Elliott Taylor

Tectonics and hydrogeology of the northern Barbados Ridge: Results from Ocean Drilling Program Leg 110

Drilling near the deformation front of the northern Barbados Ridge cored an accretionary prism consisting of imbricately thrusted Neogene hemipelagic sediments detached from little-deformed Oligocene to Campanian underthrust deposits by a decollement zone composed of lower Miocene to upper Oligocene, scaly radiolarian claystone. Biostrati-graphically defined age inversions define thrust faults in the accretionary prism that correlate between sites and are apparent on the seismic reflection sections. Two sites located 12 and 17 km west of the deformation front document continuing deformation of the accreted sediments during their uplift. Deformational features include both large- and small-scale folding and continued thrust faulting with the development of stratal disruption, cataclastic shear zones, and the proliferation of scaly fabrics. These features, resembling structures of accretionary complexes exposed on land, have developed in sediments never buried more than 400 m and retaining 40% to 50% porosity. A single oceanic reference site, located 6 km east of the deformation front, shows incipient deformation at the stratigraphic level of the decollement and pore-water chemistry anomalies both at the decollement level and in a subjacent permeable sand interval. Pore-water chemistry data from all sites define two fluid realms: one characterized by methane and chloride anomalies and located within and below the decollement zone and a second marked solely by chloride anomalies and occurring within the accretionary prism. The thermogenic methane in the decollement zone requires fluid transport many tens of kilometers arcward of the deformation front along the shallowly inclined decollement surface, with minimal leakage into the overlying accretionary prism. Chloride anomalies along faults and a permeable sand layer in the underthrust sequence may be caused by membrane filtration or smectite dewatering at depth. Low matrix permeability requires that fluid flow along faults occurs through fracture permeability. Temperature and geochemical data suggest that episodic fluid flow occurs along faults, probably as a result of deformational pumping.

Evolution of structures and fabrics in the Barbados Accretionary Prism. Insights from leg 110 of the Ocean Drilling Program

The microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo-feldspathic mylonitic shear zone (Torridon, NW Scotland) have been investigated using SEM electron channelling. Our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history. Microstructural variation is particularly pronounced at low shear strains: significant intragranular deformation occurs via a domino-faulting style process, whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures. A sudden reduction in grain size defines the transition to medium shear strains, but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin. Grains within these bands have somewhat steeper preferred dimensional orientations. These domains persist into the high shear strain mylonitic region, where they are oriented subparallel to the shear zone margin and consist of sub-20 μm grains. The microstructures suggest that the principal deformation mechanism was intracrystalline plasticity (with contributions from grain size reduction via dynamic recrystallization, grain boundary migration and grain boundary sliding). Crystal fabrics measured from the shear zone vary with position depending on the shear strain involved, and are consistent with the operation of several crystal slip systems (e.g. prism, basal, rhomb and acute rhomb planes) in a consistent direction (probably parallel to a and/or m). They also reveal the presence of Dauphine twinning and suggest that this may be a significant process in quartz deformation. A single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed. Rather, the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation. The mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution. The stability of some microstructures to higher shear strains, the exploitation of others at lower shear strains, and a continuously evolving crystal fabric, mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient.

Hydrogeochemistry in the barbados accretionary complex: Leg 110 ODP

Abstract Detailed studies of the chemical composition of interstitial waters in the sediments obtained along a drill hole transect across the accretionary prism of the Northern Barbados Ridge have revealed a complex set of processes: (1) In Plio-Pleistocene volcanic ash-bearing sediments increased concentrations of dissolved calcium and decreases in dissolved magnesium are the result of volcanic ash alteration; (2) below the decollement large concentration increases in dissolved calcium, accompanied by large decreases in sodium as well as a granual decrease in dissolved magnesium, suggest exchange of these constituents with the deeper seated rocks of layer 2 of the oceanic crust; (3) low chloride concentrations, particularly associated with the decollement zone separating the converging plates and underthrusted sandstones, and also with recent faults in the accretionary complex, suggest advective transport of low chlorinity waters from deeper within the accretionary complex. In and below the decollement zone elevated concentrations of methane of thermogenic origin have been measured.

Geotechnical sampling and testing of gas-charged marine sediments atin situ pressures

Gas-charged marine sediments were sampled using a pressure core barrel and a new technique involving standard wireline sampling procedures. Pressure core barrel sampling provides sediments atin situ pressures with no degassing. Analysis of gas content, geotechnical properties and microfabric were performed for samples atin situ pressures and compared with degassed results. Methane concentrations ranged from 3,450 to 137,140 ppm for pressurized samples. Microfabric analyses reveal highly nonoriented clay particles and grains in degassed samples. Degassing of these methane-rich sediments increases porosities and compression indices, while reducing undrained shear strengths and computed preconsolidation pressures.