Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D
[1] A numerical model incorporating experimentally determined fracture surface geometries and fracture permeability is proposed for characterizing aperture structures and fluid flow through rock fractures under confining pressures. The model was applied to artificially created granite tensile fractures with varying shear displacements (0-10 mm) and confining pressures (10-100 MPa). The findings of the study were consistent with those obtained previously, which characterized experimentally determined contact areas and changes in shear stress during the shear process. While the confining pressures considered herein are higher than those of previous studies, experimentally obtained fracture permeability is important for understanding subsurface flow, specifically the fluid flow characteristics in aperture structures under different confining pressures. Development of preferential flow paths is observed in all aperture structures, suggesting that the concept of channeling flow is applicable even under high confining pressures, as well as the existence of 3-D preferential flow paths within the subsurface fracture network.
Shear (Mode II) fractures with shear displacements of 1 and 5 mm were generated by direct shear on granite under normal stresses of 1, 20, and 60 MPa. Fracture surface mapping showed that the surface roughnesses of the shear fractures decreased with increasing shear displacement and normal stress and were smaller than those of tensile fractures reported in our previous study. Fluid flow experiments on the shear fractures provided fracture permeabilities at a wide range of confining pressures of 10–100 MPa. Nonmonotonic permeability was usually observed to decrease with increasing confining pressure. However, the permeability changes were different between the shear fractures generated at the normal stress of ≤20 and 60 MPa. In addition, obvious permeability changes with shear displacement were observed for 60 MPa, whereas no significant difference was observed for ≤20 MPa. Comparing the shear fractures with the tensile fractures having shear displacements revealed clear differences, even for equivalent shear displacements. Numerical models that were constructed using the data of the fracture surface mapping by matching their permeabilities with the experimentally evaluated fracture permeabilities revealed the development of preferential flow paths, i.e., channeling flows, for the shear fractures, providing a diversity of channeling flow in heterogeneous aperture distributions of rock fractures in the Earth's crust.
The present study evaluates aperture distributions and fluid flow characteristics for variously sized laboratory-scale granite fractures under confining stress. As a significant result of the laboratory investigation, the contact area in fracture plane was found to be virtually independent of scale. By combining this characteristic with the self-affine fractal nature of fracture surfaces, a novel method for predicting fracture aperture distributions beyond laboratory scale is developed. Validity of this method is revealed through reproduction of the results of laboratory investigation and the maximum aperture-fracture length relations, which are reported in the literature, for natural fractures. The present study finally predicts conceivable scale dependencies of fluid flows through joints (fractures without shear displacement) and faults (fractures with shear displacement). Both joint and fault aperture distributions are characterized by a scale-independent contact area, a scale-dependent geometric mean, and a scale-independent geometric standard deviation of aperture. The contact areas for joints and faults are approximately 60% and 40%. Changes in the geometric means of joint and fault apertures (μm), e m, joint and e m, fault , with fracture length (m), l, are approximated by e m, joint = 1 × 10 2 l 0.1 and e m, fault = 1 × 10 3 l 0.7 , whereas the geometric standard deviations of both joint and fault apertures are approximately 3. Fluid flows through both joints and faults are characterized by formations of preferential flow paths (i.e., channeling flows) with scale-independent flow areas of approximately 10%, whereas the joint and fault permeabilities (m 2 ), k joint and k fault , are scale dependent and are approximated as k joint = 1 × 10 À12 l 0.2 and k fault = 1 × 10 À8 l 1.1 .
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