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doi:10.2204/iodp.proc.324.103.2010

Structural geology

Cores recovered from Hole U1346A between 119.72 mbsf (Core 324-U1346A-4R) and 189.1 mbsf (Core 16R) contain structures of magmatic, alteration, and deformational origins. Structures described here are wholly brittle and are solely within the basalt complex. The main structural features are represented by amygdules (or vesicles), veins, vein networks, fractures, microfaults, and breccia. 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 structures are described followed by a discussion of the distribution and orientations of and relationships between structures and a short summary. Important observations and interpretations include

1. Preferred orientations and contact relations of planar and subplanar structures;

2. Variation in structural morphology: pattern, size, shape (roundness or sphericity), and vesicles (vesicularity); and

3. Crosscutting relationships: primary and secondary structures and structural sequence.

These are evidence for close linkages between intrusive processes, fluid flow, and brittle deformation.

Magmatic flow structures

Magmatic flow structures preserved in volcanic lavas exhibit evidence of particle rotation or plastic strain imposed by the flow of viscous magma and provide clues to movement plane and facing (Davis, 1984; Passchier and Trouw, 1996). Magmatic flow structures are only observed in interval 324-U1346A-8R-1 (Piece 2D, 43–72 cm) as flow banding in chilled margins, showing oriented elongate calcite filling parallel to the chilled margins (Fig. F16). The voids that are filled by calcite are probably oriented to the direction of pillow flow.

In addition, vesicles are the frozen records of gas bubbles in lava and therefore beneficial for use in analyzing magmatic flow kinematics. That is to say, the pattern of degassing and features of the vesicles that remain in a lava flow can yield important information on flow mechanisms and lava rheology (Walker, 1993). In Hole U1346A, filled vesicles, also called amygdules, are very common.

Chilled margins and pillow structure

Numerous chilled margins were recovered from pillow lavas and are increasingly common with depth. Chilled margins range from lobate and interfingered to sharp. Chilled margins also contain flow banding, stretched spherulites, and injections of basalt. A chilled margin is strong evidence, one of the major identifiers, of pillow structures, which are identified by many integrated indicators including spheroidal or ellipsoidal shape, radially aligned vesicles, concentric vesicular zones, radial cracking, inner glassy skin, and flow banding (Thorpe and Brown, 1985). Several typical chilled margins are observed in Hole U1346A (Fig. F35). They differ in shape and texture from the halos of some veins in intervals 324-U1346A-14R-2, 64–82 cm, and 14R-3, 21–40 cm (Fig. F36). The true dip of the chilled margins ranges from 30° to 90°. Many of these chilled margins are associated with, or highly disrupted by, diffuse veining and brecciation. Veins and breccia domains both cut and are cut by chilled margins, and alteration is enhanced at chilled margins because the opening of cracks enables water to penetrate deeply into cooling lava. When secondary joints then develop normal to these new cooling surfaces they generate the highly distinctive joint system of "pseudo-pillow lava" (Walker, 1993). We excluded some similar "pseudo-pillows" here.

Breccias/Cataclasites

Three main types of breccia are generally classified as

1. Hyaloclastite related to extrusive or intrusive activity,

2. Incipient breccia related to cooling or cracking, and

3. Hydraulic breccia related to hydrothermal activity or intrapillow fluids.

In general, incipient breccias are more common in sheet flows. However, breccias recovered from Hole U1346A are mainly associated with chilled margins; most of them are intrapillow hydraulic breccias. Observed petrofabrics include local cataclastic domains such as broken rectangular slivers of basalt and recrystallization textures of matrix such as calcite. For example, the cataclastic unit in interval 324-U1346A-16R-2, 111–116 cm, consists of rounded to angular rectangular clasts or slivers of basalt. In general, 3–4 cm of cataclasite separates basalt fragments from chilled fragments. The cataclastic zone is characterized by a complex network of tiny veins, mostly dark green, dark brown, white, and light green, on the core face (Fig. F35). Vein networks and cataclastic banding have caused incipient brecciation of the host rock, and larger fragments show only minor relative rotation.

Joints

Joints are important and common structures in the cores. They express a range of morphologies including planar, curved, irregular, and anastomosing. They can form complicated arrays including en echelon, parallel, conjugate, network, anastomosing, radial, and other structures. These are common in the cores of Hole U1346A. Most joints are related to the cooling of lava and are represented by curved, radial, Y-shaped, and irregular veins filled with secondary minerals. Joints are the major brittle fracture features, but veins are very common in fractured zones as well. Evidently, veins form where minerals precipitate in joints (Davis, 1984).

Veins

Most of the inspected pieces contain at least one vein, and veins are generally closely associated with joints such as conjugate veins (Fig. F37A). In some sets, veins have splays and intersect others with Y- and T-shaped morphologies (Fig. F37B). 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.

Veins range in width, morphology, and composition and reflect the dynamics of hydrothermal fluid flow through the oceanic crust. Vein widths are generally <5 mm (most are ~0.1 or ~3 mm wide), with a maximum width of 10 mm in interval 324-U1346A-8R-2, 31–33 cm.

Many veins, except those with well-defined walls, have halos in Hole U1346A. The color of the alteration halos reflects secondary mineral contents. Vein fill changes from oxyhydroxide and nontronite to calcite from edge to center of veins or vesicles in Hole U1346A (Fig. F37C). Calcite-rich veins commonly show polycrystalline fabrics and partly cross-fiber fabrics. Sulfides in the veins are texturally late and in many instances in the vein center. There is obviously less pyrite observed in Hole U1346A than in the igneous cores recovered from Hole 1213B at Shatsky Rise (Shipboard Scientific Party, 2002b). The mineralogical zonation from nontronite on the vein walls to calcite in the center is possibly interpreted so that veins are asymmetrically grown in the vein center toward the opposite vein wall, wherein grains are growing during incremental opening of the vein. Then, the vein minerals do not replace the wall rock, further showing the vein filling style. Some models for asymmetric vein opening suggest that fibrous and elongate minerals can track the opening history (Ramsay and Huber, 1983).

Shear veins

Unlike Expedition 309 (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006), where local cataclastic rocks were observed, faults and fault rocks were not recovered from Hole U1346A. However, several shear veins were recovered. In Section 324-U1346A-14R-1 (Fig. F38), some en echelon–aligned brown slivers are interpreted to have tracked the vein opening direction. Such structures are rare but indicate that vein opening was slightly noncoaxial, or termed a "shear vein." In the case of shear veins, vein opening was strongly oblique (Fig. F37D). Shear veins exhibit evidence for noncoaxial displacement either during or after their opening (Ramsay and Huber, 1983). Slickenfibers, also called mineral lineations, are present on some surfaces. Such planar surfaces commonly have steps in the slickenfibers, and the standard interpretation is that displacement was in the stepping-down direction (Ramsay and Huber, 1983). Only two of the five shear veins recovered exhibit sense of shear indicators, one sinistral and one dextral. Shear veins in thin section are filled with minor carbonate and smectite. Secondary minerals apparently predate much of the vein opening and provide passive markers for the opening history (Fig. F38).

Macrostructures throughout Hole U1346A

Multiple stages of intrusion and high-temperature (melt present) shear and multiple generations of flow at high and moderate temperature can be recognized using structural relationships. For example, the orientations of structures and their mutual relationships show that strain accommodated by veins, intrusions, and melt migration is intimately related to large-scale magmatic construction of the oceanic crust.

Based on the observations in Hole U1346A, the main structure is revealed to be a pile of stacked pillows whose sizes differ from 20 to ~200 cm (Figs. F39, F40, F41, F42). These pillows still preserve the shape of a flat base and a rounded top. Many pieces show typical pillow structures, having heterogeneously and/or radially distributed cracks and veins. Our statistics of structural identifiers show that the veins have steeper dip angles than those of joints in the same section (Fig. F43), and dip angles of the veins from the top to the bottom become gradually steeper (Fig. F43).

Many vesicles, amygdaloid structures dominantly filled with calcite, are layered parallel to the pillow rims. On the other hand, some of the vesicles form elongated pipe structures pointing toward the rim and are perpendicular to the chilled margins. Both types of vesicles are generally concentrated along the rim of the individual pillow units resulting in lower vesicularity in the center. Only longer pipe vesicles (>1 cm), however, are also found in the central portion of the pillows (see interval 324-U1346A-14R-1, 67–94 cm, right top in Fig. F41). These characteristic features are obviously different from the concentric variations of lobes of pahoehoe lava.

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