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International audience; Geological complexity, progressive tectonic overprint and prevailing submarine location of subduction margins generally hinder investigating the early stages of development of their oldest trench-slope basins, witnesses of subduction initiation and early history. Along the eastern North Island of New Zealand, the inner portion of the Hikurangi subduction wedge is emerged, thereby offering a unique opportunity to examine, at outcrop-scale, the tectonostratigraphic evolution of a trench-slope basin (Castlepoint trench-slope basin) that was linked to the onset of subduction. We present new occurrences of lowermost Miocene gravity-driven systems, suggesting a more intricate depositional framework to the ones previously inferred during this key period. Results show that the early stages of development of a trench-slope basin coeval with the birth of a subduction margin may deviate from traditional models either comprising (1) sustainable sediment sources connected to the basin very early in the history of the margin and (2) a sedimentation-deformation feedback mechanism promoting the long-term development of an aggradational turbidite system downslope. Analysis of the mass-transport deposits (MTDs) also revealed that the outboard migration of deformation was discontinuous and uneven along the margin during the earliest Miocene. Two major tectonic events, separated by a period of reduced tectonic activity, were recorded at the inboard border of the Castlepoint trench-slope basin, each resulting in seaward motion, oversteepening and frontal denudation of a thrust sheet. Each thrust sheet provided contrasting failed material and morphometric characteristics to the associated deposits, thereby allowing us to discriminate the nature of the nappe and related controls responsible for shedding each MTD as well as refine the timing of nappe emplacement along this part of the margin. Overall, this study draws new insights on the early structural evolution and stratigraphic infill during the birth of subduction zones, insights which may, in turn, help improve understandings of active margin settings.

Northern France presents mostly a low level of seismic hazard according to the French national seismic hazard map. Despite low instrumental seismicity rates, strong and unfrequent historical earthquakes occurred, with for instance the M~6, 1580 Strait of Dover earthquake, or the M~5, 1896 Lens-Arras earthquake, whose seismogenic sources are presumably the Sangatte and the Marqueffles Faults, respectively. Both belong to the NW-SE-directed Weald-Boulonnais-Artois structure. Moreover, the Haubourdin Fault (also named Lille-Hazebrouck Fault), at the hinge of the Mélantois anticline, and bordering the southern edge of the Lille Metropolis (1.2 millions inhabitants), is considered as potentially active during Quaternary times. All these above-mentioned faults are linked to deep Paleozoic structures in the basement that formed along the northern front of the Variscan orogeny, and that were regularly reactivated during the Mesozoic and Cenozoic. To investigate and document the possible neotectonic activity of the Artois structure and the Haubourdin Fault, and therefore improve seismic hazard assessment in northern France, we used a pluridisciplinary approach based on the analysis of 1) LiDAR dataset, 2) paleoseismological trenching, 3) OSL/14C dating, and 4) sub-surface geophysical survey.Along the Artois structure, we focused on the locality of Harnes within Lens city suburbs. A preventive archeological work unraveled a clear sub-surface deformation feature that we analyzed through several ~2m-deep trenches. Field investigations indicated that the fault presents a regular N130° strike, which is consistent with surface and subsurface regional structures, and a 25-30° southwestward dip. Reverse throw along the fault were measured to about 15-20 cm thanks to a clearly displaced coal-rich horizon, sampled for C-14 dating. Interpretation of the data is complex since the site is located in a region where glacio-tectonic processes, severe First World War bombing and subsidence due to underground mining are documented.Along the Haubourdin Fault, our analysis of high resolution LiDAR data highlighted two topographic scarps aligned along a N110°E trend, but that do not match with the fault trace extracted from the geological map. This new fault trace is confirmed by subsurface geophysical survey (electric resistivity tomography and mapping). Both scarps present contrasted uplifted blocks since the southern block is uplifted (by several meters) for the western branch, whereas the northern block is uplifted (by 1-2 m) for the eastern branch. All these new field mapping results call for a substantial revision of the fault trace in the region together with a consideration of its segmentation. Finally, two new OSL datings has been obtained from a sandy-clay layer unconformably sealing some Late Paleocene deformation in the southern Lille suburb (i.e., Villeneuve d’Ascq). It gives the first minimum age in the literature for the deformation of the Mélantois northern limb.

The North Anatolian Fault (NAF) is one of the major active structures in the Eastern Mediterranean. Its right-lateral strike-slip fault initiated in eastern Turkey 13 Ma ago. The NAF westward propagation during Neogene and Quaternary times delineates the plate boundary between Eurasia and Anatolia-Aegean. The western termination of NAF is currently located in the North Aegean Trough (NAT) where NAF displays a NE-SW direction. In the NAT, the NAF termination is located near to the Sporades Islands. In the western prolongation of this termination, there is a wide domain characterised by distributed deformation. This major extensional area is mainly constituted by the Corinth rift and the North Evia domain, our study area. The whole zone experience a relative high seismicity with strike-slip focal mechanisms, especially right-lateral displacements along NE-SW-striking faults, which are mainly located between the North Evia domain and the Southern Thessaly. Our study is mostly based on new very-high-resolution seismic reflection profiles (Sparker) acquired during the WATER surveys (Western Aegean Tectonic Evolution and Reactivations) in July-August 2017 and 2021, onboard the R/V “Téthys II”. We also analysed several seismicity catalogues in order to connect the recent structures from seismic lines to active tectonics over the region. The interpretations from these datasets emphasize the evolution of the deformation of the North Evia domain, in particular, along the NE-SW striking lineament “Lichades Area – Oreoi Channel – Skiathos Basin” (L-O-S). The deformation in the Lichades Area is dominated by numerous active normal faults striking W-E or WNW-ESE and showing metric-scale offsets (up to 5 m.) within the Holocene sequence. One of the largest sub-active to active fault is striking NE-SW, parallel to the Oreoi Channel, and thus strongly oblique to the main rift deformation. The Oreoi Channel is a marine straight linking the Lichades Area and the Skiathos Basin. The seismic profiles highlight normal faults of different ages with a NE-SW direction. In the south-east, the Oreoi Channel is delineated by the Oreoi Fault, a mainly onshore normal fault which is dipping towards north-west. The Skiathos Basin is a newly discovered structure from our seismic dataset that is separated from the Skopelos Basin by a NE-SW striking acoustic ridge. The Skiathos Basin presents two main depocenters individualized by areas of rising acoustic basement. Some normal faults, oriented NE-SW and W-E, have been identified in the basin. Finally, many earthquakes focal mechanisms located in the Skiathos Basin and the Oreoi Channel indicate strike-slip faulting, with a right-lateral motion along the NE-SW direction. This detailed structural analysis together with the synthesis of seismic activity allow to propose a tectonic map with new insights on the recent deformation of the key-area “L-O-S” in the south-western prolongation of NAF. The Skiathos basin development shows indications of transtensive deformation. The Oreoi Channel is controlled by NE-SW-striking faults with a right-lateral component and the Lichades Area displays several fault with oblique direction and pure extension. We propose that the L-O-S tectonic system prolongs the NAF system and may progressively evolve as the future plate boundary.

Deformation process and strength evolution in accretionary wedges are important factors that affect kinematics and dynamics at subduction zones. However, it is still challenging to understand the relationship between the short-term geodetic observations (coupling ratios or earthquakes) and the long-term geological structures (patterns of imbricated thrust sheets or fault networks) in natural systems. In this study, we performed 2-D, large-shortening (1 m) analog sandbox experiments to examine how the wedge deformation is affected by different décollement conditions including heterogeneities and numbers of weak layers in the incoming sediment. Four different settings (Types 1–4) of the incoming sediment layers were examined in this study. Serial side-view digital photographs were quantitatively analyzed with an open-source DIC software to characterize the accretion cycles, underthrusting/underplating, and reactivation of pre-existing thrusts (out-of-sequence thrusts).The reference models with single décollement (Type1) were dominated by periodic cycles of frontal accretion with landward propagation of strain, uplift, and reactivation of the pre-existing thrusts, which progressively increased in strength and then approached the critical state. Each cycle was composed of preparation (Phase 0), initiation (Phase 1), accretion (Phase 2), and reactivation (Phase 3). Through frontal accretion, the wedge accumulated the strain internally with landward migration of the basal coupled area along the plate interface, which caused uplift and reactivation of the landward preexisting thrusts in the wedge (hardening). When a new frontal thrust emerged at the deformation front (Phase 1), the basal coupling was suddenly lost (softening). Through this cycle, the entire accretionary wedge progressively increased in strength while experiencing hardening and softening and approached the critical state. The double décollement models (Type 2) showed a similar accretion cycle to Type 1 models, but it consisted of a combination of shallow-rooted and deep-rooted frontal thrusts, meaning that the décollement stepped up and down between the interbedded and basal weak layers. This promoted sediment underthrusting at the frontal part of the wedge during the early phase of the accretion cycle and favored the connection of pre-existing deep-rooted thrusts with shallow-rooted thrusts. A frictional interruption in the basal décollement (Type 3 or 4 models) produced a combination of a steep-taper inner wedge and a gentle-taper outer wedge, and disturbed the wavelengths of the accretion cycle. The single décollement models (Type 3) were dominated by high-angle out-of-sequence thrusts, while underplating was significantly promoted in the double décollement model (Type 4) where the interbedded décollement acted as a low-angle, smooth-surface megathrust.These results shed light on the impact of properties and homogeneity of the incoming sediment and the plate interface on the spatial and temporal evolution of internal structure and thrust activity in accretionary wedges through multiple accretion cycles. Comparisons of our results with natural subduction zones will contribute to understanding the mechanisms and dynamics of deformation process and strength evolution in natural subduction zones.

The Eastern Sardinian margin consists in a hyper-extended rifted margin, located in the western Tyrrhenian Sea, a recent back-arc basin (late Neogene). This area was affected by strong aerial erosion during the Messinian Salinity Crisis (MSC, 5.96 – 5.33 My) associated with the drop of sea level (> 1500 m) which occurred throughout the whole Mediterranean. The Gulf of Orosei and surrounding offshore areas are characterized by small and diffuse drainage systems input, where the Messinian Erosion Surface (MES) has rarely been studied while it has been in large fluvial systems (Rhone, Ebro). The MES was already found in the Cendrino valley (which flows in the Gulf) and in the East-Sardinia Basin but the link between onshore and offshore was never been studied in the area. The “METYSS 4” cruise (June 2019, R/V “Téthys II”) allowed acquiring more than 2000 km of very high-resolution (VHR) seismic data (Sparker), following a dense grid (1.5 km average profile spacing), on the Eastern Sardinian continental shelf, and especially in the Orosei Gulf area, that has been little explored until now. While the main limitation on seismic data (air-gun) interpretation is often due to the occurrence of sea bottom multiple, the limitation for Sparker data may also be due to very short shot intervals at greater water depth. The seismic trace ends where there still is signal of interest. Thus, we applied a simple method to increase investigation depth for short shot intervals (0.333 - 0.533 ms), which allowed interpretation on the continental slope. This approach consists in copying the raw data and concatenating the copied data under the raw data with a shift of 1 shot point. To constrain the MES depth on the continental slope and shelf we compared air-gun seismic data from previous METYSS surveys, where the MES has already been interpreted by a strong erosional discordance between Plio-Quaternary deposits and pre-MSC units, with the new VHR data. The restoration of the morphological features of the Orosei canyon at Messinian times shows that the former Messinian canyon network is very similar to the present-day one. The present-day canyon and its tributaries show sub-marine erosion in the talwegs. The heads of the canyons present gravitational features, highlighted by chaotic deposits near the talweg of the canyon or in-between the Plio-Quaternary strata. Offshore Arbatax, south Orosei, the seismic profiles show no significant Plio-Quaternary deposits (thickness < 0.1 sTWT), which allows polygenic pre-MSC units to occur at the seafloor. In the Gulf, we observe thick deposits (0.4 - 0.5 sTWT) on the right bank of Orosei Canyon, making it more complicated to image the MES in this area. The sedimentation rate on this margin is very low (c.a. 9 cm/ka in the Gulf of Orosei), which is consistent with previous studies on the East-Sardinian basin (3-20 cm/ka). These preliminary results will allow correlating for the first time the MES distribution from the onshore to the offshore continental slope of the Eastern Sardinian Margin in order to improve the MSC understanding in this key area.

Taphonomic analysis is a useful tool to assess the intensity of alteration of skeletal remains and to help characterize depositional conditions as well as completeness and resolution of fossil assemblages. We herein introduce TAPHOGRAPH, an Excel spreadsheet script (a R code is also available), for the production of taphonomic diagrams to characterize the taphonomy of skeletal remains. The graphical representation depicts four taphonomic factors (fragmentation, abrasion, bioerosion, and encrustation) as a cumulative curve that allows visualization and comparison of the degree and variability of taphonomic alteration for different hard part types from one or more samples in a single diagram. The TAPHOGRAPH methodology is highly flexible, and can be used to assess the relative influence of mechanical versus biological (versus chemical) taphonomic alteration. The TAPHOGRAPH approach can guide inferences about hydraulic regimes, residence time at the seafloor, and intensity of different taphonomic processes.