Rob L. Evans

Geophysical evidence from the MELT area for compositional controls on oceanic plates

Magnetotelluric and seismic data, collected during the MELT experiment at the southern East Pacific Rise, constrain the distribution of melt beneath this mid-ocean-ridge spreading centre and also the evolution of the oceanic lithosphere during its early cooling history. Here we focus on structures imaged at distances ∼100 to 350 km east of the ridge crest, corresponding to seafloor ages of ∼1.3 to 4.5 million years (Myr), where the seismic and electrical conductivity structure is nearly constant and independent of age. Beginning at a depth of about 60 km, we image a large increase in electrical conductivity and a change from isotropic to transversely anisotropic electrical structure, with higher conductivity in the direction of fast propagation for seismic waves. Conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Myr ago. We infer, however, that the structure of young oceanic plates is instead controlled by a decrease in water content above a depth of 60 km induced by the melting process beneath the spreading centre.

The remote sensing of ocean primary productivity: Use of a new data compilation to test satellite algorithms

We tested global pigment and primary productivity algorithms based on a new data compilation of over 12,000 stations occupied mostly in the northern hemisphere, from the late 1950s to 1988. The results showed high variability of the fraction of total pigment contributed by chlorophyll a (ρ), which is required for subsequent predictions of primary productivity. Two models, which predict pigment concentration normalized to attenuation length or euphotic depth, were checked against 2,800 vertical profiles of pigments (chlorophyll a, phaeopigment and total pigment). Phaeopigments consistently showed maxima at about one optical depth below the chlorophyll maxima. We also checked the global Coastal Zone Color Scanner (CZCS; daily 20km resolution) archive for data coincident with the sea truth data. A regression of satellite-derived pigment versus ship-derived pigment had a coefficient of determination (r2) of 0.40 (n=731 stations). The satellite underestimated the true pigment concentration in mesotrophic and oligotrophic waters ( 1 mg pigment m-3). The error in the satellite estimate showed no trends with time between 1978 and 1985. In general the variability of the satellite retrievals increased with pigment concentration. Several productivity algorithms were tested which utilize information on the photoadaptive parameters, biomass and optical parameters for predicting integral production. The most reliable algorithm which explained 67% of the variance in integral production for 1676 stations suggested that future success in deriving primary productivity from remotely sensed data will rely on accurate retrievals of “living” biomass from satellite data, as well as the prediction of at least one photoadaptive parameter such as maximum photosynthesis.

Melt-rich channel observed at the lithosphere–asthenosphere boundary

The lithosphere–asthenosphere boundary (LAB) separates rigid oceanic plates from the underlying warm ductile asthenosphere. Although a viscosity decrease beneath this boundary is essential for plate tectonics, a consensus on its origin remains elusive. Seismic studies identify a prominent velocity discontinuity at depths thought to coincide with the LAB but disagree on its cause, generally invoking either partial melting or a mantle dehydration boundary as explanations. Here we use sea-floor magnetotelluric data to image the electrical conductivity of the LAB beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua. Underneath the resistive oceanic lithosphere, the magnetotelluric data reveal a high-conductivity layer confined to depths of 45 to 70 kilometres. Because partial melts are stable at these depths in a warm damp mantle, we interpret the conductor to be a partially molten layer capped by an impermeable frozen lid that is the base of the lithosphere. A conductivity anisotropy parallel to plate motion indicates that this melt has been sheared into flow-aligned tube-like structures. We infer that the LAB beneath young plates consists of a thin, partially molten, channel of low viscosity that acts to decouple the overlying brittle lithosphere from the deeper convecting mantle. Because this boundary layer has the potential to behave as a lubricant to plate motion, its proximity to the trench may have implications for subduction dynamics.

Electric lithosphere of the Slave craton

The Archean Slave craton in northwestern Canada is an ideal natural laboratory for investigating lithosphere formation and evolution, and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes. Three deep-probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures. The magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity, collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity (<30 Ω·m) at depths of 80–100+ km, and is interpreted to be due to dissolved hydrogen or carbon in graphite form. This geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer. The tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630–2620 Ma.

Origin and Extent of Fresh Paleowaters on the Atlantic Continental Shelf, USA

While the existence of relatively fresh groundwater sequestered within permeable, porous sediments beneath the Atlantic continental shelf of North and South America has been known for some time, these waters have never been assessed as a potential resource. This fresh water was likely emplaced during Pleistocene sea-level low stands when the shelf was exposed to meteoric recharge and by elevated recharge in areas overrun by the Laurentide ice sheet at high latitudes. To test this hypothesis, we present results from a high-resolution paleohydrologic model of groundwater flow, heat and solute transport, ice sheet loading, and sea level fluctuations for the continental shelf from New Jersey to Maine over the last 2 million years. Our analysis suggests that the presence of fresh to brackish water within shallow Miocene sands more than 100 km offshore of New Jersey was facilitated by discharge of submarine springs along Baltimore and Hudson Canyons where these shallow aquifers crop out. Recharge rates four times modern levels were computed for portions of New Englands continental shelf that were overrun by the Laurentide ice sheet during the last glacial maximum. We estimate the volume of emplaced Pleistocene continental shelf fresh water (less than 1 ppt) to be 1300 km(3) in New England. We also present estimates of continental shelf fresh water resources for the U.S. Atlantic eastern seaboard (10(4) km(3)) and passive margins globally (3 x 10(5) km(3)). The simulation results support the hypothesis that offshore fresh water is a potentially valuable, albeit nonrenewable resource for coastal megacities faced with growing water shortages.

On the electrical nature of the axial melt zone at 13° N on the East Pacific Rise

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast-spreading center, was conducted at 13oN on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000-year-old crust. Data above ambient noise levels were obtained at ranges of up to 10 kin. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (,,,1 tim), indicating a heavily fractured, high-porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (,,,100 tim). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill-like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13oN is currently in a state of relative magmatic quiescence.

Upper mantle electrical resistivity structure beneath the central Mariana subduction system

This paper reports on a magnetotelluric (MT) survey across the central Mariana subduction system, providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back-arc spreading ridge. After calculation of MT response functions and their correction for topographic distortion, two-dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab. The resultant isotropic electrical resistivity structure contains several key features. There is an uppermost resistive layer with a thickness of up to 150 km beneath the Pacific Ocean Basin, 80–100 km beneath the Mariana Trough, and 60 km beneath the Parece Vela Basin along with a conductive mantle beneath the resistive layer. A resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge. There is no evidence for a conductive feature beneath the back-arc spreading center. Sensitivity tests were applied to these features through inversion of synthetic data. The uppermost resistive layer is the cool, dry residual from the plate accretion process. Its thickness beneath the Pacific Ocean Basin is controlled mainly by temperature, whereas the roughly constant thickness beneath the Mariana Trough and beneath the Parece Vela Basin regardless of seafloor age is controlled by composition. The conductive mantle beneath the uppermost resistive layer requires hydration of olivine and/or melting of the mantle. The resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three-dimensional and of limited size. In contrast, the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab. The resistive region beneath the back-arc spreading center can be explained by dry mantle with typical temperatures, suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge. Evidence for electrical anisotropy in the central Mariana upper mantle is weak.

Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier

Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature. The two mechanisms are not mutually exclusive. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid–melt phase. By incorporating constraints from a collocated seismic study into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

Geophysical Detection of Relict Metasomatism from an Archean (~3.5 Ga) Subduction Zone

Building Early Continents Cratons, the roots of Earths continents, have survived billions of years of accretion, volcanism, and plate motion. Due to this tumultuous history, existing evidence for how and when they formed is hard to find. C.-W. Chen et al. (p. 1089) use geophysical data collected below the Slave craton in Canada to show that subduction of lithospheric plates in the Archean may have been a major process that controlled its assembly. Spatially aligned seismic and conductive discontinuities over 100 kilometers below the surface are caused by minerals that formed from hot fluids generated as ancient crust melted at a subduction zone. Other old cratons on Earth show similar features, suggesting plate tectonics was operating at least 3.5 billion years ago. Seismic profiles of the Slave craton in Canada suggest that subduction is responsible for its formation. When plate tectonics started on Earth has been uncertain, and its role in the assembly of early continents is not well understood. By synthesizing coincident seismic and electrical profiles, we show that subduction processes formed the Archean Slave craton in Canada. The spatial overlap between a seismic discontinuity and a conductive anomaly at ~100 kilometers depth, in conjunction with the occurrence of mantle xenoliths rich in secondary minerals representative of a metasomatic front, supports cratonic assembly by subduction and accretion of lithospheric fragments. Although evidence of cratonic assembly is rarely preserved, these results suggest that plate tectonics was operating as early as Paleoarchean times, ~3.5 billion years ago (Ga).

Using CSEM techniques to map the shallow section of seafloor: From the coastline to the edges of the continental slope

Many important processes occur within the shallow section of the seafloor on the continental shelf and slope, yet conventional geophysical constraints on the physical properties within this critical boundary layer are limited. Some of the key constraints involve quantification of fluids within the seafloor, which can be provided by electrical methods. This paper reviews the application of a towed EM system to map the uppermost 20 m of seafloor in a variety of settings ranging from nearshore regions in water depths of approximately 10 m on the continental shelf out to water depths of 1300 m . The system is a mapping tool that provides areal maps of seafloor resistivity and has been used for a variety of purposes, including sedimentary characterization and facies mapping, evaluation of groundwater discharge, and mapping seafloor mounds in the Gulf of Mexico, thought to contain massive deposits of gas hydrate.