STRUCTURAL GEOLOGY

Ten basement units have been defined in Hole 1137A, including seven igneous units interpreted to be individual lava flows and three volcaniclastic sedimentary units (Units 5, 6, and 9) (Fig. F72) (see "Lithostratigraphy," "Physical Volcanology," and "Igneous Petrology"). Rocks from Site 1137 exhibit relatively few structures, and we recorded no significant tectonic features from the sediments and sedimentary rocks overlying basement. We measured the orientation, location, and mineral filling of features from all basement units (see vein-structure log on the "Core Descriptions" contents list). The massive interiors and bases of the lava flows yield the vast majority of the measurements, and most structural orientations relate to mineral-filled passive fractures and joints, although a few, very minor, normal faults are present in the core. Highly vesicular or brecciated flow tops are common features above the massive interiors of most lava flows, but the chaotic, clastic internal arrangement of these intervals does not lead to planar structural features. Hence, the relatively few orientation measurements from these intervals do not mean that these breccias are either structureless or unaltered.

Mineral-filled veins are the most common structural phenomena, and we recorded the orientation of more than 550 features from the basement units (Fig. F72). To interpret the structural data at this site, we have divided the lava flows into three packets (Units 1, 2, 3, and 4; Units 7 and 8; and Unit 10) separated by the volcanic-derived sediments. Veins are most abundant in the uppermost basement rocks (basement Unit 1; Core 183-1137A-24R) that underlie glauconitic sandy packstone of lithologic Unit III. Thick (10 mm), predominantly subhorizontal calcium carbonate veins abound in these rocks (Fig. F72). This part of the basement is the most heavily veined and is the only portion of the basement where gently dipping features dominate. The abundance of veins in this most heavily fractured portion of Hole 1137A is similar to the average vein abundance in Hole 1136A (see "Structural Geology," in the "Site 1136" chapter). In the remainder of the basement rocks, vein minerals are less prevalent even though the fracture density may be similar (cf. Units 7 and 8) (Fig. F73).

Large vesicles and other open voids in brecciated or highly vesicular portions of Units 3 and 4 are commonly filled or partly filled with well indurated, laminated siliceous mudstone (see "Alteration and Weathering"). On average, these laminations are gently inclined (true dip ~16, N = 6), requiring a subtle rotation of the lava pile if an original subhorizontal sedimentary layering is assumed.

The boundary between basement Unit 4 and the underlying volcanic sediments comprises a gently-dipping irregular contact between a cryptocrystalline to glassy chilled basalt and dark black baked silty sediments. This contact is well-preserved in the archive half of Section 183-1137A-33R-1 and dips ~20. The boundary is also clearly recognizable in the FMS logs from this hole with a true orientation of 24/076 (dip/dip direction) (see "Downhole Measurements"), and this should allow the determination of (true) orientation of structural and paleomagnetic features in both the overlying basalts and underlying sediments.

The sediments below (~10 cm) the black, baked zone are intensely fractured and brecciated by a network of predominantly subhorizontal carbonate veins. A single subvertical fracture cuts across bedding in the upper 50 cm of Unit 5 and is carbonate filled directly below the upper contact yet filled with dark green clay slightly deeper in the section. The crystal-lithic volcanic siltstones and sandstones of Unit 5 have both sedimentary bedding and cross-bedding with subhorizontal to gently dipping orientations (cf. geopetal structures described later in this section). These beds are offset by at least two high angle (65 to 70) normal faults with minor offsets (2 cm; Section 183-1137A-33R-2). Disruption of bedding at the margins of the core suggests that these minor offsets may be peripheral to a more significant fault.

Sedimentary structures in the volcanic conglomerate and sands (Unit 6) are comprehensively described elsewhere (see "Lithostratigraphy" and "Physical Volcanology"), and we observed few features of direct tectonic origin. The most prominent structures are moderately to steeply dipping, irregular carbonate veins (~2 to 10 mm) that generally do not form planar features but meander around the margins of the volcanic clasts and only rarely crosscut smaller cobbles.

Basement Units 7 and 8 are sandwiched between the volcanic conglomerate and the crystal vitric tuff (Units 6 and 9, respectively), and the massive portions of these lava flows are heavily veined with numerous moderately to steeply dipping fractures filled with dark green clay that dissect the core. Many fractures appear to have formed in conjugate sets and many very thin (0.1 to 0.5 mm) veins filled with dark green clay. Although veins abound in these units, the abundance of vein minerals (in millimeter veins per meter of core) (see Fig. F73) is low compared to the uppermost part of the basement, because the veins are generally thin (<0.5 mm). We observed rare zeolite-filled, high-angle (>75) veins in Unit 7. These veins display a staircase pattern with dilated steep ramps (1 to 2 mm) filled with pink and transparent zeolites and thin clay-lined (~0.2 mm) flats, indicating normal displacement.

The crystal vitric tuff (basement Unit 9) displays a weak, subhorizontal fabric delineated by the clayey matrix and the feldspar crystals. This fabric may be related to the compaction and alteration of the glassy tuff matrix. Flattened lithic clasts with intense green clay halos are aligned subparallel to the fabric of the tuff, and this may delineate sedimentary bedding. The most prominent features within the tuff are diffusely bounded sections of contrasting color, reflecting the presence of different clay minerals (saponite and celadonite) and their oxidation products. High-angle (>60) normal fault surfaces intersecting the core are strongly grooved, and the clay-lined surfaces are polished to a resinous luster. Trace pyrite is present on these surfaces. Also present are ragged, subhorizontal shear-zones comprising dense networks of anastomosing wispy olive-green clay veinlets.

The uppermost part of Unit 10 is brecciated, and irregular clasts of quenched lava are supported in a matrix of blue-green, indurated silty sandstone (see "Physical Volcanology"). These textures are consistent with the interaction of magma with water-saturated sediments or reworking of a flow-top breccia by sedimentary processes. Calcium carbonate and zeolites are only minor constituents of these complex breccias, partially filling open voids and vesicles. The underlying massive portion of Unit 10 is highly plagioclase phyric. The phenocrysts exhibit subtle subhorizontal alignment and are commonly replaced by calcium carbonate. Veins are uncommon in Unit 10, though in contrast to the uppermost parts of the basement, the rare carbonate veins present are inclined at steep angles (Figs. F72, F73).

Wispy segregations of mesostasis, now completely altered to clay minerals, are common in massive parts of all the lava flows (see Fig. F74). These glassy streaks are commonly oriented subparallel to vesicle trails. Rare subvertical vesicle-rich zones appear to be vesicle cylinders (see "Physical Volcanology") that cut the volcanic fabric. Both sets of features generally dip gently (~25), requiring either a nonhorizontal original orientation or a slight tilting of the basement. The orientations of both mesostasis segregations and vesicle trails are consistent throughout the lava pile. Although some evidence, from the gentle dip of the vesicle trails, the mesostasis segregations, and the included, indurated laminated sediments, suggests slight rotation of the lava flows, the subhorizontal bedding in the intercalated volcaniclastic sediments indicates negligible tectonic tilting.

Abundant geopetal structures are preserved in the igneous rocks of the basement. These features are present in vesicles and other voids that are partially or completely filled by alteration minerals, most commonly clays or banded agate (amorphous silica). There is no consistent paragenetic sequence of mineral filling, and voids with clay overlying agate, as well as vice versa, are present. These geopetal structures consistently dip horizontally or subhorizontally (<10), indicating negligible rotation of the basement after precipitation of the minerals.

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