Rocks from Site 1136 exhibited relatively few structural features. We observed no significant tectonic structures in sediments and sedimentary rocks that overlie the basaltic basement. We measured, and reported in the vein-structure log for this site (see the "Supplementary Materials" contents list), the orientation, location, and mineral filling of 315 structural features from the basaltic basement (Sections 183-1136A-15R-1 to 19R-2). Veins are the most common structural phenomena, and we present orientation information for the vast majority of veins in these cores. We also recorded the orientation of primary internal structures such as vesicle sheets coalesced into veins and wispy streaks of clay minerals replacing mesostasis. The absence of downhole logging information at this site, or a consistent structural marker plane (e.g., sedimentary bedding, magmatic/tectonic foliation) precludes reorientation of structural measurements and calculation of true-dip directions. We note, however, that the seismic reflection marking the top of basaltic crust at Site 1136 has an apparent dip of 1° (to the west) (see "Background and Objectives").
No tectonic breccias, shear zones, or ductile or cataclastic fault zones were encountered in the Hole 1136A cores. We observed only three microfaults. Two of these features are steeply dipping mineral-filled veins, which disrupt more gently dipping veins with a small (~1 to 2 cm) normal offset. The other feature is a small, steeply dipping clay vein with well-developed subhorizontal slickensides. The mineral fillings of both the displaced and offsetting veins are similar.
Veins are a common feature throughout the basaltic basement (Fig. F32). Although data are sparse because of limited basement penetration, veining and, in particular, carbonate veins are most abundant at the top of the basaltic section (Core 183-1136A-15R). The abundance of clay-filled veins is approximately constant with depth (less than a factor of 2 variation). The frequency distribution of the true dip of veins (Fig. F33) shows little discernible signal, except the hint that gently dipping veins (<30° true) are relatively more common in Unit 1. This coincides with the presence of abundant subhorizontal to gently dipping calcite-filled veins in Cores 183-1136A-15R and 16R. The number of wider, subvertical sets of conjugate veins increases near the base of Unit 2 (Cores 183-1136A-18R and 19R).
To examine possible tilting of the lava flows in response to regional or local faulting, we examined the orientation of 26 features in lava Units 1 and 2 that are interpreted to be originally subhorizontal. These features included vesicle sheets, wispy sheets of mesostasis defining a subtle foliation in the lava, and flattened vesicles. These features commonly develop with a subhorizontal, planar orientation because of flow of magma beneath a solidified upper crust. Especially in large sheet flows, the cooling front defining the base of the upper crust is generally horizontal, but exceptions can arise from significant surface topography (e.g., tumuli and inflation pits) (see "Physical Volcanology" in the "Explanatory Notes" chapter).
Inspection of the true dips of the 26 volcanic features as a function of depth in the lava flows (Fig. F34) leads to two postulations. First, the highly variable dip angles in the upper part of Unit 2 do not result from tectonics, and they probably reflect nearby topography in the upper surface of this flow. Second, both units consistently dip about 20°, perhaps related to faulting associated with formation of the Labuan Basin, which is only 8 km to the east. However, without downhole logging data, it is not possible to confirm that all dips are in the same direction, or that they are consistent with orientation of faults seen in the seismic lines (see "Background and Objectives").