|A seafloor map offshore Nicaragua showing the locations of instrument deployments and CSEm transmission towlines during the SERPENT experiment. ()|
|An electrical resistivity model of the Nicaragua subduction system derived from 2D inversion of CSEM data collected during the SERPENT experiment. ()|
SERPENT: Serpentinite, Extension, and Regional Porosity Experiment across the Nicaraguan Trench
Dan Lizarralde, WHOI Kerry Key, Steve Constable, Scripps Institution of Oceanography http://marineemlab.ucsd.edu/Projects/SERPENT
Water plays an important role in many of the processes occurring at convergent margins, as the release of water from the downgoing slab affects the rheology of the mantle, impacts seismicity, allows melting to occur more readily by lowering the solidus temperature, and alters the chemistry of arc-lavas. Yet, the amount of water entering the subduction system remains poorly constrained. One of the major uncertainties in terms of fluid inputs into the subduction factory, concerns the extent of serpentinization of the oceanic upper mantle and the volumes of water that are being carried into the subduction system through this route.
In 2010 we completed a large marine CSEM and magnetotelluric deployment off the coast of Nicaragua. Our project is the largest combined controlled-source electromagnetic (CSEM) and magnetotelluric (MT) data set ever collected on an active subduction zone. During the single 28 day research cruise aboard the R/V Melville we collected 54 stations of broadband marine magnetotelluric (MT) data and deep-towed nearly 800 km of controlled-source electromagnetic (CSEM) data. Robust multiple-station array processing of the MT data yields high quality MT responses from 10 to 20,000 s period. Two circular CSEM tows of 30 km radius were measured by special long-wire EM (LEM) sensors on the abyssal plain and the outer rise. Conventional CSEM data recorded at a broad suite of transmission frequencies along the 300 km long profile and a 50 km along strike profile provide constraints on crustal conductivity variations.
CSEM data require the porosity of the lower oceanic crust to double as it approaches the trench axis, with most of the increase concentrated in zones that correlate with seismically imaged faults.
Further into the subduction we see fluids expelled from the slab and migrating along the decollement as well as hydrofracturing the overriding plate. The fluids involved in hydrofracturing migrate upwards to locations of seeps on the seafloor.