Constraints on lithospheric mantle and crustal anisotropy in the NoMelt area from an analysis of long-period seafloor magnetotelluric data

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth, Planets and Space 69 (2017): 138, doi:10.1186/s40623-017-0724-1.Despite strong anisotropy seen in analysis of seismic data from the NoMelt experiment in 70 Ma Pacific seafloor, a previous analysis of coincident magnetotelluric (MT) data showed no evidence for anisotropy in the electrical conductivity structure of either lithosphere or asthenosphere. We revisit the MT data and use 1D anisotropic models of the lithosphere to demonstrate the limits of acceptable anisotropy within the data. We construct 1D models by varying the thickness and the degree of anisotropy within the lithosphere and conduct a series of tests to investigate what types of electrical anisotropy are compatible with the data. We find that electrical anisotropy is possible in a sheared and/or hydrous mantle within the lower lithosphere (60–90 km depth). The data are not compatible with pervasive electrical anisotropy in the crust. Causes of anisotropy within the highly resistive upper and mid-lithosphere, as seen seismically, are not expected to cause measurable impacts on MT response.RLE was supported by NSF Grant OCE-0928663

Water-rich bending faults at the Middle America Trench

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 2582–2597, doi:10.1002/2015GC005927.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.This work was supported by National Science Foundation grants OCE-0841114 and OCE-0840894, and the Seafloor Electromagnetic Methods Consortium at Scripps Institution of Oceanography.2016-02-1

Porosity and fluid budget of a water-rich megathrust revealed with electromagnetic data at the Middle America Trench

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 17 (2016): 4495–4516, doi:10.1002/2016GC006556.At convergent margins, the distribution of fluids released from the downgoing slab modulates the state of stress and seismic coupling at the megathrust plate interface. However, existing geophysical data are unable to quantify the porosity along this interface. Here we use controlled-source electromagnetic data collected across the Middle America Trench offshore Nicaragua to image the electrical conductivity structure of the outer fore arc. Our results detect a highly conductive channel, inferred to be the region around the décollement, showing the entire section of water-rich seafloor sediments underthrust with the subducting lithosphere. We use an empirical model of the electrical conductivity of porous media to quantify the channel porosity. Our estimates are consistent with sediment compaction studies, showing a rapid decay of 65%–10% porosity from the trench to 25 km landward. We constrain the channel thickness and use the porosity estimates to determine the water budget, which represents the fraction taken up by fluid. The porosity and water budget estimates show significant lateral variations that we attribute to changes in subducted sediment thickness caused by outer rise bending faults. Between 18 and 23 km from the trench, the conductive channel broadens greatly to 1.5–2 km thick, possibly due to concentrated blind faults or sediment underplating, which suggests a sudden change in hydrogeologic structure at the plate interface. The impact of the anomalous conductor on the seismic coupling and mechanical properties of the megathrust is potentially related to the discrepancy in estimated fault slip between seismic and tsunami source inversions for the 1992 Nicaragua tsunami earthquake.National Science Foundation Grant Numbers: OCE-0841114 , OCE-0840894; Scripps Institution of Oceanography2017-05-1

Correction of shallow-water electromagnetic data for noise induced by instrument motion

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Society of Exploration Geophysicists for personal use, not for redistribution. The definitive version was published in Geophysics 70 (2005): G127–G133, doi:10.1190/1.2080748.An unexpected noise source was found in magnetic and sometimes electric field data recorded on the bottom of lakes in the Archean Slave craton (NW Canada) during warm seasons. The noise is due to instrument motion and in some instances direct induction by wind-driven surface gravity waves when the lakes are not ice covered. The noise can be reduced or eliminated by pre-filtering the data with an adaptive correlation noise cancelling filter using instrument tilt records, prior to estimation of magnetotelluric (MT) response functions. Similar effects are to be expected in other shallow water environments, and the adaptive correlation canceller is a suitable method to pre-process MT data to reduce motional noise in the magnetic field. This underscores the importance of ancillary tilt measurements in shallow water MT surveys. In coastal or lake bottom surveys, special efforts to reduce hydrodynamic effects on the instrument should also be pursued.This project was funded by NSF grant EAR-9725556 and EAR-0087699. P.L. ac- knowledges the Fundacion Andes for a postdoctoral grant

Bayesian joint inversion of controlled source electromagnetic and magnetotelluric data to image freshwater aquifer offshore New Jersey

Author Posting. © The Authors, 2019. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 218(3), (2019): 1822-1837, doi: 10.1093/gji/ggz253.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.We gratefully acknowledge funding support from National Science Foundation grants 1458392 and 1459035. We thank the captain and crew of the R.V. Marcus G. Langseth for a successful cruise and the Marine EM Lab at Scripps Institution of Oceanography for providing the instrumentation. We also thank Chris Armerding, Marah Dahn, John Desanto, Jimmy Elsenbeck, Matt Folsom, Keiichi Ishizu, Jeff Pepin, Charlotte Wiman and Georgie Zelenak for participating in the cruise. We gratefully acknowledge Alberto Malinverno for the idea to use a Monte Carlo scheme to estimate the distribution of pore fluid salinity, and William Menke for many constructive conversations and suggestions

Electrical investigation of natural lawsonite and application to subduction contexts

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Solid Earth 142(2), (2019):1430-1442, doi:10.1029/2018JB016899.We report an experimental investigation of the electrical properties of natural polycrystalline lawsonite from Reed Station, CA. Lawsonite represents a particularly efficient water reservoir in subduction contexts, as it can carry about 12 wt % water and is stable over a wide pressure range. Experiments were performed from 300 to about 1325 °C and under pressure from 1 to 10 GPa using a multi‐anvil apparatus. We observe that temperature increases lawsonite conductivity until fluids escape the cell after dehydration occurs. At a fixed temperature of 500 °C, conductivity measurements during compression indicate electrical transitions at about 4.0 and 9.7 GPa that are consistent with crystallographic transitions from orthorhombic C to P and from orthorhombic to monoclinic systems, respectively. Comparison with lawsonite structure studies indicates an insignificant temperature dependence of these crystallographic transitions. We suggest that lawsonite dehydration could contribute to (but not solely explain) high conductivity anomalies observed in the Cascades by releasing aqueous fluid at a depth (~50 km) consistent with the basalt‐eclogite transition. In subduction settings where the incoming plate is older and cooler (e.g., Japan), lawsonite remains stable to great depth. In these cooler settings, lawsonite could represent a vehicle for deep water transport and the subsequent triggering of melt that would appear electrically conductive, though it is difficult to uniquely identify the contributions from lawsonite on field electrical profiles in these more deep‐seated domains.A. P. acknowledges financial support from UCSD‐SIO startup funds, NSF‐EAR Petrology and Geochemistry (grant 1551200), and NSF‐COMPRES IV EOID subaward. The use of the COMPRES Cell Assembly Project was also supported by COMPRES under NSF Cooperative Agreement EAR 1661511. Q. W. acknowledges support from NSF EAR‐1620423. We thank Kurt Leinenweber for fruitful discussion, Jake Perez for technical help in the lab, and Sabine Faulhaber (UCSD Nano‐Engineering Department) for technical assistance with SEM images and EDS analyses. We also thank two reviewers for detailed comments that improved the manuscript. All the electrical data used for Figures 4 and 5 are available in the supporting information.2019-08-2

Velocity-conductivity relations for cratonic lithosphere and their application : example of Southern Africa

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 806–827, doi:10.1002/ggge.20075.Seismic velocity is a function of bulk vibrational properties of the media, whereas electrical resistivity is most often a function of transport properties of an interconnected minor phase. In the absence of a minor conducting phase then the two should be inter-relatable primarily due to their sensitivity to temperature variation. We develop expressions between shear wave velocity and resistivity for varying temperature, composition, and water content based on knowledge from two kimberlite fields: Jagersfontein (Kaapvaal Craton) and Gibeon (Rehoboth Terrane). We test the expressions through comparison between a new high-resolution regional seismic model, derived from surface wave inversion of earthquake data from Africa and the surrounding regions, and a new electrical image from magnetotelluric (MT) data recorded in SAMTEX (Southern African Magnetotelluric Experiment). The data-defined robust linear regression between the two is found to be statistically identical to the laboratory-defined expression for 40 wt ppm water in olivine. Cluster analysis defines five clusters that are all geographically distinct and tectonically relate to (i) fast, cold, and variably wet Kaapvaal Craton, (ii) fast and wet central Botswana, (iii) slow, warm, and wet Rehoboth Terrane, (iv) moderately fast, cold, and very dry southernmost Angola Craton, and (v) slow, warm, and somewhat dry Damara Belt. From the linear regression expression and the MT image we obtain predicted seismic velocity at 100 km and compare it with that from seismic observations. The differences between the two demonstrate that the linear relationship between Vs and resistivity is appropriate for over 80% of Southern Africa. Finally, using the regressions for varying water content, we infer water content in olivine across Southern Africa.We wish to again acknowledge the three main funding agencies, the U.S. National Science Foundation’s Continental Dynamics Program (grant EAR0455242 to RLE), the South African Department of Science and Technology (grant to South African Council for Geoscience), and Science Foundation Ireland (grant 05/RGP/GEO001 to AGJ), for their support. Industry support for SAMTEX from De Beers Group Services, BHP Billiton and Rio Tinto Mining and Exploration resulted in a program far more extensive than originally conceived. S.F. has been supported by the NERC New Investigator grant NE/G000859/1. M.M. wishes to thank Science Foundation Ireland (grant 08/RFP/GEO1693 SAMTEX to AGJ) for support. J.F. wishes to thank Enterprise Ireland (grant Topo-Med to AGJ), Science Foundation Ireland (grant 10/IN.1/I3022 IRETHERMto AGJ), and the JAE-DOC Programme from Spanish CSIC, cofunded by FSE for support.2013-10-0

Robust magnetotelluric inversion

Author Posting. © The Author(s), 2014. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 196 (2014): 1365-1374, doi:10.1093/gji/ggt484.A robust magnetotelluric (MT) inversion algorithm has been developed on the basis of quantile-quantile (q-q) plotting with confidence band and statistical modelling of inversion residuals for the MT response function (apparent resistivity and phase). Once outliers in the inversion residuals are detected in the q-q plot with the confidence band and the statistical modelling with the Akaike information criterion, they are excluded from the inversion data set and a subsequent inversion is implemented with the culled data set. The exclusion of outliers and the subsequent inversion is repeated until the q-q plot is substantially linear within the confidence band, outliers predicted by the statistical modelling are unchanged from the prior inversion, and the misfit statistic is unchanged at a target level. The robust inversion algorithm was applied to synthetic data generated from a simple 2-D model and observational data from a 2-D transect in southern Africa. Outliers in the synthetic data, which come from extreme values added to the synthetic responses, produced spurious features in inversion models, but were detected by the robust algorithm and excluded to retrieve the true model. An application of the robust inversion algorithm to the field data demonstrates that the method is useful for data clean-up of outliers, which could include model as well as data inconsistency (for example, inability to fit a 2-D model to a 3-D data set), during inversion and for objectively obtaining a robust and optimal model. The present statistical method is available irrespective of the dimensionality of target structures (hence 2-D and 3-D structures) and of isotropy or anisotropy, and can operate as an external process to any inversion algorithm without modifications to the inversion program.TM was supported by the scientific program of TAIGA (trans-crustal advection and in-situ reaction of global sub-seafloor aquifer) sponsored by the MEXT of Japan, and is supported by the NIPR project KP-7. ADC is supported by US National Science Foundation (NSF) grant EAR1015185

Prediction of silicate melt viscosity from electrical conductivity : a model and its geophysical implications

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 1685–1692, doi:10.1002/ggge.20103.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).We acknowledge partial support under NASA USRA subaward 02153–04, NSF EAR 0739050, and the ASU School of Earth and Space Exploration (SESE) Exploration Postdoctoral Fellowship Program.2013-12-1