doi:10.2204/iodp.proc.324.106.2010
Structural geology
Cores recovered from Hole U1349A contain volcaniclastic sedimentation structures in the upper part of the drilled section, structures of magmatic origin and intensive alteration of the igneous basement in the lower part, and deformation through the entire set of cores. Structures described here are solely within the basalt complex between 165.06 mbsf in Core 324-U1349A-7R and 251.30 mbsf in Core 16R and exhibit wholly brittle and obvious magmatic flow structures. The main structural features are represented by two types of magmatic flow structures, subaerial and submarine sheet flows. The details of these structures are characterized by distribution patterns of amygdules (or vesicles), veins, and texture variation. All of these igneous structures include syn- to late magmatic structures that are linked to flow, cooling, and solidification of lava and postmagmatic structures that are associated with the deformation of basalt.
In the following sections, the characteristics of the major three 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, crosscutting, and contact relationships of planar and subplanar structures and variation in structural morphology such as pattern, size, shape (roundness and sphericity), and vesicles (vesicularity). 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). Both subaerial and submarine sheet flow structures are well developed in Hole U1349A and are described as follows.
Subaerial sheet flow structure
In the subaerial sheet flow, basaltic lava flows often form two main structural types, one smooth surfaced (pahoehoe) and the other rough surfaced (a'a), which are fundamentally different in their structures and flow dynamics (Walker, 1993). In this hole, a'a structure is not confirmed. The top pahoehoe surface in most sheet flows can be identified.
The subaerial sheet flow structure in this hole is developed in the upper part (Sections 324-U1349A-7R-1 through 13R-4) of the igneous basement. Reddish brown lavas showing high oxidation may indicate that these were subaerial eruptions. Some intercalated oolitic limestones, however, suggest that some of the lavas erupted in the shallow-marine environment (interval 324-U1349A-9R-1, 1–13 cm) (see "Sedimentology"). The volcaniclastic clasts above the igneous basement (above Section 324-U1349A-7R-1) are subrounded, implying the clasts were transported in an epiclastic origin or very shallow marine environment. Other indicators of subaerial environment in Hole U1349A are many hollow vesicles or spongy vesicles and no clear chilled margin in the upper part of the igneous basement. Based on Walker's criteria (Walker, 1993), these indicators provide evidence for eruption in a subaerial environment.
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 at least five sheet flows in Hole U1349A is displayed in Sections 324-U1349A-9R-1 through 10R-5 (Fig. F50).
The lava upper crust (A and B in Fig. F50) is characterized by relatively high vesicularity, hypocrystalline textures, and presence of a horizontal vesicle zone. Vesicularity decreases from top to bottom in the upper crust. Although many vesicles are filled with calcite, forming amygdules, several horizontal vesicle zones are not completely filled in Hole U1349A. 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 upward-migrating gas bubbles are trapped against the downward-growing solidification front of the upper crust of the flow. In this part of the sheet flow, no joints develop. By the top of the lava upper crust, we also observed some ropy folds in the lava core at interval 324-U1349A-11R-3, 102–120 cm (Fig. F51).
In the lava core (C in Fig. F50), vesicles are scarce, which is generally characterized by <2% vesicles, pipe vesicles, regular jointing, fine-grained groundmass, holocrystalline texture, and massive structure. Sometimes we also find typical pipe vesicles in the lava core, such as in interval 324-U1349A-13R-4, 119–126 cm. They are subvertical, ~0.5–1.0 cm in diameter, and filled with calcite in Hole U1349A.
The basal zone (D in Fig. F50) is usually 0.5–1 m thick, but it is often incompletely recovered in Cores 324-U1349A-7R through 13R. It is characterized by slight vesicularity and rare joints and subvertical or stretched pipe vesicles at the base. The basal zone usually comprises much less than 10% of the overall thickness of the sheet flow.
Submarine sheet flow structure
The submarine sheet flow structure is developed in the lower part of the igneous basement. The principal difference between underwater and subaerial volcanism is that explosive volcanism is suppressed under deep water (Walker, 1993). Submarine pillow lavas are remarkably similar to subaerial pahoehoe but are, on average, less vesicular. Small rounded flow units (pillow lavas) are important charateristics of underwater lava flows and are distingushed from subaerial flow units (e.g., pahoehoe) by continuous glassy rims (see intervals 324-U1349A-16R-6, 72–82 cm, and 16R-7, 1–10 cm) (Fig. F52). Other indicators of submarine lava flow in Hole U1349A include hyaloclastite and autobreccia (Fig. F52).
Veins
Fractures occur at discrete breaks in a rock mass in which cohesion was lost. In Hole U1349A, a considerable thickness (>0.1 mm) of filling material occupies the region between the fracture walls. There are a large number of veins in Hole U1349A. Vein widths are generally <10 mm (most are ~0.1 or ~2 mm wide). The density of veins is at least 4 veins/m. In general, the population of veins in the lava cores is greater than that in the upper crust and basal zones. Sometimes we observe a vein ribbon composed of >20 veins with 1–3 mm vein spacing, for example, at intervals 324-U1349A-10R-5, 76–79 cm, and 13R-5, 123–129 cm.
Vein texture is complex in Hole U1349A and characterized by cross-fibers composed of spherulitic or needlelike zeolites growing perpendicular to the vein wall (Thin Section 260; Sample 324-U1349A-16R-6, 7–12 cm) (Fig. F53A, F53B). The second important vein texture is polycrystalline, mainly displayed by calcite (Thin Section 254; Sample 324-U1349A-15R-4, 104–110 cm) (Fig. F53C). Sometimes we observe massive texture filled with glassy material and intravenous texture filled with disorderedly aligned calcite or zeolite (Thin Section 249; Sample 324-U1349A-14R-5, 53–54 cm) (Fig. F53D). Most of the veins accompany a thin alteration halo.
Vein geometry includes vein array and vein shape. Most veins are straight or irregular in shape. Sigmoidal-shaped veins are only seen in Section 324-U1349A-10R-4. Various characteristics of vein morphology are observed, including all vein types (conjugate, en echelon, parallel, branched, network, anastomosing, and ribbon) (Fig. F54). Generally, conjugate and en echelon veins are closely associated with shear or extensional joints at this site (e.g., interval 324-U1349A-9R-1, 126–139 cm) (Fig. F54B). In some samples, veins have splays and intersect other veins with Y-shaped (Fig. F54C, F54D) and A-shaped morphologies. 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 sections (Fig. F55).
Crosscutting relationships can also be found as displacements at vein intersections, even under the microscope, showing that jointing occurred in at least three generations resulting from a postmagmatic multiphase process. Furthermore, the horizontal or subhorizontal veins generally cut the others, for example in intervals 324-U1349A-10R-2, 25–38 cm, and 68–85 cm (Fig. F55A, F55B) and in Thin Section 221 (Sample 324-U1349A-10R-3, 43–48 cm) (Fig. F55C, F55D).
Syntaxial growth in vein kinematics is very common in the hole; antiaxial and ataxial growths are rare (Passchier and Trouw, 1996). Calcite-rich veins commonly show polycrystalline fabrics and partly syntaxial or cross-fiber fabrics. Zeolites in the veins are dominated by syntaxial growth, sometimes displaying a polycrystalline texture (Fig. F53A, F53B).
Many veins, except those that are very thin or with well-defined walls, accompany halos in Hole U1349A. The color of the alteration halos reflects secondary mineral contents. Vein-filling minerals change from calcite, zeolite, and nontronite to pyrite from early to late based on their microscopic relations in veins in Hole U1349A.
Dips of veins in the upper cores (324-U1349A-7R through 12R) become gradually steeper downhole. Dips of veins in the lower cores (324-U1349A-13R through 16R) are shallow again in the upper part of that section but gradually steepen downward (Fig. F56).
Breccias
Breccias are dominant below Core 324-U1349A-13R and show a dramatic change in magnetic susceptibility values (see "Physical properties") and color from reddish brown above interval 324-U1349A-13R-4, 1–96 cm (Fig. F57A), to greenish gray below this interval. A large change in structure occurs at interval 324-U1349A-13R-4, 1–96 cm, which is a reddish brown breccia zone that also acts as a boundary for other properties in the hole. Below this boundary, major volcanic autobreccias are characterized by a basalt clast–supported 20%–60% calcite matrix (Fig. F57B, F57C). This boundary is obviously related to hydraulic brecciation or hyaloclastite genesis. The clasts are subirregular to subrounded, with some zeolites growing perpendicular to the margins viewed under the microscope (Thin Section 246; Sample 324-U1349A-14R-4, 50–56 cm, and Thin Section 249; Sample 324-U1349A-15R-4, 104–110 cm) (Fig. F57D).