34. HIGH-RESOLUTION ELECTRICAL IMAGING IN THE NEW HEBRIDES ISLAND ARC: STRUCTURAL ANALYSIS AND STRESS STUDIES

ABSTRACT Electrical images recorded with the Formation MicroScanner in three of seven holes drilled during Leg 134 were analyzed to study the effect of the collision of the d'Entrecasteaux Ridge with the central New Hebrides Island Arc. Structural features such as bedding, faults, fractures, folds, and shear zones, automatically detected and interactively mapped from the images, were interpreted to document the deformation processes caused by the collision. At Site 829 the images show the imbricated structure of the thrust sheets in the accretionary complex. At Sites 832 and 833 the structural features observed and the compressional-stress information inferred from breakout analyses and hydraulic fractures indicate that the intra-arc North Aoba Basin is still subject to a east-northeast-west-southwest compression. This compression, which is associated with the uplift of the eastern part of the basin, is not older than latest Pliocene

34. SEISMIC PROPERTIES OF SERPENTINIZED PERIDOTITE FROM THE MARIANA FOREARC 1

ABSTRACT Compressional and shear velocities, density, and porosity were measured for 22 serpentinized peridotites recovered during ODP Leg 125. The densities of the samples vary from 2.40 to 2.86 g/cm 3 , whereas the compressional velocities at 200 MPa are between 4.60 and 6.47 km/s. A positive linear trend exists between both compressional and shear velocities and density. The high porosity in serpentinized peridotites decreases the density and seismic velocity

23. STRONTIUM ISOTOPIC COMPOSITION OF THE INTERSTITIAL WATERS FROM LEG 125: MARIANA AND BONIN FOREARCS 1

ABSTRACT The strontium isotopic data presented here are from interstitial waters squeezed from unconsolidated serpentine, an unusual type of substrate that was recovered from Mariana and Bonin forearc seamounts and has not been previously drilled by the Deep Sea Drilling Project or Ocean Drilling Program. The texture and composition of some of these serpentine deposits from Conical Seamount, located on the Mariana forearc, indicate emplacement as low-or high-viscosity, cold gravitational flows, which are therefore neither sediment nor igneous rock. The strontium isotopic ratios of the interstitial waters from the unconsolidated serpentine range from 0.70912 to 0.70525 and trend toward a relatively less radiogenic composition with increasing sub-bottom depth. These strontium isotopic ratios are derived from at least two strontium sources: seawater and igneous. The strontium isotopic gradients from the interstitial waters from the Leg 125 sites are probably the result of diffusive transport of strontium from an igneous source deep within the lithosphere that may be contaminated with subducted or underplated sediment

9. CRACK-SEAL VEINS IN UPPER LAYER 2 IN HOLE 896A 1

ABSTRACT In this paper, we present a detailed description of the morphology and microstructure of fibrous and nonfibrous veins crosscutting basaltic cores recovered from Hole 896A. The nonfibrous veins consisting of spheroidal smectite aggregates and blocky carbonate crystals indicate that minerals crystallized in open spaces during single-stage crack opening. Fibrous veins (mainly smectite + carbonate-bearing) indicate that fibers crystallized by a crack-seal mechanism, involving repeated increments of microcrack openings, followed by displacement-controlled crystal growth. We report descriptions of diagnostic features of the crack-seal veins studied. Finally, composite veins consisting of both nonfibrous and fibrous mineral infill are interpreted to result from recrystallization of fibrous minerals into blocky minerals, or to indicate a decreasing crystallization rate with respect to the fracture opening rate

Ocean Drilling Program Scientific Results Volume 148

ABSTRACT Gold contents were determined in 13 samples representing the interval from 2000 to 2111 mbsf in Hole 504B drilled during ODP Leg 148. The values range from 1.6 to 4.4 ppb, with a mean of 3.0 ppb that is similar to that for the basalts and diabases from the upper parts of the hole. A moderate level of correlation between Zn and Au concentrations observed from the upper parts of subsediment rocks is also valid for the samples from Leg 148. However, the lack of notable correlation between the intensity of alteration and Au contents observed in samples from Legs 69 and 140 is not as obvious for the deepest 300 m of the hole where the inverse correlation between Zn contents and percent alteration is strong. The problem is complicated by the discordance between the samples studied for gold concentration and materials used for shipboard geochemical investigations and determinations of intensity of alteration

Rock magnetic properties of ODP Leg 134 sites

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated by drilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides Island Arc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridge to the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanic silts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magnetic mineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone. The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titanium titanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size of titanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds. Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the cores were azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magnetic fabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) and a factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidated by scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplished primarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed

8. METASOMATIC AND METAMORPHIC PECULIARITIES OF THE DIABASE DIKE ALTERATION IN HOLE 504B, LEG 140

ABSTRACT The diabase dikes recovered during Leg 140 display four types of mineralogical alteration: (1) chlorite + amphibole + titanite; (2) chlorite + amphibole + ilmenite; (3) epidote + prehnite + anhydrite; (4) zeolites + smectite. The first two types represent both background and vein alteration. The third and fourth types are locally overprinted on the previous types. The third type exhibits oxidative features, whereas the fourth formed with cooling of the dike complex. Some veins and patches of strong alteration are surrounded by infiltrate-metasomatic halos, which suggest a difference in fluid pressure in pores and fractures. The temperature of the main alteration types ranged between 350° and 400°C. Noticeable temperature fluctuations in space and time were caused by alternating heating and cooling resulting from intrusions of younger dikes within short time intervals