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doi:10.2204/iodp.proc.309312.102.2006

Structural geology

This section outlines the techniques used for macroscopic and microscopic description of structural features observed in hard rock basement cores. Conventions for structural studies established during Leg 206 (Shipboard Scientific Party, 2003b) and other ODP hard rock drilling legs (Leg 118, Shipboard Scientific Party, 1989; Leg 131, Shipboard Scientific Party, 1991; Leg 135, Shipboard Scientific Party, 1992c; Leg 140, Shipboard Scientific Party, 1992b; Leg 141, Shipboard Scientific Party, 1992a; Leg 147, Shipboard Scientific Party, 1993b; Leg 148, Shipboard Scientific Party, 1993a; Leg 153, Shipboard Scientific Party, 1995; Leg 176, Shipboard Scientific Party, 1999; Leg 209, Shipboard Scientific Party, 2004) were generally followed during Expedition 309/312. Minor differences in protocol between Expeditions 309 and 312 are noted.

Macroscopic core description and terminology

We examined all material from both working and archive halves, although sketches of structures and orientation measurements were made from the archive half. The most representative structural features in the cores recovered during Expedition 309/312 are summarized on the VCD forms (see “Core descriptions”). For each section, more detailed structural information is described and sketched on a separate handwritten barrel sheet and in a separate spreadsheet log (see STRUCTUR.XLS in “Supplementary material”). The structure log contains data on location, orientation, and types of structures. Structural descriptions of cores recovered during Leg 206 and Expedition 309 incorporated a breccia log and a breccia list. The breccia log also contains data on clast, matrix, and cement properties of breccias. The breccia list contains data on positions of and structures in faults, microfaults, and other cataclasites. Separate breccia records were not kept during Expedition 312 because of the amount and nature of the recovered breccias. Observations of breccias were instead incorporated into the structure log.

We recorded structural data, with reference to a structures checklist (Table T5), on

• Brittle structures,
• Types of breccia, and
• Magmatic and crystal-plastic structures.

Short explanations for terms and abbreviations are given below, and several full definitions are listed in Table T6. We followed the terminology used during Leg 206 (Shipboard Scientific Party, 2003b), based mainly on Ramsay and Huber (1987), Twiss and Moores (1992), Davis (1984), and Passchier and Trouw (1996).

Brittle deformation identifiers include the following:

• Fracture (f) = brittle failure with or without displacement;
• Joint (J) = fracture with no shear displacement;
• Vein (V) = fracture filled with secondary minerals;
• Shear vein (Sv) = vein with shear displacement;
• Fault (F) = fracture with shear displacement;
• Microfault (mF) = faults with <1 mm of related width of deformation; and
• Breccia (B):
• Magmatic (Bm) = breccias containing glass or quench textures such as hyaloclastites and pillow breccia, primary matrix minerals;
• Hydrothermal (Bh) = breccias with secondary matrix or vein minerals; and
• Tectonic (Bc) = cataclasites and fault-gouges in which the matrix consists of the same material as the host rock.

Note that some breccias were classified as a combination of different types.

Magmatic and crystal-plastic structural identifiers include

• Igneous contacts (Ic) = demonstrably extrusive or intrusive contacts;
• Chilled margins (CM) = sharp gradients in grain size near igneous contacts;
• Magmatic vein (Vm) = thin, discontinuous intrusions of igneous material;
• Dike = greater than centimeter–scale tabular intrusions of igneous material;
• Magmatic fabric (M) = lineations, foliations (referred to as Mf when recognized), defined by shape-preferred orientation of primary minerals with no evidence of crystal-plastic deformation;
• Magmatic shear zone (Ms) = zones wherein the lineation or foliation indicate shear with no accompanying crystal-plastic or cataclastic deformation;
• Compositional layering (Cl) = subplanar to planar layers of finite width of similar mineralogical assemblage;
• Composition banding (Cb) = zones of contrasting compositions that are not easily distinguished, subtle compositional layers;
• Textural banding (Tb) = bands of contrasting igneous textures;
• Patches (P) = irregular areas of contrasting composition/texture;
• Crystal-plastic fabric (Cpf) = lineations or foliations defined by grains exhibiting plastic strain; and
• Alteration patches (see “Alteration”) = spherical, irregular, or elongate domains of enhanced alteration, only elongate patches were measured for orientation.

Where the identifier was unclear, details specific to structural features were illustrated with comments and sketches. Descriptions of all features were recorded using curated depths (note that the Expedition 309 breccia log is only pieces) so that “structural intervals” could be correlated with lithologic core descriptions. A designation for the color of secondary minerals found in veins or halos was included in the structure log.

The morphology of each feature (Fig. F10) was recorded in the structure log (see STRUCTUR.XLS in “Supplementary material”). Standard abbreviations are shown in Table T5. Additional terminology is defined in “Microstructure of plutonic rocks.” In gabbroic rocks, the morphology of each feature was based on the nature of its boundaries. Patches, shear zones, bands, and layers all have boundaries that are sharp, planar, irregular, or diffuse. Patches have distinctive morphologies classified as round, elongate, irregular, or amoeboid (shaped like an amoeba; a term used more widely to refer to the texture of olivine aggregates).

A semiquantitative scale of fracturing and veining intensities was used during core description. We assigned specific values to intensity estimates according to spacing of veins, volumetric occurrence of veins, percentage of matrix in cataclastic zones, and partitioning of deformation structures:

• Slight (5%–10%; = 1)
• Moderate (10%–40%; = 2)
• High (40%–70%; = 3)
• Complete (= 4)
• Evenly (E) or heterogeneously (H) distributed or localized (L)

Intensity values were assigned to section of each core and recorded in the deformation intensity log (see DEFINT.XLS in “Supplementary material”).

In the structure and breccia logs, additional comments were recorded for each structure, including the identifications of slickensides, slickenlines, vein-components, and additional morphologies that help characterize the structures.

Structural measurements

Structural features were recorded in centimeters from the top of each section. Depth of the structures was recorded as the distance from the top of the section to the top and bottom of the feature.

We measured structures on the archive half relative to the IODP core reference frame used during Leg 206. The plane normal to the axis of the borehole is referred to as the horizontal plane. On this plane, a 360° net is used with a pseudo-south (180°) pointing into the archive half and a pseudo-north (0°) pointing out of the archive half and perpendicular to the cut surface of the core (Fig. F11). The cut surface of the core, therefore, is a vertical plane striking 90°–270°.

Apparent dip angles of planar features were measured on the cut face of the archive half of the core. To obtain a true dip value, a second apparent dip reading was obtained where possible in a section perpendicular to the core face (second apparent orientation). Dip and dip direction with respect to the archive half of the core are recorded on the spreadsheet together with second plane measurements. The two apparent dips and dip directions (or one apparent direction combined with the strike) measured for each planar feature are used to calculate the true orientation. If the feature intersected the upper or lower surface of the core piece, measurements of the strike were made directly in the core reference frame and combined with the apparent dip measurements to calculate the true dip values. The “LinesToPlane” Macintosh program by S.D. Hurst was used during Expedition 309. During Expedition 312, true dip was calculated within the structure log using trigonometric functions.

For structures with shear displacements, the assignment of sinistral, dextral, normal, and reverse sense of shear is independent of any reference frame. In shear veins and faults where the direction of slip is not indicated by structural indicators, we recorded the apparent sense of shear as it appears on the cut face of the core and/or on the top or bottom side of pieces.

Microstructure of volcanic rocks

In order to better characterize different types of deformation, we studied the microstructural features of some relevant mesoscopic structures. Thin sections of basement rocks recovered during Expedition 309/312 were examined in order to

• Confirm macroscopic descriptions of brittle structures,
• Characterize the microstructure of the rocks,
• Provide information on the kinematics of brittle and brittle-ductile deformation,
• Identify time relationships between magmatic deformation and alteration processes, and
• Document major structural zones and downhole variations.

Microstructural notes were entered into a thin section description form spreadsheet (see TSECTLOG.XLS in “Supplementary material”), following the nomenclature and procedure adopted during Leg 206 for volcanic rocks. For descriptions of microstructures, we mostly used the terminology of Passchier and Trouw (1996). Shipboard thin sections were oriented; the orientation is given relative to the core reference frame and was marked on each thin section by an arrow pointing upward and a short tick pointing toward “west” from the base of the arrow. Marking two directions is necessary in order to achieve complete orientation of thin sections cut parallel to the cut surface of the core. Digital photomicrographs were taken during Expedition 309/312 to document features described in thin sections.

Microstructure of plutonic rocks

Plutonic rocks were characterized with the goal of understanding the physical processes involved in their intrusion and crystallization. The lack of distinctive magmatic or deformational structures led us to introduce several terms in the descriptions such as “bands” and “patches.” These terms are nongenetic and meant to describe inhomogeneous distributions of phases and aggregates of phases. Otherwise, descriptions of structure and microstructure of plutonic rocks used identifiers and descriptions outlined in “Macroscopic core description and terminology,” and some are tabulated in Table T7. Additional classifications and terminology were incorporated from Legs 153, 176, and 209 and Expeditions 304 and 305. These terms were used for entering data into spreadsheets of thin section descriptions (see “Thin sections” in “Core descriptions”).

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