Vein Structures of the Peru Margin, Leg 112

Abstract

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.

Layer‐parallel faults, duplexes, imbricate thrusts and vein structures of the Miura Group: Keys to understanding the Izu fore‐arc sediment accretion to the Honshu fore arc

“…As hitherto known, most of the vein structures in the world are found in the area of trench slope basins or fore-arc basins (e.g. the middle America trench landward slope off Guatemala, the Ogawa & Miyata 1985; the Izu-Bonin trench landward slope off Izu arc, Ogawa et al 1992; the Japan trench landward slope off northern Honshu, Arthur et al 1980, Leggett et al 1987, Knipe 1986; the Peru trench landward slope or basins, , Linsley-Griffin et al 1990). …”

Site 986

“…Although only few distinct silty to sandy layers have been observed within this sequence, possible mass-wasting activity is indicated by a number of sediment structures, such as inclined beds, contorted beds, and other slumping features. Slumping and sliding also can contribute to vein formation by producing sudden loading and by producing microstructures that localize veins (Lindsley-Griffin et al, 1990). The conspicuous lack of gravel within Unit III may be related to the size of …”

Deformation Structures at Site 808, Nankai Accretionary Prism, Japan

“…The origin of vein structures has been discussed by a number of workers, such as Cowan (1982), Ritger (1985), Knipe (1986), andLindsley-Griffin et al (1990), though it is by no means clear that all these workers are dealing with identical structures. The structures from Site 808 are closely similar to those described by Ogawa et al (in press), who suggested that a certain combination of extensional stress, elevated pore pressure, and lithology was required.…”