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Leg 176 Logging Summary

Shipboard Scientific Party

Background and Scientific Objectives

In this document we present a summary of the wireline logging results from ODP Leg 176. This was the second drilling leg to investigate the geological, geophysical, and geochemical processes of the lower oceanic crust at Site 735 in the Indian ocean (Figure 1).

Figure 1: Location of the Southwest Indian Ridge within the configuration of the Indian Ocean triple junction. Black rectangle shows the location of the Atlantis II fracture zone.

Site 735 is located on a shoal platform in the rift mountains of the Southwest Indian Ridge (Figure 2 ), 18 km east of the present-day axis of the Atlantis II Transform Fault. The Southwest Indian Ridge has existed since the initial breakup of Gondwana in the Mesozoic (e.g., Norton and Sclater, 1979). Shortly before 80 Ma, plate readjustment in the Indian Ocean connected the newly formed Central Indian Ridge to the Southwest Indian Ridge and the Southeast Indian Ridge to form the Indian Ocean Triple Junction (Fisher and Sclater, 1983; Sclater et al., 1981; Tapscott et al., 1980). Steady migration of the triple junction to the northeast has since created a succession of new ridge segments and fracture zones including the Atlantis II. Thus, the Atlantis II Fracture Zone and the adjacent ocean crust is entirely oceanic in origin, free from the complications of continental breakup such as those found at some equatorial fracture zones along the Mid-Atlantic Ridge (e.g., Bonatti and Honnorez, 1976).

Figure 2: Seabeam map of the Atlantis II fracture zone showing the location of Hole 735B. Seabeam data and map from Conrad survey cruise 27-09, 1986 (H. Dick, Chief Scientist, D. Gallo and R. Tyce)

In 1987, ODP Leg 118 drilled Hole 735B to a depth of 500 meters below seafloor (mbsf) and recovered coarsely crystalline gabbroic rocks from a tectonically exposed lower crustal section on a wave-cut platform that flanks the Atlantis II fracture zone on the slow-spreading SW Indian Ridge (Figure 1). Hole 735B is located on a shallow bank on the crest of a 5-km high mountain range, known as a transverse ridge, which constitutes the eastern wall of the Atlantis II transform valley (Figure 2). The hole is situated some 93 km south of the present-day Southwest Indian Ridge axis, and is 18.4 km from the inferred axis of transform faulting on the floor of the Atlantis II fracture zone (Dick et al., 1991). The bank consists of a platform, roughly 9 km long in a northsouth direction and 4 km wide, which is the shallowest of a series of uplifted blocks and connecting saddles that form a long, linear ridge parallel to the transform. The top of the platform is flat, with only about 100 m relief over 20 km2.

The primary scientific objectives of Leg 118 were:

  • determining the lateral and vertical variability of rock types on the floor of the fracture zone
  • investigating the nature and distribution of deformation in a fracture zone and determining whether there is a single slip plane, multiple planes, or penetrative slip across the entire width of the feature
  • determining the thermal structure of a transform generated crust and assessing the extent of alteration and seawater penetration
  • determining the nature and thickness of oceanic crust in the nodal basins where ridge crests meet the transform fault
  • determining the physical properties, magnetism, and seismic velocities of transform-generated crust and, particularly, to document any anisotropy in these properties

At the end of Leg 118, the cores from Hole 735B changed the general perception of the lower ocean crust at slow-spreading ridges. The data from this hole indicated that the crust formed by a complex interaction of magmatic, tectonic, and hydrothermal processes (e.g., Dick et al., 1992), quite unlike the simple large magma chamber once envisioned as the primary driver of crustal accretion (e.g., Cann, 1974). Given the typical thicknesses of seismic Layer 3, this section was not long enough to adequately characterize the lower crust, even at a very slow-spreading ridge. No other known place, however, offered the excellent drilling conditions, high recovery (approximately 87%), ease of guide-base placement, and shallow exposures of lower crust.

Building on the results from the prior scientific expedition, Leg 176 returned the Atlantis II fracture zone to deepen Hole 735B and obtain representative sections of the lower ocean crust to test the nature of the MOHO and the crust. The first major scientific objective of the return to Hole 735B was the recovery of a representative section of gabbroic Layer 3 to determine its stratigraphic variation with depth. This was accomplished by deepening the hole to 1.5 km with an overall recovery of 87%, in an environment (ultra-slow-spreading ridges) where the lower crust is believed to be only about 2 km thick. The recovery was the highest for any hard rock hole in history by a wide margin. Unfortunately, despite reaching our goal of deepening Hole 735B to a target depth of 1.5 km, we did not penetrate the crust/mantle boundary and the hole was lost after the bottom hole assembly and 1400 m of drilling pipe broke off during bad weather conditions. Fishing attempts were able to clear the uppermost 600 m of the hole for logging operations however, at end of the leg, the lower 900 m of the borehole remained obstructed. Such recovery and the good borehole conditions of the top 600 m facilitated detailed core and downhole measurements which will allow mass balances to be calculated using the chemistry of the section and the construction of more realistic models of crustal recycling and global geochemical fluxes. In the following section, a summary of the logging results and operations is given. Details of the overall results of the leg can be found in the Leg 176 scientific propspectus or in the preliminary report

Logging Operations

Initial Logging Program

After reaching Site 735, Hole 735B was re-entered, a logging bottom-hole-assembly (BHA) was set at 49 mbsf and two logging tool strings were deployed. The first tool string contained density (HLDS), porosity (APS), resistivity (DLL), spectral gamma-ray (HNGS), and temperature (TLT) probes as well as a one-arm caliper. The second logging run consisted of a natural gamma-ray (NGT), Dipole Sonic Imager (DSI) and Formation Micro scanner (FMS) probes. Operational difficulties resulted in aborting the second logging run early and limited data is available. The temperature measurements show nearly isothermal conditions throughout the top 500 m. A steady temperature decrease of approximately 0.8° C from 8.9° C at 49 mbsf to 8.1 °C at 240 mbsf, followed by a steady increase of up to 0.8° C at a depth of approximately 445 mbsf is observed. After 445 mbsf, temperature and pressure fluctuations were recorded but whether these variations were caused by a change in hydrological conditions or a tool malfunction is still in question.

The spectral gamma-ray, resistivity and porosity logs obtained with the first tool string show similar results than those recorded during Leg 118. However, results from the caliper logs show distinct differences. The HLDS and FMS logs show a variations of approximately two inches higher than the caliper measurements from Leg 118. Calibrations for the tools deployed during Leg 176 were checked after logging operations were concluded and they appear to be correct. A closer inspection of the Leg 118 caliper log suggests that these measurements were somewhat questionable because in many instances they were below the bit size specifications.

Final Logging Program

Although Hole 735B was drilled to 1508 mbsf, about 1400 m of drill pipe had broken off in the hole and only 500 m of pipe were recovered, leaving approximately 600 m of the hole open for logging. Of this, approximately 500 m had been logged during Leg 118. At the conclusion of fishing operations, the logging BHA was set at 50 mbsf and four logging tool strings were deployed during an approximate total operational time of 42 hours. The first tool string consisted of density (HLDT), caliper, porosity (APS), and spectral gamma-ray (HNGS) probes. The second logging run consisted of a Natural Gamma-Ray (NGT), Dipole Sonic Imager (DSI), General Purpose Inclinometry Tool (GPIT), and Formation Micro Scanner (FMS) probes. The third tool string was composed of Natural Gamma-Ray (NGT), General Purpose Inclinometry Tool (GPIT), and Dual Laterolog (DLL) probes. The fourth and final run consisted of the BGKT Prakla Seismos Vertical Seismic Profiling (VSP) tool. A modified Kinley sub was placed at 19.5 m above the bottom of the BHA. The inside diameter of this sub had to be modified at the beginning of the leg from 3.810 inches to 3.918 inches to allow the larger diameter VSP tool (3.85 inches) to be deployed. This sub is routinely placed in the BHA during logging operations in case there is a need to cut the logging cable and retrieve the tool. A schematic diagram of the tool strings used during this leg are shown in Figure 3.

Figure 3: Schematic diagram of the wireline logging tool string deployed during Leg 176.

Logging Results

Condition and Log Data Quality

Shipboard analysis of logging data and cores recovered during Leg 118 and Leg 176 show that Hole 735B consists of definable units and horizons, reflecting differing influences of magmatic, structural and metamorphic processes within the lower crustal gabbroic rocks drilled at this site. A selection of most of the logs acquired during Leg 176 is presented in Figures 4 through 8. The intervals identified in the logs approximately correspond to the 545 m of open hole (50-595 mbsf) above the 900 m of pipe still in the borehole and the depth below the logging BHA. Most logs were also run through the BHA to the seafloor and repeat passes were made for quality control. Two caliper logs from the FMS (Figure 4) illustrate two orthogonal dimensions of the borehole with of depth. The diameter of Hole 735B generally varies between 10.4 and 15 inches, with the largest diameters occurring at approximately 100 and 560 mbsf. Otherwise, the condition of Hole 735B is generally adequate for the acquisition of good logging data. In several sections, the borehole seems to be slightly elliptical in cross-section with only a few intervals having large systematic differences between the two calipers. These intervals are approximately at 100, 485, and 560 mbsf and have a maximum difference of 1.2 inches. The orientation of the calipers with respect to magnetic north (P1AZ) illustrates that the tool followed almost identical paths during the two full passes with the FMS. This small rate of rotation may be indicative of hole ellipticity, deviation, or directional borehole damage caused by the extensive fishing operations conducted near the end of the leg.

Figure 4: Composite illustration of Hole 735B logs showing the FMS calipers (C1 and C2) in track 1; hole deviation (DEVI), hole azimuth (HAZI) and FMS pad 1 azimuth for both passes in track 2; borehole pressure and temperature in track 3; and laterolog deep (LLd) and shallow measurements in track 4. The bottom scale in the plot axis shows the scale increments for the bottom-most curves displayed above the plot and the top scale shows the increments for the top-most curves shown above the plot.

The HLDS, HLDT and FMS logs from initial and final logging programs show variations of approximately two inches greater than the caliper measurements from Leg 118. The tools deployed during Leg 176 were re-calibrated after logging operations, and they appear to be correct. A closer inspection of the Leg 118 caliper log suggests that these measurements are somewhat questionable because in many instances they are less than the bit diameter.

Hole deviation and azimuth logs obtained with the GPIT (Figure 4) show a deviation approximately varying from 7.1° at 95 mbsf to 5.1° at 545 mbsf. Below 545 mbsf, the hole deviates more which may be a result of the fishing operations. This slight hole deviation did not affect the operation of the logging tools or the data quality. The direction of this deviation rotates from N3E at 95 mbsf to approximately N35E at 595 mbsf. The abundance of magnetic minerals in Hole 735B seems to have an influence in the azimuth measurements obtained with the GPIT magnetometer. This is evident in the inflections measured in Unit 4 from 233 to 278 mbsf. However, the azimuth values obtained in Unit 5 from 325 to 400 mbsf should be more reliable since this unit has a relatively low abundance of magnetic minerals, as already shown in the susceptibility log obtained during Leg 118 (Shipboard Scientific Party, 1989).

The quality of the raw FMS images from both passes at the end of the leg was poor and required extensive post cruise processing. In formations with resistivities greater than 10,000 ohm-m, such as these, the FMS current will tend to flow into the mud and along the borehole fluid rather than into the formation. A future alternative to this may be to increase the mud resistivity by pumping fresh water mud into the hole with the goal of increasing the mud resistivity from 0.1 ohm-m to >2 m. The poor performance of both FMS tools may have been caused by several factors:

  • the tools could not respond quickly enough to the extreme resistivity contrasts between the oxide gabbros (<10 ohm-m) and the olivine gabbros (>10,000 ohm-m)
  • the resistivity contrast between the olivine gabbros (low conductivity) and the borehole fluid (high conductivity) caused the current to travel along the borehole instead of penetrating the formation thus, causing the EMEX current to saturate the measuring electrodes
  • the low resolution of the MAXIS displays were degraded to the point where assessments of data quality control during logging operations was not possible. Extensive post-cruise data processing was performed and some useful information was obtained.

The following postcruise corrections and processing were made to the FMS data:

Speed corrections were applied to the data to correct the fact that measurements attributed to a cable depth are actually acquired at a somewhat different depth. This is essentially a depth correction but is also referred to as a speed correction because the depth error would not exist if the tool traveled at the same speed as the cable at the surface winch. The integration of the cable speed and the z-axis accelerometer data were used to estimate the speed and depth of the tool.

Because the tool was sticking for short intervals (i.e. the tool remained stationary while the cable moved for a short distance), a sticking detection threshold and recovery speed factors were applied to correct the fact that in these zones the information from the cable depth and the integration of the accelerometer data are in conflict. The former indicated that the tool is moving at a cable speed, while the latter showed that the tool velocity was not changing.

The average response of all the buttons in each pad were equalized to account for the difference in gain and offset of the pre-amplification circuits associated with each button; differences in standoff according to button location due to mismatch of borehole and pad curvature; and the difference in application pressure between pads.

A faulty button detection and correction was made specially for pad 4 because approximately 95% of the buttons failed. This correction interpolated the faulty button values using the values of adjacent good buttons.

The button response is controlled by the EMEX voltage which is applied between the button electrode and the return electrode. Because of the EMEX saturation messages recorded during this particular logging run, a voltage correction was applied where the button response was divided by the EMEX voltage channel so that the response corresponds more closely to the conductivity of the formation.

Imaging enhancing techniques were used for data display by using the method of histogram equalization. This technique enhances the depiction of details in an image by optimizing the color usage (i.e. the use of colors available with equal frequency). This technique was used in two ways for highlighting different features: static normalization, which is a global optimization with a window covering the entire logged interval, and dynamic normalization, which is a local optimization with separate normalization computations repeated at regularly spaced positions using a 5 m sliding window

Temperature Measurements

Temperature measurements obtained with the Lamont-Doherty memory temperature tool (TLT) at the beginning of the leg show that the hole is nearly isothermal for 500 m (Figure 4). A slight but steady decrease in temperature of approximately 0.8° C from 8.9° C at 49 mbsf to 8.1° C at 240 mbsf is observed and followed by a steady increase of up to 0.8° C at a depth of approximately 445 mbsf. These variations at the top of the hole may be partially attributed to a small amount of circulation (30 strokes per minute for 5 minutes) from the rigfloor while trying to pass an obstruction at the top of the hole. Below 445 mbsf, temperature and pressure fluctuations were also recorded. These fluctuations may be attributed to difficulties trying to reach the bottom of the hole. The average temperature for the entire upper 500 m of Hole 735B is 8.5° C.

Nuclear Measurements

The bulk density (RHOB) of the formation and the photoelectric factor (PEF) were measured using the HLDT (Figure 5). Density values range from 1.47 to 3.27 g/cm3 with a mean value for the entire logged section of 2.88 g/cm3. Low values are related to fractures filled with seawater or to enlarged sections of the hole. The olivine gabbros of Unit 5 exhibit a range of values from 2.25 to 2.98 g/cm3 with a mean of 2.88 g/cm3 whereas the Fe-Ti oxide gabbros of Unit 4 show a range of values from 2.95 to 3.27 g/cm3 with a mean of 3.09 g/cm3. The PEF varies from 1.10 to 9.84 barns/e- which is indicative of the lithological variations observed in this lower oceanic crustal section. Variations in the density profile correspond to variations in oxide mineralogy (Shipboard Scientific Party, 1989) and increases in porosity (Figure 5). Density values from discrete laboratory measurements show a fairly good correlation with log measurements with slightly higher values in the upper 500 m of the hole and more scatter at the bottom of the logged section.

Figure 5: Downhole logs for Hole 735B showing the Leg 118 density (118 RHOB), Leg 176 density (176 RHOB), Legs 118 and 176 discrete laboratory measurements and photoelectric effect (PEF) in track 1; array epithermal porosity (APLC), Far/Near detector porosity (FPLC) and Legs 118 and 176 discrete laboratory porosity measurements in track 2; spectral gamma-ray (HSGR) and computed gamma-ray (HCGR) in track 3; and thorium (THOR), uranium (URAN) and potassium (POTA) in track 4. The bottom scale in the plot axis shows the scale increments for the bottom-most curves displayed above the plot and the top scale shows the increments for the top-most curves shown above the plot.

The porosity measurements in the logged section of the hole show variations between 0.03 to 57.37 % with a mean value for the entire section of 3.34 %. High values correspond to borehole washouts or fractures and generally correlate with low peaks in the density log. Several isolated zones corresponding to high porosity and low density occur in the upper 450 m of the hole and were previously documented as high permeability zones from results of Leg 118 Packer experiments (Shipboard Scientific Party, 1989). Below 450 mbsf, several zones showing decreases in resistivity and density and increases in porosity and borehole size are observed (Figure 6). In addition, the results shown in Figure 7 suggest that these higher porosity and lower resistivity zones may correspond to the higher deformation intensity observed in this section of the hole. Discrete laboratory measurements do not show a good correlation with the porosity logs which may be a direct cause of sampling bias since fractured intervals are usually poorly recovered and the rocks recovered have veins and fractures which are not often sampled for measurements of physical properties.

Figure 6: Expanded section of the bottom-most 150 m of open hole logged at the end of Leg 176 showing deep (LLd) and shallow (LLs) resistivity logs in track 1; array epithermal porosity (APLC) and Legs 118 and 176 discrete laboratory porosity measurements in track 2; FMS calipers (C1 and C2) in track 3; and Legs 118 and 176 density logs (118 RHOB and 176 RHOB), Legs 118 and 176 discrete laboratory density measurements and photoelectric effect (PEF) log in track 4.

The spectral gamma-ray logs were measured with both the NGT and HNGS tools. Profiles from both tools show excellent correlations, therefore only the profiles obtained with the HGNS are shown in Figure 5 for simplicity. The total spectral gamma-ray (HSGR) varies from 1.8 to 16.9 API in the open section of Hole 735B. The contributions to the natural radioactivity observed in the HSGR and the computed gamma-ray (HCGR) vary throughout the hole. In Unit 2 (58 to 186 mbsf), most of the contributions seem to be related to an increase in thorium (THOR) content towards the base of this unit. In Unit 3 (186 to 239 mbsf), the variations are mostly caused by small increases in both potassium (POTA) and thorium. Unit 4 (250 to 278 mbsf) is characterized by sharp increases in potassium and smaller variations in thorium whereas the natural radioactivity of Unit 5 from 281 to 403 mbsf is mainly caused by increases in thorium and uranium (URAN). The base of Unit 6 (488 to 536 mbsf) is characterized by a decrease in thorium with increases in both potassium and uranium decay series. Potassium is enriched in oceanic crustal environments during low temperature oxidative alteration processes, therefore, the HSGR and potassium logs are good indicators of alteration.

Sonic Measurements

The sonic logs recorded with the DSI tool represent the first use of this tool in the lower oceanic crust. The data obtained during the second logging run at both the beginning and the end of the leg were recorded during three separate passes of the tool through the section of open hole. In total, five different modes of the DSI using different acoustic sources allowed the acquisition of both compressional and shear waveforms. Both high-frequency compressional and shear modes as well as the low-frequency dipole mode produced good sonic waveforms. Preliminary processing of compressional (DTC) and shear (DTS) travel times was completed onboard JOIDES Resolution using Slowness-Time-Coherence (STC) software on the Schlumberger MAXIS acquisition system (Kimball and Marzetta, 1984). Post cruise processing must also be applied to the dipole data to account for dispersion effects, which may reduce the travel times by 2%-6% (Brie and Saiki, 1996). The low-frequency Stoneley mode also produced high quality waveforms.

The delta-transit -time compressional (DTC) and delta-transit-time shear (DTS) logs from the monopole source and the delta-transit-time shear (DTSM) measurements from the dipole source are shown in Figure 8. The DTC trends were computed using a high-frequency source over a range from 35 to 150 µs/ft whereas the DTS trends were computed using a low-frequency source over a range from 40 to 200 µs/ft. Coherence of the transmitter and receiver combinations for the compressional- (CHTP and CHRP) and shear-wave (CHTS and CHRS) logs from the monopole source is degraded in washouts and with excursions in borehole size at several intervals.

Figure 8: Downhole logs for Hole 735B showing the FMS calipers (C1 and C2) in track 1; travel-time for compressional- (DTC) and shear-waves (DTS) generated with the DSI monopole source and shear-wave travel times produced with the DSI dipole source in track 2; transmitter and receiver coherence for the high frequency compressional-wave monopole source (CHTP and CHRP) in track 3; and transmitter and receiver coherence produced by the high frequency shear-wave monopole source (CHTS and CHRS) as well as coherence from the low frequency shear-wave dipole source (CHR2) in track 4.

The dipole-shear waveforms (CHR2) have systematically higher coherence than the high-frequency compressional and high-frequency shear waveforms, which in part is the result of less scattering from small fractures.

Compressional and shear travel-time logs show generally uniform values throughout the 550 m of section logged. Variations correlate with changes in both resistivity and density measurements. The largest variation in both compressional and shear travel times is observed at approximately 565 mbsf. This apparent low-velocity, low-density zone which also correlates with high-porosity and caliper readings, may be responsible for the reflector identified during Leg 118 (Swift et al., 1991) at this depth. The average compressional- and shear-wave velocities obtained from the monopole source are 6516 m/s and 3697 m/s. The average shear-wave velocity obtained from the dipole source is 3504 m/s. However, these values have not been corrected for dispersion effects and post cruise processing may have a significant effect on the final results. As mentioned above, the Leg 176 downhole measurements show distinct boundaries for the top six lithostratigraphic units of Hole 735B. Logging data also suggest that there are two faults centered at approximately 555 and 565 mbsf which are 2 m and 4m thick, respectively. These log measurements show reduced velocities, densities, and resistivities, and elevated porosities at these intervals. These zones also correlate with the approximate depth of a seismic reflector identified during the Leg 118 Vertical Seismic Profile (VSP) experiment. This high amplitude event occurs at approximately 560 mbsf or 1.11 s two-way traveltime (Swift et al., 1991). The origin of the reflector located at approximately 560 mbsf is also discussed below. Since the results from the Leg 176 VSP did not show any appreciable improvement over the previous experiments, the processed results from Leg 118 are shown in Figure 9. The shallowest reflector appears to correspond to the Unit I/Unit II contact whereas the next two reflectors seem to be associated with the presence of Fe-Ti oxides in Unit VI.

Figure 9: Deconvolved air-gun record section from ODP Leg 118. Horizontal scale is depth of receiver below seafloor. Timing for each seismogram has been increased by the one-way traveltime to each receiver depth. Reflections from flat, laterally continuous impedance contrasts should have the same arrival time at each receiver depth. The two panels are the same sections except that the authors' interpretation reflection events are shown on the right. (Figure and interpretation obtained from Swift et al., 1991.)

Electrical Resistivity Measurements

Electrical resistivity measurements and images were obtained with the DLL and FMS probes in Hole 735B during Leg 176, recording 2 different electrical logs and one type of formation image. The laterolog deep (LLd) and shallow (LLs) measurements (Figure 4) give similar results with the lowest values being obtained in the Fe-Ti oxide gabbros of Unit 4. Several other low-resistivity measurements were recorded throughout the upper 600 m of the hole which correlate with density and porosity variations discussed below.

A comparison of the FMS raw and processed data shown in Figure 10 illustrates the difficulties in monitoring data quality during the logging runs and the subsequent improvement. A preliminary interpretation of the processed FMS images revealed strike orientations as well as dip azimuth and magnitude for several hundred structural features. However, two main factors may influence the overall orientation of these features after more detailed postcruise interpretation and processing is performed. First, these orientations were obtained from sinusoid fits based on the assumption that the features were planar. Second, these picks may also be influenced by the high concentration and magnetization of the Fe-Ti oxide minerals present throughout the logged interval (Figure 5). The degree to which the GPIT magnetometer is influenced by the high magnetization of the oxide gabbros will be investigated at a later date. The preliminary strike orientation of the majority of the features range from 280 to 310 . The dip azimuth of these features ranges from 340 to 20 with several features also dipping from 180 to 220 and the magnitudes mostly ranging from 10 to 50 .

Figure 10: Comparison of raw and dynamic processed FMS images of gabbros from the lower oceanic crust section drilled at Hole 735B. The poor quality of the raw data monitored during acquisition was apparently due to tool sticking and specially the high resistivity of the formation (> 10,000 ohm-m) which caused the electrical current to travel along the borehole fluid and saturate the tool receivers. An alternative solution to avoid this problem would be to fill the borehole with fresh water mud in order to reduce the contrast between the conductivity of the fluid and the formation.

Examples of some of the features identified in the FMS images are displayed in Figures 11 through 13. A 28 m interval (Figure 11) from 273 to 297 mbsf shows the variations in character observed in the transition from oxide gabbros of Unit IV, through the magmatic breccia zone at the base of Unit IV, and into the olivine gabbros of Unit V. Alternating intervals of olivine and oxide gabbros are observed from 273 to 280 mbsf. The transition from the oxide gabbros into the magmatic breccia zone shows some deformation at the base of Unit IV with most of the features approximately dipping between 17 to 22 . Besides the large resistivity contrast between the interlayered subunits of Unit IV, one of the most noticeable features is the abrupt contact between the magmatic breccia zone and the olivine gabbros of Unit V at approximately 292 mbsf.

Figure 11: FMS images of lower oceanic crust gabbros from Hole 735B. The transition from iron-titanium (Fe-Ti) oxide gabbros to a magmatic breccia zone and into a unit of olivine gabbros is recorded in both the FMS and laterolog data. The Fe-Ti oxide unit shows pronounced subhorizontal igneous laminations with 20 to 100 cm thick intervals of olivine gabbros. The magmatic breccia zone is characterized by a gradual increase in resistivities due to the decrease in Fe-Ti minerals. The olivine gabbro unit is marked by high resistivities (> 10,000 ohm-m) and a lack of deformation.

Several structural features and lithological boundaries are clearly observed in the bottom most 100 m of the FMS logs are shown in Figures 12 and 13. Highly conductive zones at 557 mbsf and 566 mbsf (Figure 12) may correspond to zones of intense deformation. These 1 m and 4 m zones seem to correlate with high core fault intensity measurements (Figure 7).

Figure 7: Expanded section of the bottom-most 150 m of open hole logged at the end of Leg 176 showing deep (LLd) and shallow (LLs) resistivity logs in track 1; array epithermal porosity (APLC) log and Legs 118 and 176 discrete laboratory porosity measurements in track 2; and fault intensity measurements from core observations in track 3.

Lithological boundaries are also observed in Figure 13. Oxide gabbros in Cores 99R-6 to 101R-2 seem to correspond to a 3 m low resistivity interval ranging from 579 to 582 mbsf and a smaller 1 m interval at 590 mbsf correlates with oxide olivine gabbros recovered from Core 101R-3 through 102R-1 (Figure 13 ) by strong deformation.

Figure 12: FMS image showing a 25 m interval that includes several structural features which correlate with high core fault intensity measurements. This figure also includes a depth scale in mbsf; dynamic and static processed FMS images; borehole drift and structural dips and azimuth; deep and shallow resistivity logs from the DLL.

Figure 13: FMS image showing the bottom most 25 m logged interval that includes structural features and lithological boundaries. This figure also includes a depth scale in mbsf; dynamic and static processed FMS images; borehole drift and structural dips and azimuth; deep and shallow resistivity logs from the DLL.

Core Imaging

During Leg 176, all whole-core pieces which could be successfully rotated through 360° were imaged on the DMT Digital Color CoreScan system (Figure 14). Contiguous pieces were imaged together wherever possible. In a number of cases, pieces with lengths in excess of 1 m were broken in order to fit the core scanner. One such piece (Samples 176-735B-150R-1, 0-77cm and 176-735B-150R-1, 77-147 cm) was originally 147 cm long. Lengths of pieces too small or uneven to be scanned effectively were also measured, in order for allowance to be made for them in the total core barrel lengths.

Figure 14: DMT Color Core Scanner used during Leg 176 to obtain both unrolled (full 360° core) and slabbed (2D cut face) core images.

In total, more than 800 meters of whole core were scanned in the unrolled mode. This comprises approximately 93% of the material recovered during Leg 176. The scanned images were then integrated into core barrel lengths using the DMT CoreLog Software. The individual core images are imported into the CoreLog Software, then rotated so that the red china marker line, marked on the core by the structural geology team, is in the same orientation for each piece. The images can then be inserted together to reconstruct each core barrel length. Initial structural analysis of the images included the picking of pertinent structures, such as veins, fractures and foliations. These are plotted by the software as sinusoids, from which the dip of the feature is calculated. Examples of the unrolled scanned images are shown in Figure 15, in which 15A shows four smectite veins with dips ranging from 51 to 61 and 15B illustrates a highly foliated zone with thick mafic boundary layers. Final structural analysis and correlation with downhole logging data are currently in progress.

Figure 15: Unrolled images from Hole 735B. (A) Sample 176-735B-133R-7 (83-150). (B) Sample 176-735B-135R-1 (102-128). Dips of veins and foliations were determined using the DMT software CoreLog

Preliminary reorientention of core pieces shows a good correlation between Leg 118 Borehole Televiewer (BHTV) data, unrolled core images, and Leg 176 FMS logs (Figure 16). A westerly steeply dipping fracture is clearly identified in the core and the oriented logs. A second fracture is also identified in the FMS logs but not in the core or the BHTV log. These crosscutting fractures are dipping at 90 from each other but at this time, the lack of evidence for a second fracture in the recovered core and BHTV data prevents a classification as a conjugate pair of fractures.

Figure 16: Core reorientation using Borehole Televiewer (BHTV) and Formation MicroScanner (FMS) data. A fracture within the gabbroic cores recovered from Hole 735B has been oriented using BHTV and FMS data. The BHTV data was collected after the hole was cored to a depth of 500 m in 1987 and the FMS data was obtained ten years later after the hole was deepened to 1500 m. A second fracture that is not apparent in the BHTV or in the core images has also been identified in the FMS data. This fracture seems to be either a recent tectonic feature or a drilling induced fracture since the FMS images were obtained after the original hole was drilled and BHTV data was obtained

Approximately 48 meters of split half-core were imaged in the slabbed scan mode. This represents only 5% of the core recovered. Slabbed cores 176-735B- 89R, 90R, 93R, 94R, 95R, 97R, 107R, 108R, 109R, 110R, and 111R were imaged in entirety. Selected pieces, with structural and igneous features of particular interest, were also imaged from Cores 99R, 104R, 105R, 112R, 113R, 119R, 120R, and 121R. An example of a slabbed image is shown in Figure 17.

Figure 17: Lower oceanic crust gabbros recovered from Hole 735B show crosscutting relationships. Core photograph shows 69 cm long slabbed image displaying plagioclase (light) and clinopyroxene (dark) foliations with amphiboles veins crosscutting the primary foliation at approximately 90°

References

Bonatti, E., and Honnorez, J., 1976. Sections of the Earth's crust in the Equatorial Atlantic. J. Geophys. Res., 81:4087-4103.

Brie, A., and Saiki, Y., 1996. Practical dipole sonic dispersion correction, Trans. Soc. Expl. Geophys., 178-181.

Cann, J.R., 1974. A model for oceanic crustal structure developed. Geophys. J.R. Astr. Soc., 39:169-187.

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Logging Scientists

Gerardo J. Iturrino, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 U.S.A.
Sarah Haggas, Department of Geology, University of Leicester, Leicester LE1 7RH United Kingdom



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