Brian T. R. Lewis

Oregon Subduction Zone: Venting, Fauna, and Carbonates

Transects of the submersible Alvin across rock outcrops in the Oregon subduction zone have furnished information on the structural and stratigraphic framework of this accretionary complex. Communities of clams and tube worms, and authigenic carbonate mineral precipitates, are associated with venting sites of cool fluids located on a fault-bend anticline at a water depth of 2036 meters. The distribution of animals and carbonates suggests up-dip migration of fluids from both shallow and deep sources along permeable strata or fault zones within these clastic deposits. Methane is enriched in the water column over one vent site, and carbonate minerals and animal tissues are highly enriched in carbon-12. The animals use methane as an energy and food source in symbiosis with microorganisms. Oxidized methane is also the carbon source for the authigenic carbonates that cement the sediments of the accretionary complex. The animal communities and carbonates observed in the Oregon subduction zone occur in strata as old as 2.0 million years and provide criteria for identifying other localities where modern and ancient accreted deposits have vented methane, hydrocarbons, and other nutrient-bearing fluids.

Fine structure of the lower oceanic crust on the Cocos Plate

Abstract From seismic refraction data on the Northern Cocos Plate, it is found that the crust thickens with age. Assuming that processes of crustal formation have remained constant over at least the past 10 m.y., the seismic data indicate that the thickening is caused by a gradual transformation of the top 2 km of the upper mantle into material having crustal-like velocities. Serpentinization is a possible mechanism. The data also suggest that the crust—mantle interface may be laterally variable, in some places being relatively sharp and in others gradational. Upper mantle anisotropy appears to decrease from about 0.6 km/sec near the rise axis to 0.3 km/sec at 10 m.y.

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.

A seismic experiment at the axis of the East Pacific Rise

Abstract Data from a seismic-refraction experiment on the East Pacific Rise at the mouth of the Gulf of California indicate that the axial valley in this area is underlain by low-velocity material (possibly a magma chamber) and, in addition, that the crust thickens rapidly away from the axis. The crust apparently thickens very rapidly with age by at least 2 km within 200,000 years. Inversion of P and S travel times on crust adjacent to the axis gives a model in which the crustal velocity changes gradationally with depth and has a Poissons ratio of 0.28 in the lower crust. As part of an IPOD site survey, ocean-bottom seismometers (OBS) were placed in a sediment pond 12 km east of the East Pacific Rise at the mouth of the Gulf of California. A bottom-shot experiment suggests that P-wave velocity of the sediment is about 1.5 km/sec. Using reflection profiles and P—S conversions we can infer a shear-wave velocity and sediment thickness of 0.14 km/sec and 140 m, respectively. The P-wave velocities for the upper crust beneath the site increase rapidly from 4.6 km/sec to the 6.9 km/sec observed for the lower crust. The Poissons ratio for the lower crust is about 0.28.

Upper mantle velocities on the Northern Cocos Plate

Abstract Refraction lines on the Northern Cocos Plate between the Orozco and Clipperton Fracture Zones have been used to determine upper mantle velocities over the plate. The velocities range from 7.50 to 8.43 km/sec. Azimuthal variations are found near the rise crest with low velocities parallel to the rise crest. The low velocities increase with age, lessening the observed azimuthal dependence away from the rise crest. A low-velocity zone is found in the mantle and may extend over a considerable portion of the plate near the rise crest. A 7.1-km/sec basal crustal layer is also observed and makes up a substantial portion of the crustal thickness.

The Process of Formation of Ocean Crust

Ocean crust is the outermost layer of earth under the oceans. It is separated from the underlying mantle by a seismic transition zone called the Moho. A widely held view is that the Moho represents a petrologic change from basaltic-type rocks to a mantle composed mostly of olivine and pyroxene. According to this view, crust is formed by a steady segregation of basaltic melt, derived from partial melting of the mantle, into a crustal magma chamber wherein cooling and crystallization bring about steady-state accretion to the continuously spreading plates. There is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process. At least two other processes are more compatible with the geophysical data. In one, the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the Moho is a primary petrologic boundary. In the other, the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking; by comparison, in the mantle thermal contraction is accommodated by flow. The upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt. The lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature.

Tectonic evolution of the northern Cocos plate

Several features on the Cocos plate appear to be anomalous or of unclear origin. Specifically, the eastern extension of the Orozco Fracture Zone near lat 15°N has a northeast trend, which differs significantly from the nearly east-west motion of the Pacific and Cocos plates. Moreover, the Tehuantepec Ridge has a similar northeast strike and separates the deep Guatemala Basin on its southeast side from shallower crust to the northwest. The origin of these features can be adequately described by a small change in Cocos-Pacific plate motion and a 20° reorientation of the East Pacific Rise. This reorientation is strongly substantiated by a “fanning” of magnetic anomaly lineations over the Cocos plate between the Orozco Fracture Zone and the Tehuantepec Ridge. The reconstruction assumes that the Tehuantepec Ridge is a relict fracture zone; this interpretation is supported by recent gravity models across the ridge. The Guatemala Basin is the result of older crust formed prior to a ridge-axis jump (Clipperton Ridge to East Pacific Rise) and the reorientation of the East Pacific Rise.

A direct recording ocean bottom seismometer

We have designed a simple, cheap and reliable ocean-bottom seismometer. Signals from three-component geophones are recorded directly on magnetic tape running continuously at a speed of 1 mm s1. Time reference is derived from a temperature-compensated quartz crystal oscillator and encoded on a fourth channel as an amplitude modulation of a 20 Hz carrier. A bipolar square-root signal-compression scheme doubles the tape dynamic range to 80 db, and the available bandwidth is 2 to 100 Hz. Tape and batteries are capable of 500-hr operation, and the unique magnetic release comes close to being a fail-safe system. A heavy, high-drag concrete anchor shaped like a flower-pot provides easy launching, fast stable descent and good coupling to the ocean floor. We have had numerous successful field emplacements which have yielded good earthquake and shot-refraction data.

An ocean-bottom seismometer suitable for arrays

Abstract We have developed a cheap and simple three-component ocean-bottom seismometer. One vertical and two horizontal seismometers are levelled in a boat floating on thick silicone oil in the lower half of a buoyant spherical pressure case. Signals are compressed by recording the bipolar square root directly on magnetic tape moving at 1 mm s −1 . The nominalbandwidth is 2 to 100 Hz, and a special 24-cm reel of tape will run for 500 h. Fast emplacement is obtained by lodging the buoyant spheres in heavy flower-pot shaped concrete anchors that have stable descent characteristics. Five successful drops have been made with two prototypes in epoxy resin spheres, and clear arrivals from shots and earthquakes have been received.

Structure and subduction processes along the Oregon-Washington margin

Seismic reflection and refraction data off Washington and Oregon are used to determine the style of sediment deformation and to infer the physical properties of accreted sediments on the lower slope. Onshore-offshore seismic refraction data off Washington are used to determine the location of the “trench”, or where the plate bending starts.We find that off Washington the subduction zone is characterized by a “trench” whose physiographic expression is buried under several kilometers of sediments and is tens of kilometers landward of the lower slope, which is accreting seaward as the result of the offscraping of sediments.Seismic reflection data support previous observations that offscraping occurs along seaward and landward dipping thrust faults. Refraction data indicate that a sediment package thrust up along a seaward dipping fault (off Washington) was not measurably changed in velocity with respect to a Cascadia basin section. However a package uplifted by thrusting along a landward dipping fault (off Oregon) did have increased velocity. It is suggested that the increased velocities off Oregon could be the result of erosion and exposure of more deeply buried and compacted sediments, rather than the result of dewatering due to tectonic stress. Off Washington the sensitivity of velocity to porosity and resolution of the seismic method does not preclude dewatering due to tectonic stress, but it does limit the degree of dewatering.In the deeper parts of the lower slope section off Washington and Oregon velocities as high as 3 to 4 km/sec are found. Heat flow data indicate that the temperatures in this high velocity regime are greater than 100°C. It is hypothesized that lithification related to clay diagenesis may be partly responsible for the high velocities, rather than simply compaction. It also appears that the high velocity sediments are subducted while the unlithified low velocity sediments are offscraped.