of which closely resembled vein structures. The fracturing process during shaking may be viewed as a progressive fragmentation of the diatomite, in which new fractures form half-way between pre-existing ones. Thus fracture spacings are progressively halved. Shear zones oriented at a low angle to the shaking direction were also generated, combining with the high-angle fractures to form structures very similar to those observed in Peru margin sediments. When shaken, fines added to the diatomite segregated into planar zones that resembled ghost veins, half-way between fractures. The alternating pattern of fractures and fines indicated that a standing pressure wave had been created within the box during shaking. The fractures were created by alternating compression and extension at the antinodes, while the fines concentrated in zones of minimum grain movement around the nodal planes. This suggests that vein structures are initiated by the combined action of shear and pressure waves within a sediment. The strain waves may be seismic in origin, or may also form in downslope movement system.