Numerous well-developed vein structures occur in Leg 112 cores from the Peru margin. Mud-filled dewatering veins occur in four styles: wide discrete gashes, veins along extensional microfaults, tension-gash arrays, and injections into tension gashes about slump-fold noses. Mud veins occur at shallow depths, less than 10 meters below sea floor (mbsf) in some cases, but some also were recovered as deep as 423 mbsf. Mud vein fill is slightly different in composition than the matrix and consists of a higher proportion of very fine silica, and in some cases, more authigenic carbonate or iron sulfide, transported into the veins by fluid flow. Clay minerals and many diatom fragments that are subhorizontal outside the veins have been rotated within the veins to parallel the vein walls.Formation of mud veins occurs during sediment dewatering, perhaps initiated by the collapse of diatom frustules during compaction, or by overloading during downslope slumping and sliding. Complex multigenerational vein sets exhibit sequential development from wide discrete gashes to injections along microfaults to dense arrays of sigmoidal tension gashes, with progressive deformation of older veins during continued downslope movement. Critical factors required to produce mud veins are high sedimentation rates, relatively high porosity and resistance to compaction, and low permeability; vein preservation is enhanced by anoxic conditions that discourage bioturbation.Carbonate-filled veins in Peru margin samples include tension gashes and wide, banded zones filled with calcite or dolomite spar; these probably result from filling of open fractures. Some sparry veins are crosscut by dewatering veins filled with calcareous mud, suggesting that the carbonate veins begin to form very early, before mud interbeds have de watered.

New data indicate that the Trinity terrane of northern California is a polygenetic composite terrane rather than a single slice of oceanic lithosphere. We suggest approximately one third of the Trinity terrane consists of Silurian intrusive rocks that represent the roots of a previously unrecognized Silurian magmatic arc. Crosscutting relations and U‐Pb zircon isotopic data document at least one early Paleozoic deformation in the Trinity terrane of northern California. A ductile shear zone between Neoproterozoic metagabbro and Ordovician(?) harzburgite is intruded by the Upper Silurian China Mountain pluton. This evidence indicates a major early Paleozoic shear zone formed in the eastern Klamath Mountains after the Middle Ordovician but prior to Late Silurian plutonism.

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Impermeable, fragile sediment samples can be successfully impregnated for petrographic study by treating them with a low-viscosity epoxy resin. Before impregnation in the epoxy resin, samples are soaked in acetone until water has been completely removed. Samples are soaked in resin for several days, then cured in an oven at 50° and 70°C. The whole procedure takes 6 to 10 days. After hardening, samples are prepared for study using conventional techniques. Microstructures such as mud veins, microfaults, and delicate primary textures can be preserved well using this technique. Suess, E., von Huene, R., et al., 1990. Proc. ODP, Sci. Results, 111: College Station, TX (Ocean Drilling Program).

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