Magnetic susceptibility stratigraphy for Pleistocene and Pliocene sediments of ODP Leg 110 samples

Whole-core magnetic susceptibility measurements define a detailed stratigraphy that enables correlation between the various Pleistocene, Pliocene, and upper Miocene sections cored on ODP Leg 110, near the Tiburon Rise. The magnetic susceptibility in these sections is primarily related to the content of volcanic ash, rich in titanomagnetite, and also inversely related to calcium carbonate content. The high resolution of the susceptibility record enables correlations with a resolution of about 0.3 m of sediment thickness, and the identification of minor faults not definable by biostratigraphic means. Reverse and normal faults identified in Hole 672A are probably a result of normal oceanic sediment dewatering and compaction processes. This work indicates some of the problems of using visible ash layers as time-stratigraphic markers.

Magnetic susceptibility anisotropy in deformed sediments of ODP Leg 110

The anisotropy of magnetic susceptibility documents the generation of tectonically produced fabrics in sediments that macroscopically show no evidence of this disruption. The fabric observed in initial accretion is largely produced by overprinting of the original sedimentary susceptibility anisotropy by an E-W horizontal tectonic shortening and vertical extension. The response of the sediments to stress during initial accretion is variable, particularly near the sediment surface, and appears to reflect the inhomogeneous distribution of strain rate in the overthrust sequence. The susceptibility anisotropy of sediments possessing scaly fabric is consistent with the strong orientation of Phyllosilicates seen in thin section, producing a Kmin normal to the scalyness. The slope sediments deposited on the accreted sequence are also affected by tectonic shortening. The accreted sequences at Sites 673 and 674 show a complex history of fabric modification, with previous tectonic fabrics overprinted by later fabric modifications, pointing to continued tectonic shortening during the accretion process. The form of the susceptibility anisotropy axes at Sites 673 and 674 is consistent with NESW shortening, probably reflected in the NW-SE surface expression of the out-of-sequence thrusts. The susceptibility anisotropy appears to document a downhole change in the trend of shortening from E to W at the surface to more NESW at depth, probably as a result of the obliquely trending basement ridge, the Tiburon Rise.

Interstitial water chemistry of ODP Leg 110 holes

Major-element compositions (Cl-, SO4[2-], Ca2+, Mg2+ , Li+ , K+, Na+ , Sr2+) of interstitial waters obtained from sediment cores along the ODP Leg 110 transect across the Northern Barbados accretionary prism have shown that a complex set of geochemical processes are of importance in this area. In the volcanic ash-rich Pleistocene-Pliocene sediments, alteration reactions involving volcanic ash lead to depletions of Mg2+ and K+. This process is confirmed by the much lower than contemporaneous seawater values of the 87Sr/86Sr ratios of dissolved strontium. In the deeper sediments recovered below the zone of decollement (Sites 671 and 672) large increases in Ca2+ and gradual decreases in Mg2+ , Na+, and d18O (H2O) indicate a potential contribution to the interstitial water chemistry by exchange with underlying basement rocks. This process has been hard to confirm because the drill holes were terminated well short of reaching basement. However, the concentration gradient pattern is consistent with observations in a large number of DSDP drill holes. Finally, but most importantly, low Cl- concentrations in the decollement zone and underlying sand layers, as well as in fault zones at Sites 673 and 674, indicate dilution of interstitial waters. The potential origins of the low Cl- concentrations are discussed, though we are not able to distinguish any mechanism in particular. Our evidence supports the concept of water migration along the decollement and through the underlying sandstones as well as along recent fault zones in the accretionary complex. Interstitial water concentration depth profiles are affected by faulting, thrusting, and overturn processes in the accretionary prism. These processes have caused a diminished diffusive exchange with the overlying ocean, thus explaining increased depletions in Mg2+ and SO4[2-] in sites farther onto the accretionary prism.

Stable isotope ratios of interstitial fluids from ODP Leg 110 sites

Delta18O values of pore waters from the northern Barbados accretionary prism range from -0.3 to -3.6? and reflect pervasive reaction of volcanic ash to form smectite within the sedimentary sequence and continued low temperature alteration of basalt in the underlying ocean crust with the overprint of diffusive exchange between water in the sediment pores and the open ocean. Delta D values of pore waters in sediments sampled seaward of the deformation front drop from +5? at the sediment surface to -6? at the deepest levels sampled. These changes may also be related to alteration processes but remain largely enigmatic. Sediment deformation caused by impingement of the Caribbean plate on the Atlantic plate has instigated migration of chemically and isotopically distinct fluid along faults and coarse-grained sedimentary beds; delta18O values of pore waters are also locally affected by thrust stacking which increases diffusive pathlengths and possibly modifies diagenetic reaction rates in Pleistocene sediments. Migrating fluids are distinguished by anomalous delta18O values that are as much as 1? higher than those of surrounding fluids. Uncertainties in hydrogen isotope fractionation resulting from processes occurring under these conditions hinder identification of the hydrogen isotope composition of expelled fluid. Stable isotope analyses of pore waters help constrain the fluid migration history of the accretionary prism by limiting the source of fluids, the paths along which fluid flows, and the timing of faulting and subsequent fluid flow.

Thermal conductivity of ODP Leg 110 holes

Thirty-four sediment and mudline temperatures were collected from six drill holes on ODP Leg 110 near the toe of the Barbados accretionary complex. When combined with thermal conductivity measurements these data delineate the complicated thermal structure on the edge of this convergent margin. Surface heat-flow values from Leg 110 (calculated from geothermal gradients forced through the bottom-water temperature at mudline) of 92 to 192 mW/m**2 are 80% to 300% higher than values predicted by standard heat flow vs. age models for oceanic crust, but are compatible with earlier surface measurements made at the same latitude. Measured heat flow tends to decrease downhole at four sites, suggesting the presence of heat sources within the sediments. These results are consistent with the flow of warm fluid through the complex along sub-horizontal, high-permeability conduits, including thrust faults, the major decollement zone, and sandy intervals. Simple calculations suggest that this flow is transient, occurring on time scales of tens to tens of thousands of years. High heat flow in the vicinity of 15°30'N and not elsewhere along the deformation front suggests that the Leg 110 drill sites may be situated over a fluid discharge zone, with dewatering more active here than elsewhere along the accretionary complex.

Cenozoic calcareous nannofossils from the Lesser Antilles Forearc Region

During Leg 110 of the Ocean Drilling Program, sediment was recovered from six sites in the vicinity of the Lesser Antilles Forearc. Hole 671B, drilled near the toe of the Barbados deformation front, was the first-ever penetration of the decollement between the underthrusting Atlantic Plate and the off scraped Barbados accretionary prism. Stratigraphic repetitions in sequence associated with tectonic movement along the decollement zone, first observed on DSDP Leg 78A, were further documented at four ODP Leg 110 sites. A significant biostratigraphic inversion is present at Site 671 at 128 mbsf in which upper Miocene sediments rest atop lower Pleistocene strata. Smaller repetitions in sequence are recorded at Sites 671, 673, 674, and 676. Leg 110 sediments range from middle Eocene to early Pleistocene in age. Pliocene/Pleistocene assemblages are generally well preserved; however, Miocene assemblages have undergone extensive dissolution at all Leg 110 sites. Paleogene sediments are sometimes recrystallized and the nannofossils contained within exhibit a range in preservation from poor to good.