Antony White

Evidence for accumulated melt beneath the slow–spreading Mid–Atlantic Ridge

The analysis of data from a multi–component geophysical experiment conducted on a segment of the slow–spreading (20 mm yr-1) Mid–Atlantic Ridge shows compelling evidence for a significant crustal magma body beneath the ridge axis. The role played by a crustal magma chamber beneath the axis in determining both the chemical and physical architecture of the newly formed crust is fundamental to our understanding of the accretion of oceanic lithosphere at spreading ridges, and over the last decade subsurface geophysical techniques have successfully imaged such magma chambers beneath a number of intermediate and fast spreading (60-140 mm yr-1 full rate) ridges. However, many similar geophysical studies of slow–spreading ridges have, to date, found little or no evidence for such a magma chamber beneath them. The experiment described here was carefully targeted on a magmatically active, axial volcanic ridge (AVR) segment of the Reykjanes Ridge, centred on 57° 43′ N. It consisted of four major components: wide–angle seismic profiles using ocean bottom seismometers; seismic reflection profiles; controlled source electromagnetic sounding; and magneto–telluric sounding. Interpretation and modelling of the first three of these datasets shows that an anomalous body lies at a depth of between 2 and 3 km below the seafloor beneath the axis of the AVR. This body is characterized by anomalously low seismic P–wave velocity and electrical resistivity, and is associated with a seismic reflector. The geometry and extent of this melt body shows a number of similarities with the axial magma chambers observed beneath ridges spreading at much higher spreading rates. Magneto–telluric soundings confirm the existence of very low electrical resistivities in the crust beneath the AVR and also indicate a deeper zone of low resistivity within the upper mantle beneath the ridge.

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.

The Bouguer gravity field and crustal structure of eastern Timor

Abstract The results from a recent North—South gravity traverse across eastern Timor show that the Bouguer gravity field is characterized by a strong, 6 mGal/km, gradient on the north coast. This gradient appears to be a fundamental feature of Timor and of the Outer Banda Arc. Preliminary computer models suggest that, to a first approximation, the gradient is due to a vertical fault at the north coast of Timor separating oceanic crust from continental crust. The fit between the computed and observed gradient can be improved significantly by assuming a northward-dipping lithospheric slab, north of Timor. The model further indicates that the Australian continental crust extends at least as far as the north coast of Timor.

Episodic melt transport at mid‐ocean ridges inferred from magnetotelluric sounding

Oceanic crust is generated at mid-ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km. Geodynamic and geochemical models of upwelling, melt extraction, and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries, most of which assume steady-state or near steady-state processes. Here we present results from marine magnetotelluric (MT) measurements, carried out as part of the RAMESSES experiment on the slow spreading Reykjanes Ridge, which support a model of melt extraction and migration that is episodic, rather than steady-state.

A two-dimensional interpretation of the geomagnetic coast effect of southeast Australia, observed on land and seafloor

Abstract Data from detailed investigations of the geomagnetic coast effect across the southeast Australian continental margin are interpreted in terms of a two-dimensional numerical model. There is both land and seafloor control on this model, and both E-pol and B-pol mode responses are incorporated. The good fit to the data of a model comprising ocean water and marine sediments is improved when the sub-ocean electrical conductivity profile is allowed to differ from the continental conductivity profile. A final model is determined by an inversion procedure based on systematic search. The main characteristic of this model is that an increase in conductivity occurs at a shallower depth beneath the ocean than beneath the land (some 100 km beneath ocean, 200 km beneath land). The southeast continental margin of Australia is considered to have formed by passive rifting, at the time of the opening of the Tasman Sea. A depth of 100 km for the base of the oceanic lithosphere corresponds well to the age of rifting some 80 Ma ago. The contrast with the continental profile suggests an electrical asthenosphere relatively deeper beneath southeast Australia.

Seafloor magnetotelluric sounding above axial seamount

Axial Seamount is a large, active, ridge axis volcano located on the central segment of the Juan de Fuca Ridge in the northeast Pacific Ocean. Magnetotelluric (MT) data have been collected at three sites, approximately 4 km apart around the eastern rim of the volcano, during a 65-day deployment. MT responses, in the bandwidth of 10²–105 s, are almost isotropic, with a weakly-defined principal direction of strike parallel to the main topographic trends of Axial Seamount, and are relatively flat over the whole bandwidth. Apparent resistivities are of the order of 7–20 Ωm, and phases are as low as 30° at the short periods. Diagonal terms of the MT tensor are an order of magnitude smaller than the off-diagonal terms, suggesting that three-dimensional effects on the data are minimal. Two-dimensional inversions suggest that seafloor bathymetry and the distant coastlines have a surprisingly small effect on the MT response, and one-dimensional inversions fit the MT data to within the errors with no serial correlation in the residuals. A low crustal resistivity is the most robust part of the model, probably due to seawater in fractures and possibly due to a magma chamber. An electrical asthenosphere, although less well constrained, exists over a depth range 30–60 km, and the resistivity of this region is compatible with about 8% fraction of melt.

A sea floor magnetometer for the continental shelf

Measurement of temporal magnetic variations on the sea floor is desirable in order to extend the technique of geomagnetic depth sounding into the oceans. This paper describes a recording three-component sea floor magnetometer and its use in continental shelf depths. The orientation and tilt of the instrument on the sea floor are recorded using gelatine solutions to ‘freeze’ a compass card and a ball-bearing, respectively. A backing-off procedure initially nulls the magnetic field components along each of the three mutually orthogonal fluxgate sensors. Magnetic variations along each sensor axis are then recorded within a range of ± 300 nT of these nulled positions. The resolution is ± 1 nT, and with a power drain of 800 mW the magnetometer can record continuously for 30 days. The instrument capsule is moored to surface buoys for recovery in continental shelf applications. The buoys may have marker flags, radar reflectors or radio beacons attached to them to aid in relocation.

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.

Tectonic evolution of the southern Gawler Craton,South Australia, from electromagnetic sounding

Long-period natural-source electromagnetic data have been recorded using portable three-component magnetometers at 39 sites in 1998 and 2002 across the southern Eyre Peninsula, South Australia that forms part of the Gawler Craton. Site spacing was of order 5 km, but reduced to 1 km or less near known geological boundaries, with a total survey length of approximately 50 km. A profile trending east – west was inverted for a 2D electrical resistivity model to a depth of 20 km across the southern Eyre Peninsula. The main features from the models are: (i) on the eastern side of the Gawler Craton, the Donington Suite granitoids to the east of the Kalinjala Shear Zone are resistive (>1000 Ωm); (ii) the boundary between the Donington Suite granitoids and the Archaean Sleaford Complex, which has much lower resistivity of 10 – 100 Ωm, is almost vertical in the top 10 km and dips slightly westwards; and (iii) two very low resistivity (<1 Ωm) arcuate zones in the top 3 km of Hutchison Group sediments correlate with banded iron-formations, and are probably related to biogenic-origin graphite deposits concentrated in fold hinges. Such features suggest an extensional regime during the time period 2.00 – 1.85 Ga. We suggest that the resistivity boundary between the Donington Suite and the Archaean Sleaford Complex represents a growth fault, typical for rift systems that evolve into a half-graben structure. In the graben basin, low-resistivity shallow-marine Hutchison Group sediments were deposited. Folding of the sediments during the Kimban Orogeny between 1.74 and 1.70 Ga has led to migration of graphite to the fold hinges resulting in linear zones of very low resistivity that correlate with banded iron-formation magnetic anomalies.

Marine self-potential gradient exploration of the continental margin

The self-potential (SP) method for mineral exploration is seldom used on land, primarily because of electrode noise problems and nonunique interpretations. Marine measurements of the horizontal gradient of the SP field, on the other hand, are relatively simple to make with an array of electrodes towed behind a ship. With low ship speeds of 5 to 10 km/hour, dense spatial sampling (∼1 m) can be obtained with resolution of better than 1 μV/m. In this paper we report on gradient SP data recorded on the continental shelf of South Australia by a horizontal array of towed electrodes approximately 20 m above the seafloor. Ocean waves and swells with periods of 5 to 15 s yielded large amplitude signals ±20 μV/m, but subseafloor mineralization produced SP gradient anomalies of ±50 μV/m and widths of 2 km or more in a number of parallel traverses. Integrating the observed SP gradients along each line delineated SP anomalies of amplitude up to −100 mV. Self-potential and magnetic anomaly data show limited spatial cor...