Bending-related faulting and mantle serpentinization at the Nicaraguan subduction zone

At a convergent margin large amounts of structurally bound water are carried into the Earth’s interior and - as the subducting plate descends and the temperature rises - are driven off to some extent into the mantle wedge, where they are thought to trigger intermediate-depth earthquakes in the Wadati-Benioff zone and melting under volcanic arcs. However, a largely uncertain fraction outlasts sub-arc fluid release and hence enters the deeper mantle, which leads to a connection between the oceans and the Earth’s deep water cycle. Thus, a detailed knowledge of the water budget of a subduction zone is not only important to understand arc volcanism, but as well to comprehend the chemical development of the Earth’s mantle. For this purpose, profound information about the amount of water that is subducted along with the oceanic plate is indispensable. The present thesis uses geophysical methods to determine the degree of hydration of the Cocos Plate offshore Nicaragua, which is subducted beneath the Caribbean Plate. In general it was assumed that structured water is transported into the slab in sediments and the upper crust only, though in recent years growing evidence suggested that lower crust and upper mantle might contain capacious amounts of fluids as well, since the bending of the incoming oceanic plate leads to a reactivation or creation of normal faults (bend-faults), which are visible in batrymetric data and have been inferred to cut deep enough into the plate to provide a pathway for seawater to penetrate into the lithosphere, changing ”dry” peridotites to ”wet” serpentinites, which contain up to 13% of water. Such a mechanism could transport much more fluids into the earth’s interior than any other considered possibility. However, the cutting depth of these bend-faults and hence the depth that seawater could penetrate into the mantle was not well-defined, for one reason since focal depth of earthquakes associated with the bend-faults were poorly known. Yet previous studies assumed cutting depths such that serpentinization is firstly restricted by its thermal limit of 600± C. This study uses openly accessible, global broadband data of earthquakes offshore Central America as well as an unique dataset from a local long-period earthquake monitoring network offshore Nicaragua, to determine typical focal depths off earthquakes at the trench-outer rise and further relates these focal depths to the cutting depths of bend-faults. In addition, a full 3d-tomographic inversion that consistently integrates seismic airgun blasts and local as well as regional seismicity, could show reduced seismic mantle velocities at the outer rise and nearby the deep sea trench with an evolutionary trend towards it. Best explained is this by a fractured and ii partly serpentinized lithosphere. The use of regional sources (i.e. earthquakes in distances of ¸200 km from the seismic network) in the tomographic inversion process made it possible, for the first time, to reflect the entire brittle lithosphere. In a second approach, relative arrival times of large earthquakes that occurred during the deployment of the seismic network were investigated. Again, it could be shown that seismic mantle velocities decrease in accordance with the onset of bend-faults in the bathymetry. But not only seismic velocities decrease nearby the trench, the average moment magnitude of outer rise earthquakes does as well, though the number of events increases significantly. We explain this a weakened lithosphere and hence a reduced yield strain, which again suggests an occurrence of serpentinite. However, tomographic images suggest that the area of reduced seismic velocities and in turn possible serpentinization does not reach the cutting depth of bend-faults nor the depth of the 600± C isotherm. Focal mechanisms of several earthquakes were determined via moment tensor inversion and forward modelling respectively and it could be shown that where seismic velocities are reduced only tensional ruptures occur, which allow for water infiltration, meanwhile the area beneath is dominated by compressional rupture behaviour, which presents a barrier for seawater. This result does not only confirm and enlarge flexure models of subducting plates [Chapple and Forsyth, 1979; Christensen and Ruff, 1988], but also establishes a coherent connection between stress distribution in the incoming plate and penetration depth of seawater and is the first study in this vein.