Previous   |    Next

doi:10.2204/iodp.proc.335.103.2012

Structural geology

The following observations were made on cores from Hole 1256D recovered during Expedition 312 between 1406.1 and 1507.1 mbsf (core), as well as cores (between 1507.1 and 1521.6 mbsf) and cuttings and cobbles from the lower part of the hole recovered during Expedition 335. Gabbroic and basaltic rocks with granoblastic textures (referred to hereafter as “plutonic rocks”) display structures of magmatic, alteration, and metamorphic origins (Fig. F57). Structures occur both within granoblastic dikes and gabbroic rocks. All planar orientations are recorded as dip direction (000–360) and dip (0°–90°).

Reevaluation of macroscopic structures in the Hole 1256D plutonic section

We redescribed structures observed in the Expedition 312 plutonic cores to ensure consistency in the description for Hole 1256D plutonic rocks, following techniques and methods used to describe and document structures reported in “Structural geology” in the “Methods” chapter (Expedition 335 Scientists, 2012b).

The Hole 1256D plutonic section drilled during Expedition 312 (1406.1–1507.1 mbsf) comprises two gabbro units (Gabbro 1 and Gabbro 2) and basaltic rocks interpreted to be dike screens with granoblastic texture (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008). Observations were made and structural orientation measured on the archive section halves of Sections 312-1256D-213R-1 through 234R-1. The newly obtained results were compared with previous structural logs recorded during Expedition 312 (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”). Where discrepancies were found between the new observations and the Expedition 312 structure logs, further analyses were made on the archive section halves. The structure data log for the Expedition 312 plutonic section was revised to generate a coherent data set for the Hole 1256D plutonic section (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”).

In general, newly made observations and measurements were consistent with the Expedition 312 structural descriptions. Noticeable changes or additions are

• Orientations of newly identified internal boundaries in Gabbro 1 and Gabbro 2 (see “Igneous petrology”);

• Two-dimensional quantitative estimates of shape-preferred orientation (SPO) on oriented pieces of Gabbro 1; and

• Relogged brittle structures introducing the concept of subhorizontal irregular fractures (SIFs).

All structural observations were incorporated to generate the Structural synthesis column (Fig. F58). We also attempted to restore orientations of important structures into the geographic reference frame using available paleomagnetic data (see “Structural geology” in the “Methods” chapter [Expedition 335 Scientists, 2012b]) (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”). It must be noted that significant uncertainties in the reliability of reorientation of individual structures are likely, relating to assumptions made in the reorientation process. However, spatial coherence within different classes of reoriented data (e.g., fractures) suggests that the distributions of reoriented data provide first-order insights into primary structural trends in the geographic reference frame.

Some important new observations and reinterpretations of Expedition 312 cores made during Expedition 335 include the following:

• The tentative reoriented azimuths of the contact between sheeted Dike Screen 1 and Gabbro 2 and the internal contact between Units 81 and 82 of Gabbro 1 suggest that these units are steeply dipping toward the west-southwest (i.e., toward the spreading ridge). Other igneous contacts of minor igneous veins display moderate dips in various directions (Fig. F59).

• Veins in the Expedition 312 cores are variably oriented but are generally clustered into two populations (Fig. F60). One population is steeply dipping and the other is shallowly dipping; both have similar northwest–southeast strikes and dip mainly to the southwest (Fig. F61). Dark green veins tend to have steeper dips.

• The concept of SIFs was introduced to try to constrain drilling-induced fractures. Downhole fracture density is variable, with boundary zones between the gabbros and granoblastic dikes displaying higher fracture densities.

• Gabbro 1 consists of three distinct zones marked by different magnetic susceptibility and SPO of leucocratic patches. The distribution of plagioclase-rich patches in Gabbro 1 is consistent with porous melt flow through a crystal mush.

Structural measurements

During the redescription of Expedition 312 cores, a few mistakes in structural measurements and one computational error were identified in the previous structural logs. The error occurred with the computation program that translates two apparent dip and direction measurements to dip and dip direction of a planar structure, which was systematically used throughout Expedition 312. The mistakes were corrected in the revised structural data log (see STRUCTURE_LOGS_312_335 in DESCRIPTIONS in “Supplementary material”) and uploaded into DESClogik.

Igneous contacts

Igneous contacts measured range from boundaries between units to magmatic veins (see “Igneous petrology” for discussion). The orientation of each contact was measured and recorded in the igneous petrology workbook on the Contacts tab. In the Expedition 312 core there are four major contacts in the plutonic section, where only two occur in oriented core pieces. The only geographically reoriented contact is the upper boundary of Gabbro 2 (Fig. F58). The upper boundary of Gabbro 1 was observed in an oriented piece, but no stable principal component of remanence was obtained; hence this boundary cannot be reoriented. The lower contact of Gabbro 1 was not recovered. The lower contact of Gabbro 2 was recovered, but the piece is not oriented.

The geographically reorientated azimuth of the contact between Dike Screen 1 and Gabbro 2 is 76°/255 on average (n = 3). The geographically reoriented azimuth of the internal contact between Units 81 and 82 in Gabbro 1 is 69°/237 (n = 1). The orientations of the preserved contacts suggest that they steeply dip toward the west-southwest (i.e., toward the spreading ridge). Other igneous contacts of minor igneous veins dip moderately to various directions (Fig. F59).

There are 10 oriented contacts between units within Gabbros 1 and 2 and within Dike Screen 1 (unit/unit contacts), which vary in orientation. Contacts range in dip from 15° to 72°, with the majority of the dips lying between 15° and 45° (n = 9). Contacts between magmatic veins and their host rocks are usually planar and sharp.

Magmatic and crystal-plastic fabrics

Magmatic fabrics, such as magmatic banding or textural banding, were remeasured. No major change was made in the description of Expedition 312, and data were uploaded into DESClogik, together with information from two-dimensional SPO analysis (see “Quantitative estimation of shape-preferred orientation of Gabbro 1”). Crystal-plastic fabrics (see STRUCTURE_SKETCHES_335 in DESCRIPTIONS in “Supplementary material”) were reexamined, but detailed observation was not possible without the shipboard thin sections.

Quantitative estimation of shape-preferred orientation of
Gabbro 1

The macroscopic fabric of gabbros in the plutonic section of Hole 1256D is overall isotropic, and in order to detect any possible downhole variation in magmatic fabric, we used an image analysis method to quantify SPO (Launeau et al., 1990) (see “Structural geology” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). SPO was measured on zones with leucocratic patches from Gabbro 1 in the interval from 1410.9 to 1452.5 mbsf (Figs. F62, F63, F64). These leucocratic zones correspond to plagioclase-rich patches and clusters and are defined, for image analysis, by a digital threshold number > 60 on the grayscale image.

Gabbro 1 was separated into three zones based on variations in magnetic susceptibility and in the percentage of leucocratic patches. The modal percentage of leucocratic patches in Zone 1 of Gabbro 1, from 1410.9 to 1416.2 mbsf, is relatively large: 8.4% ± 3.2% (Fig. F64). Zones 2 and 3 of Gabbro 1, from 1416.2 to 1452.5 mbsf, show smaller and fewer disseminated leucocratic clusters that account for 3.3% ± 2.3% of the rock. At 1422.47 mbsf (Sample 312-1256D-217R-1, 87–92 cm [Piece 21]), a plagioclase-rich zone shows a higher percentage of leucocratic clusters (20.6%) characterized by euhedral plagioclase grains. These variations suggest that Gabbro 1 may consist of three distinct petrological units: Zone 1 is defined by larger and more abundant leucocratic patches than Zones 2 and 3. All zones are characterized by a broad upward increase in percentage of leucocratic patches. Zone 2 is defined by a statistically higher average SPO shape ratio (1.044) than Zone 2 (1.032) and Zone 1 (1.016). The boundaries between zones were determined independently of petrological units, yet these limits correspond to the Unit 1256D-85/86A and 89A/89C boundaries (Fig. F58). SPO provides important information on internal boundaries in Gabbro 1. The leucocratic patches, characterized by a lower anorthite content measured in plagioclase grains compared to groundmass plagioclase (Koepke et al., 2008), are interpreted as late-stage felsic melts. The increase in percentage of these leucocratic patches toward the top of Gabbro 1 most likely reflects the accumulation and upward movement of felsic melts that typically have a lower density than the surrounding crystal mush.

The SPO shape ratio of the leucocratic patches is overall very small and confirms the macroscopically isotropic fabric of Gabbro 1 (Fig. F64). The shape ratio varies between 1.001 and 1.140, corresponding to SPO shape ratio ranks between 0 and 1 (isotropic to weak; average ≈ 1.030). The most anisotropic intervals of Gabbro 1 are 1417–1420 and 1443–1452 mbsf. These two intervals correspond to the lower part of Zone 1 and the lower part of Zone 3, respectively. Elsewhere, the SPO shape ratio is < 1.050, corresponding to an SPO shape ratio rank of 0. Overall, the shape ratio shows a weak negative correlation with the percentage of leucocratic patches (Fig. F65). The interpretation of this correlation remains speculative because the SPO shape ratio is overall very small (average = 1.030) and because SPO analysis was performed in two dimensions only. One possible explanation would be that some compaction took place in the magmatic crystal mush before complete solidification. This mechanism would explain why shape ratios are larger toward the base of Zones 1 and 3.

Most of the steeply dipping leucocratic zones occur in Zone 1 of Gabbro 1 between 1410.9 and 1419.0 mbsf. However, this zone also exhibits 20% of gently dipping zones, which gives the rock a reticulated fabric with intertwined steep and shallow fabrics (Fig. F63). Zone 3 of Gabbro 1, between 1445 and 1452 mbsf, is characterized by great variation of SPO orientation. This variation could be explained by incipient segregation of plagioclase-rich melt through a weakly organized crystal mush (Fig. F66).

Vein orientation

Several veins measured in Expedition 312 cores seem to have a systematic distribution in orientation. Several core pieces were tentatively geographically reoriented based on paleomagnetic data (see “Structural geology” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). All of the geographically reoriented veins were plotted in a stereonet (Fig. F60B). They have variable orientations but generally form two populations. One population is steeply dipping (>60°) and the other is shallowly dipping (<30°), and both have similar northwest–southeast strikes. This is mirrored in Figure F60A, where all dips, reoriented or not, show the dip distribution of various types of veins (grouped by color). Dark green veins tend to have steeper dips.

Geographically reoriented data were used to plot several rose diagrams to show the vein strikes in the three major units: Gabbro 1, Dike Screen 1, and Gabbro 2 (Fig. F61). Veins in Gabbro 1 and Gabbro 2 have similar bulk orientations, with some scatter. Veins in Dike Screen 1 have northwest–southeast orientations, slightly different from those in the gabbro units. The vein azimuth is plotted using the right-hand rule, so dip direction is always to the right of the strike direction. Most veins, therefore, would strike northwest–southeast and dip toward the southwest.

Brittle deformation

Macroscopic brittle structures are present in all core sections. The majority of brittle features were classified as SIFs (see “Structural geology” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). SIFs are mostly exhibited on piece end surfaces, and it is unclear whether these fractures are drilling induced or whether they reflect a preexisting plane of weakness in the rocks. High densities of SIFs near the upper contact between the granoblastic dikes and Gabbro 1 may correlate with closely spaced subhorizontal features observed in Formation MicroScanner (FMS) images (see Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006) and therefore are not drilling induced. Several nonmineralized steeply dipping fractures were also observed. The distribution of fracture orientations is bimodal, with a common occurrence of dip angles near 20° and a higher frequency of dip angles between 70° and 90° (Fig. F67). The majority of geographically reoriented steep fractures tend to dip to the southwest.

No semibrittle features or fault structures were identified from macroscopic observations. Fractures other than SIFs tend to be steeply dipping. Many of these steep open fractures have mineralized surfaces, indicating places where the rock has broken along a vein. Small confined joints or cracks that do not fully penetrate the core pieces are very common in the granoblastic dikes and are also present but less common in the gabbros. Unoriented pieces of cuttings and cobbles from the hole also contain some small irregular joints. These tend to be very fine and have irregular and splayed morphologies. Gabbro 2 contains some intervals of recovered gravel-sized pieces that may represent rocks with a very high fracture density.

Downhole fracture intensity is variable, with the zones between the gabbro and granoblastic dikes having higher fracture densities (Fig. F68). For instance, the high fracture densities seen in the intervals near 1410 and 1460 mbsf and just above 1500 mbsf are all associated with major lithological boundaries. The highest occurrence of SIFs is near the top of Gabbro 1, which is reflected in the small size of recovered pieces. Non-SIF brittle features appear to be slightly more abundant in gabbro than in granoblastic material.

Expedition 335 plutonic section

The following observations were made on cores (1507.1–1521.6 mbsf), cuttings, and cobbles recovered during Expedition 335. The recovered material mostly consists of aphyric basalt with a granoblastic texture with minor amounts of dioritic rocks and olivine gabbros. Observations were made on the archive section halves of Sections 312-1256D-235R-1 through 239R-1, and structural measurements were done on oriented pieces (see STRUCTURE _SKETCHES_335 in DESCRIPTIONS in “Supplementary material”). During Expedition 335, numerous cuttings and cobbles were recovered that carry important information about the crosscutting relationships of several types of veins. Details specific to structural features were illustrated with comments and sketches (see STRUCTURE _SKETCHES_335 in DESCRIPTIONS in “Supplementary material”). Structural orientations were measured on large cobbles, with respect to arbitrarily chosen x-, y-, and z-axes. Cubes for magnetic measurements were sampled from the same cobbles using the same reference frame to reorient the measured structures. Thin sections were made from cores, cuttings, and cobbles to confirm macroscopic descriptions of structures and to characterize the microstructures (see STRUCTURE_SKETCHES _335 in DESCRIPTIONS in “Supplementary material”). Important structural observations are as follows:

• Vein crosscutting relationships indicate that magmatic veins and orthopyroxene veins occurred first and were followed by actinolite needle veins, anhedral hornblende veins, and thin, black anhedral actinolite(-chlorite) veins, in that order.

• The irregular shapes of quartz-bearing magmatic veins and pyroxene veins that were cut by amphibole veins with alteration halos illustrate the interplay between magmatic, metamorphic, fluid flow, and brittle deformation processes.

Gabbros

Overall, gabbros display equilibrated magmatic microstructures. Samples of olivine gabbronorite sampled during Runs 11 and 20 display a magmatic isotropic fabric characterized by equant granular microstructure, subrounded grains, and lobate grain boundaries (Sample 335-U1256D-Run 11-EXJB; Thin Section 29). Olivine and pyroxenes are systematically surrounded by abundant small grains of oxides (Fig. F69). The large difference in grain size between these grains and magmatic phases such as olivine, pyroxene, and plagioclase indicates that these oxides are not primary but, to the contrary, result from alteration. Microstructures indicative of crystal-plastic deformation are very rare and limited, when present, to tapered twins in plagioclase.

Basaltic granoblastic dikes

Basaltic dikes are host rocks to Gabbro 1 and Gabbro 2, and within tens of meters above, below, and in between contacts, they display equilibrated and equigranular microstructures, referred to as granoblastic (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Dziony et al., 2008). These microstructures, characteristic of high-grade static metamorphism, have been attributed to the thermal overprint caused by emplacement and cooling of relatively large gabbro bodies (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006; Koepke et al., 2008). Recrystallization is pervasive and affects all parts of the basaltic dikes, including narrow pyroxene domains interpreted as recrystallized hydrothermal veins (Koepke et al., 2008; France et al., 2009). Overall, magmatic structures have been extensively overprinted (Fig. F70). However, in several samples, magmatic foliation marked by recrystallized plagioclase laths may persist (Fig. F71). Rare relics of plastic deformation can also be observed in a few large plagioclase grains (Sample 335-1256D-235R-1, 11–12 cm; Thin Section 2). One recrystallized medium-grained sample displays strong banding and plagioclase foliation (Sample 335-1256D-Run12-RCJB-Rock Q; Thin Section 25) (Fig. F71).

Magmatic veins

A 15 mm thick magmatic leucocratic vein of quartz diorite (Section 335-U1256D-235R-1, 23–25 cm [Piece 5]; Thin Section 3) cuts through granoblastic dikes at 1507.34 mbsf. The quartz-diorite displays a magmatic microstructure with lobate grain boundaries along plagioclase/quartz contacts. Quartz shows weak undulose extinction.

Vein crosscutting relationships

There are several types of veins in the Expedition 335 cores, cuttings, and cobbles. In this section, the relationships between different vein types and especially their crosscutting relationships are discussed. The five main vein types are

1. Magmatic veins defined by plagioclase and pyroxene ± amphibole ± quartz;

2. Pyroxene veins defined by orthopyroxene or clinopyroxene ± plagioclase;

3. Amphibole veins defined by green actinolite intergrown needles;

4. Black amphibole veins defined by anhedral, diffuse actinolite (±chlorite) crystals; and

5. Green to colorless amphibole veins defined by anhedral, diffuse hornblende crystals.

(See “Veins and vein-related alteration halos” in “Alteration.”)

Several thin sections exemplify different crosscutting relationships between veins: Samples 335-1256D-Run12-RCJB-Rock B (Thin Section 21; Fig. F72), 335-1256D-Run12-RCJB-Rock C (Thin Section 22; Fig. F73), and 335-1256D-238R-1, 24 cm (Thin Section 6; Fig. F74). The crosscutting relationships depicted in Sample 335-1256D-Run12-RCJB-Rock B (Thin Section 21) indicate the following sequence: orthopyroxene and magmatic veins occurred first, then the actinolite needle veins, and last the smaller anhedral, diffuse actinolite veins. The crosscutting relationships exhibited in Sample 335-1256D-Run12-RCJB-Rock C (Thin Section 22) indicate diffuse background alteration occurred before coherent actinolite veins formed. Actinolite veins were then cut by thin, black actinolite veins. The crosscutting relationships in Sample 335-1256D-238R-1, 24 cm (Thin Section 6), indicate the following sequence: the larger actinolite needle vein came first, then the anhedral hornblende vein, and all are crosscut by the conjugate black, anhedral actinolite veins.

Brittle structure

Piece sizes in the recovered cores were typically <10 cm. There is a common occurrence of SIFs, and several nonmineralized steeply dipping fractures were observed. Fracture density is essentially uniform in the recovered intervals. Section 335-1256D-236R-1 contains brittle structures. In this section, Piece 1, 0–5.5 cm, contains an igneous contact that is crosscut by a small fault with ~2 mm of offset (Fig. F75). This piece also contains evidence of deformation (possibly submagmatic fractures) in the alteration halo around the contact. The random orientation of prehnite and chlorite along the fault surface suggests that this mineralization postdates faulting. A lower interval in the same core (Sample 236R-1, 43–68 cm) contains a set of shear veins with an uncertain amount of offset. In nonmineralized steeply dipping fractures, slickensides in the form of chlorite lineations on the fracture surfaces of the shear veins indicate that the motion of shearing is approximately parallel to the strike of the vein (Fig. F76).

Top of page   |    Previous   |    Next