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doi:10.2204/iodp.proc.344.102.2013

Structural geology

Our methods for documenting the structural geology of Expedition 344 cores largely followed those used by Expedition 334 structural geologists (see “Structural geology” in the Expedition 334 “Methods” chapter [Expedition 334 Scientists, 2011]). We documented deformation observed on the working half of the split cores by classifying structures, determining the depth extent, measuring orientation data, and recording the sense of displacement. The collected data were hand-logged onto a printed form at the core table and then typed into both a spreadsheet and DESClogik. Where possible, orientation data were also corrected for rotation related to drilling on the basis of paleomagnetic declination and inclination information.

Structural data acquisition and orientation measurements

Each structure was recorded manually on a description table sheet modified from that used during Expedition 334 (Fig. F6). Core measurements followed those made during IODP Expeditions 315 and 316, which in turn were based on previous ODP procedures developed at the Nankai accretionary margin (Legs 131 and 190). We used a plastic protractor for orientation measurements (Fig. F7). Using the working half of the split core provided greater flexibility in removing—and cutting, if necessary—pieces of the core for measurements.

Orientations of planar and linear features in cored materials were determined relative to the core axis, which represents the vertical axis in the core reference frame, and the double line marked on the working half of the split core liner, which represents 0° (and 360°) in the plane perpendicular to the core axis (Fig. F8). To determine the orientation of a planar structural element, two apparent dips of the element were measured in the core reference frame and converted to a plane represented by dip angle and either a strike or dip direction (Fig. F9). One apparent dip is usually represented by the intersection of the planar feature with the split face of the core and is quantified by measuring the dip direction and angle in the core reference frame (β₁; Fig. F10). Typical apparent dip measurements have a trend of 90° or 270° and range in plunge from 0° to 90°. The second apparent dip is usually represented by the intersection of the planar feature and a cut or fractured surface at a high angle to the split face of the core (β₂; Fig. F10). In most cases, this was a surface either parallel or perpendicular to the core axis. In the former cases, the apparent dip lineation would trend 000° or 180° and plunge from 0° to 90°; in the latter cases, the trend would range from 000° to 360° and plunge 0°. Linear features observed in the cores were always associated with planar structures (e.g., striations on faults), and their orientations were determined by measuring either the rake (or pitch) on the associated plane or the trend and plunge in the core reference frame. During Expedition 344, we measured rake for striations on the fault surface (Fig. F11) and azimuth and plunge for other lineations (e.g., fold axes). All data were recorded on the log sheet with appropriate depths and descriptive information.

Paleomagnetic correction

Paleomagnetic data were used during Expedition 344 to restore the structural data with respect to the geographic reference system (Fig. F9). Especially in the XCB and RCB intervals, drilling-induced biscuiting of the cores and individual rotation of each core biscuit require paleomagnetic reorientation to correct the structural data. Because APC cores are continuous, we used the orientation data from discrete paleomagnetic samples or from the orientation tool attached to the APC to restore all structural data collected from the same core. Because each biscuit can rotate independently in the XCB and RCB cores, a discrete paleomagnetic correction for each biscuit was needed. We therefore recorded intact and undisturbed intervals without internal biscuiting for paleomagnetic correction when we identified structures. We collected paleomagnetic measurement samples from the same coherent core piece in which structures were measured. After paleomagnetic measurements were made, we oriented the structural data using the declination of the characteristic remanence of magnetization (see “Paleomagnetism”).

Description and classification of structures

We constructed a structural geology template for DESClogik that facilitated the description and classification of observed structures. For clarity, we defined the terminology used to describe fault-related rocks, as well as the basis for differentiating natural structures from drilling-induced features.

Faults were classified into several categories based on the sense of fault slip and their structural characteristics. The sense of the fault slip was identified using offsets of markers (e.g., bedding and older faults) across the fault plane and predominantly by slicken steps. A fault with cohesiveness across the fault zone was described as a healed fault. Zones of dense fault distribution and intense deformation were described as brecciated zones and fractured zones. Fractured zones are moderately deformed zones where the size of fragments is usually bigger than the width of the core; brecciated zones are intensively deformed zones fragmented into centimeter-size and smaller fragments, containing a few larger fragments. A zone of strong localized deformation delimited by material with lower finite deformation was described as a shear zone.

In the basement rocks, igneous rocks and tectonic mélanges were expected to correspond to the onshore geological map of Osa Peninsula. The lithology and mineralogy of the vein minerals were described by the petrologists, and orientations of the veins, foliations, and other structural features in the igneous rocks were measured by the shipboard structural geologists.

Structures can sometimes be disturbed by drilling-induced deformation such as flow-in structures in APC cores and biscuiting, fracturing, faulting, and rotation of fragments in XCB and RCB cores. In the cases where structures have been disturbed by flow-in on >60% of the cross section of the core, we excluded measurements because of the intense disturbance (bending, rotation, etc.) of these structures. We used paleomagnetic data to correct for possible rotations of the fault orientation caused by coring. When multiple orientation measurements were plotted in stereographic projection, natural faults were expected to display preferred orientations that were possibly related to tectonic stress orientations, whereas drilling-induced fractures were expected to yield random orientation distributions.

Calculation of plane orientation

For planar structures (e.g., bedding or faults), two apparent dips on two different surfaces (e.g., the split core surface, which is east–west vertical, and the horizontal or north–south vertical surface) were measured in the core reference frame as azimuths (measured clockwise from north, looking down) and plunges (Figs. F8, F9, F10). A coordinate system was defined in such a way that the positive x-, y-, and z-directions coincide with north, east, and vertical downward, respectively. If the azimuths and plunges of the two apparent dips are given as (α₁, β₁) and (α₂, β₂), respectively, as in Figure F10, then the unit vectors representing these two lines, v₁ and v₂, are

and

.

The unit vector normal to the plane, v_n (Fig. F10), is then defined as

,

where

.

The azimuth, α_n, and plunge, β_n, of v_n are given by

.

The dip direction, α_d, and dip angle, β, of this plane are α_n and 90° + β_n, respectively, when β_n < 0° (Fig. F12). They are α_n ± 180° and 90° – β_n, respectively, when β_n ≥ 0°. The right-hand rule strike of this plane, α_s, is then given by α_d – 90°.

Calculation of slickenline rake

For a fault with striations, the apparent rake angle of the striation, ϕ_a, was measured on the fault surface from either the 90° or 270° direction of the split-core surface trace (Figs. F9, F11). Fault orientation was measured as described above. Provided that v_n and v_c are unit vectors normal to the fault and split core surfaces, respectively, the unit vector of the intersection line, v_i, is perpendicular to both v_n and v_c (Fig. F11) and is therefore defined as

,

where

and

.

Knowing the right-hand rule strike of the fault plane, α_s, the unit vector, v_s, toward this direction is then

.

The rake angle of the intersection line, ϕ_i, measured from the strike direction, is given by

because

The rake angle of the striation, ϕ, from the strike direction is ϕ_i ± ϕ_a, depending on the direction from which the apparent rake was measured and the dip direction of the fault (Fig. F13). ϕ_a should be subtracted from ϕ_i when the fault plane dips west and ϕ_a was measured from either the top or 90° direction or when the fault plane dips east and ϕ_a was measured from either the bottom or 90° direction. On the other hand, ϕ_a should be added to ϕ_i when the fault plane dips east and ϕ_a was measured from either the top or 270° direction or when the fault plane dips west and ϕ_a was measured from either the bottom or 270° direction.

Azimuth correction using paleomagnetic data

Provided that a core is vertical, its magnetization is primary, and its bedding is horizontal, its paleomagnetic declination (α_p) indicates magnetic north when its inclination (β_p) ≥ 0°, whereas α_p indicates magnetic south when β_p < 0° (Figs. F9, F14). The dip direction and strike of a plane in the geographic reference frame, α_d* and α_s*, are therefore

and

when β_p ≥ 0° and are

and

when β_p < 0°.

If the core was complete and continuous, one paleomagnetism sample per section (1.5 m) was deemed sufficient. If the core was discontinuous, then each part of the core that was continuous and structurally important had to contain a paleomagnetism sample. Paleomagnetism samples were taken as cubic or cylindrical samples close to the measured faults (usually within 5 cm) and from a coherent interval that included the fault. We avoided core fragments which were so small that a spin around an axis significantly deviating from the core axis could have taken place (e.g., fragments of brecciated segments of the core).

DESClogik

DESClogik is an application used to store visual (macroscopic and/or microscopic) descriptions of core structures at a given section index. During this expedition, only the locations of structural features, calculated orientations in the core reference frame, and restored orientation based on the paleomagnetic data were input into DESClogik, and orientation data management and planar fabric analysis were recorded on a spreadsheet, as described above.

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