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doi:10.2204/iodp.proc.330.107.2012

Structural geology

Structures observed, measured, and described at Site U1376 on Burton Guyot are sedimentary bedding, geopetals, veins, vein networks, fractures, magmatic flow textures (flow alignment and vesicle bands), and igneous contacts (Fig. F31). The characteristics, orientations, and distribution of these structures at Site U1376 are described below.

Throughout Hole U1376A, veins (N = 1190, with 1489 individual features) and vein networks (N = 280, with 1995 individual veinlets) are clearly the dominant structural features (Figs. F31, F32, F33, F34, F35). At least two generations of vein growth are indicated in some samples, exemplified by Thin Section 252, which shows an earlier, more coarsely crystalline calcite vein that is crosscut by a later carbonate vein containing cross fibers (Fig. F35A). Hole U1376A has the highest vein density of any hole drilled in the Louisville Seamounts, with a maximum density of 39 veins per meter (Fig. F32), as compared to the previous maximum of 19 veins per meter at Site U1374. Veins are particularly abundant from 42 to 63 mbsf (upper portions of Unit III) and from 146 to 178 mbsf (lower portions of Unit IV), with typically >20 veins per meter over these intervals (Fig. F32). Veins are also commonly wider than those observed at other Louisville drill sites, with numerous veins between 5 and 10 mm wide (Fig. F34) and some as wide as 30 mm, indicating a higher fluid flow in this part of Burton Guyot than that at previous sites. However, similar amounts of veins were observed at the Emperor Seamounts (Tarduno, Duncan, Scholl, et al., 2002). In further contrast to previous sites, veins and vein networks are abundant in hyaloclastites and breccia in addition to the rheologically hard lava units (Fig. F32). Indeed, the highest density of veins per meter in Hole U1376A is found within hyaloclastite units (Fig. F32). These factors taken together indicate that the processes of fluid flow active in this part of Burton Guyot were substantially different than those at the other Louisville sites.

Veins and vein networks at Site U1376 are overwhelmingly shallowly dipping (Figs. F33, F34), with a much higher proportion of subhorizontal veins compared to other Louisville sites. The most common dip angles are 25°–30° for veins (Fig. F33C) and 10°–15° for vein networks (Fig. F33D). This dominance of shallowly dipping veins, combined with their commonly “stepped” habit (Fig. F34) and the occurrence of vertical or near-vertical cross fibers (Fig. F35), requires that the minimum principal stress axis is tension in a subvertical direction (i.e., normal to the vein orientation) (Ramsay and Huber, 1987).

We propose two alternative mechanisms to produce the abundant subhorizontal veins observed at Site U1376. Both mechanisms are related to the position of Site U1376 near the center of Burton Guyot, as opposed to the guyot edge for the other deep Louisville holes. In the first scenario, gravitational unloading during subaerial (wave) erosion of the volcano to sea level (i.e., forming a flat-topped guyot) would cause decompression, with the minimum component of stress in a subvertical direction. Because Site U1376 is located in the center of the seamount, the amount of overburden removed would have been greater than that for other sites drilled in the Louisville Seamounts, leading to a greater abundance of veins. Alternatively, a mechanism for uplift on oceanic volcanoes is magma inflation during growth (Chadwick et al., 2006). Similar to the erosion mechanism, the location of Site U1376 within the core of Burton Guyot would have enhanced the amount of uplift at this site relative to the other Louisville sites. Injection of magma may also have enhanced fluid circulation within the seamount, as may have occurred around 170 mbsf, where numerous veins are located proximal to the dike of lithologic Unit 41.

Fractures are sparse in Hole U1376A, with only 38 recorded in total (Figs. F31, F36). It is likely that most fractures formed were subsequently filled with the abundant vein material that pervades this part of Burton Guyot. The few fractures present are located mostly in massive parts of Units III and IV. These have gentle to moderate dips at a maximum of 20°–30° (Fig. F33F).

Sedimentary rocks in Unit I contain prominent bedding (see “Sedimentology”), and the orientations of these layers were recorded in collaboration with the onboard sedimentologists. Dip angles vary mostly from 0° to 20°, with a maximum at 15° (Fig. F33A). If the cores can be corrected for drilling-induced rotation, these structural measurements on bedding potentially will yield the paleodeposition direction(s) for these sediments, which will be useful for the sedimentologic reconstruction of this site. Other implications of these dip angles are discussed in “Sedimentology.”

Geopetal structures were observed from 23 to 85.5 mbsf, in the limestone of Subunit IIA, the conglomerate of Subunit IIB, and the volcanic basement (Table T7; Fig. F37). Spectacular large geopetals are present in fossils from Subunit IIB (Figs. F8, F9). Geopetal infills are overwhelmingly horizontal, indicating that this part of the seamount experienced negligible tilting after these geopetals were filled.

The lowermost portion of Hole U1376A is cut by two aphyric sheet intrusions (Fig. F10). The first (112–112.78 mbsf; lithologic Unit 20) has chilled margins and baked contacts preserved on both the upper and lower margins (Fig. F19). These baked contacts are <1 cm wide, indicating relatively low levels of heat flow from the dike into the country rock, which is consistent with the narrow width of this intrusion. The dike margins are moderately dipping (Fig. F19), but the orientation of vesicle bands from the center of this intrusion (Fig. F19) indicate that magma flow in this dike was subvertical to vertical.

The lower dike (170.80–179.76 mbsf; lithologic Unit 41) is visually different from other sheet intrusions observed at Site U1374 on Rigil Guyot. It is aphyric but also vesicle poor and cut by a dense vein network of thin (typically 0.1 mm wide), closely spaced veins, often with ~5 veins per centimeter. The vein orientations in this unit are also different from other rocks in Hole U1376A, with vertical veins dominating (Fig. F33E). These vertical features potentially represent small, infilled cooling or contraction joints subparallel to the dike margins, or they could be related to the direction of magma flow in the intrusion. The upper baked contact of this dike is <1 cm wide and steeply dipping. The lower contact was not recovered.

Magmatic foliations were recorded in only four intervals in Hole U1376A (Table T8), which is considerably less frequent than at other Louisville sites. Only one of these occurrences is from an in situ lava (109.3 mbsf; Unit III), indicating that magma flow during crystallization was not a common process in the center of Burton Guyot, as opposed to the seamount flanks sampled in Holes U1372A, U1373A, and U1374A at Canopus and Rigil Guyots. The conglomerate clasts in Subunit IIB indicate that such lava flow processes are present elsewhere on Burton Guyot.

Summary

Structural features at Site U1376 are dominated by veins (N = 1190, with 1489 individual features) and vein networks (N = 280 with 1995 individual veinlets). Veins are found in lavas flows, breccia, and hyaloclastites, with a maximum density of 39 veins per meter. The maximum vein width is 30 mm, and numerous veins are 5–10 mm wide, which is wider than at previous sites, indicating higher levels of fluid flow at Burton Guyot compared to other Louisville sites. Veins are dominantly shallowly dipping, with near-vertical fibrous carbonate crystal growth, indicating that the direction of minimum principal stress was subvertical during vein formation. Potential explanations for this stress regime are uplift and inflation of this part of the seamount during magma injection or decompression due to erosion of the guyot top. A total of 153 cases of sedimentary bedding are present, with the dominant dip angle being 15°. Geopetal structures are overwhelmingly horizontal, indicating that this part of the seamount has not been tilted since deposition of the geopetal infilling material. Only four intervals of magmatic foliation are present (three of which are in conglomerate clasts), demonstrating that significant magma flow was uncommon in the center of Burton Guyot.

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