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[1] In this work we investigate the crustal and tectonic structures of the Central Tyrrhenian back-arc basin combining refraction and wide-angle reflection seismic (WAS), gravity, and multichannel seismic (MCS) reflection data, acquired during the MEDOC (MEDiterráneo OCcidental)-2010 survey along a transect crossing the entire basin from Sardinia to Campania at 40°N. The results presented include a~450 km long 2-D P wave velocity model, obtained by the traveltime inversion of the WAS data, a coincident density model, and a MCS poststack time-migrated profile. We interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity-derived density models with existing compilations for continental crust, oceanic crust, and exhumed mantle. The first domain includes the continental crust of Sardinia and the conjugate Campania margin. In the Sardinia margin, extension has thinned the crust from~20 km under the coastline to~13 km~60 km seaward. Similarly, the Campania margin is also affected by strong extensional deformation. The second domain, under the Cornaglia Terrace and its conjugate Campania Terrace, appears to be oceanic in nature. However, it shows differences with respect to the reference Atlantic oceanic crust and agrees with that generated in back-arc oceanic settings. The velocities-depth relationships and lack of Moho reflections in seismic records of the third domain (i.e., the Magnaghi and Vavilov basins) support a basement fundamentally made of mantle rocks. The large seamounts of the third domain (e.g., Vavilov) are underlain by 10-20 km wide, relatively low-velocity anomalies interpreted as magmatic bodies locally intruding the mantle.

S U M M A R YNew marine geophysical data recorded across the Tonga-Kermadec subduction zone are used to image deformation and seismic velocity structures of the forearc and Pacific Plate where the Louisville Ridge seamount chain subducts. Due to the obliquity of the Louisville Ridge to the trench and the fast 128 mm yr −1 south-southwest migration of the ridge-trench collision zone, post-, current and pre-seamount subduction deformation can be investigated between 23• S and 28• S. We combine our interpretations from the collision zone with previous results from the post-and pre-collision zones to define the along-arc variation in deformation due to seamount subduction. In the pre-collision zone the lower-trench slope is steep, the mid-trench slope has ∼3-km-thick stratified sediments and gravitational collapse of the trench slope is associated with basal erosion by subducting horst and graben structures on the Pacific Plate. This collapse indicates that tectonic erosion is a normal process affecting this generally sediment starved subduction system. In the collision zone the trench-slope decreases compared to the north and south, and rotation of the forearc is manifest as a steep plate boundary fault and arcward dipping sediment in a 12-km-wide, ∼2-km-deep mid-slope basin. A ∼3 km step increase in depth of the middle and lower crustal isovelocity contours below the basin indicates the extent of crustal deformation on the trench slope. At the leading edge of the overriding plate, upper crustal P-wave velocities are ∼4.0 km s −1 and indicate the trench fill material is of seamount origin. Osbourn Seamount on the outer rise has extensional faulting on its western slope and mass wasting of the seamount provides the low V p material to the trench. In the post-collision zone to the north, the trench slope is smooth, the trench is deep, and the crystalline crust thins at the leading edge of the overriding plate where V p is low, ∼5.5 km s −1 . These characteristics are attributed to a greater degree of extensional collapse of the forearc in the wake of seamount subduction. The northern end of a seismic gap lies at the transition from the smooth lowertrench slope of the post-collision zone, to the block faulted and elevated lower-trench slope in the collision zone, suggesting a causative link between the collapse of the forearc and seismogenesis. Along the forearc, the transient effects of a north-to-south progression of ridge subduction are preserved in the geomorphology, whereas longer-term effects may be recorded in the ∼80 km offset in trench strike at the collision zone itself.

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15Mantle hydration (serpentinisation) at magma-poor rifted margins is thought to play a key role in 16 controlling the kinematics of low-angle faults and thus, hyperextension and crustal breakup. 17However, because geophysical data principally provide observations of the final structure of a margin, 18 little is known about the evolution of serpentinisation and how this governs tectonics during 19 hyperextension. Here we present new observational evidence on how crustal strain-dependent 20 serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of 21 the Porcupine Basin, offshore Ireland. We present three new P-wave seismic velocity models that 22show the seismic structure of the uppermost lithosphere and the geometry of the Moho across and 23 along the basin axis. We use neighbouring seismic reflection lines to our tomographic models to 24 estimate crustal stretching (βc) of ~2.5 in the north at 52.5 o N and > 10 in the south at 51.7 o N. These 25 values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting may 26 have progressed to crustal breakup in the southern part of the study area. We observed a decrease in 27 mantle velocities across the basin axis from east to west. These variations occur in a region where βc 28 is within the range at which crustal embrittlement and serpentinisation are possible (βc 3-4). Across 29 the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount 30 of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that 31 2 the degree of serpentinisation, together with the amount of crustal faulting, increases southwards 32 along the basin axis. Seismic reflection lines show a major detachment fault surface that grows 33 southwards along the basin axis and is only visible where the inferred degree of serpentinisation is > 34 15 %. This observation is consistent with laboratory measurements that show that at this degree of 35 serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. Based on 36 these results, we propose two alternative formation models for the Porcupine Basin. The first involves 37 a northward propagation of the hyperextension processes, while the second model suggests higher 38 extension rates in the centre of the basin than in the north. Both scenarios postulate that the amount 39 of crustal strain determines the extent and degree of serpentinisation, which eventually controls the 40 development of detachments faults with advanced stretching. 41 42 3