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

High-resolution resistivity imaging of marine gas hydrate structures by combined inversion of CSEM towed and ocean-bottom receiver data

We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. 2.5-D CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilized to map and monitor seafloor mineralization, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems

Marine electromagnetic experiment across the Nicaragua Trench : Imaging water-rich faults and melt-rich asthenosphere

Electromagnetic (EM) methods have been widely applied in exploration geophysics and to study tectonics for several decades. Electrical resistivity, or its reciprocal conductivity, is a physical quantity that varies by several orders of magnitude. Bulk resistivity is highly dependent on the presence of fluids and ore bodies. While EM is primarily used to map the geoelectrical structures of terrestrial environments, advances over the last two decades in instrument technology and computing software have not only made marine EM experiments viable but also routine and reliable. In this dissertation, I explore the utility of the marine magnetotelluric and controlled- source electromagnetic techniques for probing subduction zone processes. In the Spring of 2010, the Scripps Institution of Oceanography Marine EM group ventured on the R/V Melville to conduct the Serpentinite, Extension and Regional Porosity Experiment across the Nicaragua Trench (SERPENT). Over the course of 28 days, 54 sites of broadband marine magnetotelluric (MT) and 800 km of marine controlled-source electromagnetic (CSEM) data were collected, culminating in the first CSEM survey and the largest marine EM dataset at a subduction zone to date. In this dissertation, I perform regularized two-dimensional inversions on the marine MT and CSEM data from SERPENT to model the electrical resistivity structure of the crust and upper mantle. The MT data revealed an unexpected conductive channel at a depth interval of 45-70 km. I apply measurements from laboratory studies and find that only partial melt can account for the electrical signature of the conductor. I conclude that the anomalous channel is a sheared partial melt layer at the lithosphere- asthenosphere boundary that decouples the lithosphere from the deeper mantle. The CSEM data image sub-vertical conductive channels that correlate with outer rise fault scarps, providing the first observation to confirm bending faults behave as fluid pathways. I use Archie's law to infer porosity and find that the crust subducts significantly more pore water than previously thought. The CSEM data also image the complete subduction of the incoming sediments along the megathrust plate interface, providing the first large-scale estimates of porosity at the megathrust. At 20 km into the forearc, a conductive anomaly extends from the plate interface into the overlying crust beneath a high concentration of active seafloor seeps, possibly imaging both the origin and migratory pathway of fluids escaping along the margin seafloor. The location of the anomaly correlates with a section of the seafloor that exhibits steepened bathymetric slope, suggesting a sediment underplating mechanism as its caus

Constraining Magma Reservoir Conditions by Integrating Thermodynamic Petrological Models and Bulk Resistivity From Magnetotellurics

Abstract Magnetotelluric (MT) data image the bulk resistivity of the subsurface which can be used to infer magma reservoir conditions beneath volcanoes. The bulk resistivity of magma depends primarily on the melt volume fraction, temperature, and water content. These variables are coupled thermodynamically, yet mixing relations for bulk resistivity implicitly treat them as independent. Here, we use a parameterization of the rhyolite‐MELTS thermodynamic model to constrain relationships between melt fraction, temperature, dissolved water content and bulk resistivity for rhyolitic magmas. This method results in MT interpretations which are (a) thermodynamically consistent at near‐equilibrium conditions, (b) independent of temperature and water content estimates derived from erupted products, and (c) able to consider saturated melts containing a volatile (i.e., aqueous fluid) phase. The utility of the method is demonstrated with three case studies of silicic systems: Mono Basin, Newberry volcano and the Laguna del Maule Volcanic Field (LdMVF). The moderately conductive feature at Mono Basin can be explained by under‐saturated partial melt (6–15 vol%) at <775°C, indicating relatively stable magma storage conditions since the last eruption. However, a relatively resistive feature at Newberry Volcano requires lower temperatures (<750°C) than previous estimates, suggesting that the system has cooled since the last eruption. A conductive feature at the LdMVF cannot be explained by saturated or under‐saturated rhyolitic melt and requires additional conductive phases. These results demonstrate the potential of this new method to reduce uncertainty in MT interpretations and highlight the need for additional coupling strategies between petrology, geophysics, and thermo‐mechanical models to better understand magmatic systems

Magnetotelluric Data and Resistivity Model for Shumagin Gap, Alaska

Marine magnetotelluric data and 2-D electrical resistivity model for a profile through the Shumagin Gap, Alaska. See readme.txt file for details

High-resolution resistivity imaging of marine gas hydrate structures by combined inversion of CSEM towed and ocean-bottom receiver data

We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. Two-dimensional CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems