Vein structures that have been described from the slope apron sediments of modern accretionary prisms represent brittle failure caused by an effective tensile stress. The effective stress arises from abnormal pore-water pressure acting in opposition to the compressional stresses on the sediment. Thus, even though σ3 (the minimum stress) may be compressional, if the pore water pressure is of greater magnitude than σ3, the resultant effective stress will be tensional, and failure (opening of the veins) will occur in the σ1–σ2 plane. As the veins open, the pore water can escape into the veins and eventually out of the sediment along more permeable (coarser) beds. The initial movement of pore water into the veins may entrain finer grains from the host rock near the vein walls, resulting in the observed finer grained vein filling. Then, as the veins close as pore water escapes, the elongate and platy grains may be rotated into near parallelism with the vein walls. The tributaries that are observed at the ends of some veins may be a result of continued dewatering after the veins have closed, as the pore water is forced to diffuse back into the sediment after being concentrated into an efficient flow by the parallel preferred orientation of the platy mineral grains in the vein.

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