Structural Evolution of Upper Layer 2, Hole 896A

Abstract

An analysis of the state of stress in the upper ocean crust shows that high hydrostatic pore-fluid pressures will result from the thick overlying water column, providing that the upper crust is sufficiently permeable. High pore-fluid pore pressures will result in very low effective stresses in the upper ocean crust. Such conditions favor deformation by formation of extension fractures and mixed mode (combined extension and shear) fractures rather than by frictional sliding on faults. High pore-fluid pressures are considered very likely to have produced many of the structures in Hole 896A, including jigsaw-puzzle breccias and associated vein networks, cement-supported breccias, and fibrous veins. These structures record an overall volume increase, and their geometry and offset markers indicate formation as extensional fractures or, in some cases, mixed mode fracturing. The geometry and textures of jigsaw-puzzle breccias and associated vein networks suggest formation by longitudinal (axial) splitting, a type of brittle rock failure that occurs in rock deformation experiments done under stress conditions similar to those in the upper ocean crust (σ 2 ' and σ 3 ' < 0). Breccias can form by disaggregation during volume increase associated with extension or mixed mode fractures. The textures of many breccias formed in this way are very similar to those of sedimentary breccias and thus the sedimentary vs. tectonic origin of many breccias in the drill core is uncertain, especially because the margins of the breccias are not usually preserved in core. The fibrous veins generally postdate jigsaw-puzzle breccias and vein networks. They probably formed by repeated cycles of hydrofracturing and sealing of fractures (crack-seal mechanism). The extension directions indicated by both fibrous and nonfibrous veins are highly variable in orientation, even within a single piece, and vein networks suggest simultaneous extension in several directions. In addition, small faults having fibrous slickensides occurring below 334 m below seafloor indicate normal, reverse, and oblique slip. The style of brittle deformation observed in Hole 896A should produce microseismicity characterized by a nonuniform stress field and non-double-couple earthquakes. Such microseismicity characterized geothermal activity in the post-rifting stage at Krafla, Iceland.

Deformation pattern in a massive ponded lava flow at ODP‐IODP Site 1256: A core and log approach

“…Pink dashed line marks the depth at which density, velocity, and porosity sharply change and are not correlated with core fracturing spikes, which occur slightly deeper, at 294 mbsf (see text). 2008GC002346 rected; see ''ncFFR'' in Figures 12 and 13) data obtained in this study by core analysis. Such a comparison is possible because the resolution (0.5-10 cm) of core data is comparable with the scale of physical properties measurements.…”

“…Pink dashed line marks the sharp change in porosity and velocity at 322 mbsf, where no cores were recovered (see text). 2008GC002346 at 322 mbsf, but at such depth there was no recovery of cores. Below this depth, the lava pond displays a trend of decreasing porosity, toward values as low as 3%, and slight increasing velocity, accompanied by a general decrease in fracturing.…”

Physical properties of young (3.5 Ma) oceanic crust from the eastern flank of the Juan de Fuca Ridge: Comparison of wireline and core measurements with global data

“…This knoll was interpreted as a small volcanic cone emplaced off-axis [Swift et al, 1998]. Drilling results of Hole 896A show that fracturing is stronger and alteration is more pervasive than seen in neighboring Hole 504B [e.g., Harper and Tartarotti, 1996;Wilkens and Salisbury, 1996;Bartetzko et al, 2002]. Much like the basement surrounding Site U1301, basement at Site 896 was likely one of the last areas of seafloor to be buried by accumulating sediments.…”

Site 1256

“…Good examples of incipient breccias occur in intervals 309-1256D-83R-2 (Piece 5, 16-21 cm) and 108R-2 (Pieces 2-6, 4-30 cm), but such features occur irregularly in Cores 78R through 128R, mostly in small unoriented pieces. These characteristics suggest in situ formation, with fragmentation occurring along veins grading into a vein network that isolates clasts of intact basalt (Harper and Tartarotti, 1996). Disaggregation of the host basalt would be aided by contractive cooling joints.…”