Magnetotelluric investigation of fluid- and melt-involved processes in the Central American subduction zone
Final Report Abstract
Long-period magnetotellurics (MT) is one of the methods of choice to image fluids and melts in subduction zones. This ranges from fluid release at the interface between overriding and sub-ducting plates to the source region, the ascend paths and storage volumes (magma chambers) of partial melts associated with active volcanism. Here we describe an experiment carried out at the Central American subduction zone formed by subduction of the Cocos and the Caribbean Plates, respectively. The highly active volcanic arc in NW Costa Rica and in NW Nicaragua was traversed by two MT profiles, each extending from the coastlines of the Pacific Ocean to the backarc. The field work was supported by the Instituto Costarricense de Electricidad (ICE, San José) and the Instituto Nicaragüense de Estudios Territoriales (INETER, Managua). The Costa Rica profile was later extended with ocean bottom instruments across the Middle America Trench by Geomar (Kiel). Strike and dimensionality analysis of the Costa Rica data encouraged a two-dimensional (2D) inversion. In the upper crust, the mafic Nicoya Block is imaged as a poor conductor, while the Tempisque basin in the forearc and the San Carlos Basin in the backarc are clearly displayed as low-resistivity structures. A possible magma chamber beneath Tenorio volcano is modeled, however with limited resolution due to the lack of suitable sites in the volcano’s rainforest. The deeper crust shows a conductive feature in the forearc just before the volcanic arc which is interpreted as a fluid repository originating from the downgoing slab; their fate and connection to the volcanoes remains unclear for the time being. The lower backarc crust is also characterized by low resistivities, consistent with standard models of backarc spreading and volcanism. The main result of the offshore study is the detection of a high-conductivity zone in the upper mantle of the oceanic plate which could be caused by partial melts in the asthenosphere. The Nicaragua data at first glance allowed a 2D interpretation, too. However, the main result of 2D inversion was a series of conductors in the lower crust which we largely attribute to artifacts originating from 3D anomalies off-profile. The assumption of structural anisotropy was tested with a large number of forward models but did not prove to be successful. The solution came with the availability of full 3D codes: After 3D inversion, the series of conductors converged into a single conductor just beneath the volcanic arc plus a plausible backarc zone of reduced resistivity. Furthermore, the Nicaraguan Depression is imaged as a good conductor extending to approximately 2 km depth. A deep magma reservoir beneath the arc as in Nicaragua could not be found in NW Costa Rica. This may be also be reflected in the higher activity in the Nicaraguan Maribios Range compared to the volcanoes of Guanacaste, Costa Rica. Our work in Guanacaste is now being extended by ICE, particularly aiming at resolution of near-surface magma chambers and geothermal potential.
Publications
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First Deep Electrical Resistivity Section of Costa Rica, International Workshop Electromagnetic Induction in the Erath, International Workshop on Electromagnetic Induction in the Earth, Beijing, 2008
Brasse, H., Kapinos, G., Mütschard, L., Alvarado, G. & Worzewski, T.
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(2009): Deep electrical resistivity structure of northwestern Costa Rica, Geophys. Res. Lett., 36, L02310
Brasse, H., Kapinos, G., Mütschard, L., Alvarado, G.E., Worzewski, T., and Jegen, M.
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Amphibious MT Experiment on Costa Rican Subduction Zone to investigate Deep Electrical Resistivity Structure, DGG- Tagung, Kiel, 2009
Worzewski, T., Jegen, M.D., Brasse, H., Taylor, W
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Investigating the Central American volcanic arc with long-period and broad-band magnetotellurics, IAGA- Meeting, Sopron, 2009
Brasse, H., Mütschard, L., Muñoz, A., Alvarado, G. & Worzewski, T.
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Magnetotelluric images of the Central American margin in Nicaragua and Costa Rica, AGU Fall Meeting, San Francisco, 2009
Mütschard, L., Brasse, H. & Worzewski, T.
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Magnetotelluric images of Costa Rica and Nicaragua, AGU Joint Assembly, Foz do Iguassu, 2010
Brasse, H., Worzewski, T.
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The Cycle of Hydration and Fluid Release in the Costa Rican Subduction Zone imaged through electromagnetic soundings: Where has all the water gone? AGU Fall Meeting, San Francisco, 2010
Worzewski, T., Jegen, M.D., Kopp, H., Brasse, H., Taylor, W.
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(2011): Electromagnetic Images of the South and Central American Subduction Zones, in: The Earth's Magnetic Interior (eds. E. Petrovsky, E. Herrero-Bervera, T. Harinarayana and D. Ivers), IAGA Special Sopron Book Series 1, 43-81, Springer, Berlin
Brasse, H.
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(2011): Magnetotelluric Image of the Fluid Cycle in the Costa Rican Subduction Zone, Nature Geosci.
Worzewski, T., M. Jegen, H. Kopp, H. Brasse, and W. Taylor
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(2015): Deepcrustal magma reservoirs beneath the Nicaraguan volcanic arc, revealed by 2-D and semi 3-D inversion of magnetotelluric data, Phys. Earth Planet. Inter., 248
Brasse, H., Schäfer, A., Díaz, D., Alvarado, G., Muñoz, A. & Mütschard, L.
