[1] We analyzed the structure and evolution of the external Calabrian Arc (CA) subduction complex through an integrated geophysical approach involving multichannel and single-channel seismic data at different scales. Pre-stack depth migrated crustal-scale seismic profiles have been used to reconstruct the overall geometry of the subduction complex, i.e., depth of the basal detachment, geometry and structural style of different tectonic domains, and location and geometry of major faults. High-resolution multichannel seismic (MCS) and sub-bottom CHIRP profiles acquired in key areas during a recent cruise, as well as multibeam data, integrate deep data and constrain the fine structure of the accretionary wedge as well as the activity of individual fault strands. We identified four main morpho-structural domains in the subduction complex: 1) the post-Messinian accretionary wedge; 2) a slope terrace; 3) the pre-Messinian accretionary wedge and 4) the inner plateau. Variation of structural style and seafloor morphology in these domains are related to different tectonic processes, such as frontal accretion, out-of-sequence thrusting, underplating and complex faulting. The CA subduction complex is segmented longitudinally into two different lobes characterized by different structural style, deformation rates and basal detachment depths. They are delimited by a NW/SE deformation zone that accommodates differential movements of the Calabrian and the Peloritan portions of CA and represent a recent phase of plate re-organization in the central Mediterranean. Although shallow thrust-type seismicity along the CA is lacking, we identified active deformation of the shallowest sedimentary units at the wedge front and in the inner portions of the subduction complex. This implies that subduction could be active but aseismic or with a locked fault plane. On the other hand, if underthrusting of the African plate has stopped recently, active shortening may be accommodated through more distributed deformation. Our findings have consequences on seismic hazard, since we identified tectonic structures likely to have caused large earthquakes in the past and to be the source regions for future events.

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Water and carbon are transferred from the ocean to the mantle in a process that 22 alters mantle peridotite to create serpentinite and supports diverse 23 ecosystems 1 . Serpentinised mantle rocks are found beneath the seafloor at slow-to 24 ultraslow-spreading mid-ocean ridges 1 and are thought to be present at about half 25 the world's rifted margins 2,3 . Serpentinite is also inferred to exist in the downgoing 26

[1] Active gas venting occurs on the uppermost continental slope off west Svalbard, close to and upslope from the present-day intersection of the base of methane hydrate stability (BMHS) with the seabed in about 400 m water depth in the inter-fan region between the Kongsfjorden and Isfjorden cross-shelf troughs. From an integrated analysis of high-resolution, two-dimensional, pre-stack migrated seismic reflection profiles and multibeam bathymetric data, we map out a bottom simulating reflector (BSR) in the inter-fan region and analyze the subsurface gas migration and accumulation. Gas seeps mostly occur in the zone from which the BMHS at the seabed has retreated over the recent past as a consequence of a bottom water temperature rise of 1 C. The overall margin-parallel alignment of the gas seeps is not related to fault-controlled gas migration, as seismic evidence of faults is absent. There is no evidence for a BSR close to the gas flare region in the upper slope but numerous gas pockets exist directly below the predicted BMHS. While the contour following trend of the gas seeps could be a consequence of retreat of the landward limit of the BMHS and gas hydrate dissociation, the scattered distribution of seeps within the probable hydrate dissociation corridor and the occurrence of a cluster of seeps outside the predicted BMHS limit and near the shelf break indicate the role of lithological heterogeneity in focusing gas migration.Citation: Sarkar, S., C. Berndt, T. A. Minshull, G. K. Westbrook, D. Klaeschen, D. G. Masson, A. Chabert, and K. E. Thatcher (2012), Seismic evidence for shallow gas-escape features associated with a retreating gas hydrate zone offshore west Svalbard,