Graham Heinson

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.

Crustal architecture of the Capricorn Orogen, Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.

Magnetotelluric evidence for a deep-crustal mineralizing system beneath the Olympic Dam iron oxide copper-gold deposit, southern Australia

The iron oxide copper-gold Olympic Dam deposit, situated along the margin of the Proterozoic Gawler craton, South Australia, is the worlds largest uranium deposit and sixth-largest copper deposit; it also contains significant reserves of gold, silver, and rare earth elements. Gaining a better understanding of the mechanisms for genesis of the economic liberalization is fundamental for defining exploration models in similar crustal settings. To delineate crustal structures that may constrain mineral system fluid pathways, coincident deep crustal seismic and magnetotelluric (MT) transects were obtained along a 220 km section that crosses Olympic Dam and the major crustal boundaries. In this paper we present results from 58 long-period (10–104 s) MT sites, with site spacing of 5– 10 km. A two-dimensional inversion of MT data from 33 sites to a depth of 100 km shows four notable features: (1) sedimentary cover sequences with low resistivity ( 1000 Ω·m) Archean crustal core from a more conductive crust and mantle to the north (typically <500 Ω·m); (3) to the north of Olympic Dam, the upper-middle crust to ∼20 km is quite resistive (∼1000 Ω·m), but the lower crust is much more conductive (<100 Ω·m); and (4) beneath Olympic Dam, we image a low-resistivity region (<100 Ω·m) throughout the crust, coincident with a seismically transparent region. We argue that the cause of the low-resistivity and low-reflectivity region beneath Olympic Dam may be due to the upward movement of CO2-bearing volatiles near the time of deposit formation that precipitated conductive graphite liberalization along grain boundaries, simultaneously annihilating acoustic impedance boundaries. The source of the volatiles may be from the mantle degassing or retrograde metamorphism of the lower crust associated with Proterozoic crustal deformation.

ELECTROMAGNETIC STUDIES OF THE LITHOSPHERE AND ASTHENOSPHERE

In geodynamic models of the Earths interior, the lithosphere and asthenosphere are defined in terms of their rheology. Lithosphere has high viscosity, and can be divided into an elastic region at temperatures below 350 °C and an anelastic region above 650 °C. Beneath the lithosphere lies the ductile asthenosphere, with one- to two-orders of magnitude lower viscosity. Asthenosphere represents the location in the mantle where the melting point (solidus) is most closely approached, and sometimes intersected. Seismic, gravity and isostatic observations provide constraints on lithosphere-asthenosphere structure in terms of shear-rigidity, density and viscosity, which are all rheological properties. In particular, seismic shear- and surface-wave analyses produce estimates of a low-velocity zone (LVZ) asthenosphere at depths comparable to the predicted rheological transitions. Heat flow measurements on the ocean floor also provide a measure of the thermal structure of the lithosphere.Electromagnetic (EM) observations provide complementary information on lithosphere-asthenosphere structure in terms of electrical conductivity. Laboratory studies of mantle minerals show that EM observations are very sensitive to the presence of melt or volatiles. A high conductivity zone (HCZ) in the upper mantle therefore represents an electrical asthenosphere (containing melt and/or volatile) that may be distinct from a rheological asthenosphere and the LVZ. Additionally, the vector propagation of EM fields in the Earth provides information on anisotropic conduction in the lithosphere and asthenosphere. In the last decade, numerous EM studies have focussed on the delineation of an HCZ in the upper mantle, and the determination of melt/volatile fractions and the dynamics of the lithosphere-asthenosphere. Such HCZs have been imaged under a variety of tectonic zones, including mid-ocean ridges and continental rifts, but Archaean shields show little evidence of an HCZ, implying that the geotherm is always below the mantle solidus. Anisotropy in the conductivity of oceanic and continental lithosphere has also been detected, but it is not clear if the HCZ is also anisotropic. Although much progress has been made, these results have raised new and interesting questions of asthenosphere melt/volatiles porosity and permeability, and lithosphere-upper mantle heterogeneity. It is likely that in the next decade EM will continue to make a significant contribution to our understanding of plate tectonic processes.

An application of thin-sheet electromagnetic modelling to the Tasman Sea

Abstract The method of thin-sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric (MT) data observed on the floor of the Tasman Sea, between Australia and New Zealand. A technique to remove 3D distortion from the observed seafloor MT data is developed from the thin-sheet model MT responses; the ‘de-distorted’ data then appear isotropic, and are reinverted using 1D procedures. This approach thus assists interpretation through a combination of 3D forward-modelling procedures and 1D inversion. A seafloor sediment layer is shown to have a strong effect on observed seafloor MT data. Also, an exercise is carried out to compare the results given by the vertical gradient sounding (VGS) method with seafloor MT data. The electrical conductivity structure beneath the Tasman Sea is analysed in terms of an upper-mantle lithosphere and asthenosphere, and a highly conducting lower mantle. The observed anisotropy in seafloor MT data is diagnostic of a lithospheric conductivity of less than 10−4 S m−1. Conductivity rises by two orders of magnitude below 80 km, to greater than 10−2 S m−1, and is probably related to an asthenospheric layer in the upper mantle. Seismic interpretations similarly place a low-velocity zone below a depth of 70 km. At a depth of approximately 400 km, the conductivity is approximately 1 S m−1, consistent with global estimates for the lower mantle. There is little evidence for major change with age in the structure across the Tasman Sea.

Magnetotelluric constraints on subduction polarity: Reversing reconstruction models for Proterozoic Australia

Two-dimensional, lithospheric-scale magnetotelluric imaging in the central Australian Proterozoic has constrained the large-scale architecture of terrane assembly during Paleoproterozoic accretion and collision. The comparatively conductive North Australian craton, consisting of rocks between ~2500 and 1730 Ma in age, has been imaged to extend for 150 km under the 1690–1620 Ma Warumpi Province, which forms part of a large, comparatively juvenile terrane in central-southern Australia. Collision between the North Australia craton and Warumpi Province occurred ca. 1640 Ma. The boundary between these domains is modeled to be subvertical at crustal scale, but dips south at ~45° in the mantle to depths of 150 km. We interpret this geometry to reflect lithospheric-scale underthrusting of the North Australian craton beneath the Warumpi Province, and suggest that it provides a first-order constraint on subduction polarity during collision ca. 1640 Ma. In contrast, most contemporary models for the evolution of Paleoproterozoic Australia propose that the North Australian craton was located on the overriding plate of a long-lived (ca. 1800–1550 Ma) north-directed subduction system. The polarity of these models is not consistent with the lithospheric-scale geophysical architecture.

Geophysical methods in saline groundwater studies: locating perched water tables and fresh-water lenses

A fast and informative tool was required for the delineation of stratigraphy and hydrology and for monitoring salinisation effects around the Stockyard Plains Disposal Basin, which is used as a storage lake for salt water pumped from aquifers close to the Murray River near the town of Waikerie, South Australia. Such salinisation effects include the formation of a perched saline watertable close to the disposal basin and consequent threats to local fresh-water lenses. Ground penetrating radar, direct current (DC) resistivity, time-domain EM (TEM) and low induction number frequency-domain EM (FEM) were compared in terms of efficiency and effectiveness at two survey locations near the Stockyard Plains Disposal Basin. Low induction number FEM measurements were fast to implement but suffered from a nonlinear response with ground conductivity in high conductivity areas. Radar methods were ineffective due to high signal attenuation in the highly saline environment. Fast time-sampling TEM successfully depicted a perched saline water table related to leakage from the disposal basin, and was found to be the most useful technique for delineation of hydrogeology due to its high vertical resolution. DC resistivity was the slowest technique, but was found useful in imaging a fresh-water lens. Results from this study suggest that TEM and DC resistivity methods are useful tools for both fresh-water detection and hydrogeology monitoring in saline groundwater environments.

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.

Magnetotelluric imaging of upper crustal partial melt at Tendaho graben in Afar, Ethiopia

We report on a recent magnetotelluric (MT) survey across the Manda Hararo magmatic segment (MHMS) within the Tendaho graben in the Afar Depression in northeastern Ethiopia. Twenty-two broadband MT sites with ∼1 km station spacing were deployed along a profile with the recorded data covering a period range from 0.003 s to 1000 s. A two-dimensional (2-D) resistivity model reveals an upper crustal fracture zone (fault) and partial melt with resistivity of 1–10Ωm at a depth of >1 km. The partial melt has a maximum horizontal width of 15 km and extends to a depth of 15 km within the Afar Stratoid Series basalts. We estimate a melt fraction of about 13% based on geochemical and borehole data, and bulk resistivity from the 2-D MT inversion model. The interpreted upper crustal partial melt may have been formed by either a magma intrusion from mantle sources or a large volume of continental crust that has been fluxed by a small amount of mantle melt and heat. Within the MHMS and Tendaho graben, a magma intrusion is a plausible explanation for the upper crustal conductor. The inferred presence of a conductive fracture zone or fault with hydrothermal fluid and shallow heat sourcing magma reservoir also makes the Tendaho graben a promising prospect for the development of conventional hydrothermal geothermal energy.