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 (σ2' and σ3' < 0). Breccias can form by disaggregatio n 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 hydrofracturi ng 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.