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doi:10.2204/iodp.proc.330.105.2012

Structural geology

Structures observed, measured, and described at Site U1374 on the western side of Rigil Guyot are sedimentary bedding, geopetals, veins, vein networks, fractures, magmatic flow textures, vesicle bands, and igneous contacts (Fig. F51). The characteristics, orientations, and distribution of these structures at Site U1374 are described below.

Sedimentary rocks are common in the upper parts of Hole U1374A, with thick intervals from 0–16.70 mbsf (Units I and II), 37.60–41.84 mbsf (Unit VII), 63.67–84.70 mbsf (Unit IX), and 109.87–116.45 mbsf (Unit XI) (Fig. F4). Thin sedimentary layers are also interbedded between volcanic units deeper in the hole. Many of these sedimentary units 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 33°, with a single ~10 cm wide sedimentary zone at 162.3 mbsf (Unit XIII) having an extremely steep dip of 55° (Fig. F52A, F52D). Excluding Unit II, most sedimentary beds have dips of 20°–25° (Fig. F52B), which is likely close to the maximum angle of repose for water-saturated sand-size sediments. In contrast, bedding in Unit II is much shallower, dominantly between 0° and 5° (Fig. F52C), which reflects the onset of deposition of the seamount’s sedimentary cover in a subtidal to intertidal environment following the initial drowning of the former volcanic island at Site U1374 (see “Sedimentology”). Dip angles within Unit II also change systematically with depth (Fig. F52E). From the bottom upward, Unit II starts with a single relatively steep dip at 15.06 mbsf (arrowed) before abruptly dropping to near-horizontal dips (12.82–11.36 mbsf), followed by a gradual increase of dip angles upsequence (Fig. F52E). These changes in dip angles correspond with other sedimentologic variations in the stratigraphic subunits of Unit II (see “Sedimentology”). If the core can be corrected for drilling-induced core rotation, structural measurements on bedding will yield the paleodeposition direction(s) for these sediments, which will be useful for the sedimentologic reconstruction of this site.

Lower in the sequence, from 16.8 to 139.3 mbsf, 35 geopetal structures were observed within sediments, volcanic breccia, lavas, and peperitic horizons (Table T9; Fig. F53). Except for a single geopetal at 29.6 mbsf where the underlying 6 cm of sediment has clearly slumped, all geopetal infills are horizontal, indicating that this part of the seamount experienced little or no tilting after the geopetals were filled.

The lowermost third of Hole U1374A is cut by at least 10 sheet intrusions of variably vesicular aphyric basalt (Fig. F18). Because of excellent recovery in these cores, in many cases the chilled margins and baked contacts were preserved on both the upper and lower contacts (Figs. F54, F55). The widths of the baked contacts vary from <1 to >7 cm, indicating considerable variability in the amount of heat supplied by the different intrusions. This variability is likely controlled, at least in part, by the length of time during which magma flowed through the intrusion. Other variables include the magma temperature, the type of country rock, and the degree of water saturation in the country rock.

The chilled contacts and baked margins have dips ranging from ~25° to 90° (Fig. F56C), indicating they are moderate to steep intrusions. Within these intrusions there are numerous vesicle bands with high- and low-density vesicle concentrations (Figs. F54, F55). These features likely formed by volatile segregation during magma flow. Near the edge of the intrusions, the vesicle bands run parallel to the chilled margins (Figs. F54, F55). Toward the center of the intrusions, however, the vesicle bands become much steeper. This is the case even for intrusions with moderately dipping contacts, indicating that most of the magma flow within the intrusions was subvertical to vertical (Fig. F56D). The sheet intrusions also contain zones of magmatic foliation within the crystalline groundmass (Table T10). Where magmatic foliations and vesicle bands occur adjacent to each other, both features yield (sub)parallel directions.

Lithologic Units 146 and 148 in stratigraphic Unit XIX at the base of Hole U1374A yielded both subhorizontal and subvertical magmatic foliations within 1–2 m of core. As such, these units may not be sheet intrusions, where unidirectional flow is typical. These units could instead represent inflated lava flows, formed in a similar manner to Unit VII of Hole U1373A on the other side of Rigil Guyot. Both lithologic Units 146 and 148 are highly fractured (Fig. F57), in stark contrast to the clearly intrusive sheets that occur from 318 to 496 mbsf but again similar to Unit VII in Hole U1373A. Inspection of the upper margin is the best way to distinguish if these units represent flows or intrusions. Unfortunately, the upper margin for Unit 146 was not recovered and the upper contact for 148 is inconclusive. Because there is no conclusive evidence for these units to be flows, and given the steep margins for Unit 146 observed during downhole logging (see “Downhole logging”), it was decided that these units are more likely intrusions. Nonetheless, if these units do instead represent flows, they will provide the stratigraphically lowest samples for paleomagnetic, geochronological, geochemical, and petrologic analyses for this seamount.

Magmatic foliations were also observed in numerous lava flows in the upper parts of Hole U1374A (Table T10). These lavas generally yielded gently dipping flow orientations (Fig. F56E). Thin section inspection revealed that the magmatic foliations are defined by aligned plagioclase laths within the groundmass (Fig. F58). Many samples with flow textures also contain aligned titanomagnetite crystals within the groundmass (Fig. F58C, F58D). Such samples have moderate to strong anisotropy of magnetic susceptibility (see “Paleomagnetism”). After correction for drilling-induced core rotation, these flow orientation structures, including vesicle bands and magmatic foliations, will be useful for the volcanological reconstruction of Rigil Guyot.

Throughout Hole U1374A, veins (N = 916 intervals, with 1229 individual veins) and vein networks (N = 515, with 3225 individual veinlets) are clearly the dominant structural features (Fig. F51). Veins and vein networks are distinctly more frequent in the lava flow units and intrusive sheets (Fig. F59). The highest density of veins is found in the lava flows of Units IV and VI, with as many as 14 and 18 veins per meter, respectively (Fig. F59). This vein density is midway between the most vein-rich units in Holes U1372A and U1373A. In contrast to these previously drilled sites, an appreciable number of veins and vein networks is also present within volcanic breccia units, with typically 2–4 veins per meter (Fig. F59). Most of the veins in breccia units are still restricted to the rheologically harder and larger clasts, although very rarely veins also occur within the breccia matrix (Fig. F60). The maximum vein width at Site U1374 is 25 mm, which is wider than at previous Louisville sites. Nonetheless, veins wider than 10 mm are very uncommon, and the overall average vein size (0.8 mm) is still relatively small compared to other seamount settings (e.g., Emperor Seamounts; Tarduno, Duncan, Scholl, et al., 2002), implying relatively low levels of fluid flow at Site U1374. Vein dips are generally steep, with a maximum of 60°–65°, although numerous veins have both steeper and shallower angles (Fig. F56A).

Fractures (N = 356) are sparse to nonexistent in most of the breccia in Hole U1374A (Fig. F57). The number of fractures is higher in lava flow units, but the number of fractures per meter in lavas of Hole U1374A is low compared to Sites U1372 and U1373. The only exceptions are lithologic Units 146 and 148 in Unit XIX, which have >130 fractures in 14 m (~9 fractures per meter) and are thus comparable to the highly fractured lowermost Unit VII of Site U1373 (11 fractures per meter). Fractures have a broad range of dip angles ranging from subhorizontal to vertical, with maxima at 85°–90°, 60°–65°, and 15°–20° (Fig. F56B).

Summary

Structural features at Site U1374 are dominated by veins (N = 1229), vein networks (N = 3225 individual veinlets), and fractures (N = 356). Veins are found mostly within lava flows, although veins also occur in larger fragments in the volcanic breccia units and rarely within the breccia matrix. The maximum vein width is 25 mm, but most are considerably smaller, with an average width of 0.8 mm. Fractures are most common in massive units, especially in the lowermost 14 m of the hole, with >14 fractures per meter recorded. Structural measurements were also undertaken for 46 examples of sedimentary bedding, 35 geopetals, 18 igneous contacts, 35 vesicle bands, and 80 cases of flow textures. The dip angles of bedding change downhole and correspond to important lithofacies variations described in “Sedimentology.” Geopetal structures are horizontal, indicating that this part of the seamount has not been tilted since deposition of the geopetal infilling material. Several lava flow units have flow textures with subhorizontal or shallowly dipping orientations. Excellent examples of baked contacts and chilled margins were recorded from ~335 to 500 mbsf in a series of steeply dipping sheet intrusions. These intrusions also contain steeply inclined vesicle bands or flow textures, which indicate that magma flow in these intrusions was subvertical to vertical.

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