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doi:10.2204/iodp.proc.330.103.2012

Structural geology

In Hole U1372A, consolidated sediment (breccia and conglomerate) was first recovered at 13.50 mbsf, with igneous basement commencing below 45.6 mbsf (Core 330-U1372A-8R). Structures drilled on Canopus Guyot represent syn- to late- and postmagmatic features, including magmatic flow alignments, igneous contacts, fractures, veins, vein networks, and geopetal structures. The characteristics, orientations, and distribution of these structures are described below.

Fractures (N = 112) and veins and vein networks (N = 422) are the dominant structural features in the igneous cores of Hole U1372A (Fig. F44). Both veins and fractures occur predominantly in the lava flow units, with few or no fractures or veins within volcaniclastics, breccia, or interbedded volcanogenic sediment (Figs. F45, F46). This distribution likely resulted from differences in rock rheology, with lavas being comparatively impermeable, strong, and brittle and thus concentrating strain (fracturing) and fluids (veining) along zones of weakness. In contrast, porous volcaniclastic rocks, breccia, and sediment were likely able to deform relatively uniformly via compaction (similar to processes in a sedimentary pile), with their higher porosities enabling easier fluid flow without requiring concentration of fluids into veins. Veins that are present within volcaniclastic units are often strongly concentrated along unit boundaries (Fig. F46).

Veins (and individual veinlets within vein networks) have an average width of 1 mm and a maximum width of 10 mm. These widths are quite narrow compared to other submarine volcanic environments, such as the Emperor Seamounts (average widths of ~2–4 mm; Tarduno, Duncan, Scholl, et al., 2002), and indicate that the volume of fluid passing through rocks of Hole U1372A might have been lower than in the Emperor Seamount volcanic structures.

Dip and dip direction were measured for 174 veins and 43 fractures. For unoriented core fragments or veins/fractures that were irregular or curved, no measurement was possible. From the available structural measurements it is clear that most fractures in the seamount basement have shallow dips, with two main groupings: one at ~20° and a conjugate set with moderately steeper dips of 40°–60° (Fig. F47). The predominance of shallow to moderately dipping features is likely influenced by the gently dipping character of lava flows on the volcano flank. However, at least in part, the predominance of shallow fracture dips may reflect a sampling bias produced by rotary coring because steeply dipping fractures (i.e., those aligned down the long axis of the core barrel) make the rock more susceptible to fragmentation during drilling, which in turn would cause poorer recovery of these steep features.

One of the most important structural features identified in Hole U1372A were 17 geopetal structures (Fig. F48), which were recorded at 14.50, 14.90, 17.15, 19.30, 22.10, 26.36, 46.06–50.31, 62.16, and 146.00 mbsf. Geopetals are vesicles or voids that are partially or completely filled by one or more generations of sediment and often also by chemical precipitates (e.g., calcite) during later stages of fluid flow (Fig. F48). Geopetals provide a clear indication of whether the cored rock sequences were tilted after the sediment or precipitates were originally deposited horizontally in the vesicles or voids. Horizontal geopetal infills occur between 14.50 and 62.16 mbsf in stratigraphic Units II (breccia/conglomerate) and III (sparsely olivine-phyric peperitic basalt) (Fig. F48B, F48C). These features indicate that the seamount as a whole experienced little or no reorientation postemplacement, either by incipient subduction of this seamount into the nearby Tonga-Kermadec Trench, or by flank collapses. This observation is crucial because it indicates that paleomagnetic directions were determined on lavas that are still in their in situ position.

In contrast, geopetal structures in the lower parts of the sequence (146.00 mbsf; glomerophyric basalt lava flow in volcaniclastic rocks of stratigraphic Unit XIV) are tilted up to 9° from horizontal (Fig. F48D). These volcaniclastics likely experienced a small amount of rotation or slumping soon after deposition, as is typical during or closely following eruption and emplacement of volcaniclastic and hyaloclastic rocks (e.g., Surtsey, Iceland; Jakobsson et al. 2009). These geopetal structures at 146.00 mbsf, however, do not indicate that the whole seamount has tilted, because geopetal structures higher in the sequence, in the younger units, are still in their in situ positions (Fig. F48B, F48C).

Several of the recovered lava flows have moderate to strong macroscopic and microscopic flow alignment (N = 12; Table T8), predominantly defined by alignment of plagioclase laths (Fig. F49). Lavas with flow textures are particularly concentrated in the subaerial portion of the sequence (stratigraphic Units VI and X). These lavas with flow textures contain alignments of elongate titanomagnetite crystals (Fig. F49), which yield a higher degree of magnetic susceptibility anisotropy (see “Paleomagnetism”). In certain sections, especially Sections 330-U1372A-13R-1 through 13R-4 where this flow alignment is particularly strong, fractures and veins are largely controlled by this texture, with similar dips and dip directions to the flow texture.

If core barrel rotation could be corrected and removed, these flow textures would yield the direction in which the lava flowed, which would be useful for volcanological reconstruction of this site.

Summary

Structures at Canopus Guyot in Hole U1372A are veins, vein networks, fractures, aligned vesicles, and geopetals. The location, size, and orientation of these features were measured where possible. A large portion of veins and fractures occurs in massive lava flow units, with relatively few veins and fractures in the volcaniclastic units low in the sequence. The majority of fractures and veins within the seamount basement have shallow dips (~20°), with a conjugate set having moderately steeper dips (40°–60°). The occurrence of horizontally oriented geopetals between 14.50 and 62.16 mbsf indicates that these upper units are in situ and have not been tilted since formation of the seamount. Several flows have moderate to strong mineral alignment, both in samples and thin sections, indicating subhorizontal flow textures within these lavas.

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