Robert A. Sohn

Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge

Newly acquired seismic refraction and microearthquake data from the Trans-Atlantic Geotraverse (TAG) segment of the Mid-Atlantic Ridge at 26°N reveal for the first time the geometry and seismic character of an active oceanic detachment fault. Hypocenters from 19,232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an ∼15-km-long, dome-shaped fault surface that penetrates to depths >7 km below the seafloor on a steeply dipping (∼70°) interface. A tomographic model of compressional-wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall, which appears to roll over to a shallow dip of 20° ± 5° and become aseismic at a depth of ∼3 km. Outboard of the detachment the exhumed lithosphere is deformed by ridge-parallel, antithetical normal faulting. Our results suggest that hydrothermal fluids at the TAG field exploit the detachment fault to extract heat from a region near the crust-mantle interface over long periods of time.

Seismic and hydrothermal evidence for a cracking event on the East Pacific Rise crest at 9° 50′ N

Interaction between the hydrothermal system and the axial magma chamber at a mid-ocean ridge spreading centre takes place in a boundary layer of crust that separates circulating sea water from basaltic melt. The nature of heat flow through this region is critical because it determines the pressure–temperature conditions of the water–rock interaction and regulates the total heat flux through the system. Here we combine seismic, thermal and chemical time-series data from high-temperature vents on the East Pacific Rise axis at 9° 50.2′ N to link a microearthquake swarm with changes measured in vent fluids. Four days after the earthquake swarm opened fractures near the base of the circulation system, a sudden increase in fluid temperature in the overlying ‘Bio9’ black-smoker vent was observed. Temperatures peaked at the vent 11 days after the swarm and gradually declined back to just above pre-swarm levels (365 °C) over the next 70 days. These observations are consistent with the Bio9 hydrothermal system tapping a previously isolated region of crust, and an upflow fluid residence time of 4 days, compared to previous lower-resolution estimates of 3 years or less.

Explosive volcanism on the ultraslow-spreading Gakkel ridge, Arctic Ocean

Roughly 60% of the Earth’s outer surface is composed of oceanic crust formed by volcanic processes at mid-ocean ridges. Although only a small fraction of this vast volcanic terrain has been visually surveyed or sampled, the available evidence suggests that explosive eruptions are rare on mid-ocean ridges, particularly at depths below the critical point for seawater (3,000 m). A pyroclastic deposit has never been observed on the sea floor below 3,000 m, presumably because the volatile content of mid-ocean-ridge basalts is generally too low to produce the gas fractions required for fragmenting a magma at such high hydrostatic pressure. We employed new deep submergence technologies during an International Polar Year expedition to the Gakkel ridge in the Arctic Basin at 85° E, to acquire photographic and video images of ‘zero-age’ volcanic terrain on this remote, ice-covered ridge. Here we present images revealing that the axial valley at 4,000 m water depth is blanketed with unconsolidated pyroclastic deposits, including bubble wall fragments (limu o Pele), covering a large (>10 km2) area. At least 13.5 wt% CO2 is necessary to fragment magma at these depths, which is about tenfold the highest values previously measured in a mid-ocean-ridge basalt. These observations raise important questions about the accumulation and discharge of magmatic volatiles at ultraslow spreading rates on the Gakkel ridge and demonstrate that large-scale pyroclastic activity is possible along even the deepest portions of the global mid-ocean ridge volcanic system.

Magma storage beneath Axial volcano on the Juan de Fuca mid-ocean ridge

Axial volcano, which is located near the intersection of the Juan de Fuca ridge and the Cobb–Eickelberg seamount chain beneath the northeast Pacific Ocean, is a locus of volcanic activity thought to be associated with the Cobb hotspot. The volcano rises 700 metres above the ridge, has substantial rift zones extending about 50 kilometres to the north and south, and has erupted as recently as 1998 (ref. 2). Here we present seismological data that constrain the three-dimensional velocity structure beneath the volcano. We image a large low-velocity zone in the crust, consisting of a shallow magma chamber and a more diffuse reservoir in the lower crust, and estimate the total magma volume in the system to be between 5 and 21 km3. This volume is two orders of magnitude larger than the amount of melt emplaced during the most recent eruption (0.1–0.2 km3). We therefore infer that such volcanic events remove only a small portion of the reservoir that they tap, which must accordingly be long-lived compared to the eruption cycle. On the basis of magma flux estimates, we estimate the crustal residence time of melt in the volcanic system to be a few hundred to a few thousand years.

A microearthquake survey of the high-temperature vent fields on the volcanically active East Pacific Rise (9° 50'N)

A 3 month deployment of nine ocean bottom seismometers (OBS) on the axis of the East Pacific Rise at 9°50′N detected 283 local microearthquakes in the spring of 1995. The earthquakes exhibit small, uniform seismic moments of 1014–1016 dyne-cm (). Accurate locations were determined for 147 of the earthquakes, with hypocenters clustered beneath two high-temperature hydrothermal fields (Bio9/P and Tube Worm Pillar/Y) at depths <1.4 km beneath the seafloor. Waveform cross-correlation techniques relocated 76 events, 65 of which had lateral standard errors <100 m. Relocated hypocenters lie along two vertical columns from 0.7 to 1.1 km depth. A continuous level of seismic activity was observed beneath the Tube Worm Pillar and Y vent fields, while activity beneath the Bio9 and P vent fields was dominated by a vigorous swarm of 162 events during 3 hours that triggered a 7°C temperature increase in Bio9 exit fluids. The close correlation between the hydrothermal systems and the observed seismicity suggests the microearthquakes were generated by thermal strain associated with cooling of the shallow crust and that the water-rock reaction zone is also a seismogenic zone. The earthquake depths suggest that brittle deformation on the rise axis is restricted to the upper kilometer of crust.

Crustal structure of the Trans-Atlantic Geotraverse (TAG) segment (Mid-Atlantic Ridge, 26°10′N): Implications for the nature of hydrothermal circulation and detachment faulting at slow spreading ridges

New seismic refraction data reveal that hydrothermal circulation at the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge at 26°10′N is not driven by energy extracted from shallow or mid-crustal magmatic intrusions. Our results show that the TAG hydrothermal field is underlain by rocks with high seismic velocities typical of lower crustal gabbros and partially serpentinized peridotites at depth as shallow as 1 km, and we find no evidence for low seismic velocities associated with mid-crustal magma chambers. Our tomographic images support the hypothesis of Tivey et al. (2003) that the TAG field is located on the hanging wall of a detachment fault, and constrain the complex, dome-shaped subsurface geometry of the fault system. Modeling of our seismic velocity profiles indicates that the porosity of the detachment footwall increases after rotation during exhumation, which may enhance footwall cooling. However, heat extracted from the footwall is insufficient for sustaining long-term, high-temperature, hydrothermal circulation at TAG. These constraints indicate that the primary heat source for the TAG hydrothermal system must be a deep magma reservoir at or below the base of the crust.

Deep sea underwater robotic exploration in the ice-covered Arctic ocean with AUVs

The Arctic seafloor remains one of the last unexplored areas on Earth. Exploration of this unique environment using standard remotely operated oceanographic tools has been obstructed by the dense Arctic ice cover. In the summer of 2007 the Arctic Gakkel Vents Expedition (AGAVE) was conducted with the express intention of understanding aspects of the marine biology, chemistry and geology associated with hydrothermal venting on the section of the mid-ocean ridge known as the Gakkel Ridge. Unlike previous research expeditions to the Arctic the focus was on high resolution imaging and sampling of the deep seafloor. To accomplish our goals we designed two new Autonomous Underwater Vehicles (AUVs) named Jaguar and Puma, which performed a total of nine dives at depths of up to 4062m. These AUVs were used in combination with a towed vehicle and a conventional CTD (conductivity, temperature and depth) program to characterize the seafloor. This paper describes the design decisions and operational changes required to ensure useful service, and facilitate deployment, operation, and recovery in the unique Arctic environment.

Assessment of decadal-scale ecological change at a deep Mid-Atlantic hydrothermal vent and reproductive time-series in the shrimp Rimicaris exoculata

This study presents a comparison of distribution and abundance of dominant megafaunal species at the TAG hydrothermal mound on the Mid-Atlantic Ridge from 1994 to 2004. A geographical information system (GIS) database was compiled from georeferenced observations of faunal abundances at 534 locations on the TAG hydrothermal mound, determined by image analysis of ROV dive footage from November 2004. These data are compared with observations from submersible dives in 1994 to assess changes in the extent and population density of aggregations of the shrimp Rimicaris exoculata at the central black smokers of TAG. The GIS database was also used to assess changes in abundance and distribution of the anemone Maractis rimicarivora by simulating the path of a biotransect conducted in 1994 and 1995. There was no evidence of a decline in the extent of shrimp aggregations at the central black smokers of TAG between 1994 and 2004. This result indicates that occasional exposure to high-intensity submersible lighting, which took place during several scientific expeditions in the intervening period, does not pose an immediate conservation threat to populations of R. exoculata. Similarly, there were no significant differences in the distribution and abundance of anemones between 1994 and 2004. These results indicate a constancy in the identity, distribution and abundance of dominant species at TAG that contrasts with other vent sites where quantitative time-series have been established. The reproductive pattern of R. exoculata was also examined by dissection and direct measurement of oocytes from females collected in September 1994 and November 2004, providing the first comparison of reproductive development in samples from different months for this species. There was no significant difference in oocyte size–frequency distributions of females collected in these samples, indicating a lack of seasonal reproduction in R. exoculata.

Three-dimensional tomographic velocity structure of upper crust, Coaxial segment, Juan de Fuca Ridge: Implications for on-axis evolution and hydrothermal circulation

Three-dimensional models of compressional velocity and azimuthal anisotropy from tomographic inversions using 23,564 ocean bottom seismometer P wave arrivals define systematic lateral variations in seismic structure of the CoAxial segment of the Juan de Fuca Ridge (JdFR). Over much of the segment the across-axis structure is roughly axisymmetric, characterized by a progressive increase in dike velocities moving away from the ridge axis. This trend is most apparent in the basal dikes, where on-axis velocities are about 800 m/s slower than those measured elsewhere within the rift valley. The on-axis sheeted dikes also exhibit ridge-oriented azimuthal anisotropy, with a peak-to-peak amplitude of about 600 m/s. Outboard of the rift valley, beneath ridge flanks with fault scarps, velocities in the upper 1500 m of crust are reduced. The maximum amplitude of this anomaly is about 700 m/s, located near the top of the sheeted dikes. Variations in the three-dimensional velocity model are believed to reflect changes in crustal porosity, from which we infer an axisymmetric porosity model for seismic layer 2 of the CoAxial segment. As the crust ages, the evolution of layer 2 porosity could occur in the following way: (1) the porosity of zero-age, on-axis dikes is set at formation by the contraction of molten material, (2) hydrothermal alteration fills pore spaces as the dikes move away from the center of the axial valley, and (3) normal faulting on the ridge flank scarps opens fractures and increases porosity of the upper dikes as they move off-axis. At the north end of the segment, dike velocities are several hundred meters per second slower, on average, and the across-axis structure is lost. The transition from a coherent, aligned seismic structure to a less distinct pattern with reduced velocities may represent a shift from magmatic to amagmatic extension moving away from the Cobb hotspot on the ridge axis. The porosity structure we have derived for the CoAxial segment suggests an alternative to the usual hydrothermal circulation model of cross-axis convection cells. A circulation model with along-axis convection cells located entirely within the axial valley appears to be more compatible with our data.

Melt migration in plume–ridge systems

Abstract We assess the potential for melt migration, separate from solid flow, to accommodate the transport of plume-signature material from off-axis mantle plumes to nearby mid-ocean ridges. We use a boundary-element method to estimate the solid pressures induced by buoyant plume flow, and find that the solid pressure gradients are small compared to melt buoyancy, suggesting that melt streamlines in mantle plumes are essentially vertical. We combine our plume pressure solutions with analytical pressure fields for ridge corner flow and find that the combined plume–ridge pressure field is not sufficient to drive porous flow in the upper mantle over the distances (hundreds of km) observed in many natural systems. We also examine melt transport via porous flow in a melt-rich layer at the base of the lithosphere for plume–ridge systems, and find that melts can traverse plume–ridge offsets of several hundred kilometers in a few hundred thousand years, or less. Our results suggest that plume signatures observed in ridge basalts can be explained by lateral migration of plume melts in a sub-lithospheric channel augmented by solid flow pressure gradients. We apply our models to the Galapagos plume–ridge system and find that melt migration, as opposed to solid flow, provides a means to explain many aspects of the observed chemical anomalies on the Galapagos Spreading Center, including the position of the maximum anomalies due north of the archipelago, the symmetric pattern, and the gradual along-axis gradient.