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doi:10.2204/iodp.proc.330.108.2012

Structural geology

Structures observed, measured, and described at Site U1377 on Hadar Guyot are geopetals, veins, vein networks, fractures, vesicle bands, and igneous contacts (Fig. F21). The characteristics, orientations, and distribution of these structures at Site U1377 are described below.

In Hole U1377A, veins (N = 135; 208 individual features), vein networks (N = 43; 358 individual veinlets), and vesicle bands (N = 38) are the dominant structural features (Fig. F21A). These features are also dominant in Hole U1377B, where 224 veins (306 individual features), 59 vein networks (432 individual features), and 38 vesicle bands were recorded (Fig. F21B).

Veins are abundant in the recovered basement from both Holes U1377A and U1377B (Fig. F22). The highest vein density (34 veins per meter) in Hole U1377A was observed at 15.1–16.6 mbsf (lithologic Unit 1), whereas most of the recovered intervals have 5–25 veins per meter. In Hole U1377B, the maximum vein density is 26 veins per meter at 20–21 mbsf (lithologic Unit 2), whereas other recovered intervals typically have 10–24 veins per meter. Average vein widths in Holes U1377A and U1377B are 0.6 and 1.1 mm, and the maximum vein widths in these holes are 9 and 12 mm, respectively. Thus, Site U1377 has a similar average vein width to Sites U1372, U1373, and U1374 but smaller average width compared to Site U1376. The vein density at Site U1377 is intermediate between Site U1376 (which has a very high vein density) and the other Louisville sites (which have generally lower vein densities), indicating only intermediate amounts of fluid flow in this part of Hadar Guyot compared to the other sites. The veins are moderately to steeply dipping in both Holes U1377A and U1377B (Fig. F23A, F23D).

Fractures are sparse at Site U1377, with only five observed in Hole U1377A and none observed in Hole U1377B (Fig. F21). In Hole U1377A they occur in the aphyric rocks of lithologic Unit 3, with three oriented features having moderately steep dips of 50°–60° (Fig. F23B).

In Hole U1377B a single horizontal geopetal structure is present at 0.8 mbsf (Fig. F24), providing evidence that this part of Hadar Guyot has not been tilted since deposition of the geopetal infilling material.

The lower part of Hole U1377B has several glassy chilled margins that are interpreted as the quenched rims of pillow lavas (see “Igneous petrology and volcanology”). These chilled margins have moderate to steep dips that range from 45° to 90° (Fig. F23E). Subparallel to these chilled margins are zones of vesicle banding (Fig. F10), which also yield moderate to steep dips (Fig. F23F).

Two zones of glass in Hole U1377B do not simply represent quenching on pillow rims. In interval 330-U1377B-5R-2A, 96–114 cm (Fig. F12), moderately dipping vesicle bands on the top right of the image are truncated by glassy material. This glass progressively grades into more crystalline material toward the base of this interval, with vesicle banding parallel to the edges of the glass. This crosscutting relationship indicates that molten material from below was intruded into the rock above. In interval 330-U1377B-4R-5A, 1–34 cm, angular fragments of trachybasalt are surrounded by glassy material (Fig. F11). Both of these examples indicate forceful injection of magma after the surrounding material cooled. Although these particular lithologic units have not yet been analyzed chemically, this behavior may be a result of the more evolved—and therefore more viscous—magma (of trachybasalt composition) found elsewhere at Site U1377 (see “Geochemistry”).

Vesicle bands in Hole U1377A are typically steeply dipping or subhorizontal (Fig. F23C). The vesicle bands in Hole U1377A are visually similar to those recorded in Hole U1377B, with the exception that no glassy chilled margins were found. The vesicle bands in Hole U1377A were potentially formed in the same manner as those in Hole U1377B (as pillows), but chilled margins were not recovered because of the low recovery in Hole U1377A.

Summary

Structural features at Site U1377 are dominated by veins, with as many as 34 and 26 veins per meter recorded in Holes U1377A and U1377B, respectively. The average vein widths in Holes U1377A and U1377B are 0.6 and 1.1 mm, respectively, and the maximum vein widths are 9 and 12 mm. Site U1377 therefore has similar vein widths as Sites U1372, U1373, and U1374 (but with somewhat higher vein density than these sites) and fewer and smaller veins than Site U1376. The veins have moderate to steep dips in both Holes U1377A and U1377B. A single horizontal geopetal structure at 0.8 mbsf in Hole U1377A provides evidence that this part of Hadar Guyot has not been tilted since deposition of the geopetal infilling material. In Hole U1377B glassy chilled contacts and vesicle bands have moderate to steep dips. Glass also occurs in Hole U1377B along the edge of a small intrusion as well as in the matrix of a breccia, indicating the forceful local injection and rapid cooling of new magma of trachybasalt composition into already solidified rocks.

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