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doi:10.2204/iodp.proc.324.107.2010

Structual geology

Cores recovered from Hole U1350A between 143.10 mbsf in Core 324-U1350A-6R and 315.8 mbsf in Core 324-U1350A-26R contain structures of magmatic, alteration, and deformational origins. The main structural features are represented by pillow structures, veins, and joints. Igneous structures include syn- to late-magmatic structures that are linked to flow, expansion, and contraction of lava.

In the following sections, the characteristics of these structures are described, followed by a description of the distribution and orientations of and relationships between structures. Important observations and interpretations include (1) preferred orientations and contact relations of planar and subplanar structures; (2) variation in structural morphology, such as pattern, size, shape (roundness or sphericity), and vesicles (vesicularity); and (3) crosscutting relationships: primary and secondary structures and structural sequence.

Magmatic flow structures

Magmatic flow structures are seen both within pillows (intrapillow) and in surrounding rocks (interpillow). They include rotation or plastic strain imposed by the flow of viscous magma. Vesicles are the frozen records of gas bubbles in pillow lavas, which record the pattern of degassing and, once filled with secondary minerals, become amygdules. In Hole U1350A, amygdules are commonly radially oriented within the pillows.

Intrapillow structures

Chilled margins

A chilled margin is one of the major identifiers of pillow structures (Fig. F32). Numerous chilled margins were recovered from pillow lavas in the hole and increase in number with depth. Chilled margins have a typical spheroidal shape, radially aligned vesicles, concentric vesicular zones, radial or network cracking, and a glassy skin (Thorpe and Brown, 1985). Several typical chilled margins are observed in pillow breccia clasts found within hyaloclastic intervals 324-U1350A-19R-2, 11–23 cm; 19R-2, 69–79 cm; and 19R-2, 79–82 cm (Figs. F32, F33). Typical chilled margins in the piled-up pillow sequence can be found in intervals 324-U1350A-15R-2, 92.5–98.5 cm; 20R-1, 100.0–105.5 cm; and 20R-1, 105.5–113.5 cm (Fig. F34).

The estimated size and geometry of pillows inferred by the roundness of chilled margins range from 5 cm in diameter above Core 324-U1350A-22R to 20–80 cm in diameter from Cores 22R through 26R. For example, the curvature of the chilled margins seen on pieces from Core 26R varies between 30° and 90°, implying smaller diameter pillows in this hole interval, whereas the curvature of the chilled margins seen on pieces from Core 24R is lower, ranging between 5° and 20°, implying their origin from larger diameter pillows. On the other hand, because pillows are seldom ideally round but rather tubelike or elliptical, it cannot be ruled out that the chilled margins of both cores stem from pillows that have similar diameters but are differently oriented within the strata penetrated by the drill hole. However, this assumption appears not very likely.

Radially aligned and pipe vesicles

Radially aligned vesicles pointing toward the rim, perpendicular to the chilled margins, can be seen in the pillows throughout the hole (Fig. F34). Pipe vesicles or vesicle cylinders are mainly developed in the central part of larger pillow and sheet flow units, as typically seen in intervals 324-U1350A-12R-1, 36.5–57.0 cm; 13R-1, 13.5–56.5 cm; 14R-2, 13.0–36.5 cm; 17R-1, 79–85 cm; 17R-1, 106–113 cm; and 18R-1, 77–102 cm (Fig. F35).

Interpillow structures

Interpillow structures are divided into two types of rocks: hyaloclastic breccias and sedimentary rocks with distinct or weak bedding (Fig. F36). Both are separated by chilled margins from the intrapillow structures, but sometimes we can observe some sediments entering the pillow along fractures. The triangular shape of the sedimentary rock implies the interaction of three pillow rims at interval 324-U1350A-26R-4, 69–77 cm. Baked margins of sediment indicate that the basaltic lavas were intruded into the sediments (Fig. F36A). Hyaloclastites in the hole are very dark, brecciated, and glassy in Core 324-U1350A-24R (Fig. F36B) and interval 324-U1350A-19R-2, 79–92 cm.

Joints

Joints are important and frequently found in the lower part of Hole U1350A below Core 324-U1350A-22R. Their morphologies are planar, curved, or irregular. They also form complicated arrays including en echelon, parallel, conjugate, radial, and network. Jointing in basalt is often related to three synmagmatic mechanisms: flowing, expansion (inflation), and contraction (cooling) of basalt lava with time (Walker, 1993), and one postmagmatic mechanism, tectonic deformation. The synmagmatic joints are often curved in shape, columnar- or prismatic-jointed, or V-shaped. Except for a few curved joints filled with calcite, the geometry of most joints seems to be mainly unrelated to the cracking and cooling of pillow lava. Most joints exhibit no regular distribution or any correlation to pillows or sheet flow morphologies, and their dips do not apparently correlate with rock properties, rock units, or vesicularity (Fig. F37).

Veins

There are a large number of veins in Hole U1350A. Vein widths are generally <10 mm, and most are from 0.1 to ~2 mm wide. Pyrite veins are darker and thinner than calcite veins.

Veins generally form later than joints. Some of them are filled with secondary minerals. As observed for joints, dips of veins are irregular through the hole (Fig. F37). Most veins are formed in the large pillow and sheet flow units. Vein-filling minerals are generally calcite and pyrite. Most pyrite veins are very thin, between 0.1 and 1 mm. A few exceptions, such veins as thick as 2 mm, occur in intervals 324-U1350A-16R-1 (Piece 5A) and 17R-1 (Piece 1). Syntaxial vein growths are very common. Calcite-rich veins commonly show polycrystalline fabrics and partly syntaxial or cross-fiber fabrics as well. A few veins show antiaxial fabrics, such as in Thin Section 267 (Sample 324-U1350A-8R-1, 54–58 cm).

Under the microscope, crosscutting relationships can also be found as displacements at vein intersections. The horizontal or subhorizontal veins generally cut the subvertical veins, for example in Thin Sections 287 (Sample 324-U1350A-16R-2, 26–29 cm) and 289 (Sample 324-U1350A-16R-3, 71–74 cm), showing horizontal syntaxial growth of fibrous calcite and vein crosscutting and indicating lateral compression (Fig. F38A, F38B).

The density of veins in the recovered cores is at least 3 veins/m. Most veins are straight or display planar shape. However, some zigzag veins are also seen in Sections 324-U1350A-22R-4 and 24R-5. Various characteristics of vein arrays are observed, including conjugate or en echelon, parallel, Y-shaped or branched, network, zigzag, and anastomosing veins. Conjugate and en echelon veins are generally closely associated with shear or extensional joints at this site, for example in interval 324-U1350A-22R-5, 92–99 cm. In some samples, veins have splays and intersect others with Y-shaped ends, for example in intervals 324-U1350A-22R-5, 78–80 cm, and 22R-5, 104–106 cm. Sometimes both filling minerals and texture change along the vein strike, for example, from calcite to pyrite in mineralogy and from cross-fiber to polycrystalline in filling pattern.

Overall structure

Based on observations in Hole U1350A, the overall structure of Hole U1350A cores can be separated into two parts by the hyaloclastite (Unit III) of Core 324-U1350A-24R: the upper part, characterized by a stacked sheet flow unit with intercalated pillows, and the lower part, characterized by piled-up pillows whose sizes range from ~20 to 80 cm. Both parts are mainly characterized by synmagmatic pillow structure, including intra- and interpillow structures. In the upper part of the hole, interpillow structures are found in hyaloclastic breccias or calcite fillings. In the lower part, interpillow structures are seen in limestones displaying weak bedding. Intrapillow structures throughout the hole are similar in all pillows but differ in size from ~10 to ~80 cm in diameter, depending on pillow size. Structural observations were made on thin glassy margins, radially aligned vesicles, concentric vesicular zones, and spheroidal shapes. Synmagmatic structural features are represented by amygdaloidal and vein structures, including vein networks.

The postmagmatic structures include conjugate veins and joints. Dip angles of both veins and joints in the hole show no correlation with depth. However, the number of veins and/or joints is higher in the lower part of the hole (Fig. F39).

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