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