Fred N. Spiess

East pacific rise: hot springs and geophysical experiments.

Hydrothermal vents jetting out water at 380� � 30�C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Gal�pagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

Hydrothermal heat flux of the “black smoker” vents on the East Pacific Rise

Abstract Active hydrothermal vents have been discovered on the East Pacific Rise at 21°N [1]. The most spectacular of the vents jet out 350°C water at flow rates of several meters per second. The heat loss associated with a single vent of this type is three to six times the total theoretical heat loss for a 1-km segment of ridge out to 1 m.y. age. This underscores the importance of hydrothermal circulation in the heat budget of mid-ocean ridges. It also requires that vent activity of this type be highly episodic rather than steady state.

Methane-rich plumes on the Carolina continental rise: Associations with gas hydrates

Seafloor venting of microbial gases occurs at 2167 m water depth over the Blake Ridge diapir-Gas-rich plumes were identified acoustically in the water column up to 320 m above a pockmarked sea floor associated with active chemosynthetic biological communities. Plumes and venting fluids emanate from near a small fault that extends downward toward a dome in the bottom-simulating reflector, indicating that fluid and/or gas migration is associated with gas hydrate bearing sediment below. These plumes might be caused by gas bubbles or buoyant dumps of gas hydrate that float upward from the seafloor. 18 refs., 3 figs.

Precise GPS/Acoustic positioning of seafloor reference points for tectonic studies

Abstract Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earths surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy). The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of ±0.8 cm north and ±3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island.

East Pacific Rise Crest: A Near-Bottom Geophysical Profile

A deep-towed magnetometer profile made across the East Pacific Rise crest shows many anomalies with about 1000-gamma amplitudes and 500-meter wavelengths and has larger amplitude changes corresponding to magnetic field reversals. This profile across contacts between normal and reversely magnetized crustal blocks is interpreted to place an upper limit of 4700 years on the time required for field reversals and an upper limit of 280 meters on the width of the intrusion center at the rise crest. This intrusion center may occasionally shift several kilometers laterally with respect to the rise axis. The magnetometer records are compatible with the hypothesis that the magnetic field has undergone many fluctuations of short period and small intensity in the past 2 million years. Sediment accumulation increases from less than 2 meters at the rise crest axis to about 20 meters at the western end and 10 meters at the eastern end of the profile. This increase in accutmiulation appears to be the result of ocean-floor spreading.

Broadband seismology in the oceans: Lessons from the Ocean Seismic Network Pilot Experiment

The fundamental objective of the Ocean Seismic Network Pilot Experiment (OSNPE)—which was carried out over a period of about 4 months at a site 225 km southwest of Oahu, Hawaii—was to learn how to make high-quality, broadband seismic measurements in the deep oceans. The OSNPE results demonstrate that broadband data of quality similar to that of quiet land stations can be acquired with seafloor seismographs, but that the location of the seismometer—whether it be on the seafloor, surficially buried within the seabed, or in a deep borehole—has a profound effect on data quality. At long-periods ( 0.1 Hz), data quality was best for a seismometer deployed 242 m below the seafloor in a borehole.

Deep tow studies of the Tamayo transform fault

The Tamayo transform fault occurs at the north end of the East Pacific Rise where it enters the Gulf of California. The two deep-tow surveys reported here show that the transform fault zone changes significantly as a function of distance from the spreading center intersections. At site 1, near the intersection, one side of the fault is young and the fault zone is narrow and well-defined. Strike slip occurs in a zone approximately 1-km wide suggesting a correspondingly narrow zone of decoupling between the Pacific and North American plates. On the young side of the strike-slip zone, normal faults occur along shear zones which are 45°–50° oblique to the transform strike. They occur parallel to the short axis of the strain ellipse for transform fault strain here, i.e., perpendicular to the least compressive stress. The transform walls are formed by normal faulting as has been pointed out in previous detailed surveys. Here, however, the age contrast of 2.5 m.y. across the transform valley is apparent in the morphology of the normal fault scarps. While the scarps are steep and well-defined on the young side, the scarps on the older side have gradual 10°–30° slopes and appear to be primarily talus ramps. Apparently, the scarps have been tectonically eroded by continued strike slip activity after the initial stages of normal faulting. Thus, transform valleys should be quite asymmetric in cross-section where there is a significant age contrast and one side is less than approximately 0.5 m.y. old. Also, along older sections of the transform valley walls, normal faulting may not be at all obvious due to degradation of the scarps by tectonic erosion. This phenomenon makes the likelihood of transform faults providing ‘windows’ into the oceanic crust most unlikely except in special cases.The picture of transform deformation is more complex at site 2 in the central portion of the fault where both sides of the fault are greater than 1 m.y. old. Here the transform valley is wider (25–30 km as opposed to 2–5 km). There is no clear simple zone of strike slip tectonics. In fact, the only clear evidence for deformation is the intrusion of magmatic or serpentinite diapirs through the sediments of the transform valley floor. The diapirs have deformed the turbidite layers flooring the valley and in one carefully studied case the turbidite sequence has been uplifted, perched atop the diapir. The pattern of deformation on this outcropping diapir shows radial and concentric fractures which can be modeled by a vertical intrusion circular in plan view. Magnetic studies limit the possible composition to basalt or serpentinite. A 60-km-long median ridge is also likely to be the product of intrusion along the transform fault. The survey at site 2 pointed out the importance of vertical tectonics in the transform valley floor and in particular the importance of diapiric intrusions of either basaltic or serpentinite composition.Based on initial boundary conditions and present tectonic elements in the Tamayo fault zone, a possible history of the mouth of the Gulf of California is outlined. The median ridge was emplaced starting approximately 0.8 m.y. ago by regional extension across the transform fault, the result of ‘leaky’ transform faulting. The diapirs occur along a possible ‘relay’ zone of extension midway along the fault which began approximately 0.15 m.y. ago. The extension in this case is parallel to the trend of the transform fault, is still occurring at present, and may evolve into a true spreading center.

Abyssal bedforms explored with a deeply towed instrument package

Abstract Several types of abyssal bedforms have been discovered during surveys with a deeply towed instrument package at water depths of 1.5–6 km in the Pacific and Atlantic Oceans. Cores and current-meter records obtained at the same sites provide data for interpreting their dynamics. Wave and current ripples are best portrayed in bottom photographs, but medium-scale bedforms, including sand waves, mud waves and erosional furrows, are described by interpreting high-resolution side-looking sonar records. The largest examples affect surface-ship echograms, though their shape and structure can seldom be resolved without near-bottom observations. Wave ripples are common on the slopes of seamounts and ridges, while current ripples and sand waves occur beneath some fast thermohaline currents whose beds are shallower than the foraminiferal compensation depth. Depositional and erosional bedforms in cohesive sediment have been found beneath the deepest thermohaline currents; they may be restricted to areas where the flow is unusually steady in direction.

Suboceanic Geodetic Measurements

This paper reviews the needs for geodetic ties among seafloor points and between such points and others on land, as well as the environmental constraints on potential systems for making such measurements. Underwater acoustic techniques provide the best opportunities, and an essential element on which to build acoustic systems-a precision-transponder system capable of making travel-time measurements over ranges of several kilometers with an accuracy of a few microseconds-is described. Finally, three particular systems are discussed. One uses direct transmission between transponders to achieve one part in 105 accuracy over ranges from a few meters to about a kilometer. The second, using a larger transponder network and an intermediate vehicle, can achieve similar accuracy over ranges up to about 10 km. The third is a composite acoustic global positioning system (GPS) which should be able to achieve subdecimeter accuracy over ranges of a few hundred kilometers.

Near-bottom geophysical study of the Mid-Atlantic Ridge median valley near lat 37° N: Preliminary observations

Near-bottom geophysical observations made over the median valley of the Mid-Atlantic Ridge near lat 37°N show that the median valley can be divided into four provinces: the outer walls, terraces, inner walls, and floor. Block faulting along normal faults with as much as 600 m throw is the dominant mechanism for creating median-valley relief, particularly at the inner and outer walls. Most of these faults dip toward the median-valley axis an average of 45°. Many of the block tops slope 3° to 10° away from the valley axis. Some of the major fault scarps are linear for more than 20 km. Large-scale block faulting occurs less than 1 km from the axis of spreading. Inversion of near-bottom magnetic anomalies suggests that the principal zone of intrusion may be no wider than 1 to 2 km and is along the valley axis. This spreading center is marked by either a central ridge or central depression, which suggests either discrete magma sources or discrete taps of a linear source beneath the median-valley axis. The central ridge is a volcanic construction, although faulting and (or) volcano-tectonic uplift may contribute to its relief.