James S. McClain

Physical characteristics of the Endeavour Ridge hydrothermal plume during July 1988

Abstract We conducted CTD-transmissometer tows from 8 to 26 July, 1988 within 15 km of the central hydrothermal vent site ( ≈ 47°57′N, 129°06′W) on the Endeavour segment of Juan de Fuca Ridge. Anomalies of temperature, salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site. As a result of widespread plume dispersion, background values of potential temperature, salinity and light attenuation below the 1900 m depth exceeded those for “pristine” ambient waters by 0.05°C, 0.05 psu and 0.03 m −1 , respectively. Maximum plume anomalies relative to the background waters were of the order of 0.10°C, 0.010 psu and 0.10 m −1 at core depths of 2000–2050 m. Heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation (particle) anomalies were confined to within 2.5 km of the vent site. Based on the background water property anomalies and moored current meter records, the mean (time-averaged) heat fluxes for the survey region were+2.3(±1.5) × 10 8 W in the along-ridge direction (20°T) and−7.7(±4.7) × 10 8 W in the cross-ridge direction (110°T). Mean along- and cross-ridge salt fluxes were+7(±5)and−25(±15)kg s −1 ; mean particle fluxes were+0.09(±0.06)and−0.29(±0.18)kg s −1 . Estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1.2(±0.6)×10 10 W and corresponding particulate fluxes as high as6(±3)kg s −1 .

A two‐dimensional tomographic study of the Clipperton transform fault

From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to Investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray-tracing problem is a hybrid of the graph (or shortest path) method and a ray-bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6–5.9 km under the transform fault to produce the best fit of the PmP phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.

Contrast in crustal structure across the Clipperton transform fault from travel time tomography

A three-dimensional (3-D) seismic refraction study of the Clipperton transform fault, northern East Pacific Rise, reveals anomalously low compressional velocities from the seafloor to the Moho. We attribute this low-velocity anomaly to intensive brittle deformation, caused by transpression across this active strike-slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, but to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure corresponds well with the differences in mid-ocean ridge morphology across the Clipperton transform. We conclude that the amount of fracturing of the upper crust, which largely controls seismic velocity variations, is strongly dependent on the shallow temperature structure at the ridge axis. Intermittent supply of magma to the shallow crust north of the Clipperton transform allows seawater to penetrate deeper, and the cooler crust is brittle to a greater depth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of ∼0.3 km are not required by our data. The absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short-lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies.

Major off-axis hydrothermal activity on the northern Gorda Ridge

The first hydrothermal field on the northern Gorda Ridge, the Sea Cliff hydrothermal field, was discovered and geologic controls of hydrothermal activity in the rift valley were investigated on a dive series using the DSV Sea Cliff. The Sea Cliff hydrothermal field was discovered where predicted at the intersection of axis-oblique and axis-parallel faults at the south end of a linear ridge at mid-depth (2700 m) on on the east wall. Preliminary mapping and sampling of the field reveal: a setting nested on nearly sediment-free fault blocks 300 m above the rift valley floor 2.6 km from the axis; a spectrum of venting types from seeps to black smokers; high conductive heat flow estimated to be equivalent to the convective flux of multiple black smokers through areas of the sea floor sealed by a caprock of clastic breccia primarily derived from basalt with siliceous cement and barite pore fillings; and a vent biota with Juan de Fuca Ridge affinities. These findings demonstrate the importance of off-axis hydrothermal activity and the role of the intersection of tectonic lineations in controlling hydrothermal sites at sea-floor spreading centers.

SEISMICITY AND TREMOR IN A SUBMARINE HYDROTHERMAL FIELD : THE NORTHERN JUAN DE FUCA RIDGE

We have made the first multi-year deployments of a small number of ocean bottom seismometers on a mid-ocean ridge. We located our instruments in and around a vigorous hydrothermal vent field on the Endeavour Segment of the Juan de Fuca Ridge. The deployments have revealed a consistent pattern of activity comprising several types of events. Most abundant are swarms of extremely small microearthquakes that we suggest are associated with cracking of the oceanic crust. We also observe local tectonic earthquakes that occur about four times per day. Preliminary hypocentral locations place these tectonic events away from the ridge axis and outside the hydrothermal field. The seismicity on the Endeavour Segment is remarkably different from the southern Juan de Fuca Ridge where little, if any, tectonic activity is occurring. We speculate that the Endeavour and southern Juan de Fuca Ridge are in two different phases of their long-term evolution. The latter has only rare activity that is only beginning to focus hydrothermal circulation along faults. In contrast, the Endeavour Segment is in an active tectonic phase; and the faults have had time to develop robust hydrothermal circulation, resulting in large hydrothermal vent deposits.

Thickening of the oceanic crust with age

The most widely accepted models for ocean-crust formation and composition fail to predict the thickening of the oceanic crust with age, yet such thickening has been described in the Pacific Ocean for years. To reconcile this apparent long-standing conflict, we have reexamined the evidence for the thickening of the Pacific crust by using a statistical treatment of a large number of seismic profiles and by doing a detailed reanalysis of several profiles fanning a transect across the southern Pacific Ocean. From the statistical studies, we find that crust younger than 30 Ma has a mean thickness of 5.67 km, whereas crust between 30 and 100 Ma has a mean thickness of 6.01 km. This 0.34-km difference, although statistically significant, is far less than that reported in several previous studies. Our results from the South Pacific transect suggest that changes in the crustal thickness are not systematic, and from both studies it appears that crustal thickening is not particularly important. The small thickening that we do observe is probably the result of isolated processes that are not active under the oceanic crust as a whole. As a result, arguments favoring a large component of serpentinite in the crust cannot be based upon evidence for crustal thickening beneath the Pacific Ocean.

Velocity structure from forward modeling of the eastern ridge‐transform intersection area of the Clipperton Fracture Zone, East Pacific Rise

In the spring of 1994, we undertook an extensive geophysical study of the Clipperton Fracture Zone (FZ) on the fast spreading East Pacific Rise. The Clipperton Area Seismic Study to Investigate Compensation experiment (CLASSIC) included surveys to examine the deep structures associated with the fracture zone and adjacent northern ridge segment. In this paper, we report the results from five seismic profiles acquired over the eastern ridge-transform intersection (RTI), including profiles over the RTI high, the northern ridge segment, and the eastern transform region. The travel time data for crustal phases, Moho reflections, and mantle phases were modeled using two-dimensional ray tracing. Seismic profiles reveal that the crust is similar in thickness north and south of the Clipperton FZ, despite differences in axial topography that have previously been interpreted in terms of differences in magma supply. When compared to older crust, the northern ridge axis is characterized by lower seismic velocities and higher attenuation. In our model, a low-velocity zone exists beneath the ridge axis, probably associated with a zone of partial melt and/or very high temperatures. Within the transform zone, we find that the southeastern trough is underlain by nearly normal crustal structure. The crust is slightly thinner than the adjacent aseismic extension but not enough to compensate for the depths of the trough. Toward the RTI, the trough is replaced by an intersection high which appears underlain by a thickened crust, and a thicker upper crustal section. Both characteristics indicate that the intersection high is a volcanic feature produced by excess volcanism at the intersection. The volcanism acts to “fill in” the transform trough, creating the thicker crust that extends under the eastern aseismic extension of the transform. Our results show that the northern ridge segment, often identified as magma-starved, displays the crustal thickness and apparent signal attenuation characteristic of a plentiful, but perhaps episodic, magma supply.

The morphology and structure of the West O’Gorman Fracture Zone

The West O’Gorman Fracture Zone is an unusual feature that lies between the Mathematician Ridge and the East Pacific Rise on crust generated on the East Pacific Rise between 4 and 9 million years ago. We made a reconnaissance gravity, magnetic and Sea Beam study of the zone with particular emphasis on its eastern (youngest) portion. That region is characterized by an elongate main trough, a prominent median ridge and other, smaller ridges and troughs. The structure has the appearance of large-offset fracture zone, possibly in a slow spreading environment. However, magnetic anomalies indicate that the offset, if any, is quite small, and the spreading rate during formation was fast. In addition, the magnetic profiles do not support earlier models for a difference in spreading rate north and south of the fracture. The morphology of the fracture zone suggests that flexure may be responsible for some of the topography; but gravity studies indicate some of the most prominent features of the fracture zone are at least partially compensated. The main trough is underlain by a thin crust (or high density body), similar to large-offset fracture zones in the Atlantic, while the median ridge is underlain by a thickened crust. Sea Beam data does not unambiguously resolve between volcanism or serpentinization of the upper mantle as a mechanism for isostatic compensation.Why the West O’Gorman exists remains enigmatic, but we speculate that the topographic expression of a fracture zone does not require a transform offset during formation. Perhaps the spreading ridge was magma starved for some reason, resulting in a thin crust that allowed water to penetrate and serpentinize portions of the upper mantle.

Hydrothermal activity on the Gorda Ridge

Near-bottom plumes of materials indicative of discharge of metal-rich hot springs were discovered at sites on the Gorda Ridge by a research team of government and university scientists on a cruise of the National Oceanic and Atmospheric Administration (NOAA) ship Surveyor during May 1985 as part of the NOAA Vents Program. The Gorda Ridge, off northern California and Oregon, is the only seafloor spreading center within the proclaimed 200-mile U.S. Exclusive Economic Zone (370 km wide) of the conterminous United States and is one of the last oceanic ridges to be explored for metal-rich hot springs. One reason for this neglect is that the Gorda Ridge is slow spreading, with half-rates ranging from 1.1 cm/yr in the southern portion to 2.2 cm/yr in the northern portion. Slow spreading centers have not been fully evaluated with regard to hydrothermal activity by many members of the research community, who have concentrated their attention on the faster spreading East Pacific Rise to the south and the Juan de Fuca Ridge to the north of the Gorda Ridge.

Project Title: Geothermal Play Fairway Analysis of Potential Geothermal Resources in NE California, NW Nevada, and Southern Oregon: A Transition between Extension

Play Fairway analysis is a powerful tool in the petroleum industry for reducing drilling risk. It relies on general models for sedimentary depositional systems (at the basin scale) and applies all available data to identify weighted combinations of characteristics that can be used to predict the locations where drilling is likely to lead to successful fossil fuel extraction. This project represents an effort to apply the same approach to geothermal resource exploration and assessment. Geothermal systems do not have the same level of basin-wide coherence as oilor gas-bearing formations, but rather are often controlled by very localized characteristics. However, all geothermal fields must include elements of a heat source, fluids, and a permeability structure that may vary systemically over a region. This is where Geothermal Play Fairway Analysis (GPFA) may be a valuable tool. To this end, we are applying GPFA to a region where both volcanically and extensionally hosted systems are known, and examine the nature of the transition between these different geothermal play types. For this effort we have chosen a region that spans the boundary between volcanic systems and extensional systems (Figure 1) in northeastern California, northwestern Nevada and southern Oregon. Our approach in Phase I of this study is to utilize two “end member” locations: Medicine Lake Volcano (CA), which is a volcanic-hosted system (Cumming and Mackie, 2010), and the San Emidio geothermal field, which is controlled by extensional tectonics (Rhodes et al., 2010). The important GPFA characteristics of these two play types will be interpolated (in a general sense) to at least two