Description: Mud extrusion is frequently observed as a dewatering phenomenon in compressional tectonic settings such as subduction zones. Along the Middle American Trench, several of these features have been recently discovered. This paper presents a heat flow study of actively venting Mound Culebra, offshore Nicoya Peninsula, and is complemented by data from geophysical surveys and coring. The mud diapir is characterised by methane emission and authigenic carbonate formation at its crest, and is composed of overconsolidated scaly clays and clast-bearing muds. Compared with the conductive background heat flow, the flux through the mud dome is elevated by 10–20 mW/m2, possibly related to advection of heat by fluids rising from greater depth. Decreased chlorinity in the pore waters from gravity cores may support a deep-seated fluid origin. Geothermal measurements across the mound and temperature measurements made with outriggers on gravity corers were corrected for the effects of thermal refraction, forced by the topography of the mound. Corrected values roughly correlate with the topography, suggesting advection of heat by fluids rising through the mound, thereby generating the prominent methane anomaly over the dome and nurturing vent biota. However, elevated values occur also to the southeast of the mound. We believe that the overconsolidated clays and carbonates on the crest form an almost impermeable lid. Fluids rising from depth underneath the dome are therefore partially channelled towards the flanks of the mound.

Description: ottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.

Description: Sediment cover over mid-ocean ridges is expected generally to thicken with seafloor age and distance from spreading center, reflecting symmetric sediment accumulation on both flanks of the ridge. In high quality reflection seismic records and sediment echosounding measurements recently collected across the East Pacific Rise we find a strong asymmetric distribution of sediments. On the eastern flank in the EXCO (Exchange between Crust and Ocean) area at 15°S sediment thickness increases only slowly with distance from the spreading axis, and hence crustal age, to about 15 m on 4.5 Ma old crust and 30 m on 7 Ma old crust. Sediments are draping the basement rather than ponding. On the western flank sediment was sampled that is already 70 m thick on 4.5 Ma old crust and up to 150 m on about 7 Ma old crust. Sediment ponds imply efficient transport by gravitationally driven turbidity currents. Sediment accumulation on the western ridge flanks and the rather flat seafloor indicate a redistribution of sediments. Accumulation of sediments corresponds with the extreme asymmetry of a helium plume at 15°S in the South Pacific. A tongue of high 3He extending westward from the rise near 2500–2700 m depth and a corresponding tongue of high temperature suggesting that the helium plume introduced by hydrothermal activity on the EPR spreading axis is being carried westward by abyssal currents. Fall-out of hydrothermal plumes may contribute and intensify sedimentation on the western flanks. However, it is reasonable to hypothesize that hydrothermal plumes are important agents in the dispersal of the larvae of hydrothermal vent fauna and may be responsible for the enhancement of pelagic zooplankton biomass resulting in a larger mass of pelagic rain.

Description: The structure of the Mid-Atlantic Ridge at 5°S was investigated during a recent cruise with the FS Meteor. A major dextral transform fault (hereafter the 5°S FZ) offsets the ridge left-laterally by 80 km. Just south of the transform and to the west of the median valley, the inside corner (IC – the region bounded by the ridge and the active transform) is marked by a major massif, characterized by a corrugated upper surface. Fossil IC massifs can also be identified further to the west. Unusually, a massif almost as high as the IC massif also characterizes the outside corner (OC) south of the inactive fracture zone and to the east of the median valley. This OC massif has axis-parallel dimensions identical to the IC massif and both are bounded on their sides closest to the spreading axis by abrupt, steep slopes. An axial volcanic ridge is well developed in the median valley both south of the IC/OC massifs and in an abandoned rift valley to the east of the OC massif, but is absent along the new ridge-axis segment between the IC and OC massifs. Wide-angle seismic data show that between the massifs, the crust of the median valley thins markedly towards the FZ. These observations are consistent with the formation of the OC massif by the rifting of an IC core complex and the development of a new spreading centre between the IC and OC massifs. The split IC massif presents an opportunity to study the internal structure of the footwall of a detachment fault, from the corrugated fault surface to deeper beneath the fault, without recourse to drilling. Preliminary dredging recovered gabbros from the scarp slope of the rifted IC massif, and serpentinites and gabbros from the intersection of this scarp with the corrugated surface. This is compatible with a concentration of serpentinites along the detachment surface, even where the massif internally is largely plutonic in nature.

Description: A detailed bathymetric and magnetic survey of the eastern flank of the East Pacific Rise at 14°14′S covering seafloor ages of 0–10 Ma has been carried out and used, along with a flowline profile on the conjugate western ridge flank, to reveal the spreading history and the temporal ridge crest segmentation. Additional information from basaltic lavas is included to study the relationship between physical and magmatic segment boundaries. The sequence of magnetic reversals indicates a total spreading rate of 150 mm/yr since 10 Ma. Symmetric spreading, however, occurred only since 2.8 Ma. Between 7 and 2.8 Ma spreading was asymmetric, with a higher spreading rate toward the east. Migration events of at least five overlapping spreading centres (OSC) left discordant zones on the Nazca plate consisting of hummocky basins and motley texture of curved lineations striking a few degrees oblique to the strike of the ridge crest. Four of the OSCs were right-stepping and migrated northward and one was left-stepping and migrated southward. By transferring Pacific lithosphere to the Nazca plate, these migration events may account for most of the asymmetric accretion observed. The basaltic samples from the eastern flank have been analysed and back tracked to the position of eruption on the ridge crest. In terms of their geochemical signature (Mg# 0.41–0.68) the samples reveal that the magmatic segment boundary between the Garrett transform and 14°30′S has remained stationary over the last 10 Myr and therefore provide no evidence for a link between magmatic and physical segmentation. We therefore propose that migrating non-transform ridge axis discontinuities are governed by propagating giant cracks; as a crack front advances a melt reservoir is tapped and magma rises passively into the crack and erupts subsequently on the seafloor. Some of the OSCs seem to have originated close to transform faults and therefore argue that far-field stresses, perhaps caused by the evolution of the Bauer microplate, rather than mantle upwelling create non-transform ridge axis discontinuities.