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

Seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches

Water transported by slabs into the mantle at subduction zones plays key roles in tectonics, magmatism, fluid and volatiles fluxes, and most likely in the chemical evolution of the Earth's oceans and mantle. Yet, incorporation of water into oceanic plates before subduction is a poorly understood process. Several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and/or slab pull, and display anomalously low seismic velocities. The low velocities are interpreted to arise from a combination of fluid-filled fractures associated to normal faulting and mineral transformation by hydration. Mantle hydration by transformation of nominally dry peridotite to water-rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle. The depth of fracturing by normal-fault earthquakes is usually not well constrained, but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle, restrained only by serpentine stability. Yet, interpretation of deep intraplate mineral alteration remains speculative because active-source seismic experiments have sampled only the uppermost few kilometers of mantle, leaving the depth-extent of anomalous velocities and their relation to faulting unconstrained. Here we use a joint inversion of active-source seismic data, and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor. We found that anomalous velocities are restrained to the depth of normal-fault micro-earthquake activity recorded in the network, and are considerably shallower than either the rupture depth of teleseismic, normal-fault earthquakes, or the limit of serpentine stability. Extensional micro-earthquakes indicate that each fault in the region slips every 2–3 months which may facilitate regular water percolation. Deeper, teleseismic earthquakes are comparatively infrequent, and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study. Our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration

Centroid depth and mechanism of trench-outer rise

Trench-outer rise earthquakes occur by reactivation or creation of normal faults caused as the oceanic lithosphere approaches a subduction zone and bends into the deep-sea trench. These faults may cut deep enough into the mantle to allow sea water to penetrate into the lithosphere, causing serpentinization. The amount of water carried into the mantle is linked to the maximum depth that the tensional faults cut into the lithosphere, which in turn is directly linked to the maximum focal depths of outer rise normal faulting earthquakes. We analysed teleseismic P and S waves of seven earthquakes from the trench-outer rise offshore of Central America using teleseismic waveform inversion of broad-band data. For the computation of Green's functions for waveform inversion, probabilistic earthquake locations were calculated. To study the rupture process, earthquake centroid depths and focal mechanisms for a sequence of subevents were calculated. Both, hypocentral depths from the relocation process and the estimated centroid depths from the waveform inversion show that all events occur at shallow depths (<30 km). Furthermore, the locations of the subevents relative to each other suggest that fault planes for Mw∼ 6 are in the order of 50 km in length and only 5–10 km in width. Rupture generally propagates downdip and the focal mechanisms change for most events from normal faulting to strike-slip or oblique thrusting with time. The depth at which this mechanism change is observed may represent the depth of the nodal plane between tensional and compressional regions in the incoming plate