Lamont-Doherty Earth Observatory, Columbia University
Serpentinite, Extension and Regional Porosity Experiment across the Nicaraguan Trench (SERPENT)
This project is the first to use controlled-source electromagnetic (CSEM) imaging to image the fluid content of the incoming plate and forearc crust at a subduction zone. In 2010 we collected broadband magnetotelluric (MT) and CSEM data along a 300 km long profile crossing the Middle America Trench offshore Nicaragua. CSEM deep-tows along the entire profile imaged porosity variations associated with lithospheric bending and cracking near the trench and mapped a water-rich plate boundary beneath the continental slope. Circular CSEM tows measured the anisotropic fabric of upper mantle conductivity in order to constrain the extent of serpentinization. The deeper sensing MT data discovered a high conductivity partially molten zone in the asthenosphere beneath the oceanic plate.
Regional tectonic map. The Cocos oceanic plate subducts at a rate that increases from 40 mm/yr in the northwest to 90 mm/yr in the southeast. The black dashed line marks the boundary that separates the Cocos crustal origin between the East Pacific Rise (EPR) and the Cocos-Nazca spreading center (CNS). The survey profile (boxed) is located on EPR-sourced crust and crosses the MAT off Nicaragua.Map of the electromagnetic survey. The faulted seafloor fabric is clearly seen in the high-resolution bathymetric map. Solid black squares show the location of ocean bottom electromagnetic receivers whose data we consider here. The black square outlined in white is a LEM collocated with a standard receiver. Data from white squares outlined in black were excluded from modeling. The orange line shows the CSEM transmitter towpath and the blue and red circles are the LEM anisotropy tows. The black dashed line and black dotted line represent the location where our data observe the onset of hydration in the layer of dikes and gabbros at 80 km and 60 km seaward of the trench, respectively.The electrical structure of the Middle America Trench from nonlinear inversion of deep-towed CSEM data. The vertical cross section shows the electrical resistivity structure and the stitched top plot shows seafloor bathymetry. The dark blue cubes show the location of EM receivers. The blue cylinders show the location of active seafloor seeps. The incoming Cocos plate develops several steeply dipping bending faults that correlate with subvertical conductive channels, which significantly hydrate the oceanic crust [Naif et al., 2015]. The channel of low resistivity within the fore-arc margin is caused by subducted sediments. The 1992 tsunami earthquake ruptured this section of the megathrust.Resistivity model obtained from anisotropic inversion of the seafloor magnetotelluric data. At the top is the surface view; inverted triangles denote seafloor magnetotelluric station locations. a) The electrical resistivity in the direction parallel to plate motion (ry). Blue and red colours corresponding to resistive and conductive (less resistive) features, respectively. The dark red line is a model of the top of the subducting slab. Earthquake hypocentres from up to 50 km off-axis are shown as black circles. The region enclosed by the dashed black line is where the model is at least 1.5 times more conductive in the direction parallel to plate motion. b) Resistivity ratio for the plate-motion-parallel (ry) to trench- axis-parallel (rx) model components. The colour scale gives log(ry/rx), and the plot shows the strong anisotropy of the conductive layer at 45–70 km depth (red regions 150 km offshore). Although the lithosphere above shows a strong anisotropy, we warn that this is not well constrained, because the magnetotelluric method is primarily sensitive to conductive rather than resistive features. The deeper mantle beneath the conductive layer is isotropic, suggesting it is not being sheared.
