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doi:10.2204/iodp.proc.330.104.2012

Structural geology

At Site U1373 on the eastern side of Rigil Guyot, structures represent syn- to late- and postmagmatic features, comprising magmatic flow alignments/foliations, fractures, veins, vein networks, and geopetal structures (Fig. F34; Table T7). The characteristics, orientations, and distribution of these structures are described below.

Geopetal structures were observed at 2.9, 9.7–10.8, 16.5, 25.9, and 36.9 mbsf (Fig. F35). All geopetals have horizontal infilling, implying that this part of the seamount experienced little or no reorientation after these geopetals were filled. The paleomagnetic directions (see “Paleomagnetism”) were therefore determined on lavas that are still in their in situ position.

Fractures (N = 110) and veins and vein networks (N = 290) are the dominant structural features at Site U1373 (Fig. F34). Both veins and fractures were found predominantly in the lava flows of Units IV and VII, with fewer fractures or veins in breccia or interbedded sediment (Figs. F36, F37). Rocks at Site U1373 host considerably more veins and fractures per meter than those at Site U1372, and hence this part of Rigil Guyot likely experienced more fluid flow than Site U1372 on Canopus Guyot. Fractures are especially abundant in the lowermost Unit VII, with as many as 11 fractures per meter (Figs. F36, F37), which is more than twice the density observed in other fractured rocks at Sites U1372 and U1373.

Unit VII also has moderate to strong magmatic foliation, which is visible in both hand specimen and in thin section. The magmatic foliation is defined by aligned plagioclase laths (Fig. F38). In contrast to Site U1372, elongated and aligned Fe-Ti oxides were not observed in the groundmass. Foliation directions within this unit vary from subhorizontal to subvertical, often over a scale of 1 m or less. These frequent direction changes in magmatic foliation are best explained by a sheet flow that experienced several horizontal injections of fresh lava, causing horizontal flow alignment, followed by flow inflation or vertical breakout events to produce the vertical mineral alignment. This explanation of repeated flow inflation is consistent with the thick nature of Unit VII (>22 m recovered) (Fig. F36). Repeated flow inflation events also explain why this unit is relatively homogeneous and fine grained. If this unit instead represented a dike or ponded lava flow, such a thick unit would be expected to have a coarser grain size toward the center of the unit, which was not observed. In certain sections, especially Sections 330-U1373A-13R-1 through 13R-4, where flow alignment is particularly strong, fractures and veins have similar orientations to the flow texture. If the pieces could be corrected for core barrel rotation, these flow textures would yield the direction of the lava flows, which would be useful for volcanological reconstruction of this site.

Dip and dip direction were measured for 143 veins, 57 fractures, and 24 magmatic foliations (Fig. F39). Fractures are dominantly steeply dipping, with a pronounced maximum of 70°–75°. There is also a conjugate set of fractures with shallower dips of 10°–15°. Most vein dips are between 10° and 40°, with a maximum at ~25°, although several veins have steeper dips of 50°–75° (maximum at 55°) and probably represent conjugate structures. The veins are likely healed fractures.

At Site U1373, single veins (and individual veinlets within vein networks) have an average width of 1 mm and a maximum width of 9 mm. These widths are similar to those observed at Site U1372 and are relatively narrow compared to those at other submarine volcanic environments, such as the Emperor Seamounts (~2–4 mm; Tarduno, Duncan, Scholl, et al., 2002). The comparatively narrow vein widths indicate that the volume of fluid passing through the rocks of Site U1373 might have been lower than in Emperor Seamount drill sites.

Summary

Structures found at Site U1373 are veins, vein networks, fractures, magmatic foliations, and geopetals. The location, size, and orientation of these features were measured where possible. The occurrence of horizontally oriented geopetal structures indicate that these units are in situ and have not been tilted since deposition. A large portion of veins and fractures occur in hard lava flows of the lower units, with relatively few in breccia or volcaniclastic sediments. There is a bimodal distribution of dip angles for veins and fractures, with maxima of 70° and 15° for fractures and 25° and 55° for veins. Unit VII has moderate to strong flow textures that change direction from subhorizontal to subvertical, often within 1 m intervals. These changes in flow textures in Unit VII are best explained by a single thick lava flow unit that underwent several episodes of lava injection and flow inflation.

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