Evans Rob L.

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Evans

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Rob L.

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0000-0001-5585-0684

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Azimuthal seismic anisotropy of 70-ma Pacific-plate upper mantle.

(American Geophysical Union, 2019-01-28) Mark, Hannah ; Lizarralde, Daniel ; Collins, John ; Miller, Nathaniel C. ; Hirth, Greg ; Gaherty, James B. ; Evans, Rob L.

Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2‐D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70‐Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2‐D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.
Geophysical evidence from the MELT area for compositional controls on oceanic plates

( 2005-06-29) Evans, Rob L. ; Hirth, Greg ; Baba, Kiyoshi ; Forsyth, Donald W. ; Chave, Alan D. ; Mackie, Randall L.

Magnetotelluric (MT) and seismic data, collected during the MELT experiment at the Southern East Pacific Rise (SEPR) constrain the distribution of melt beneath this mid-ocean-ridge spreading center and also the evolution of the oceanic lithosphere during its early cooling history. In this paper, we focus on structure imaged at distances ~100 to 350 km east of the ridge crest, corresponding to seafloor ages of ~1.3 to 4.5 Ma, where the seismic and electrical conductivity structure is nearly constant, independent of age. Beginning at a depth of about 60 km, there is 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. Because conductive cooling models predict structure that increases in depth with age, extending to about 30 km at 4.5 Ma, we infer that the structure of young oceanic plates is instead controlled by a decrease in water content above 60 km induced by the melting process beneath the spreading center.
Water in cratonic lithosphere : calibrating laboratory-determined models of electrical conductivity of mantle minerals using geophysical and petrological observations

(American Geophysical Union, 2012-06-14) Jones, Alan G. ; Fullea, Javier ; Evans, Rob L. ; Muller, Mark R.

Measurements of electrical conductivity of “slightly damp” mantle minerals from different laboratories are inconsistent, requiring geophysicists to make choices between them when interpreting their electrical observations. These choices lead to dramatically different conclusions about the amount of water in the mantle, resulting in conflicting conclusions regarding rheological conditions; this impacts on our understanding of mantle convection, among other processes. To attempt to reconcile these differences, we test the laboratory-derived proton conduction models by choosing the simplest petrological scenario possible – cratonic lithosphere – from two locations in southern Africa where we have the most complete knowledge. We compare and contrast the models with field observations of electrical conductivity and of the amount of water in olivine and show that none of the models for proton conduction in olivine proposed by three laboratories are consistent with the field observations. We derive statistically model parameters of the general proton conduction equation that satisfy the observations. The pre-exponent dry proton conduction term (σ0) and the activation enthalpy (ΔHwet) are derived with tight bounds, and are both within the broader 2σ errors of the different laboratory measurements. The two other terms used by the experimentalists, one to describe proton hopping (exponent r on pre-exponent water content Cw) and the other to describe H2O concentration-dependent activation enthalpy (term αCw1/3 added to the activation energy), are less well defined and further field geophysical and petrological observations are required, especially in regions of higher temperature and higher water content.
Prediction of silicate melt viscosity from electrical conductivity : a model and its geophysical implications

(John Wiley & Sons, 2013-06-12) Pommier, Anne ; Evans, Rob L. ; Key, Kerry ; Tyburczy, James A. ; Mackwell, Stephen ; Elsenbeck, James R.

Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction ~ >0.7).
Conductivity structure of the lithosphere-asthenosphere boundary beneath the eastern North American margin

(John Wiley & Sons, 2017-02-25) Attias, Eric ; Evans, Rob L. ; Naif, Samer ; Elsenbeck, James R. ; Key, Kerry

Tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere-asthenosphere boundary (LAB), yet this boundary remains enigmatic regarding its properties and geometry. The processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies. Here we employ marine magnetotelluric (MT) data acquired along a ∼135 km long profile, offshore Martha's Vineyard, New England, USA, to image the electrical conductivity structure beneath the New England continental margin for the first time. We invert the data using two different MT 2-D inversion algorithms and present a series of models that are obtained using three different parameterizations: fully unconstrained, unconstrained with an imposed LAB discontinuity and a priori constrained lithosphere resistivity. This suite of models infers variability in the depth of the LAB, with an average depth of 115 km at the eastern North America passive margin. Models robustly detect a ∼350 Ωm lithospheric anomalous conductivity zone (LACZ) that extends vertically through the entire lithosphere. Our preferred conductivity model is consistent with regional P-to-S receiver function data, shear-wave velocity, gravity anomalies, and prominent geological features. We propose that the LACZ is indicative of paleolithospheric thinning, either resulting from kimberlite intrusions associated with rifting and the New England Great Meteor hot spot track, or from shear-driven localized deformation related to rifting.
Geophysical and geochemical survey of a large marine pockmark on the Malin Shelf, Ireland

(American Geophysical Union, 2012-01-19) Szpak, Michal T. ; Monteys, Xavier ; O'Reilly, S. ; Simpson, A. J. ; Garcia, Xavier ; Evans, Rob L. ; Allen, C. C. R. ; McNally, D. J. ; Courtier-Murias, D. ; Kelleher, B. P.

Marine pockmarks are a specific type of seabed geological setting resembling craters or pits and are considered seabed surface expressions of fluid flow in the subsurface. A large composite pockmark on the Malin Shelf, off the northern coast of Ireland was surveyed and ground truthed to assess its activity and investigate fluid related processes in the subsurface. Geophysical (including acoustic and electromagnetic) data confirmed the subsurface presence of signatures typical of fluids within the sediment. Shallow seismic profiling revealed a large shallow gas pocket and typical gas related indicators such as acoustic blanking and enhanced reflectors present underneath and around the large pockmark. Sulphate profiles indicate that gas from the shallow reservoir has been migrating upwards, at least recently. However, there are no chimney structures observed in the sub-bottom data and the migration pathways are not apparent. Electromagnetic data show slightly elevated electrical conductivity on the edges of the pockmarks and a drop below regional levels within the confines of the pockmark, suggesting changes in physical properties of the sediment. Nuclear Magnetic Resonance (NMR) experiments were employed to characterize the organic component of sediments from selected depths. Very strong microbial signatures were evident in all NMR spectra but microbes outside the pockmark appear to be much more active than inside. These observations coincide with spikes in conductivity and the lateral gas bearing body suggesting that there is an increase in microbial activity and biomass when gas is present.
Seismological evidence for girdled olivine lattice‐preferred orientation in oceanic lithosphere and implications for mantle deformation processes during seafloor spreading

(American Geophysical Union, 2022-10-03) Russell, Joshua B. ; Gaherty, James B. ; Mark, Hannah F. ; Hirth, Greg ; Hansen, Lars N. ; Lizarralde, Daniel ; Collins, John A. ; Evans, Rob L.

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep.
Bayesian joint inversion of controlled source electromagnetic and magnetotelluric data to image freshwater aquifer offshore New Jersey

(Oxford University Press, 2019-06-03) Blatter, Daniel ; Key, Kerry ; Ray, Anandaroop ; Gustafson, Chloe ; Evans, Rob L.

Joint inversion of multiple electromagnetic data sets, such as controlled source electromagnetic and magnetotelluric data, has the potential to significantly reduce uncertainty in the inverted electrical resistivity when the two data sets contain complementary information about the subsurface. However, evaluating quantitatively the model uncertainty reduction is made difficult by the fact that conventional inversion methods—using gradients and model regularization—typically produce just one model, with no associated estimate of model parameter uncertainty. Bayesian inverse methods can provide quantitative estimates of inverted model parameter uncertainty by generating an ensemble of models, sampled proportional to data fit. The resulting posterior distribution represents a combination of a priori assumptions about the model parameters and information contained in field data. Bayesian inversion is therefore able to quantify the impact of jointly inverting multiple data sets by using the statistical information contained in the posterior distribution. We illustrate, for synthetic data generated from a simple 1-D model, the shape of parameter space compatible with controlled source electromagnetic and magnetotelluric data, separately and jointly. We also demonstrate that when data sets contain complementary information about the model, the region of parameter space compatible with the joint data set is less than or equal to the intersection of the regions compatible with the individual data sets. We adapt a trans-dimensional Markov chain Monte Carlo algorithm for jointly inverting multiple electromagnetic data sets for 1-D earth models and apply it to surface-towed controlled source electromagnetic and magnetotelluric data collected offshore New Jersey, USA, to evaluate the extent of a low salinity aquifer within the continental shelf. Our inversion results identify a region of high resistivity of varying depth and thickness in the upper 500 m of the continental shelf, corroborating results from a previous study that used regularized, gradient-based inversion methods. We evaluate the joint model parameter uncertainty in comparison to the uncertainty obtained from the individual data sets and demonstrate quantitatively that joint inversion offers reduced uncertainty. In addition, we show how the Bayesian model ensemble can subsequently be used to derive uncertainty estimates of pore water salinity within the low salinity aquifer.
Lithospheric structures and Precambrian terrane boundaries in northeastern Botswana revealed through magnetotelluric profiling as part of the Southern African Magnetotelluric Experiment

(American Geophysical Union, 2011-02-03) Miensopust, Marion P. ; Jones, Alan G. ; Muller, Mark R. ; Garcia, Xavier ; Evans, Rob L.

Within the framework of the Southern African Magnetotelluric Experiment a focused study was undertaken to gain improved knowledge of the lithospheric geometries and structures of the westerly extension of the Zimbabwe craton (ZIM) into Botswana, with the overarching aim of increasing our understanding of southern African tectonics. The area of interest is located in northeastern Botswana, where Kalahari sands cover most of the geological terranes and very little is known about lithospheric structures and thicknesses. Some of the regional-scale terrane boundary locations, defined based on potential field data, are not sufficiently accurate for local-scale studies. Investigation of the NNW-SSE orientated, 600 km long ZIM line profile crossing the Zimbabwe craton, Magondi mobile belt, and Ghanzi-Chobe belt showed that the Zimbabwe craton is characterized by thick (∼220 km) resistive lithosphere, consistent with geochemical and geothermal estimates from kimberlite samples of the nearby Orapa and Letlhakane pipes (∼175 km west of the profile). The lithospheric mantle of the Ghanzi-Chobe belt is resistive, but its lithosphere is only about 180 km thick. At crustal depths a northward dipping boundary between the Ghanzi-Chobe and the Magondi belts is identified, and two middle to lower crustal conductors are discovered in the Magondi belt. The crustal terrane boundary between the Magondi and Ghanzi-Chobe belts is found to be located further to the north, and the southwestern boundary of the Zimbabwe craton might be further to the west, than previously inferred from the regional potential field data.
High-resolution constraints on pacific upper mantle petrofabric inferred from surface-wave anisotropy.

(American Geophysical Union, 2018-12-26) Russell, Joshua B. ; Gaherty, James B. ; Lin, Pei-Ying Patty ; Lizarralde, Daniel ; Collins, John A. ; Hirth, Greg ; Evans, Rob L.

Lithospheric seismic anisotropy illuminates mid‐ocean ridge dynamics and the thermal evolution of oceanic plates. We utilize short‐period (5–7.5 s) ambient‐noise surface waves and 15‐ to 150‐s Rayleigh waves measured across the NoMelt ocean‐bottom array to invert for the complete radial and azimuthal anisotropy in the upper ∼35 km of ∼70‐Ma Pacific lithospheric mantle, and azimuthal anisotropy through the underlying asthenosphere. Strong azimuthal variations in Rayleigh‐ and Love‐wave velocity are observed, including the first clearly measured Love‐wave 2θ and 4θ variations. Inversion of averaged dispersion requires radial anisotropy in the shallow mantle (2‐3%) and the lower crust (4‐5%), with horizontal velocities (VSH) faster than vertical velocities (VSV). Azimuthal anisotropy is strong in the mantle, with 4.5–6% 2θ variation in VSV with fast propagation parallel to the fossil‐spreading direction (FSD), and 2–2.5% 4θ variation in VSH with a fast direction 45° from FSD. The relative behavior of 2θ, 4θ, and radial anisotropy in the mantle are consistent with ophiolite petrofabrics, linking outcrop and surface‐wave length scales. VSV remains fast parallel to FSD to ∼80 km depth where the direction changes, suggesting spreading‐dominated deformation at the ridge. The transition at ∼80 km perhaps marks the dehydration boundary and base of the lithosphere. Azimuthal anisotropy strength increases from the Moho to ∼30 km depth, consistent with flow models of passive upwelling at the ridge. Strong azimuthal anisotropy suggests extremely coherent olivine fabric. Weaker radial anisotropy implies slightly nonhorizontal fabric or the presence of alternative (so‐called E‐type) peridotite fabric. Presence of radial anisotropy in the crust suggests subhorizontal layering and/or shearing during crustal accretion.
Evaluating models for lithospheric loss and intraplate volcanism beneath the Central Appalachian Mountains

(American Geophysical Union, 2021-09-16) Long, Maureen D. ; Wagner, Lara S. ; King, Scott D. ; Evans, Rob L. ; Mazza, Sarah E. ; Byrnes, Joseph S. ; Johnson, Elizabeth A. ; Kirby, Eric ; Bezada, Maximiliano J. ; Gazel, Esteban ; Miller, Scott R. ; Aragon, John C. ; Liu, Shangxin

The eastern margin of North America has been shaped by a series of tectonic events including the Paleozoic Appalachian Orogeny and the breakup of Pangea during the Mesozoic. For the past ∼200 Ma, eastern North America has been a passive continental margin; however, there is evidence in the Central Appalachian Mountains for post-rifting modification of lithospheric structure. This evidence includes two co-located pulses of magmatism that post-date the rifting event (at 152 and 47 Ma) along with low seismic velocities, high seismic attenuation, and high electrical conductivity in the upper mantle. Here, we synthesize and evaluate constraints on the lithospheric evolution of the Central Appalachian Mountains. These include tomographic imaging of seismic velocities, seismic and electrical conductivity imaging along the Mid-Atlantic Geophysical Integrative Collaboration array, gravity and heat flow measurements, geochemical and petrological examination of Jurassic and Eocene magmatic rocks, and estimates of erosion rates from geomorphological data. We discuss and evaluate a set of possible mechanisms for lithospheric loss and intraplate volcanism beneath the region. Taken together, recent observations provide compelling evidence for lithospheric loss beneath the Central Appalachians; while they cannot uniquely identify the processes associated with this loss, they narrow the range of plausible models, with important implications for our understanding of intraplate volcanism and the evolution of continental lithosphere. Our preferred models invoke a combination of (perhaps episodic) lithospheric loss via Rayleigh-Taylor instabilities and subsequent small-scale mantle flow in combination with shear-driven upwelling that maintains the region of thin lithosphere and causes partial melting in the asthenosphere.
Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds

( 2008-08-12) Jones, Alan G. ; Evans, Rob L. ; Eaton, David W.

Can mineral physics and mixing theories explain field observations of seismic velocity and electrical conductivity, and is there an advantage to combining seismological and electromagnetic techniques? These two questions are at the heart of this paper. Using phenomologically-derived state equations for individual minerals coupled with multi-phase, Hashin-Shtrikman extremal-bound theory we derive the likely shear and compressional velocities and electrical conductivity at three depths, 100 km, 150 km and 200 km, beneath the central part of the Slave craton and beneath the Kimberley region of the Kaapvaal craton based on known petrologically-observed mineral abundances and magnesium numbers, combined with estimates of temperatures and pressures. We demonstrate that there are measurable differences between the physical properties of the two lithospheres for the upper depths, primarily due to the different ambient temperature, but that differences in velocity are negligibly small at 200 km. We also show that there is an advantage to combining seismic and electromagnetic data, given that conductivity is exponentially dependent on temperature whereas the shear and bulk moduli have only a linear dependence in cratonic lithospheric rocks. Focussing on a known discontinuity between harzburgite-dominated and lherzolitic mantle in the Slave craton at a depth of about 160 km, we demonstrate that the amplitude of compressional (P) wave to shear (S) wave conversions would be very weak, and so explanations for the seismological (receiver function) observations must either appeal to effects we have not considered (perhaps anisotropy), or imply that the laboratory data require further refinement.
Constraints on the depth, thickness, and strength of the G Discontinuity in the Central Pacific from S Receiver Functions

(American Geophysical Union, 2021-03-09) Mark, Hannah F. ; Collins, John A. ; Lizarralde, Daniel ; Hirth, Greg ; Gaherty, James B. ; Evans, Rob L. ; Behn, Mark D.

The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere-asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.
Offshore freshened groundwater in continental margins

(American Geophysical Union, 2020-11-20) Micallef, Aaron ; Person, Mark ; Berndt, Christian ; Bertoni, Claudia ; Cohen, Denis ; Dugan, Brandon ; Evans, Rob L. ; Haroon, Amir ; Hensen, Christian ; Jegen, Marion ; Key, Kerry ; Kooi, Henk ; Liebetrau, Volker ; Lofi, Johanna ; Mailloux, Brian J. ; Martin-Nagle, Renée ; Michael, Holly A. ; Müller, Thomas ; Schmidt, Mark ; Schwalenberg, Katrin ; Trembath-Reichert, Elizabeth ; Weymer, Bradley ; Zhang, Yipeng ; Thomas, Ariel T.

First reported in the 1960s, offshore freshened groundwater (OFG) has now been documented in most continental margins around the world. In this review we compile a database documenting OFG occurrences and analyze it to establish the general characteristics and controlling factors. We also assess methods used to map and characterize OFG, identify major knowledge gaps, and propose strategies to address them. OFG has a global volume of 1 × 106 km3; it predominantly occurs within 55 km of the coast and down to a water depth of 100 m. OFG is mainly hosted within siliciclastic aquifers on passive margins and recharged by meteoric water during Pleistocene sea level lowstands. Key factors influencing OFG distribution are topography-driven flow, salinization via haline convection, permeability contrasts, and the continuity/connectivity of permeable and confining strata. Geochemical and stable isotope measurements of pore waters from boreholes have provided insights into OFG emplacement mechanisms, while recent advances in seismic reflection profiling, electromagnetic surveying, and numerical models have improved our understanding of OFG geometry and controls. Key knowledge gaps, such as the extent and function of OFG, and the timing of their emplacement, can be addressed by the application of isotopic age tracers, joint inversion of electromagnetic and seismic reflection data, and development of three-dimensional hydrological models. We show that such advances, combined with site-specific modeling, are necessary to assess the potential use of OFG as an unconventional source of water and its role in sub-seafloor geomicrobiology.
Electrical lithosphere beneath the Kaapvaal craton, southern Africa

(American Geophysical Union, 2011-04-20) Evans, Rob L. ; Jones, Alan G. ; Garcia, Xavier ; Muller, Mark R. ; Hamilton, Mark P. ; Evans, Shane ; Fourie, C. J. S. ; Spratt, Jessica ; Webb, Susan J. ; Jelsma, Hielke ; Hutchins, David A.

A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200–250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly “damp” (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab.
Electrical structure beneath the northern MELT line on the East Pacific Rise at 15°45′S

(American Geophysical Union, 2006-11-16) Baba, Kiyoshi ; Tarits, Pascal ; Chave, Alan D. ; Evans, Rob L. ; Hirth, Greg ; Mackie, Randall L.

The electrical structure of the upper mantle beneath the East Pacific Rise (EPR) at 15°45′S is imaged by inverting seafloor magnetotelluric data obtained during the Mantle ELectromagnetic and Tomography (MELT) experiment. The electrical conductivity model shows no evidence for a conductive region immediately beneath the ridge, in contrast to the model previously obtained beneath the EPR at 17°S. This observation can be explained by differences in current melt production along the ridge, consistent with other observations. The mantle to the east of the ridge at 60 –100 km depth is anisotropic, with higher conductivity in the spreading direction compared to the along-strike direction, similar to the 17°S region. The high conductivity in the spreading direction can be explained by a hydrated mantle with strain-induced lattice preferred orientation of olivine or by partial melt preferentially connected in the spreading direction.
Area selection for diamonds using magnetotellurics : examples from southern Africa

( 2009-06-05) Jones, Alan G. ; Evans, Rob L. ; Muller, Mark R. ; Hamilton, Mark P. ; Miensopust, Marion P. ; Garcia, Xavier ; Cole, Patrick ; Ngwisanyi, Tiyapo ; Hutchins, David A. ; Fourie, C. J. S. ; Jelsma, Hielke ; Aravanis, Theo ; Pettit, Wayne ; Webb, Susan J. ; Webb, Jan ; Collins, Louise ; Hogg, Colin ; Horan, Clare ; Spratt, Jessica ; Wallace, Gerry ; Chave, Alan D. ; Cole, Janine ; Stettler, Raimund ; Tshoso, G. ; Mountford, Andy ; Cunion, Ed ; Khoza, T. David ; Share, Pieter-Ewald ; SAMTEX Team

Southern Africa, particularly the Kaapvaal Craton, is one of the world’s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to the original four through the course of the experiment, SAMTEX was enlarged from two to four phases of acquisition, and extended to cover much of Botswana and Namibia. The complete SAMTEX dataset now comprises MT data from over 675 distinct locations in an area of over one million square kilometres, making SAMTEX the largest regional-scale MT experiment conducted to date. Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km, constructed through approximate one-dimensional methods, map resistive regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth Terrain. Known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions. Comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive, and low velocity regions that are conductive. In particular, the electrical resistivity of the sub-continental lithospheric mantle of the Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix property, namely temperature, and to a lesser degree by iron content and composition, and is not controlled by contributions from interconnected conducting minor phases, such as graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations between velocity and resistivity valid, and a robust regression between the two gives an approximate relationship of Vs [m/s] = 0.045*log(resistivity [ohm.m]).
Water-rich bending faults at the Middle America Trench

(John Wiley & Sons, 2015-08-16) Naif, Samer ; Key, Kerry ; Constable, Steven ; Evans, Rob L.

The portion of the Central American margin that encompasses Nicaragua is considered to represent an end-member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge.
Aquifer systems extending far offshore on the US Atlantic margin.

(Nature Research, 2019-06-18) Gustafson, Chloe ; Key, Kerry ; Evans, Rob L.

Low-salinity submarine groundwater contained within continental shelves is a global phenomenon. Mechanisms for emplacing offshore groundwater include glacial processes that drove water into exposed continental shelves during sea-level low stands and active connections to onshore hydrologic systems. While low-salinity groundwater is thought to be abundant, its distribution and volume worldwide is poorly understood due to the limited number of observations. Here we image laterally continuous aquifers extending 90 km offshore New Jersey and Martha’s Vineyard, Massachusetts, on the U.S. Atlantic margin using new shallow water electromagnetic geophysical methods. Our data provide more continuous constraints on offshore groundwater than previous models and present evidence for a connection between the modern onshore hydrologic system and offshore aquifers. We identify clinoforms as a previously unknown structural control on the lateral extent of low-salinity groundwater and potentially a control on where low-salinity water rises into the seafloor. Our data suggest a continuous submarine aquifer system spans at least 350 km of the U.S. Atlantic coast and contains about 2800 km3 of low-salinity groundwater. Our findings can be used to improve models of past glacial, eustatic, tectonic, and geomorphic processes on continental shelves and provide insight into shelf geochemistry, biogeochemical cycles, and the deep biosphere.
Using CSEM techniques to map the shallow section of seafloor : from the coastline to the edges of the continental slope

(Society of Exploration Geophysicists, 2007-03-01) Evans, Rob L.

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.

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