The subduction of the oceanic spreading center at the Chile Triple Junction is marked by a substantial thermal perturbation and marked changes in the hydrogeologic and aqueous geochemical regimes in the overthrust plate. Ridge subduction substantially changes the fluid chemistry in the wedge through variably hydrating the oceanic basement, accretionary wedge, and continental backstop. This generates positive anomalies in salinity and chloride values with respect to sea water. The wedge immediately above the subducted ridge also experiences greatly enhance diagenesis and cementation together with the influx of primordial mantle derived 4 He. Linear temperature and pore fluid chemistry profiles suggest a predominantly diffusive/conductive regime predominates in the interior eastern portion of the wedge and continental backstop region. In contrast, a vigorous and transient hydrogeolgic system within 5 km of the toe of the wedge at both Sites 859 and 863 generates spatially narrow, large, and complex anomalies in temperature and fluid chemistry. At the toe the vigorous hydrogeologic system may be variably influenced by the episodic expulsion of fluid from both the deeper parts of the wedge and oceanic basement driven convection systems. Structural and diagenetic observations are also consistent with a hydrogeologic regime that both evolves with time and that is dominated by episodic processes. In particular, studies of cements, mineralized veins, deformation bands, and Fe sulfide distribution suggest that above the subducting ridge (i.e., Site 863) the lithification in the wedge is greatly enhanced and that and periods of enhanced fluid expulsion are associated with local hydrofracture and dilation episodes.

Deformation bands, with good macroscopic and microscopic indicators of shear are the clearest macroscopic manifestation of accretionary deformation from ODP Site 860. Backscattered electron SEM studies of deformation bands show that these structures are characterized by low porosity, an increase in matrix:grain ratios and grain-size reduction. Locally grain alignment fabrics are observed and some deformation bands show evidence of injection of foreign material and diagenetic changes. Strain magnitudes from space and shape fabrics are small, both inside and outside deformation bands. We propose that deformation bands develop during transient cycles of dilation, shear and fluid flow associated with accretionary deformation.

Petrographic analysis of textures and microstructures in muds and mudstones from the Chile Triple Junction reveals that formation of framboidal pyrite begins in the top few meters below sea floor (mbsf), and continues during and after the formation of such features as vein structures, faults, and microfaults. Pyrite formation is strongly influenced by primary porosity, as shown by the many microfossils filled by pyrite and by major concentrations of pyrite localized within coarser grain sizes. Some microfossil shells are coated and outlined by pyrite, and many calcareous microfossils are partially dolomitized. Glauconite and phosphate occur as clasts in sand-and silt-sized lithologies. Rare phosphatic concretions form in place within organic-rich muds. Cataclastic fabrics form along some microfaults, but other planar structures exhibit little or no deformation. Microfaults, fractures, and vein structures play an important part in the transport of fluids during diagenesis and lithification.

Deformation bands, with good macroscopic and microscopic indicators of shear are the clearest macroscopic manifestation of accretionary deformation from ODP Site 860. Backscattered electron SEM studies of deformation bands show that these structures are characterized by low porosity, an increase in matrix:grain ratios and grain-size reduction. Locally grain alignment fabrics are observed and some deformation bands show evidence of injection of foreign material and diagenetic changes. Strain magnitudes from space and shape fabrics are small, both inside and outside deformation bands. We propose that deformation bands develop during transient cycles of dilation, shear and fluid flow associated with accretionary deformation.

Microtextural studies show that vertically bedded sandstones drilled at ODP Site 863 have undergone at least two phases of diagenetic change. Early cements of intergrown filamentous smectites and euhedral K/Ca zeolites are postdated by a later carbonate cement. Dissolution of clasts, most notably glassy volcanic lithics, is associated with the early diagenetic phase and may extend through to the later phase. Formation of Fe-sulfides occurs throughout the diagenetic evolution. The early clay-zeolite cement partially or fully occludes pore spaces. Zeolites are rare at depths shallower than 408 mbsf. There is a sharp compositional change at 474 mbsf from K-rich clinoptilolites to more Ca-rich heulandites. Carbonate cementation is strongly controlled by the pre-existing permeability, governed in turn by sandstone texture and the earlier clay-zeolite cementation. The carbonate preferentially cemented coarser, more permeable units and the cement has a dilational fracture morphology in siltstones and in many well-cemented sandstones. We postulate that the fluids that precipitated the carbonate in part generated their own transient permeability through fracture under high pore-fluid pressures. The degree of diagenetic alteration observed at Site 863 is unique for a forearc site and is postulated to reflect the influence of the subducting mid-ocean ridge. The clear separation of the early clay-zeolite cements and the later carbonate cements may reflect two different fluid sources associated with the hydrothermal effects of ridge subduction upon an overlying forearc sedimentary sequence. It is postulated that the early clay-zeolite cements relate to continuing accretionary deformation and related circulation of accretionary fluids with the additional thermal effect of the subduction of the ridge or associated hot, young crust. Carbonate cementation may reflect penetration of the forearc by ridge related fluids.