Philippe A. Pezard

Accretion, structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations

Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Due to the observed zonation with depth of alteration processes, and its relation to lava morphologies, the 650-m-thick effusive section penetrated in Hole 504B is postulated to be emplaced as the result of two main volcanic sequences. Massive lava flows are interpreted as corresponding to the onset of these sequences emplaced on the floor of the axial graben. The underlying lava made of structures with large porosity values and numerous cm-scale fractures is thus necessarily accreted at the end of the previous volcanic episode. On top of such high heterogeneous and porous intervals, the thick lava flows constitute crustal permeability barriers, thereby constraining the circulation of hydrothermal fluids.Accreted in the near vicinity of the magma chamber, the lower section is that exposed to the most intense hydrothermal circulation (such as black smokers activity). Once capped by a massive flow at the onset of the second volcanic phase, the lower interval is hydrologically separated from ocean-waters. A reducing environment develops then below it resulting, for example, in the precipitation of sulfides. Today, whereas the interval corresponding to the first volcanic episode is sealed by alteration minerals, the second-one is still open to fluid circulation in its upper section. Thus, upper part of the volcanic edifice is potentially never exposed to fluids reaching deep into the crust, while the lower one is near the ridge axis.Considering that most of the extrusives are emplaced within a narrow volcanic zone, the first unit extruded for a given vertical cross-section is necessarily emplaced at the ridge-axis. In Hole 504B, the 250-m-thickTransition Zone from dikes to extrusives is interpreted as the relict massive unit flooding the axial graben at the onset of the first volcanic sequence, and later ruptured by numerous dikes. Further from the axis, the same massive unit constitutes a potential permeability cap for vertical crustal sections accreted earlier. Also, the upper 50 meters of the basement might be considered as the far-end expression of massive outpours extruded near the ridge-axis.

Downhole images: Electrical scanning reveals the nature of subsurface oceanic crust

High-resolution electrical images of oceanic sediments exposed by drilling are permitting scientists to make detailed evaluation of the record preserved in the rocks, particularly in intervals where little core was recovered. The images are generated from measurements taken with a slimhole Formation Micro-Scanner (FMS), developed by Schlumberger specifically for the Ocean Drilling Program (ODP). The new measurement technique was used in May 1989 on ODP Leg 126 in two holes drilled in the Izu-Bonin intra-oceanic volcanic arc (Figure 1), which extends south of Honshu to Iwo Jima, Japan.

Ocean floor volcanism: constraints from the integration of core and downhole logging measurements

Abstract The volcanic architecture of oceanic crust records the diversity in volcanic activity during its development in the neovolcanic zone of individual ridge systems. Potentially there exists a spectrum of lithological architectures which may primarily be related to the spreading rate and the dynamics of individual magma chambers along different ridges. Recent studies have emphasized the observable spatial variations within different neovolcanic zones, although direct extrapolation into the third dimension can only be achieved by the use of drilling results. To study the structure of the volcanic layer it is essential that individual lithologies (sheet flows, pillow lavas and/or breccias) can be discriminated from the core and/or logging results and mapped within the borehole. Unfortunately a problem with the drilling of the volcanic basement during the Ocean Drilling Program has been the generally low (typically c. 25%) and biased core recoveries, which produce an erroneous picture of the lithological diversity of the volcanics. This problem is further compounded by the difficulty in determining the volcanic stratigraphy, particularly when the key information is lost during coring (i.e. boundaries/contacts). Downhole logging provides near continuous records of the physical/chemical properties of the borehole which when integrated with core measurements, yield a detailed picture of the architecture of the volcanic layer. Logging results from ODP Hole 896A are of sufficient quality that sheet flows, pillow lavas and brecciated units can be discriminated and mapped effecively within the borehole. From their distribution it is evident that sheet flows become more abundant in the lower part of the hole, which probably correlates with ridge axis volcanism whereas, the predominance of pillow lava flows (<340 mbsf (metres below sea floor)) in the upper part of the hole, is probably related to off-axis volcanism within the neovolcanic zone.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, I: stratigraphy of eruptive products and accretion processes

Abstract Hole 504B is located about 200 km south of the Costa Rica Rift and constitutes the reference section for the structure of the upper oceanic crust. Compared to core, the continuous electrical resistivity (at m scale) and the high-resolution electrical images (at cm scale) recorded in Hole 504B, provide a continuous and detailed lithostratigraphic description of the effusive section at Site 504. Flow thicknesses measured from cm scale FMS images average 0.5 (±0.1) m. The massive units, known to bound fluid circulation at large scale into the crust, are constituted with a series of 20 to 50 individual flows. If Site 504 was created over two volcanic cycles, each volcanic cycle allows the emplacement of [0.60 (±0.30)] × 106 m3 of magma per m along the ridge axis. This computation leads to an estimate of magma volume for a single eruption of [0.003 (± 0.001)] × 106 m3 per m along the ridge axis, and eventually, a gradient in magma pressure within the magma chamber lens of 52 (±26) MPa, appropriate for one eruption.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, III: faulting and fluid circulation. Constraints from integration of FMS images, geophysical logs and core data

Abstract Downhole geophysical logs and high-resolution electrical images (FMS) from DSDP/ODP Hole 504B are analysed in combination with core data to obtain an integrated description of oceanic faults met in the hole. About 34 500 fractures were mapped from FMS images over 1672 m of basement. The fracture distribution from FMS confirms the presence of a main fault zone between 800 and 1100 mbsf (metres below sea floor), elsewhere detected from seismic data as well as magnetic, acoustic, and electrical resistivity measurements. The fracture density profile reveals the presence of two other highly fractured zones, (1) between 400 and 575 mbsf and (2) close to the bottom of Hole 504B (1700 to 2100 mbsf). Consequently, we infer that Site 504 was submitted first to an extensional stress regime near the ridge axis, with circulation of high-temperature fluids and pervasive alteration of the basalts. This initial phase is associated with the main fault met in Hole 504B. Similar but less developed deformation was generated off-axis, with lower-temperature parageneses, such as that cored between 400 and 575 mbsf. The present compressional to strike-slip stress regime is expressed in subhorizontal fracturing detected in discrete zones, such as within the main fault zone and the lower fracture zone (1700 to 2100 mbsf) in Hole 504B.

Basin plain turbidite succession of the Oligocene Izu-Bonin intraoceanic forearc basin

Abstract The sedimentary succession of intraoceanic forearc basins is poorly known. Boreholes from ODP Leg 126 rectify this by providing long sections through a volcaniclastic, basin plain, predominantly turbidite succession in the Izu-Bonin intraoceanic forearc south of Japan. Cores and continuous Formation MicroScanner (Schlumberger) images form the basis of long bed by bed sections that indicate the style of filling of the forearc basin, the high rate of supply of eruptive products from the Izu-Bonin arc, and the relative frequency of initiation of turbidity currents of various sizes. Turbidites and related debris flow deposits range in thickness from approximately the 2.5 cm resolution of the Formation MicroScanner tool to 10–15 m. Bed thicknesses are distributed according to a power law with an exponent of about 1.0. Upwards thickening or thinning sequences are absent. Groups of thick and very thick beds may reflect global sea-level lowstands, particularly at 30 Ma, periods of increased tectonic uplift, or periods of more intense volcanism. The very thickest beds, with maximum recurrence intervals of 0.3–1 million years, may have been deposited from flows triggered by powerful subduction zone earthquakes, or may simply be the result of the failure of unusually large accumulations of volcaniclastic sand and ash on the flanks of arc volcanoes.

Evolution of the Izu-Bonin intraoceanic forearc basin, western Pacific, from cores and FMS images

Abstract One of the objectives of Ocean Drilling Program (ODP) Leg 126 was to investigate the origina nd evolution of the Izu-Bonin arc and forearc, both products of the subduction of Pacific lithosphere under the Philippine Sea Plate. Within the forearc basin, a full set of downhole measurements was recorded in two deep holes (792E and 793B). In addition, borehole electrical images were obtained (for the first time in the context of ODP) with the Formation MicroScanner (FMS*). The main result of the drilling is that the forearc basin formed between 31.0 and 24.0 Ma by separation of a formerly contiguous frontal and outer arc high. The cored material shows a characteristic pattern of volcanogenic input, from turbidites and debris flows produced by volcanism and erosion of surrounding highs. The short rifting period is characterized by high sedimentation rate (300 m/Ma). In this context, the high resolution of FMS images was used to analyze the sedimentary processes associated with the deposition of deep-water volcaniclastics. The images reveal fine details of turbidite sequences that dip at low angles due to recent tectonics. An FMS-based sedimentary log was calibrated from cores and prepared for each of the two holes, providing continuous bed-by-bed sections and permitting the investigation of trends in bed thicknesses. Palaeocurrent data were obtained from the analysis of ripple marks. During early basin history (30.2 to 29.5 Ma), the main sediment source was located to the east, in the vicinity of the modern outer arc high, with a secondary transport-mode oriented northward, along the basin axis. In the shallower section, emplaced at a lower rate from 28.9 to 27.3 Ma, axial transport from the north dominates a small component of flow from the western margin of the basin. Initiated by rifting of the arc during the Oligocene time, basin development was followed by periods characterized also by extensional tectonics. Postdepositional extensional deformations such as normal microfaults, conjugate high-angle fractures, and dewatering veinlets were identified in the core and on FMS images. The orientation of the stress field within the arc and forearc was obtained from the analysis of borehole ellipticity. The results confirm models of stress distribution in forearc-arc-back arc regions. In particular, a rotation of the maximum horizontal stress trajectory in the overlying plate was observed, in a direction orthogonal to the plate boundary. In spite of a 90° clockwise rotation of the Philippine Sea plate since Oligocene time, the orientation of the stress field seems to have remained stable with respect to the trench axis over this period.

Borehole images of the ocean crust: case histories from the Ocean Drilling Program

Abstract Downhole logging is an integral part of the Ocean Drilling Program (ODP), although the choice of tool deployment is primarily a function of the scientific objectives of an individual leg. Following the successful deployment of the Formation Microscanner (FMS) during Leg 126, FMS images have been used to constrain a variety of sedimentary, igneous, metamorphic and tectonic questions in the ocean. However, it is only recently, that the full potential of FMS images and other logging data has been employed in the study of volcanology, which provides important constraints on the evolution of the volcanic pile when fully integrated with core based measurements. Furthermore, many of the holes drilled into the volcanics often have incomplete core recovery and here the logging results can be used to constrain the lithological, chemical and mineralogical variations in such intervals. Using the log based stratigraphy from crust created at slow (Hole 395A) and intermediate (Holes 504B and 896A) spreading centres, it is evident that the proportions of volcanic rock types (pillows/flows/breccias) correlate with the spreading rate. This relationship is suggested from submersible observations, but the ODP data provide additional data to test such hypotheses and to also extended and test it within the third dimension.

Multi-scalar structure at DSDP/ODP Site 504, Costa Rica Rift, II: fracturing and alteration. An integrated study from core, downhole measurements and borehole wall images

Abstract We used a database derived from the integration of core material and geophysical downhole measurements in order to investigate the relationships between fracturing and alteration in the volcanic section of DSDP/ODP Hole 504B. The studied crustal section (from top of the basement to 1000 mbsf (metres below sea floor)) consists of low resistivity/high porosity pillow lavas associated with breccias and rubble material, alternating with high resistivity/low porosity massive basalt flows. A positive correlation between DLL (Dual Laterolog)-derived porosity and occurrence of breccias in the core suggests that breccias more than fractures contribute to the electrical resistivity signal. A structural analysis performed from core suggests that most fractures and veins are steeply dipping, and may represent tectonic features or cracks due to contractional cooling of the crust, the latter being more abundant in pillows. Fractures and veins recorded on core tend to be clustered in massive units or thin flows. This result may derive from criteria adopted during structural measurements and must be taken with care. The natural radioactivity (GR) profile delineates two main alteration zones in the volcanic section: an oxidizing zone with increased potassium above, and a reducing one without K gain below. Most of the GR maxima are found to be correlated with celadonite-bearing alteration halos. GR minima are frequently located at the boundaries between domains of contrasting fracture orientation, where metasomatic reactions may have occurred due to contrasting permeability.

Petrophysical estimation from downhole Mineralogy logs

Abstract A number of physical and chemical properties of rocks, many of which are important in petrophysics are simple, usually linear functions, of a rock’s mineralogy. Examples include matrix density, porosity, magnetic susceptibility and cation exchange capacity. Mineralogy logs can be obtained directly from geochemical logs through an inversion process and, in turn, estimates of petrophysical properties such as those noted above can be obtained. The accuracy of such estimates is directly dependent on the quality of both the inversion and the geochemical measurements. Examples from producing oil fields and from deep-sea environments are used to show that accurate estimates of derived parameters can be obtained provided that appropriate inversion procedures are adopted, and that these estimates are generally better than those obtained by conventional log analysis. Logs of some parameters, such as cation exchange capacity, cannot be obtained except through the mineral inversion.