Mid-ocean ridges spreading at ultraslow rates of <20 mm yr-1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2-5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s-1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P-to S-wave velocity (Vp/Vs) ratio. Here, we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow spreading Mid-Cayman Spreading Centre. High Vp/Vs ratios of >1.9 and continuously increasing P-wave velocity, changing from 4 km s-1 at the seafloor to >7.4 km s-1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough provides therefore a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle to ~25% of the seafloor. About 60% of the Earth's surface is oceanic crust and new seafloor is continually created along the ~65,000 km long mid-ocean ridge (MOR) system 1. Most oceanic crust has a relatively uniform

. (2017) 'Magmatic-tectonic conditions for hydrothermal venting on an ultraslow-spread oceanic core complex. ', Geology., 45 (9). pp. 839-842. Further information on publisher's website:https://doi.org/10.1130/G39045.1Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. CaySeis experiment, we determined the seismic velocities in the large massif beneath the 20 VDVF. We propose that this massif was produced by a pulse of on-axis magmatism ~2 21Mya, which was then followed by exhumation, cooling, and fracturing. A low seismic 22

Abstract. 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.

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[1] Geophysical investigations of the northern Hikurangi subduction zone northeast of New Zealand, image fore-arc and surrounding upper lithospheric structures. A seismic velocity (Vp) field is determined from seismic wide-angle data, and our structural interpretation is supported by multichannel seismic reflection stratigraphy and gravity and magnetic modeling. We found that the subducting Hikurangi Plateau carries about 2 km of sediments above a 2 km mixed layer of volcaniclastics, limestone, and chert. The upper plateau crust is characterized by Vp = 4.9-6.7 km/s overlying the lower crust with Vp > 7.1 km/s. Gravity modeling yields a plateau thickness around 10 km. The reactivated Raukumara fore-arc basin is >10 km deep, deposited on 5-10 km thick Australian crust. The fore-arc mantle of Vp > 8 km/s appears unaffected by subduction hydration processes. The East Cape Ridge fore-arc high is underlain by a 3.5 km deep strongly magnetic (3.3 A/m) high-velocity zone, interpreted as part of the onshore Matakaoa volcanic allochthon and/or uplifted Raukumara Basin basement of probable oceanic crustal origin. Beneath the trench slope, we interpret low-seismic-velocity, high-attenuation, low-density fore-arc material as accreted and recycled, suggesting that underplating and uplift destabilizes East Cape Ridge, triggering two-sided mass wasting. Mass balance calculations indicate that the proposed accreted and recycled material represents 25-100% of all incoming sediment, and any remainder could be accounted for through erosion of older accreted material into surrounding basins. We suggest that continental mass flux into the mantle at subduction zones may be significantly overestimated because crustal underplating beneath fore-arc highs have not properly been accounted for. Citation: Scherwath, M., et al. (2010), Fore-arc deformation and underplating at the northern Hikurangi margin, New Zealand,