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doi:10.2204/iodp.proc.324.104.2010

Structural geology

Cores recovered from the igneous basement in Hole U1347A between 157.4 mbsf in Core 324-U1347A-11R and 315.15 mbsf in Core 29R contain structures of magmatic, alteration, and deformational origins. Structures described here are solely within the basalt complex and exhibit wholly brittle and magmatic flow structures. The main structural features are represented by two types of magmatic flow structures, pillows and sheet flows. The details of these structures are characterized by distribution patterns of amygdules (or vesicles), veins, vein networks and fractures, and texture variation. All of these igneous structures include syn- to late magmatic structures that are linked to flow, cooling, and solidification of lava.

In the following sections, the characteristics of the major two types of structures are described, followed by a discussion of distribution, orientations, and relationships between structures and a short summary. Important observations and interpretations include

• Preferred orientations and contact relationships of planar and subplanar structures;

• Variation in structural morphology, such as pattern, size, shape (roundness or sphericity), and vesicles (vesicularity); and

• Crosscutting relationships, such as primary and secondary structures and structural sequence.

These are evidence for close linkages between extrusive processes, fluid flow, brittle deformation, and paleostress field.

Magmatic flow structures

Magmatic flow structures exhibit evidence of particle rotation or plastic strain imposed by the flow of viscous magma and provide clues to movement plane and direction (Davis, 1984; Passchier and Trouw, 1996).

Sheet flow structure

A sheet flow is generally composed of three parts: lava upper crust, lava core, and basal zone (Keszthelyi et al., 1999). The distinctive and typical structure of a sheet flow in Hole U1347A is displayed in Sections 324-U1347A-16R-1 through 16R-5 (Fig. F43).

The upper crust has relatively high vesicularity, irregular and hackly jointing, and hypocrystalline textures. At the top of the upper crust, some ropy folds were also observed (see "Igneous petrology"). Vesicularity decreases from top to bottom in the upper crust. Although many vesicles are filled by calcite, forming amygdules, several horizontal vesicle zones are not filled in Hole U1347A (Fig. F43). The size distribution and shape of the vesicles in the upper crust show that the horizontal zones of vesicles can be interpreted to form when the upward migrating gas bubbles become trapped against the downward growing solidification front of the upper crust of the flow. Irregular jointing is often related to jostling of the brittle, chilled lava that takes place during inflation (Fig. F43).

The lava core is generally characterized by <2% vesicles, pipe vesicles and regular jointing, fine- to medium-grained holocrystalline texture, and massive structure. Sometimes we also find several typical vesicle cylinders in the lava core, such as in interval 324-U1347A-18R-3 (Piece 1, 5–55 cm). Vesicle cylinders are the most characteristic structure of a vapor differentiation processes (Goff, 1977, 1996; Caroff et al., 2000). They are vertical tubes, 1–2 cm in diameter, filled with residual liquid and bubbles in Hole U1347A. The vesicle cylinder in interval 324-U1347A-18R-3 (Piece 1, 5–55 cm) extends from ~37 cm above the flow base (i.e., the top of interval 324-U1347A-18R-3 [Piece 3, 92–95 cm] to the bottom of the upper chilled crust).

The basal zone is usually 0.5–1 m thick but is often incompletely recovered (e.g., Sections 324-U1347A-16R-1 to 16R-5) (Fig. F43). This zone is characterized by being slightly vesicular, having little poorly developed jointing or subvertical or stretched pipe vesicles at the base. It usually comprises much less than 10% of the overall sheet flow thickness (Fig. F43).

Pillow structure

Amygdules in pillows are not common in Hole U1347A. Numerous pillow lavas in this hole have rounded chilled margins without radial cracking, concentric vesicular zones, or radially aligned vesicles. Chilled margins show that individual pillow sizes are from 10 to 50 cm (Fig. F44). Under the microscope, the chilled margins show flow banding and stretched spherulites. The other major identifiers of pillow structures include spheroidal or ellipsoidal shape, inner glassy skin, and flow banding (Thorpe and Brown, 1985). Several layers of pillow lavas, with typical chilled margins, are observed in Hole U1347A, such as at interval 324-U1347A-21R-4, 94–104 cm (Fig. F45).

Brittle fracturing

Fractures occur at discrete breaks in a rock mass where cohesion was lost, including four kinds of structures in Hole U1347A:

1. Microfaults and faults in which the two sides are displaced relative to each other,

2. Joints in which the two sides show no differential displacement (relative to the naked eye),

3. Healed or sealed joints in which fluids passing through the rock have partially or completely joined together the adjacent sides by the deposition of crystalline material, and

4. Veins in which a considerable thickness (>1 mm) of filling material occupies the region between the fracture walls (Ramsay and Huber, 1987).

In this report, we consider veins with >0.1 mm thickness of filling material.

Microfaults

A microfault is a hole-scale observable fracture with a small displacement or tiny offset, including microcracks fractions of a millimeter long seen in thin sections of rock under the microscope (Ramsay and Huber, 1987). Here we adopted the principle of Davis (1984) that a microfault is a fracture with an amount of shear separation <1 cm. In general, the displacement or offset is difficult to estimate because of core breakage. However, the slickenlines on the fault plane surfaces (slickenside) display some oriented fiber minerals in intervals 324-U1347A-29R-4, 110–124 cm (315.48–315.70 mbsf), and 29R-5, 46–56 cm (314.75–314.88 mbsf) (Fig. F46). The dip angles are very steep, >70°. When the striations plunge steeply on the slickenside surface, we can identify it as a normal microfault in combination with striation and congruous steps (Hancock, 1985), but their original orientations must be constrained with the help of FMS logging data.

Joints

Joints are common in Hole U1347A cores. Some joints are related to the cooling of lava and are represented by curved, concentric, and irregular veins filled with secondary minerals. However, most joints in the rocks of Hole U1347A are conjugate joints related to postmagmatic deformation rather than cooling. Generally, dips of joints in the middle and lower cores (324-U1347A-17R through 29R) are steeper than those in the upper cores (11R through 16R) (Fig. F47).

On a meter scale, moreover, it seems that the steeper joints in each unit are correlated with lower vesicularity and smaller groundmass grain size (Figs. F47, F48). Joints are very useful in interpreting the stress and strain conditions of past deformational events (Davis, 1984).

Veins

Veins are defined as a sealed fracture by Ramsay and Huber (1987). Vein widths are generally <5 mm (most are ~0.1 or ~1 mm wide) in Hole U1347A. The density of veins in Hole U1347A is ~3 veins/m (also see "Alteration and metamorphic petrology").

Veins are generally closely associated with joints. Conjugate joint planes are also generally shear veins at this site (e.g, interval 324-U1347A-27R-6, 70–95 cm) (Fig. F49). In general, dips of veins in the middle and lower cores (17R through 29R) are steeper than those in the upper cores (11R through 16R) (Fig. F47), as well as those of the joints in Hole U1347A. The steepness of veins in each unit or in Cores 17R, 18R, 20R, 21R, 24R, and 26R appears to be correlated with lower vesicularity and smaller groundmass grain size (Figs. F47, F48). In some samples, veins have splays and intersect others with Y- and T-shaped morphologies (Fig. F49). These intersections can be complex, with changes in mineralogy along the length of the vein. Furthermore, many veins that apparently merge in hand samples have crosscutting relationships when observed in thin section. Crosscutting relationships can also be found as displacements at vein intersections, even under the microscope, showing that jointing is a multiphase process.

In Hole U1347A many veins, except those with well-defined walls, have halos. The color of the alteration halos reflects secondary mineral contents. Vein-filling minerals change from pyrite and nontronite to calcite from edge to center of veins or vesicles in Hole U1347A. Calcite-rich veins commonly show polycrystalline fabrics and partly syntaxial or cross-fiber fabrics. Sulfides in the veins or vesicles are texturally late in many instances.

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