The basement rocks recovered from Hole 1138A displayed relatively few structural features. The lava flows that comprise Units 3-22 have brecciated flow tops and thin massive interiors. Veins are the most common structure, and they are dominantly in the more highly fractured massive portions of basement Units 3-22 (see "Igneous Petrology"). The true dip, vein abundance, and fracture density of oriented features from the basement of Hole 1138A (Fig. F70) are recorded in the vein-structure log for this Site (see the "Core Descriptions" contents list).
The uppermost portions of Unit 1 include intensely weathered igneous rocks, completely altered to brown to gray to green clays, but rare pieces retain a relict igneous texture. We could not establish the orientation of either veins or the prominent flow-banding because loose felsic pebbles and other altered lava types from beneath the altered top of Unit 1 do not include any oriented pieces.
Unit 2 contains multiple beds or regular layers of highly altered volcaniclastic material with variable concentrations of pumice, lithic clasts, and ash (see "Physical Volcanology"). Bedding is not strongly developed in most of this unit, but diffuse laminations, flattened clasts, and rare scoured bases of the debris and/or pyroclastic flows indicate a subhorizontal orientation for the sediments. The ashes and other clay-rich horizons fracture along subconchoidal partings.
The structure in the subaerial basaltic lavas in Hole 1138A (Units 3-22) is monotonous and follows a regular pattern of brecciated flow tops, which overlie minor, more massive interiors hosting numerous veins (clay > zeolite > calcite). Few structures provide evidence for significant tectonic disturbance, and slickensides are rare on either vein surfaces or on the rims of breccia clasts. A minor interval of tectonic breccia, comprising fractured altered glass within a zeolite-clay matrix, is present at the contact between Units 3 and 4 (Interval 183-1138A-80R-1 [Pieces 16-19, 118-154 cm]) (Fig. F70). Three other subplanar contacts between basement units or subunits were recovered intact, and these structures all have moderate dips (Table T15).
We recorded the orientation of >350 veins and their mineral fillings from the basaltic lava flows (Fig. F70) (see also "Alteration and Weathering"). Vein abundance and fracture density generally decrease with depth (Fig. F70). Clay minerals fill two-thirds of the veins, whereas minor (4%) calcite veins are present at the top of the basaltic section (Unit 3) (Sections 183-1138A-80R-1 to 80R-3) and zeolite-filled veins (~30%) are in most of the underlying lava units. The clay veins do not dip at any preferred angle throughout the section, but the carbonate veins most commonly dip gently. Zeolite veins dip moderately to steeply (Figs. F70, F71).
Geopetal structures are rare (N = 6) in Hole 1138A, and the boundaries between the different mineral fillings (clays, calcite, and zeolite) in these features are subhorizontal (<10° dip), although a consistent mineral paragenesis is not present. Fine wisps of mesostasis altered to clay minerals, and veins formed by the linking of vesicle trails subparallel to the mesostasis fabric, dip gently to moderately (average dip = 33°, N = 33).