Description: The subduction of partially serpentinized oceanic mantle may potentially be the key geologic process leading to the regassing of Earth’s mantle and also has important consequences for subduction zone processes such as element cycling, slab deformation, and intermediate-depth seismicity. Little is known about the quantity of water that is retained in the slab during mantle serpentinization. Recent studies using thermodynamical and/or experimental models of subduction zone processes have assumed that the mantle is uniformly serpentinized to a depth determined from the equilibrium stability of serpentine minerals in P-T space. This approach yields an incomplete picture of the pattern of serpentinization that may occur during bending-related faulting; an initial state that is essential for quantifying subsequent dehydration processes. In order to provide further constraints on the pattern of hydration and the amount of water trapped in the subducting mantle, we build a 2-D reactive-flow model incorporating the kinetic rate-dependence of serpentinization based on experimental results. After simulating hydration processes at the trench outer-rise, we find that the water content in serpentinized mantle strongly depends on the age of the subducting lithosphere and subduction rate, with values ranging between 1.8x105 and 4.0x106 kgm-2 reactive water uptake into the subducting mantle column. Serpentinization also results in a reduction in surface heat flux towards the trench caused by advective downflow of seawater into the reaction region. Observed heat flow reductions are larger than the reduction due to the minimum-water downflow needed for partial serpentinization, predicting that active hydrothermal vents and chemosynthetic communities should also be associated with bend-fault serpentinization. Model results agree with previous studies that the lower plane of double Benioff zones can be generated due to dehydration of serpentinized mantle at depth. The depth-dependent pattern of serpentinization including reaction kinetics predicts a separation between the two Benioff planes consistent with seismic observations.

Description: The understanding of the Earth’s water cycle is inherently linked to the subduction of water at deep sea trenches. The transfer of water into the deep Earth’s interior is related to the alteration and hydration of the incoming lithosphere. The release of water from subducting lithospheres affects the composition of the mantle wedge, enhances partial melting and triggers intermediate-depth earthquakes. Water is transferred with the incoming plate into the subduction zone as water trapped in sediments and open void spaces in the igneous crust and as chemically bound water in hydrous minerals in sediments and oceanic crust (Jarrad, 2003). However, if water reaches upper mantle rocks, significant amounts can be transferred into the deep subduction zone as water-bearing mineral serpentine (Peacock, 2004). Serpentinites have nearly the same chemical composition as mantle peridotite except that they contain approximately 13 wt% water in mineral structures. Seismic refraction and wide-angle data were collected at a number of active continental margins in the trench-outer rise to investigate the impact of bending related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction. Surveys provided data from offshore of Nicaragua (Grevemeyer et al., 2007; Ivandic et al., 2008), Chile (Contreras-Reyes et al., 2008), and Tonga (Contreras-Reyes et al., 2011). At all settings tomographic joint inversion of seismic refraction and wide-angle reflection data yielded anomalously low seismic P-wave velocities in the crust and uppermost mantle seaward of the trench axis. Crustal velocities are reduced by 0.2-0.8 km/s compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.4-7.8 km/s and hence 5-12% lower than the typical velocity of mantle peridotite. These systematic changes in P-wave velocity from the outer rise towards the trench axis indicate an evolutionary process in the subducting slab consistent with percolation of seawater through the faulted and fractured lithosphere and serpentinization of mantle peridotites. The observed velocity reduction suggests that mantle serpentinization reaches 12-25%. Thus, processes occurring in the trench-outer rise affect indeed the Earth’s water cycle and indicate that significant amount of waters are transferred into the subducting lithosphere and hence carried to the deep Earth interior. References Contreras-Reyes, E., Grevemeyer, I., Flueh, E.R., and Reichert, C. (2008), Upper lithospheric structure of the subduction zone offshore of southern Arauco peninsula, Chile, at 38°S, J. Geophys. Res., 113, B07303, doi:10.1029/2007JB005569. Contreras-Reyes, E., Grevemeyer, I., Watts, A.B., Flueh, E.R., Peirce, C., Moeller, S., and Papenberg, C., 2011. Deep seismic structure of the Tonga subduction zone: Implications for mantle hydration, tectonic erosion, and arc magmatism, J. Geophys. Res., 116, doi:10.1029/2011JB008434. Grevemeyer, I., Ranero, C.R., Flueh, E., Kläschen, D., Bialas, J. (2007). Passive and active seismological study of bendingrelated faulting and mantle serpentinization at the Middle America trench. Earth Planet. Sci. Lett. 258, 528-542. Ivandic, M., Grevemeyer, I., Berhorst, A., Flueh, E.R. and McIntosh, K. (2008), Impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of Nicaragua, J. Geophys. Res., 113, B05410, doi:10.1029/2007JB005291. Jarrad, R.D. (2003). Subduction fluxes of water, carbon dioxid, chlorine, and potassium. Geochemistry, Geophysics, Geosystems 4: doi: 10.1029/2002GC000392. Peacock, S.M. (2004). Insight into the hydrogeology and alteration of oceanic lithosphere based on subduction zones and arc volcanisms. In: Davis E.E, Elderfield, H. (Eds.), Hydrogeology of Oceanic Lithosphere. Cambridge University Press, pp. 659-676.

Description: The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the Western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines–Calabrian subduction zone. Late-Oligocene-to-Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet it remains unclear if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a 130 km long seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, a Priority Programme of the German Research Foundation (DFG) and the German component of the European AlpArray initiative, and trends in a NE–SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data were recorded on the newly developed GEOLOG recorder, designed at GEOMAR, and are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Mohorovičić discontinuity (Moho) reflection. The main features share several characteristics (e.g. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are complemented by gravity data and yield a ∼ 6–8 km thick sedimentary cover and the seismic Moho at 11–13 km depth below the sea surface. Our study reveals that the oceanic domain does not extend as far north as previously assumed. Whether Oligocene–Miocene extension led to extremely thinned continental crust or exhumed subcontinental mantle remains unclear. A low grade of mantle serpentinisation indicates a high rate of syn-rift sedimentation. However, rifting failed before oceanic spreading was initiated, and continental crust thickens towards the NE within the northern Ligurian Basin.

Description: The Liguro-Provençal basin was formed as a back-arc basin of the retreating Calabrian-Apennines subduction zone during the Oligocene and Miocene. The resulting rotation of the Corsica-Sardinia block is associated with rifting, shaping the Ligurian Sea. It is still debated whether oceanic or atypical oceanic crust was formed or if the crust is continental and experienced extreme thinning during the opening of the basin. We invert velocity models using an amphibious network of seismic stations, including 22 broadband Ocean Bottom Seismometers (OBS) to investigate the lithospheric structure of the Ligurian sea. The instruments were installed in the Ligurian Sea for eight months between June 2017 and February 2018 as part of the AlpArray seismic network. Because of additional noise sources in the ocean, OBS data are rarely used for ambient noise studies. However, we attentively pre-process the data, including corrections for instrument tilt and seafloor compliance. We took extra care to exclude higher modes of the ambient-noise Rayleigh waves. We calculate daily cross-correlation functions for the LOBSTER array and surrounding land stations. Additionally, we correlate short time windows that include teleseismic earthquakes that allow us to derive surface wave group velocities for longer periods than using ambient noise only. Group velocity maps are obtained by inverting Green’s functions derived from the cross-correlation of ambient noise and teleseismic events, respectively. We then used the resulting 3D group velocity information to calculate 1D depth inversions for S-wave velocities. The shear-wave velocity results show a deepening of the Moho from 12 km at the southwestern basin centre to 20–25 km at the Ligurian coast in the northeast and over 30 km at the Provençal coast. We find no hint on mantle serpentinisation and no evidence for an Alpine slab, at least down to depths of 25 km. However, we see a separation of the southwestern and northeastern Ligurian Basin that coincides with the promoted prolongation of the Alpine front.

Description: The northern margin of the Ligurian Basin shows notable seismicity at the Alpine front, including frequent magnitude 4 events. Seismicity decreases offshore towards the Basin centre and Corsica, revealing a diffuse distribution of low magnitude earthquakes. We analyse data of the amphibious AlpArray seismic network with focus on the offshore component, the AlpArray OBS network, consisting of 24 broadband ocean bottom seismometers deployed for eight months, to reveal the seismicity and depth distribution of micro-earthquakes beneath the Ligurian Sea. Two clusters occurred between ~10 km to ~16 km depth below sea surface, within the lower crust and uppermost mantle. Thrust faulting focal mechanisms indicate compression and an inversion of the Ligurian Basin, which is an abandoned Oligocene rift basin. The Basin inversion is suggested to be related to the Africa-Europe plate convergence. The locations and focal mechanisms of seismicity suggest reactivation of pre-existing rift structures. Slightly different striking directions of faults in the basin centre compared to faults further east and hence away from the abandoned rift may mimic the counter-clockwise rotation of the Corsica-Sardinia block during ~20–16 Ma. The observed cluster events support the hypothesis of strengthening of crust and uppermost mantle during rifting related extension and thinning of continental crust.